PLANT DISEASE RESISTANCE GENES: STRUCTURE, FUNCTION AND EVOLUTION
J JONES, CM THOMAS, J PARKER, K HAMMOND-KOSACK, M COLEMAN, E VAN DER BIEZEN, W DURRANT, T ROMEIS and P PIEDRAS
Sainsbury Laboratory, John lnnes Centre, Colney Lane, Norwich NR4 7UH, UK
Background and objectives
NBS/LRR genes confer resistance to viruses, bacteria, fungi and nematodes. This class probably recognizes Avr gene-dependent molecules that enter the plant cell. Two other classes of R gene carry extracytoplasmic (e) LRRs and probably recognize Avr gene-specified molecules delivered from outside the plant cell. Xa21 encodes LRRs and a cytoplasmic protein kinase domain, whereas Cf-2, Cf-4, Cf-5 and Cf-9 carry membrane-anchored eLRRs. Since many R genes have been isolated, attention has shifted to what and how their products recognize, and how signals are transduced to initiate defence mechanisms.
We study tomato Cf-2, Cf-4, Cf-5 and Cf-9 genes conferring resistance to specific races of Cladosporium fulvum. Cf-4 and Cf-9 genes also confer responsiveness to the C. fulvum Avr4 and Avr9 peptides, respectively.
Results and conclusions
Cf-gene function is being investigated at the whole plant and cell culture levels. Salicylic acid (SA) is dispensable for full resistance, though SA is induced late in the Cf-dependent response to Avr peptides, and may play a role in some aspects of the response. Cf-9 is active on transfer to tobacco, and transgenic tobacco suspension cultures respond to Avr9 by producing reactive oxygen species (ROS). This has permitted an investigation into the role of various signalling molecules, particularly protein kinases, in Cf-9-dependent ROS production. The availability of active suspension culture lines carrying a c-myc-tagged Cf-9 should facilitate the search for proteins or peptides that associate with Cf-9 during or before activation. ROS production is inhibited by diphenylene iodonium (DPI) which inhibits the mammalian neutrophil superoxide synthesizing NADPH oxidase. Arabidopsis carries six homologues of the mammalian NADPH oxidase subunit, and these genes may encode a component of the ROS-producing machinery.