Search

The Cf genes in tomatoes are responsible for resistance against Cladosporidium fulvum. There are several copies of this gene and 11 of these copies were analyzed for amino acid composition. Results revealed seven hypervariable amino acid residues in the leucine-rich domain. The predicted structure of the protein exposes these residues to the solvent. Further analysis of this domain reveals selection for sequence diversification.

The host gene Prf was found to encode a protein with leucine-zipper, nucleotide-binding and leucine-rich repeat motifs in tomato. Four prf mutant alleles were found to carry alterations within the Prf coding sequence, and a genomic fragment containing Prf was found to complement a prf mutant tomato line for sensitivity to the insecticide Fenthion and for resistance to Pst strains expressing avrPto. In addition, Prf was found to reside in the center of the Pto gene cluster, 500 bp from the Fen gene and 24 kb from the Pto gene.

Resistance to plant diseases may be guided by gene-for-gene interaction in which a single resistance gene (Pto) responds to the expression of a specific avirulence gene in the pathogen. In a tomato plant affected by bacterial speck disease, the Pto gene is associated with a hypersensitive response upon exposure to the organophosphorous insecticide fenthion. The Pto gene and fenthion-sensitivity gene Fen encode similar protein kinases indicating that both participate in signal transduction involved in disease resistance.

Experimental investigations reveal that tomato lines having Mi, a nematode-resistance gene, exhibits resistance to Macrosiphum euphorbiae, a potato aphid. This aphid resistance is connected to the nematode resistance, as revealed by their association in near-isogenic lines. A single major locus, constituting the aphid resistance, is linked to Mi genetically, as revealed by F2 population analysis.

In tomato, Ve is implicated in race-specific resistance to infection by Verticillium species causing crop disease. Characterization of the Ve locus involved positional cloning and isolation of two closely linked inverted genes. Expression of individual Ve genes in susceptible potato plants conferred resistance to an aggressive race 1 isolate of Verticillium albo-atrum. The deduced primary structure of Ve1 and Ve2 included a hydrophobic N-terminal signal peptide, leucine-rich repeats containing 28 or 35 potential glycosylation sites, a hydrophobic membrane-spanning domain, and a C-terminal domain with the mammalian E/DXXXL[Phi] or YXX[Phi] endocytosis signals ([Phi] is an amino acid with a hydrophobic side chain). A leucine zipper-like sequence occurs in the hydrophobic N-terminal signal peptide of Ve1 and a Pro-Glu-Ser-Thr (PEST)-like sequence resides in the C-terminal domain of Ve2. These structures suggest that the Ve genes encode a class of cell-surface glycoproteins with receptor-mediated endocytosis-like signals and leucine zipper or PEST sequences.

The plant-intracellular interaction of the avirulence protein AvrPto of Pseudomonas syringae pathovar tomato, the agent of bacterial speck disease, and the corresponding tomato resistance protein Pto triggers responses leading to disease resistance. Pto, a serine/threonine protein kinase, also interacts with a putative downstream kinase, Pto-interactor 1, as well as with members of a family of transcription factors Pto-interactors 4, 5, and 6. These proteins are likely involved, respectively, in a phosphorylation cascade resulting in hypersensitive cell death, and in defense gene activation. The mechanism by which the interaction of AvrPto and Pto initiates defense response signaling is not known. To pursue the hypothesis that tertiary interactions are involved, we modified the yeast two-hybrid protein interaction trap and conducted a search for tomato proteins that interact with Pto only in the presence of AvrPto. Five classes of AvrPto-dependent Pto interactors were isolated, and their interaction specificity confirmed. Also, to shed light on a recently demonstrated virulence activity of AvrPto, we conducted a standard two-hybrid screen for tomato proteins in addition to Pto that interact with AvrPto: i.e., potential virulence targets or modifiers of AvrPto. By constructing an N-terminal rather than a C-terminal fusion of AvrPto to the LexA DNA binding domain, we were able to overcome autoactivation by AvrPto and identify four classes of specific AvrPto-interacting proteins.

Genetic analysis of plant-pathogen interactions has demonstrated that resistance to infection is often determined by the interaction of dominant plant resistance (R) genes and dominant pathogen-encoded avirulence (Avr) genes. It was postulated that R genes encode receptors for Avr determinants. A large number of R genes and their cognate Avr genes have now been analyzed at the molecular level. R gene loci are extremely polymorphic, particularly in sequences encoding amino acids of the leucine-rich repeat motif. A major challenge is to determine how Avr perception by R proteins triggers the plant defense response. Mutational analysis has identified several genes required for the function of specific R proteins. Here we report the identification of Rcr3, a tomato gene required specifically for Cf-2-mediated resistance. We propose that Avr products interact with host proteins to promote disease, and that R proteins 'guard' these host components and initiate Avr-dependent plant defense responses.

The cloning of maize genes reveals the functioning of disease resistance genes of plants at molecular levels. The maize plant resists the fungal pathogen which infects the plant by degrading the fungal toxin. The plant resistance gene in the tomato is also cloned, which indicates the presence of gene-to-gene resistance. The tomato plant resistance gene identifies the pathogen's avirulence and produces the hypersensitive response. The response triggers the production anti-microbial compounds.