Trends in Genetics
ReviewInterplay of signaling pathways in plant disease resistance
Section snippets
R gene-dependent signaling and potentiation mechanisms
Plant R proteins confer specific recognition of a pathogen avr gene product but must also serve as a signal transducer to elicit downstream defenses. A large number of plant R genes specifying resistance to bacterial, fungal, viral or nematode pathogens and aphids, have now been cloned3, 4. Surprisingly, despite the widely different modes of pathogen colonization, analysis of the structural features of R proteins reveals the existence of only a limited number of sequence motifs. These include
Salicylic acid and plant defense
The small defense signaling compound SA has previously been shown to play a central role in plant disease resistance, both in the establishment of SAR and the elaboration of local defense responses in the attacked tissue. Analyses of an expanding panel of Arabidopsis disease resistance mutants with respect to SA levels, as well as the isolation of mutants with defects in SA accumulation, are helping to further elucidate the role of SA in plant resistance.
Mutations in the Arabidopsis gene, PAD4,
Overlapping requirements for jasmonate, ethylene and camalexin in resistance
Several independent analyses have now established a requirement for the plant wound response regulator, JA, in defenses against certain pathogens39. The involvement of another plant hormone, ethylene, in plant–pathogen interactions has also been vigorously debated. Recently, however, it has become evident that participation of ethylene may lie more in the control of disease symptom expression than in determining absolute plant resistance or susceptibility phenotypes. For example, reduced
Interplay of plant defense signaling pathways
The previous sections have illustrated the multiplicity of signaling processes that impinge on plant defense and that signal relay may depend on the particular R protein type conferring resistance, as well as the lifestyle of the pathogen. The question of how different signaling circuits cross-talk with each other now arises. Recent analyses illustrate the plant’s ability to fine-tune responses to particular pathogens in order to activate appropriate subsets of downstream defenses.
As outlined
Conclusion
Phenotypic and molecular analyses of plant mutants compromised in disease resistance have provided a first glimpse of the complexities of pathway utilization and signal communication in plant–pathogen recognition. This complexity no doubt equips the plant with the flexibility to respond to a particular pathogen by activating appropriate subsets of defenses and suppressing inappropriate responses. Further studies have extended the roles of previously characterized genes, such as a requirement
Acknowledgements
The authors thank colleagues at The Sainsbury Laboratory for helpful discussions and Ken Shirasu and Paul Shulze-Lefert for providing pictures of the barley–Erysiphe interaction. Work in J.E.P.’s group at The Sainsbury Laboratory is funded by The Gatsby Charitable Foundation and grants from The Biotechnology and Biological Sciences Research Council and EEC.
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