Molecular and genetic mechanisms of plant disease resistance and pathogen virulence

Plant R genes mediate resistance to virulent phytopathogenic bacteria by recognizing specific avirulence (avr) genes of the pathogen. Recognition activates defense responses and cell death manifested as the hypersensitive response (HR). Absence of an R and/or avr gene permits a virulent pathogen to cause disease. In Arabidopsis, mutually exclusive sets of R genes rely on either EDS1 or NDR1 gene functions to effect resistance. These two signaling pathways are distinguished by R gene structure rather than pathogen class. R genes containing a TIR domain (similarity to Drosophila T&barbelow;oll and mammalian I&barbelow;nterleukin-1 R&barbelow;eceptors) depend on Eds1. R genes that depend on NDR1 contain a CC domain (C&barbelow;oiled C&barbelow;oil). Additional signaling pathways are indicated since some CC containing R genes are independent of EDS1 and NDR1. The RPS4 R gene of Arabidopsis specifies resistance to bacterial pathogens bearing the avrRps4 from Pseudomonas syringae pv. pisi. The Arabidopsis ecotype RLD, which lacks RPS4 function, and eds1 mutants are susceptible to P. s. pv. pisi isolates and P. s. pv. tomato DC3000 bearing avrRps4 . I purified AvrRps4 protein and used it to generate polyclonal antisera from rabbits that specifically detects this bacterial protein. A dexamethasone inducible promoter system driving expression of avrRps4 was transformed into Arabidopsis in order to enable chemical activation of an RPS4 dependent HR. Low level expression of avrRps4 in plants with RPS4 function activates enhanced resistance to virulent DC3000 and Peronospora parasitica (downy mildew), but also stunts plant growth. Screening of 32,000 EMS mutagenized M2 plants for susceptibility to DC3000 bearing avrRps4 generated at least 35 mutant alleles of rps4, pad4, rpr2 , and two new loci. The avrRx5 gene from Xanthomonas campestris pv. vesicatoria that is recognized in Nicotiana benthamiana was cloned and found to act as a virulence factor. X. c. v. strains lacking avrRx5 , while not recognized by N. benthamiana remain unable to cause disease, indicating that host susceptibility factors are required for the establishment of disease. Agrobacterium-mediated transient expression of avrRx5 in planta cannot provoke an HR unless the Type III secretion dependent avrH activity is provided from X. c. pvs. vesicatoria or campestris.