WO2012067999A2

WO2012067999A2 - Pathogen-derived effectors and their use in enhancing plant resistance to pathogens, insect pests and freezing stress

Info

Publication number: WO2012067999A2
Application number: PCT/US2011/060567
Authority: WO
Inventors: Nickolas J. Panopoulos; Antigoni Stylianou Papanikolaou; Panagiotis Sarris; Nickolas Skandalis
Original assignee: Panopoulos Nickolas J; Antigoni Stylianou Papanikolaou; Panagiotis Sarris; Nickolas Skandalis
Priority date: 2010-11-15
Filing date: 2011-11-14
Publication date: 2012-05-24
Also published as: WO2012067999A3

Abstract

Method for producing transgenic plants that are resistant to at least one pathogen and one insect pest are provided. Plants expressing a pathogen-derived effector that are resistant to at least one pathogen, at least one insect pest, at freezing stress are provided. Genes and gene sequences comprising pathogen-derived effectors chosen as to not trigger the hypersensitive response and to be capable of enhancing gene silencing on a given plant are also provided, as well as plants, plant parts, plant cells, seeds, and non-human host cells expressing such genes.

Description

PATHOGEN-DERIVED EFFECTORS AND THEIR USE IN ENHANCING PLANT RESISTANCE TO PATHOGENS, INSECT PESTS AND FREEZING STRESS

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated bv reference in its entirety. Said ASCII copy, created on December 21. 201 1. is named 09887514.txt and is 66.663 bvtes in size.

FEELD OF THE INVENTION

The present invention relates to plant biotechnology and the production of transgenic plants. More specifically, this invention relates to methods of enhancing tolerance/resistance to insect, disease and freezing stress in plants of industrial interest by transforming a plant cell with a polynucleotide encoding HopX l protein or an active fragment or homologue thereof and regenerating the transformed cell into a plant.

BACKGROUND

W itefiies are among the most invasive and economically damaging insects to plant agriculture, spanning food and fiber crops, fruit trees, and nursery grown ornamentals and energy crops, with one of the species, Bemissia tabaci, being able to infest more than 500 plant species. Whitefiies transmit a large number of viruses (Geminiviruses) to many plants and have become a major constraint to agricultural development in tropical and subtropical regions of the world.

The predominant methods of control for whitefiies and the virus diseases they transmit have been based mainly on limitation of the insect/vector populations through the application of insecticides, and by using various types of physical barriers to reduce the spread of whitefly- transmitted viruses. However, under conditions of high insect pressure, none of these measures suffice to prevent virus spread (Lapidot 2002). Other methods include biological control using entomopathogenic fungi and conventional breeding of resistant crops, but the success of these methods is very limited.

Transgenic plants carrying gene sequences of endogenous or heterologous origin or even synthetic sequences are often practiced by plant molecular biologists (Aragao and Faria, 2009; Akad et al., 2007; Edelbaum et al., 2009; Czosnek, 2010; Yang et al., 2004). Examples of insect resistant transgenic plants produced by such methods include U.S. Pat. Nos. 5,767,372 and 5,500,365. Examples of virus resistant transgenic plants produced by such methods include U.S. Pat. App. No. 201001 15665. However, none of them provide effective resistance against whitefly infestation.

SUMMARY

It is an object of the present invention to provide transgenic plants exhibiting enhanced tolerance to freezing, at least one pest insect specie/biotype, or at least one disease compared to non-transformed plants of the same species.

It is another object of the present invention to provide plant seeds capable of growing plants having an enhanced tolerance to freezing, at least one pest insect specie/biotype, or at least one disease.

It is another object of the present invention to provide tissue cultures capable of regenerating plants having an enhanced tolerance to freezing, at least one pest insect

specie/biotype, or at least one disease. It is further an object of the present invention to provides methods of producing transgenic plants having an enhanced tolerance to freezing, at least one pest insect

specie/biotype, or at least one disease.

To accomplish these and other objectives, the present invention provides transgenic plants comprising at least one cell transformed with a polynucleotide encoding a HopXl protein, an active fragment or homologue thereof. In one embodiment, the polynucleotide encodes a HopXl protein of the phytopathogenic bacterium Pseudomonas syringae pv. tomato DC3001. In one embodiment, the HopXl protein has an amino acid sequence as set forth is SEQ ED NO: 1. In one embodiment, the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO: l . In one embodiment, the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:2. In one embodiment, the

polynucleotide comprises a nucleic acid sequence having at least 70%» homology to SEQ ID NO:3.

In one embodiment, the HopXl protein, its active fragment or homologue is capable of enhancing RNA-mediated gene silencing in the transgenic plants. In a one embodiment, the HopXl protein, its active fragment or homologue is expressed in the transgenic plant in a constitutive manner. In a further embodiment, the HopXl protein, its active fragment or homologue is expressed in leaves.

In one embodiment, as compared to a non-transgenic plant, the transgenic plant has an enhanced tolerance to at least one insect specie/biotype chosen from the Whiteflies family, while the insect specie/biotype is capable of transmitting at least one plant pathogenic virus. In one embodiment, the transgenic plant has an enhanced tolerance to at least one disease as compared to a non-transgenic plant, wherein a plant pathogenic agent of the disease is Agrobacterium tumefaciens. In one embodiment, the transgenic plant has an enhanced tolerance to freezing as compared to a non-transgenic plant.

In one embodiment, the transgenic plant is selected from the group consisting of tomato, tobacco, cucumber, prune, potato, bean, barley, soybean, pea, beet, grapevine, petunia, abutilon, melon, watermelon, okra, cotton, cassava, wheat, maize, rice, cotton, citrus and cabbage. In one embodiment, the transgenic plant is selected form a group consisting of ornamental,

horticultural, or field crop species.

Another embodiment of the present invention provides a plant seed produced by the transgenic plant, while a plant grown from said seed has an enhanced tolerance to at least one insect specie/biotype as compared to a non-transgenic plant. In one embodiment, a plant grown from the seed has an enhanced tolerance to at least one disease as compared to a non-transgenic plant. In one embodiment, a plant grown from said seed has an enhanced tolerance to freezing as compared to non-transgenic plant.

Another embodiment of the present invention provides a tissue culture comprising at least one cell transformed with a polynucleotide encoding a HopX l protein, its active fragment or homologue, or a protoplast derived therefrom, while the tissue culture regenerates plants having an enhanced tolerance to at least one insect specie/biotype as compared to a non-transgenic plant. In one embodiment, the tissue culture regenerates plants having an enhanced tolerance to at least one disease as compared to a non-transgenic plant. In one embodiment, the tissue culture regenerates plants having an enhanced tolerance to freezing as compared to non-transgenic plant. A further embodiment provides the plant regenerated from the above tissue culture.

Another embodiment of the present invention provides a method of producing a transgenic plant having an enhanced tolerance to freezing, at least one insect specie/biotype, or at least one disease, comprising (a) transforming a plant cell with a polynucleotide encoding HopXl protein or an active fragment or homologue thereof; and (b) regenerating the transformed cell into a plant, wherein the regenerated plant has an enhanced tolerance to at least one insect, at least one disease, or freezing as compared to a corresponding non-transgenic plant.

In one embodiment, the polynucleotide encodes a HopXl protein from the bacterium Pseudomonas syringae pv tomato DC3001. In one embodiment, the HopXl has an amino acid sequence as set forth is SEQ ID NO: 1. In one embodiment, the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO: l . In one embodiment, the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:2. In one embodiment, the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:3.

In another embodiment, the polynucleotide further comprises a regulatory element selected from the group consisting of an enhancer, a promoter, and a transcription termination sequence. In a further embodiment, the promoter is selected from the group consisting of a constitutive promoter, an induced promoter and a tissue-specific promoter. In a still further embodiment, the promoter is derived from the Cauliflower mosaic virus DNA.

In one embodiment, wherein the regenerated plant has an enhanced tolerance to a plant pathogenic virus transmitted by a whitefly. In one embodiment, the regenerated plant has an enhanced tolerance to a disease caused by Agrobacterium tumefaciens. In one embodiment, the regenerated plant has an enhanced tolerance to freezing.

BRIEF DESCRIPTION OF FIGURES Figure 1 illustrates one embodiment, showing a scheme of Agrobacterium tumefaciens binary vector pAKT27-CaMv-35S-hopXl -Toes used to transform N. benthamiana plants with the hopXl gene from Pseudomonas syringae pv. tomato DC3001. The plant expression cassette 35S::hopXl::ocs was cloned in pART27 vector. 35S: promoter of CaMV-35S mRNA; ocs:: transcription terminator sequence of the octopine synthase gene.

Figure 2 illustrate one embodiment, showing an agarose gel electrophoresis gel of the pART21-CaMv-35S-hopXl-Tocs pART27 transformation vector after digestion with Xhol and Xbal restriction endonucleases. Pstl-digested λ DNA is used as a molecular weight marker. The presence of a fragment of the expected size for hopXl (lower band) and the vector backbone DNA (higher band) is shown.

Figure 3 illustrate one embodiment, showing the results of Northern and Southern blot hybridization of total RNA (left two panels) and DNA (right 3 panels), respectively. The nucleic acids were extracted from transgenic To N. benthamiana (35S::hopXl) leaf tissue. The hopXl gene was used as probe. Arrows in the northern blot panels indicate the position of the approximately 1400 nt, which is the expected size of the hopXl transcript.

Figure 4 illustrate one embodiment, showing the results of Northern (upper panels) and Southern (lower panels) blot hybridization of total RNA and DNA, respectively, from transgenic Ti N. benthamiana (35S::hopXl) leaf tissue. The hopXl gene was used as probe.

Figure 5 illustrate one embodiment, showing the result of PCR amplification, which shows the presence of hopXl and the absence of virG DNA sequences in DNA extracted from transgenic Ti N. benthamiana (35S::hopXl) leaf tissue.

hopXl primers: upper: 5'-GACATAGCTAGCTCGAGGAAAGTATTT-3' iSEO ID NO: m lower: 5'-GGCGCTTATTCTAGACGTACGTACT-3' iSEO ΓΡ NO: 28). v rG primers: upper: 5 ' -GCCGGGGCG AGACC ATAGG-3 ' iSEO ID NO: 29^:

lower: 5'-CGCACGCGCAAGGCAACC-3' iSEO ID NO: 30).

Figure 6 illustrate one embodiment, showing the result of reverse transcription PCR, which shows the presence of hopXl RNA transcripts in total R A extracts from Tl N.

benthamiana (35S::hopXl) leaf tissue.

hopXl primers: upper : 5 '-G AC ATAGCTAGCTCG AGGAAAGTATTT-3 ' iSEO ID NO:

221;

lower: 5'-GGCGCTTATTCTAGACGTACGTACT-3' fSEO ID NO: 28). actin primers: upper: 5 ' -TTAACTCTTAAATACCC AATTGAGC AT-3 ' (SEP ID NO: lower: 5 ' -G AC AGCCTGAATAGC AACATAC ATAG-3 ' iSEO ID NO:

3J}.

Figure 7 illustrate one embodiment, showing the reduced crown gall tumor development in transgenic N. benthamiana plants expressing the hopXl gene. A - transgenic line 8b II, B - control: untransformed (wild type, w.t.) N. benthamiana plant, C - transgenic line l id IV. Stems of wild type N. benthamiana were stab-inoculated with ^. tumefaciens A281 {A. tumefaciens C58C1 [pTiBo542] which induces crown gall tumors). Four stems were inoculated for each treatment and all experiments were repeated at least four times. Large crown gall tumors were observed in all wild type N. benthamiana plants. In contrast, no or vestigial gall formation was observed in the two transgenic N. benthamiana lines expressing the hopXl gene that were tested. Stems were photographed 18 days after inoculation. Figure 8 illustrate one embodiment, showing representative photographs of leaves from N. benthamiana wild type (w.t) and 35S::hopXl (Tl generation) plants, which show the differences in whitefly infestation 54 days after transplantation.

Figure 9 illustrate one embodiment, showing a quantitative assessment of whitefly infestation on transgenic and wild type N. benthamiana plants. Upper panel (A): whitefly nymphal/pupae and adults per sq. cm of leaf area. The two transgenic 35S::hopXl plants shown had the maximum numbers of nymphal/pupae stages and adults ever observed in infested leaves (the values for all other leaves were zero). Lower panel (B): Pooled data from the experiments shown in the upper panel showing the standard error bars.

Figure 10 illustrate one embodiment, showing the levels of whitefly infestation in transgenic N. benthamiana 35S::hopXl plants (Tl generation) (A, B, C panels) photographed 48 days after transplantation and in N. benthamiana wild type (w.t.) plants photographed 36 days after transplantation and kept under the same conditions. White fly infestation is clearly visible in the untransformed (w.t.) but is virtually absent from the transgenic plants.

Figure 1 1 illustrate one embodiment, showing whitefly infestation in different transgenic N. benthamiana 35S::hopXl lines (Tl generation) (in A, B, D, E, F, G) photographed 48 days after transplantation; and N. benthamiana wild type (w.t.) plants photographed 36 days after transplantation (in C). White fly infestation is clearly visible in the w.t. but is absent from the transgenic plants.

Figure 12 illustrate one embodiment, showing infestation by whiteflies of two N.

benthamiana 35S::hopXl (Tl) lines and N. benthamiana untransformed (w.t.) plants that were placed side by side in physical contact for 5 days to observe possible cross-infestation by white flies. The transgenic lines remained virtually un-infested. Figure 13 illustrate one embodiment, showing observations of whitefly infestation in leaves ofN benthamiana untransformed (w.t.) and 35S::hopXl (Tl ) plants viewed under a dissecting stereomicroscope.

Figure 14 illustrate one embodiment, showing the recovery of transgenic N. benthamiana 35::hopXl plants from freezing injury (-4°C for 18 hours approximately), photographed 4 days later. Wild type plants in the same growth room never recovered.

DETAILED DESCRIPTION

The present invention provides plants with enhanced tolerance to diseases and insects, including but not limited to insects that are important vectors of plant viruses.

The strategy employed to develop the plants of the present invention is based on a novel concept, taking advantage of the finding that some bacterial type III effector proteins enhance the RNA-mediated gene silencing mechanisms in plants in which they are not recognized by R gene receptors and, consequently do not trigger the hypersensitive response. Only a subset of the effectors tested from various pathovars of Pseudomonas syringae have such activity, while the majority do not appear to do so. A subset of such effectors from various pathovars of

Pseudomonas syringae have such activity, while the majority do not appear to do so. This property is in contrast to certain proteins from viral pathogens that are known to act as silencing suppressors. Many phytobacterial effectors are multifunctional proteins, with distinct cellular functions associated with different structural domains and the silencing enhancer activity is independent of effector- receptor recognition. Based on this finding, we reason that the silencing enhancing effectors need not elicit a hypersensitive response.

The unanticipated finding that the particular effector proteins once they are produced endogenously in plants, can provide resistance to at least one pathogen and one insect pest that were tested enables a novel technological exploitation of this and other bacterial effector proteins that act as silencing enhancers in crop protection. RNA-mediated gene silencing is involved in the orchestration of plant resistance responses but these effector proteins, while not triggering the silencing mechanism by themselves, they enhance its efficiency. It follows that to the extend that the gene silencing mechanism is necessary to suppress plant defense responses to pathogen/pest attack, it's enhancement would logically lead to, or increase native disease/pest resistance/tolerance in the plant. There is evidence that silencing RNA molecules that are transcribed in the host can affect gene expression in the parasite. Inactivating parasite genes via cross-family gene silencing could be an effective approach for engineering resistance against parasites and we speculate that such mechanisms probably apply to insect pests as well (Price, and Gatehouse, 2008).

In contrast to the hitherto known methods of breeding for resistant plants to pathogens and insects, the present invention utilizes new proteins, endowed with unique functions. Aside from triggering R-gene mediated resistance in plant genotypes that have functionally complementary R genes, the silencing-enhancing effectors engage the RNA-mediated gene silencing mechanism in plants even when effector-R gene recognition is not operative.

Thus, the present invention provides a transgenic plant comprising at least one cell transformed with a polynucleotide encoding the HopXl protein, or a silencing-enhancing R-gene "blind", variant or homologue thereof. According to certain embodiments, the transgenic plant has an enhanced tolerance to at least one disease caused by a bacterium and to infestation by at least one insect, compared to a non-transgenic plant.

The concept of the present invention has been exemplified by expressing a gene encoding HopX l from Pseudomonas syringae pv. tomato DC3001 in the model plant Nicotiana benthamiana, and testing these plants for resistance to whitefly infestation and crown gall in similar growth room/greenhouse environments.

DEFINITION

The term "plant" is used herein in its broadest sense. It includes, but is not limited to, tomato, tobacco, cucumber, prune, potato, bean, barley, soybean, pea, beet, grapevine, petunia, abutilon, melon, watermelon, okra, cotton, cassava, wheat, maize, rice, cotton, citrus and cabbage. It can also be any ornamental, horticultural, or field crop species. Moreover, any plant susceptible to a crown gall, whiteflies and/or whitefiy-transmitted viruses is included. It also refers to a plurality of plant cells that are largely differentiated into a structure that is present at any stage of a plant's development.

As used herein, the term "HopX l " refers to a family of proteins found in Pseudomonas syringae strains. The HopXl protein is a type III effector protein typically present in phytopathogenic Gram negative bacteria. The sequence of the HopXl from the Pseudomonas syringae pv. phaseolicola 1448A can be found in GenBank accession number AAZ37209. The polynucleotide encoding HopXl is capable of enhancing RNA-mediated silencing, for which the functional part of the HopXl is not the domain(s) interacting with plant R gene receptors.

Accordingly, any polynucleotide encoding HopX l having a silencing enhancer activity can be employed and such polynucleotide can encode a fragment or homologue of HopXl as long as the silencing enhancer activity of the protein is preserved. Examples of HopX l protein and polynucleotides encoding same are listed in SEQ ID NO: l , SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO: 10, SEQ ID NO: 1 1 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, SEQ ID NO:20, SEQ ID NO:21 , SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, and SEQ ID NO:26.

The terms "HopXl homologue" refers to proteins and polynucleotides encoding same that are at least 70%, at least 75%, at least 80%, at least 85%, at least 95% or more homologous to HopXl proteins found in SEQ ID NO: l, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ED NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO: 10, SEQ ID NO: 1 1 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, SEQ ID NO: 16, SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19, SEQ ID NO:20, SEQ ID NO:21 , SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, and SEQ ID NO:26.

The term "homology", as used herein, refers to a degree of sequence similarity in terms of shared amino acid or nucleotide sequences. There may be partial homology or complete homology (i.e., identity). For amino acid sequence homology, amino acid similarity matrices may be used as are known in different bioinformatics programs (e.g. BLAST, Smith Waterman). Different results may be obtained when performing a particular search with a different matrix. Homologous peptide or polypeptides are characterized by one or more amino acid substitutions, insertions or deletions, such as, but not limited to, conservative substitutions, provided that these changes do not affect the biological activity of the peptide or polypeptide as described herein.

Degrees of homology for nucleotide sequences are based upon identity matches with penalties made for gaps or insertions required to optimize the alignment, as is well known in the art (e.g. Altschul et al. 1990; Altschul et al. 1997). The degree of sequence homology is presented in terms of percentage, e.g. "70% homology". As used herein, the term "at least" with regard to a certain degree of homology encompasses any degree of homology from the specified percentage up to 100%. The term "HopXl fragment" as used herein refers to sub-sequences (fragments) of the HopXl protein that still capable of enhancing RNA-mediated gene silencing. The fragment may correspond to a contiguous amino acid sequence of the complete protein, or it may be formed from non-contiguous amino acids sequences.

The term "insect" refers to any pest insect capable of causing damage to agriculture through feeding on crops or transmitting virus. Included here is whitefly.

The terms "whitefly" refers to the insect family oiAleyrodidae, which are small hemipterans, including but not limited to, the Bemissia (abaci species complex, which is composed of over 40 biotypes, and the species Trialeurodes vaporariorum, the greenhouse whitefly. Whiteflies typically feed on the underside of plant leaves. It is to be understood that the taxonomy of whiteflies undergoes periodic revisions, the species/genus names are given merely as a set of examples and additional whitefly names and biotypes are explicitly encompassed in the present invention.

The term "disease" refers to plant diseases caused by pathogens and environmental conditions. Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Included here is crown gall disease caused by a Gram negative soil bacterium, Agrobacterium tumefaciens. Agrobacterium tumefaciens is a pathogenic alphaproteobacterium of the family Rhizobiaceae.

The terms "plant having an enhanced tolerance" refer to a plant having an increased tolerance as compared to a susceptible plant. The increased tolerance may be examined by deliberately allowing the plant to interact with pest insects, pathogenic agents, or environmental stress. Plants showing lower symptom intensity or damage caused by the stress compared to susceptible plant, according to a symptom scale, are defined as plants resistant to the pest insects, pathogenic agents, or environmental stress.

These terms do not necessarily mean total immunity from all damages by insects, diseases or freezing stress but merely a decrease as compared to non-manipulated control. Most importantly, the terms refer to plants having similar yields compared to non-infected plants. To the contrary, susceptible plants are defined by an almost total yield loss as a result of insects, diseases or freezing stress.

The tolerant plants as defined above can be transgenic. Resistance can be a stable trait, which can be inherited to the offspring population.

The term "gene" refers to a nucleic acid (e.g., DNA or NA) sequence that comprises coding sequences necessary for the production of RNA or a polypeptide. A polypeptide can be encoded by a full-length coding sequence or by any part thereof. The term "parts thereof when used in reference to a gene refers to fragments of that gene, particularly fragment encoding a HopXl protein fragment as is defined hereinabove. The fragments may range in size from a few nucleotides to the entire gene sequence minus one nucleotide. Thus, "a nucleic acid sequence comprising at least a part of a gene" may comprise fragments of the gene or the entire gene.

The term "gene" also encompasses the coding regions of a structural gene and includes sequences located adjacent to the coding region on both the 5' and 3' ends for a distance of about 1 kb on either end such that the gene corresponds to the length of the full-length mRNA. The sequences which are located 5' of the coding region and which are present on the mRNA are referred to as 5' non-translated (or untranslated) sequences (5' UTR). The sequences which are located 3' or downstream of the coding region and which are present on the mRNA are referred to as 3' non-translated (or untranslated) sequences (3' UTR). The term "nucleic acid" as used herein refers to RNA or DNA that is linear or branched, single or double stranded, or a hybrid thereof. The term also encompasses RNA/DNA hybrids.

The term "construct" as used herein refers to an artificially assembled or isolated nucleic acid molecule which includes the gene of interest. In general a construct may include the gene or genes of interest, a marker gene which in some cases can also be the gene of interest and appropriate regulatory sequences. It should be appreciated that the inclusion of regulatory sequences in a construct is optional, for example, such sequences may not be required in situations where the regulatory sequences of a host cell are to be used. The term construct includes vectors but should not be seen as being limited thereto.

The term "operably linked" refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is regulated by the other. For example, a promoter is operably linked with a coding sequence when it is capable of regulating the expression of that coding sequence (i.e., that the coding sequence is under the transcriptional control of the promoter). Coding sequences can be operably linked to regulatory sequences in a sense or antisense orientation. In another example, the complementary RNA regions of the invention-can be operably linked, either directly or indirectly, 5' to the target mRNA, or 3' to the target mRNA, or within the target mRNA, or a first complementary region is 5' and its complement is 3' to the target mRNA.

The terms "promoter" as used herein refers to a DNA sequence that is located at the 5' end (i.e. precedes) the protein coding region of a DNA polymer. The location of most promoters known in nature precedes the transcribed region. The promoter functions as a switch, activating the expression of a gene. If the gene is activated, it is said to be transcribed, or participating in transcription. Transcription involves the synthesis of mRNA from the gene. The promoter, therefore, serves as a transcriptional regulatory element and also provides a site for initiation of transcription of the gene into mRNA.

Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as "constitutive promoters". Promoters that derive gene expression in a specific tissue are called "tissue specific promoters. Tissue specific promoters can be expressed constitutively or their expression may require a specific induction. New promoters of various types useful in plant cells are constantly being discovered; numerous examples may be found in the compilation by Okamuro and Goldberg (1989) (see also Shahmuradov, 2003).

As used herein, the term an "enhancer" refers to a DNA sequence which can stimulate promoter activity, and may be an innate element of the promoter or a heterologous element inserted to enhance the level or tissue-specificity of a promoter. The term "silencing enhancer" or "silencing enhancement" refer to a potentiation of the gene silencing process, meaning made to happen earlier or to a greater degree, once they are triggered by appropriate signal(s) in an organism such as a plant.

The terms "encoding" and "coding" refer to the process by which a gene, through the mechanisms of transcription and translation, provides information to a cell from which a series of amino acids can be assembled into a specific amino acid sequence to produce an active enzyme. Because of the degeneracy of the genetic code, certain base changes in DNA sequence do not change the amino acid sequence of a protein. It is therefore understood that modifications in the DNA sequence encoding transcription factors which do not substantially affect the functional properties of the protein are contemplated.

The term "expression", as used herein, refers to the production of a functional end- product e.g., an mRNA or a protein. The terms "heterologous gene" or "exogenous genes" refer to a gene encoding a factor that is not in its natural environment (i.e., has been altered by the hand of man). For example, a heterologous gene includes a gene from one species introduced into another species. A heterologous gene also includes a gene native to an organism that has been altered in some way (e.g., mutated, added in multiple copies, linked to a non-native promoter or enhancer sequence, etc.). Heterologous genes may comprise plant gene sequences that comprise cDNA forms of a plant gene; the cDNA sequences may be expressed in either a sense (to produce mRNA) or anti- sense orientation (to produce an anti-sense NA transcript that is complementary to the mRNA transcript). Heterologous plant genes are distinguished from endogenous plant genes in that the heterologous gene sequences are typically joined to nucleotide sequences comprising regulatory elements such as promoters that are not found naturally associated with the gene for the protein encoded by the heterologous gene or with plant gene sequences in the chromosome, or are associated with portions of the chromosome not found in nature (e.g., genes expressed in loci where the gene is not normally expressed). A plant gene endogenous to a particular plant species (endogenous plant gene) is a gene which is naturally found in that plant species or which can be introduced in that plant species by conventional breeding.

The terms "polypeptide," "peptide" and "protein" are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residue is an artificial chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers.

The term "transgenic" when used in reference to a plant or seed (i.e., a "transgenic plant" or a "transgenic seed") refers to a plant or seed that contains at least one heterologous gene in one or more of its cells. The term "transgenic plant material" refers broadly to a plant, a plant structure, a plant tissue, a plant seed or a plant cell that contains at least one heterologous gene in at least one of its cells.

The terms "transform" refers to the introduction and integration of one or more exogenous polynucleotides into the genome of a cell. Transformation of a cell may be detected by Northern Blot, Southern blot, PCR and/or RT-PCR.

The term "tissue culture" refers to plant tissues propagated under sterile conditions, often for producing clones of a plant. Plant tissue culture relies on the fact that many plant cells have the ability to regenerate a whole plant. Single cells, plant cells without cell walls (protoplasts), pieces of leaves, or roots can often be used to generate a new plant on culture media given the required nutrients and plant hormones.

All technical terms used herein are terms commonly used in biochemistry, molecular biology and agriculture, and can be understood by one of ordinary skill in the art to which this invention belongs. Technical terms can be found in: Molecular Cloning: A Laboratory Manual, 3rd ed., vol. 1 -3, ed. Sambrook and Russell, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2001 ; Current Protocols in Molecular Biology, ed. Ausubel et al., Greene Publishing Associates and Wiley-Interscience, New York, 1988 (with periodic updates); Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in

Molecular Biology, 5th ed., vol. 1-2, ed. Ausubel et al., John Wiley & Sons, Inc., 2002; Genome Analysis: A Laboratory Manual, vol. 1-2, ed. Green et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1997. Methodology involving plant biology techniques is described herein and is described in detail in treatises such as Methods in Plant Molecular Biology: A Laboratory Course Manual, ed. Maliga et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1995. Various techniques using PCR are described in Innis et al., PCR Protocols: A Guide to Methods and Applications, Academic Press, San Diego, 1990 and in Dieffenbach and Dveksler, PCR Primer: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2003. PCR-primer pairs can be derived from known sequences by known techniques such as using computer programs intended for that purpose, Primer, Version 0.5, 1991 , Whitehead Institute for Biomedical Research, Cambridge, MA. Methods for chemical synthesis of nucleic acids are discussed, for example, in Beaucage and Caruthers, 1981 , Tetra. Letts. 22: 1859-1862, and Matteucci and Caruthers, 1981 J. Am. Chem. Soc. 103: 3185.

Restriction enzyme digestions, phosphorylations, ligations and transformations were done as described in Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed. (1989), Cold Spring Harbor Laboratory Press. All reagents and materials used for the growth and

maintenance of bacterial cells were obtained from Aldrich Chemicals (Milwaukee, WI), DIFCO Laboratories (Detroit, MI), Invitrogen (Gaithersburg, MD), or Sigma Chemical Company (St. Louis, MO), unless otherwise specified.

DNA CONSTRUCTS

In accordance with one aspect of the invention, a hopXl gene or hopXl homologous gene sequence is incorporated into a DNA construct that is suitable for plant transformation. Such a DNA construct can be used to modify hopXl expression in plants, as described above.

Accordingly, DNA constructs are provided comprising a hopXl gene sequence or hopXl homologous gene sequence, under the control of a promoter, such as any of those mentioned above, so that the construct can generate RNA in a host plant cell.

Recombinant DNA constructs may be made using standard techniques. For example, the DNA sequence for transcription may be obtained by treating a vector containing said sequence with restriction enzymes to cut out the appropriate segment. The DNA sequence for transcription may also be generated by annealing and ligating synthetic oligonucleotides or by using synthetic oligonucleotides in a polymerase chain reaction (PCR) to give suitable restriction sites at each end. The DNA sequence then is cloned into a vector containing upstream promoter and downstream terminator sequences.

The expression vectors of the invention may also contain termination sequences, which are positioned downstream of the nucleic acid molecules of the invention, such that transcription of mRNA is terminated, and polyA sequences added. Exemplary of such terminators are the cauliflower mosaic virus CaMV 35S terminator and the octopine synthase gene T-ocs terminator. The expression vector may also contain enhancers, start codons, splicing signal sequences, and targeting sequences.

Expression vectors of the invention may also contain a selection marker by which transformed plant cells can be identified in culture. The marker may be associated with the heterologous nucleic acid molecule, i.e., the gene operably linked to a promoter. As used herein, the term "marker" refers to a gene encoding a trait or a phenotype that permits the selection of, or the screening for, a plant or plant cell containing the marker. Usually, the marker gene will encode antibiotic or herbicide resistance. This allows for selection of transformed cells from among cells that are not transformed or transfected.

Examples of suitable selectable markers include adenosine deaminase, dihydrofolate reductase, hygromycin-B-phosphotransferase, thymidne kinase, xanthine-guanine phospho- ribosyltransferase, glyphosate and glufosinate resistance and amino-glycoside 3'-0- phosphotranserase (kanamycin, neomycin and G418 resistance). These markers include resistance to G418, hygromycin, bleomycin, kanamycin, and gentamicin. The construct may also contain the selectable marker gene Bar that confers resistance to herbicidal phosphinothricin analogs like ammonium gluphosinate (Thompson et al., 1987) or phosphomannose isomerase (He et al., 2004). Other suitable selection markers are known to the person skilled in the art.

Replication sequences, of bacterial or viral origin, may also be included to allow the vector to be cloned in a bacterial or phage host. Preferably, a broad host range prokaryotic origin of replication is used. A selectable marker for bacteria may be included to allow selection of bacterial cells bearing the desired construct. Suitable prokaryotic selectable markers also include resistance to antibiotics such as kanamycin or tetracycline.

Other DNA sequences encoding additional functions may also be present in the vector, as is known in the art. For instance, when Agrobacterium is the host, T-DNA sequences may be included to facilitate the subsequent transfer to and incorporation into plant chromosomes. PLANT CELL TRANSFORMATION

Constructs according to the invention may be used to transform any plant cell, using a suitable transformation technique. Both monocotyledon and dicotyledonous angiosperm or gymnosperm plant cells may be transformed in various ways known to the art. For example, see Klein et al., 1993; Bechtold et al., 1993; Bent et al., 1986; Paszowski et al., 1984; Sagi et al., 1994.

Agrobacterium species such as A. t mefaciens and . rhizogenes can be used, for example, in accordance with Nagel et al., 1990. Agrobacterium may be transformed with a plant expression vector via electroporation, followed by introduction of the Agrobacterium into plant cells via the well known leaf-disk method. Additional methods include, but are not limited to, particle gun bombardment, calcium phosphate precipitation, polyethylene glycol fusion, transfer into germinating pollen grains, direct transformation (Lorz et al., 1985), and other methods known to the art. Use of a selection marker, such as kanamycin resistance, allows quick identification of successfully transformed cells.

The Agrobacterium transformation methods discussed above are known to be useful for transforming dicots. For transformation of cereal monocots using Agrobacterium, see de la Pena et al., 1987; Rhodes et al., 1988; and Shimamato et al., 1989, all of which are incorporated by reference. See also Bechtold et al., 1994, showing the use of vacuum infiltration for

Agrobacterium-mediated transformation.

The presence of a protein, polypeptide, or nucleic acid molecule in a particular cell can be measured to determine if, for example, a cell has been successfully transformed according to methods well known in the art. Known techniques include, but are not limited to, Northern blot, Southern blot, PCR and RT-PCR.

REGENERATING TRANSGENIC PLANT

Successfully transformed plant tissue can be collected, placed on rooting and then to shooting medium, grown to a desirable height, and transferred to soil. T i seeds can be produced from self-pollinating T₀ lines, and germinated on a medium containing at least one selection marker. A transgenic plant can be produced by transplanting Ti seedlings in soil after a desirable period of time after germination.

INSECT TOLERANCE

Transgenic plants of the invention are characterized by enhanced tolerance to insects infestation. The phrase "enhanced tolerance to insects" refers to a transgenic plant that shows little or no damages from insect infestation after insect exposure, when compared to a wild-type or non-transformed plant of the same species subjected to the same level of insect exposure that shows significant damages from insect infestation. The phrase "insect exposure" refers to exposing the transgenic plants to pest insects, such as whiteflies, or insect-infested plants, under a condition and for a period of time for which wild-type plants are expected to be infested. Increase in tolerance to insects infestation is assessed by comparing, visually or through a microscope, the wild-type plants and the transgenic plants.

DISEASE TOLERANCE

Transgenic plants of the invention are also characterized by enhanced disease tolerance. The phrase "enhanced disease tolerance" refers to a transgenic plant that survives exposure to a plant pathogenic agent and maintains its normal phenotype after survival, when compared to a wild-type or non-transformed plant of the same species that does not survive attack by the same plant pathogenic agent, or shows significant symptoms caused by the plant pathogenic agent. The phrase "plant pathogenic agent" refers to any organism capable of causing infectious disease on plant, including fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. The word "significant symptoms" refers to a significant departure from a normal phenotype of a plant, indicating the presence of a plant disease.

FREEZING TOLERANCE

Transgenic plants of the invention are characterized by enhanced freezing stress tolerance. The phrase "enhanced freezing stress tolerance" refers to a transgenic plant that survives exposure to freezing stress and maintains its normal phenotype after survival, when compared to a wild-type or non-transformed plant of the same species that does not survive the freezing stress, or shows significant freeze damage. The phrase "freezing stress" indicates exposure of a plant to a temperature in a range between 0° and -30°C for 2 to 72 hours, followed by a 4 to 8 hour recovery period at 4°C, before transfer into the greenhouse at 22°C. Increase in cold tolerance is assessed by scoring the number of transgenic plants surviving the freezing stress after 1 to 5 days in the greenhouse, compared to the number of wild-type or non-transformed plants of the same species. Increase in cold tolerance can also be assessed by scoring the freezing damage to leaves and shoots after exposure to freezing stress.

Specific examples are presented below of methods for obtaining DNA constructs comprising HopXl genes or HopXl homologous gene sequences, as well as for introducing the target genes, via Agrobacterium, to produce plant transformants. These examples are meant to be exemplary and not as limitations on the present invention.

EXAMPLE 1

Preparation of DNA Constructs Containing the HopXl Gene Cloning of the HopX l gene was accomplished by PCR using total DNA extracted form Pseudomonas syringae pv. tomato strain DC3001 (Landgraf et ai, 2006, Mo I. Plant Pathol. 7, 355-64), using primers designed according to the published sequence of the synonymous gene from Pseudomonas syringae pv. tomato strain DC3000 (Accession No. AAO59038). The PCR product was cloned between the 35S CaMV promoter and octopine synthase transcriptional terminator of the vector pART7, Then promoter-effector-terminator cassette was sub-cloned onto the Agrobacterium Binary Vector pART27, producing the transformation vector pART27- CaMv-35S-hopXl-T-ocs used to transform tomato plants with the hopXl gene from

Pseudomonas syringae pv. tomato DC3001. As shown in figure 1 , the plant expression cassette 35S::hopXl::ocs was cloned in pART27 vector (35S:promoter of CaMV-35S mRNA; ocs:: transcription terminator sequence of the octopine synthase gene). The pART27 vector is described in A. P. Gleave, Plant Molecular Biology 20: 1203-1207 (1992). The recombinant vector was checked for its molecular structure by digestion with Xhol and Xbal restriction endonucleases, as shown in Figure 2, and by sequencing the expression cassette. The hopXl nucleotide and predicted and the predicted protein product were 100% identical to the gene/protein from strain DC3000 of Pseudomonas syringae pv. tomato strain DC3000.

EXAMPLE 2

Producing Transgenic N. benthamiana Plants Expressing the HopXl Gene

The transformation vector was transferred to Agrobacterium strain C58C 1 by electroporation. The resulting strain was used to transform surface sterilized leaf discs of N. benthamiana. Selection for transformation was done on medium containing kanamycin (150 μg/ml). Kanamycin-resistant shoots were collected, placed on rooting and then to shooting medium, grown to height of 5 to 6 cm, and transferred to soil. T o lines were self-pollinated and T i seeds germinated on MS medium containing 50 μg/ml of kanamycin. T i seedlings were transplanted in soil one month after germination.

30 μg of tomato total genomic DNA was extracted as described in Bernatzky and Tanksley, Mol. Gen. Gen. 203 :8-14 (1986), digested to completion with EcoRI, subjected to electrophoresis on a 0.8% agarose gel, and transferred to Hybond N ⁺ membranes (Amersham, UK). ³²P-dCTP labeled hopXl DNA used as a probe was prepared by random primer labeling. Hybridization was carried out at 65°C for 18 h. The blot was washed with 0.1 xSSC at 65°C and exposed for 72 h at -70°C using an intensifying screen and Kodak Biomax film. As shown in figure 3 and 4, Northern and Southern blot hybridization of total RNA and DNA extracted from leaves of transgenic (T₀ and T_t generations) ofN. benthamiana (35S::hopXl) plants revealed the presence and expression of the hopXl gene. PCR and RT-PCR amplification was carried out to confirm the presence of hopXl and the absence of virG DNA sequences and to detect presence of hopXl mRNA transcript, in transgenic T| N. benthamiana (35S::hopXl) leaf tissue.

hopXl primers: upper: 5'-GACATAGCTAGCTCGAGGAA AfiTATTT-3¹ iSEO ID NO: m

lower: 5'-GGCGCTTATTr.TAGAGGTACGTACT-3' (SEP ID NO: 28 . virG primers: upper: 5^?-GCCGGGGCGAGACCATAGG-3' iSEO ID NO: 29 :

lower: S'-CGCACGCGflA AGGflAAr.C-T fSEO ID NO: 30V

As shown in figure 5, PCR analysis conforms the presence of hopXl and absence of virG DNA sequences in DNA extracted from transgenic Ti N. benthamiana (35S::hopXl) leaf tissue. As shown in Figure 6, reverse transcription PCR confirms the presence of hopXl RNA transcripts in total RNA extracts from TI N. benthamiana (35S::hopXl) leaf tissue.

EXAMPLE 3

Transgenic N. benthamiana Plants Have Enhanced Tolerance to Whitefly Infestation

N. benthamiana plants were maintained in a plant growth room at 25 °C and in 16h- light/8h-dark photoperiod. In one instance, the plants became infested by whiteflies.

Subsequently, un-infested plants were produced and placed in a controlled chamber along with whitefly-infested source plants and maintained for several weeks under the above stated conditions. Infestation was recorded both visually and by taking photographs of whole plants and leaves. Individual leaves were also detached and placed photographed under a dissecting microscope and larvae and adult numbers were counted and quantified.

As shown in figures 8-13, hopXl -expressing N. benthamiana plants developed tolerance to whitefly infestation. Figures 8 shows representative photographs of leaves from N. benthamiana wild type (w.t) and 35S::hopXl (Tl generation) plants, illustrating that white fly infestation is clearly visible in w.t. but is absent from the transgenic plants. As shown in figure 9, a quantitative assessment of whitefly infestation on transgenic and wild type N. benthamiana plants is given. Observations on insect infestation of the two transgenic 35S::hopXl plants were only made on regions of leaves exhibiting infestation by nymphae/pupae and adults. Some regions of leaves from transgenic 35S::hopXl plants exhibited no insect infestation. The pooled data from the experiments indicate statistically significant differences between the transformed and untransformed plants in insect colonization and reproduction. As shown in figure 12, two transformed N. benthamiana 35S::hopXl (Tl) plants and untransformed N. benthamiana (w.t.) plants were placed side by side in physical contact for 5 days to observe possible cross- infestation by white flies. The transgenic lines remained uninfested.

EXAMPLE 4

Transgenic N. benthamiana Plants Have Enhanced Tolerance to Crown Gall Tumor

Four stems were inoculated for each treatment. And all experiments were repeated at least four times by puncturing the stem with a plastic pipette tip containing 10 μΐ of

Agrobacterium cell suspension grown overnight in LB medium, spun and resuspended in sterile distilled water to a cell density of 10⁷ colony forming units/ml and determined

spectrophotometrically by reference to a standard curve. Crown gall tumor formation was recorded 18 days after inoculation.

As shown in figure 7, reduced crown gall tumor development is apparent in stems of transgenic N. benthamiana plants expressing the hopXl gene, compared to the untransformed controls. Stems of 2 month-old plants were inoculated with A tumefaciens A281 (A.

tumefacie s C58C1 [pTiBo542] which induces crown gall tumors. Large crown gall tumors were observed in all wild type N. benthamiana plants. In contrast, no or vestigial gall formation was observed in the two transgenic N. benthamiana lines expressing the hopXl gene.

EXAMPLE 5

Transgenic N. benthamiana Plants Have Enhanced Tolerance to Freezing

During one experiment plants from several N. benthamiana transgenic hopXl plant lines along with untransformed control plants were maintained in pots that were kept in a plant growth room at 24 °C and 16 hr-light 8-hr day/night photoperiod. Unexpectedly, the temperature control system malfunctioned during the night and the growth room temperature dropped to below zero levels, as evidenced from the frozen or flaccid condition of nearly all plants the next day, at which time the growth room temperature control system was turned off and remained so until it was repaired some days later. Daily observations during this period informed the inventors that several plants were recovering gradually from the freezing injury. At 3-4 days after the incident a majority of transgenic plants had fully recovered while none of the untransformed plants ever recovered from the freezing injury. The plants were photographed at 4 days after the freezing incident and shown in Figure 14.

CITED REFERENCES

Akad et al., Making a friend from a foe: expressing a GroEL gene from the whitefly Bemisia tabaci in the phloem of tomato plants confers resistance to tomato yellow leaf curl virus. Archiv. Virol. 152(7): 1323-1339 (2007).

Altschul et al., J. Mol. Biol 215(3):403-10 (1990).

Altschul et zl, Nucleic Acids Res. 25:3389-3402 (1997).

Aragao et al. First transgenic geminivirus-resistant plant in the field. Nat. Biotechnol. 27,1086- 1088 (2009).

Beaucage and Caruthers, Tetra. Letts. 22: 1859- 1862 (1981 ). Bechtold et al., C. R. Acad. Sci. Paris 316: 1 194- 1 199 (1993). Bent et al, Mol. Gen. Genet. 204:383-396 (1986). Bernatzky and Tanksley, Mol. Gen. Genet. 203:8-14 (1986)

Brown, The Bemisia tabaci complex: genetic and phenotypic variability drives begomovirus spread and virus diversification. Plant Disease APS-Net Feature (2007).

Brown, Current status of Bemisia tabaci as a plant pest and virus vector in agroecosystems worldwide. FAO Plant Prot Bull 42( 12):332 (1994).

Burr et al. Bacterial resistance in grapevine. U.S. Patent No. 6, 172,280 (1989).

Chu et al. Cryptic Invasion of the Exotic Bemisia tabaci Biotype Q Occurred Widespread in Shandong Province of China. Florida Entomologist 93:203-207 (2010).

Czosnek, Virus tolerant plants and methods of producing same. U.S. Patent Pub. No.

20100115665.

De Barro et al, Bemisia argentifolii is a race of B. tabaci (Hemiptera: Aleyrodidae): the molecular genetic differentiation of B. tabaci populations around the world. Bull. Entomol. Res. 95: 193-203 (2005).

De la Pena et al. Nature 325:274-276 (1987).

Diaz-Pendon et al. Tomato yellow leaf curl viruses: menage a trois between the virus complex, the plant and the whitefly vector. Mol. Plant Pathol. 1 1 :441_^450 (2010).

Dieffenbach and Dveksler, PCR Primer: A Laboratory Manual, 2nd ed. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y, 2003. Dhakar et al., Virus resistance and gene silencing in plants infected with begomovirus. J. Applied Sci. 10: 1787-1791 (2010).

Dunoyer et al., Induction, suppression and requirement of RNA silencing pathways in virulent Agrobacterium tumefaciens infections. Nat. Genet. 38:258-63 (2006).

Edelbaum et al., Expressing a whitefly GroEL protein in Nicotiana benthamiana plants confers tolerance to tomato yellow leaf curl virus and cucumber mosaic virus, but not to grapevine virus A or tobacco mosaic virus. Arch. Virol. 154:399-407 (2009).

Ellendorff et al., RNA silencing is required for Arabidopsis defense against Verticillium wilt disease. J. Exp. Bot. 60: 591-602 (2009)

Escobar and Dandekar, Agrobacterium tumefaciens as an agent of disease. Trends Plant Sci. 8:380-386 (2003).

Escobar et al., RNAi-mediated oncogene silencing confers resistance to crown gall

tumorigenesis. PNAS 98: 13437-13442 (2001).

Escobar et al., Silencing crown gall disease in walnut (Juglans regia L.). Plant Sci. 163:591-597 (2002).

Freitas-Astua et al., Traditional and transgenic strategies fro controlling tomato-infecting Begomoviruses. Fitopatol. Basil. 27(5) (2002).

Gelvin, Agrobacterium and plant genes involved in T-DNA transfer and integration. Annu. Rev. Plant Physiol. Plant Mol. Biol. 51 :223-256 (2000).

Gleave, Plant Molecular Biology 20: 1203-1207 ( 1992).

He et al., Phosphomannose-isomerase [pmi) gene as a selectable marker for rice transformation via Agrobacterium. Plant Science 166: 17-22 (2004).

Innis et al., PCR Protocols:A Guide to Methods and Applications, Academic Press, San Diego, 1990.

Jones, Plant viruses transmitted by whitefiies. Euro. J. Plant Pathol. 109: 195-219(2003).

Lapidot and Friedman, Breeding for resistance to whitefly transmitted geminiviruses. Ann. Appl. Biol. 140:109-127 (2002).

Klein et al., Biotechnology 4:583-590 (1993).

Lorz et al., Mol. Genet. 199: 179-182 (1985). McCullen and Binns, Agrobacterium tumefaciens and plant cell Interactions and activities required for Interkingdom macromolecular transfer Annu. Rev. Cell Dev. Biol. 22: 101-27

(2006) .

Morales, Conventional breeding for resistance to Bemissia tabaci-transmitted geminiviruses. Crop Protect. 20:825-834 (2001).

Morales et al., The ecology and epidemiology of whitefly-transmitted viruses in Latin America. Virus Res. 100:57-65 (2004).

Naranjo and Ellsworth, Challenges and opportunities for pest management of Bemisia in the New Century. Special Issue. Crop Prot. 20:707-869 (2001).

Prins et al., Strategies for antiviral resistance in transgenic plants. Mol. Plant Pathol. 9:73-83 (2008).

Matteucci and Caruthers, J. Am. Chem. Soc. 103:3185 (1981). Nagel et al., Microbiol Lett. 67:325 (1990).

Nimchuk et al., The HopX (AvrPphE) Family of Pseudomonas syringae Type III Effectors Require a Catalytic Triad and a Novel N-Terminal Domain for Function. MPMI 20:346-357

(2007) .

Okamuro and Goldberg, The Biochemistry of Plants: A comprehensive Treatise. Vol. 15 Molecular Biology Academic Press 1 -82 (1989).

Paszowski et al., EMBO J. 3:2717-2722 (1984).

Price and Gatehouse, Trends Biotechnol. 26, 393^100 (2008)

Rhodes et al., Science 240:204-207 (1988).

Sagi et al., Plant Cell Rep. 13:262-266 (1 94).

Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd ed., Cold Spring Harbor Laboratory Press ( 1989).

Sarris et al, Mol Plant Microbe Interact. 24(8):907-17 (201 1). Shahmuradov et al., Nucleic Acids Research 31 : 1 14-1 17 (2003). Shimamato et al., Nature 328:274-276 (1989).

Tamarzizt et al., Use of Tomato leaf curl virus (TYLCV) truncated Rep gene sequence to engineer TYLCV resistance in tomato plants. Acta. Virol. 53:99-104 (2009). Tomilov et al., The Plant Journal 56, 389-397 (2008).

Tzfira and Citovsky, Partners-in-infection: host proteins involved in the transformation of plant cells by Agrobacterium. Trends Cell Biol. 12: 121-128 (2002).

Vanderschuren et al., Engineering resistance to geminiviruses - review and perspectives. Plant Biotechnol. J. 5 :207-220 (2007).

Wintermantel, Emergence of greenhouse whitefly (Trialeurodes vaporariorum) transmitted Criniviruses as threats to vegetable and fruit production in North America. APSnet Features Online (2004).

Wisler et al., Ecology and epidemiology of whitefly-transmitted closteroviruses. Plant Dis. 82:270-280 ( 1998).

Wraight et al., Evaluation of the entomopathogenic fungi, Beauveria bassiana and Paecilomyces fumosoroseus, for microbial control of the silverleaf whitefly, Bemisia argentifolii. Biol. Control 17:203-217 (2000).

Yang et al, Use of Tomato yellow leaf curl virus (TYLCV) Rep Gene Sequences to Engineer TYLCV Resistance in Tomato. Phytopathology 94(5):490-496 (2004).

Current Protocols in Molecular Biology, ed. Ausubel et al., Greene Publishing Associates and Wiley-Interscience, New York, 1988 (with periodic updates);

Genome Analysis: A Laboratory Manual, vol. 1 -2, ed. Green et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1997.

Methods in Plant Molecular Biology: A Laboratory Course Manual, ed. Maliga et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1995.

Molecular Cloning: A Laboratory Manual, 3rd ed., vol. 1 -3, ed. Sambrook and Russell, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2001 ;

Primer, Version 0.5, 1991 , Whitehead Institute for Biomedical Research, Cambridge, MA.

Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in Molecular Biology, 5th ed., vol. 1-2, ed. Ausubel et al., John Wiley & Sons, Inc., 2002;

Claims

WHAT IS CLAIMED IS:

1. A transgenic plant comprising at least one cell transformed with a polynucleotide encoding a HopXl protein, an active fragment or homologue thereof.

2. The transgenic plant of claim 1 , wherein the HopXl protein, the active fragment or homologue thereof is capable of enhancing R A-mediated gene silencing in plants.

3. The transgenic plant of claim 1 , wherein the polynucleotide encodes a HopXl protein of the phytopathogenic bacterium Pseudomonas syringae pv. tomato DC3001.

4. The transgenic plant of claim 1 , wherein the HopXl protein has an amino acid sequence as set forth is SEQ ID O:l .

5. The transgenic plant of claim 1, wherein the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO: 1.

6. The transgenic plant of claim 1 , wherein the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:2.

7. The transgenic plant of claim 1 , wherein the polynucleotide comprises a nucleic acid sequence having at least 70%> homology to SEQ ID NO:3.

8. The transgenic plant of claim 1 , wherein the HopXl protein or the active fragment or homologue thereof is expressed in said plant in a constitutive manner.

9. The transgenic plant of claim 8, wherein the HopXl protein or the active fragment or homologue thereof is expressed in leaves.

10. The transgenic plant of claim 1, wherein said plant has an enhanced tolerance to at least one insect specie/biotype as compared to a non-transgenic plant, wherein the insect

specie/biotype is from within the Whiteflies family and wherein the insect specie/biotype is capable of transmitting at least one plant pathogenic virus.

11. The transgenic plant of claim 1 , wherein said plant has an enhanced tolerance to at least one disease as compared to a non-transgenic plant, wherein a plant pathogenic agent of the disease is Agrobacterium tumefaciens.

12. The transgenic plant of claim 1 , wherein said plant has an enhanced tolerance to freezing as compared to a non-transgenic plant.

13 The transgenic plant of claim 1, wherein said plant is selected from the group consisting of tomato, tobacco, cucumber, prune, potato, bean, barley, soybean, pea, beet, grapevine, petunia, abutilon, melon, watermelon, okra, cotton, cassava, wheat, maize, rice, cotton and cabbage.

14. The transgenic plant of claim 1, wherein said plant is selected form a group consisting of ornamental, horticultural, or field crop species.

15. A plant seed produced by the transgenic plant of claim 1 , wherem a plant grown from said seed has an enhanced tolerance to at least one insect specie/biotype as compared to a non- transgenic plant.

16. A plant seed produced by the transgenic plant of claim 1 , wherein a plant grown from said seed has an enhanced tolerance to at least one disease as compared to a non-transgenic plant.

17. A plant seed produced by the transgenic plant of claim 1 , wherein a plant grown from said seed has an enhanced tolerance to freezing as compared to non-transgenic plant.

18. A tissue culture comprising at least one transformed cell of claim 1 or a protoplast derived therefrom, wherein the tissue culture regenerates plants having an enhanced tolerance to at least one insect specie/biotype as compared to a non-transgenic plant.

19. A tissue culture comprising at least one transformed cell of claim 1 or a protoplast derived therefrom, wherein the tissue culture regenerates plants having an enhanced tolerance to at least one disease as compared to a non-transgenic plant.

20. A tissue culture comprising at least one transformed cell of claim 1 or a protoplast derived therefrom, wherein the tissue culture regenerates plants having an enhanced tolerance to freezing as compared to non-transgenic plant.

21. A plant regenerated from the tissue culture of claims 17, 18, or 19.

22. A method of producing a transgenic plant having an enhanced tolerance to freezing, at least one insect specie/biotype, or at least one disease, comprising (a) transforming a plant cell with a polynucleotide encoding HopXl protein or an active fragment or homologue thereof; and (b) regenerating the transformed cell into a plant, wherein the regenerated plant has an enhanced tolerance to at least one insect, at least one disease, or freezing as compared to a corresponding non-transgenic plant.

23. The method of claim 22, wherein the polynucleotide encodes a HopXl protein from the bacterium Pseudomonas syringae pv tomato DC3001.

24. The method of claim 22, wherein the HopXl has an amino acid sequence as set forth is SEQ ID NO: l.

25. The method of claim 22, wherein the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO: 1.

26. The method of claim 22, wherein the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:2.

27. The method of claim 22, wherein the polynucleotide comprises a nucleic acid sequence having at least 70% homology to SEQ ID NO:3.

28. The method of claim 22, wherein the polynucleotide further comprises a regulatory element selected from the group consisting of an enhancer, a promoter, and a transcription termination sequence.

29. The method of claim 28, wherein the promoter is selected from the group consisting of a constitutive promoter, an induced promoter and a tissue-specific promoter.

30. The method of claim 29, wherein the promoter is derived from the Cauliflower mosaic virus DNA.

31. The method of claim 22, wherein the regenerated plant has an enhanced tolerance to a plant pathogenic virus transmitted by a whitefly.

32. The method of claim 22, wherein the regenerated plant has an enhanced tolerance to a disease caused by Agrobacterium tumefaciens.

33. The method of claim 22, wherein the regenerated plant has an enhanced tolerance to freezing.