We have employed Arabidopsis thaliana as a model host plant to genetically dissect the molecular pathways leading to disease resistance. A. thaliana accession Col-0 is susceptible to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000 but resistant in a race-specific manner to DC3000 carrying any one of the cloned avirulence genes avrB, avrRpml, avrRpt2, and avrPph3. Fast-neutron-mutagenized Col-0 M2 seed was screened to identify mutants susceptible to DC3000(avrB). Disease assays and analysis ofin planta bacterial growth identified one mutant, ndrl-l (nonrace-specific disease resistance), that was susceptible to DC3000 expressing any one of the four avirulence genes tested. Interestingly, a hypersensitive-like response was still induced by several of the strains. The ndrl-l mutation also rendered the plant susceptible to several avirulent isolates of the fungal pathogen Peronospora parasitica. Genetic analysis of ndrl-1 demonstrated that the mutation segregated as a single recessive locus; located on chromosome III. Characterization of the ndrl-1 mutation suggests that a common step exists in pathways of resistance to two unrelated pathogens.

Plant disease resistance (R) genes confer an ability to resist infection by pathogens expressing specific corresponding avirulence genes. In Arabidopsis thaliana, resistance to both bacterial and fungal pathogens, mediated by several R gene products, requires the NDR1 gene. Positional cloning was used to isolate NDR1, which encodes a 660-base pair open reading frame. The predicted 219-amino acid sequence suggests that NDR1 may be associated with a membrane. NDR1 expression is induced in response to pathogen challenge and may function to integrate various pathogen recognition signals.

SummaryA new reverse genetics method has been developed to identify and isolate deletion mutants for targeted plant genes. Deletion mutant libraries are generated using fast neutron bombardment. DNA samples extracted from the deletion libraries are used to screen for deletion mutants by polymerase chain reaction (PCR) using speci®c primers¯anking the targeted genes. By adjusting PCR conditions to preferentially amplify the deletion alleles, deletion mutants were identi®ed in pools of DNA samples, each pool containing DNA from 2592 mutant lines. Deletion mutants were obtained for 84% of targeted loci from an Arabidopsis population of 51 840 lines. Using a similar approach, a deletion mutant for a rice gene was identi®ed. Thus we demonstrate that it is possible to apply this method to plant species other than Arabidopsis. As fast neutron mutagenesis is highly ef®cient, it is practical to develop deletion mutant populations with more complete coverage of the genome than obtained with methods based on insertional mutagenesis. Because fast neutron mutagenesis is applicable to all plant genetic systems, this method has the potential to enable reverse genetics for a wide range of plant species.

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Summary The rice resistance gene Xa21 confers resistance against the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). The molecular genetic mechanism controlling the integration of the Xa21‐mediated disease resistance response with the developmental program in rice is under study in this model system. Reproducible means of infecting plants at certain developmental stages were designed based on the timing of full expansion of the leaf. Xa21‐resistance progressively increases from the susceptible juvenile leaf 2 stage through later stages, with 100% resistance at the adult leaf 9/10 stage. We found that Xa21 expression is independent of plant developmental stage, infection with Xoo, or wounding. Expression of the Xa21 gene transcript is not correlated with expression of Xa21 disease resistance indicating that the developmental regulation of Xa21‐resistance is either controlled post‐transcriptionally or by other factors.