1.1.19
INVOLVEMENT OF PROTEIN KINASES IN PSEUDOMONAS SYRINGAE AVRB-ELICITED PLANT RESISTANCE RESPONSE

S GOPALAN, L MUNCIE and SY HE

MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA

Background and objectives
Plants resist many phytopathogens with a rapid-cell-death response at the site of infection, termed the hypersensitive response (HR). HR is associated with the restriction of multiplication and spread of the infecting pathogen. According to the gene-for-gene hypothesis, recognition of a pathogen elicitor, encoded by an avirulence (avr) gene, by the product of a corresponding plant-resistance (R) gene, results in resistance. Recently, we [1] and others have shown that some bacterial avr gene products elicit HR cell death when expressed inside the plant cells. It is speculated that these bacterial avr gene products serve as ligands to trigger the plant disease-resistance response. Pseudomonas syringae pv. tomato avirulence protein AvrPto has been shown to bind to Pto, a kinase, in the yeast two-hybrid system. We are investigating the molecular basis of recognition of P. syringae pv. glycinea AvrB by Arabidopsis thaliana.

Results and conclusions
We have found three lines of evidence implicating a role for protein kinases in the recognition of AvrB in Arabidopsis. (i) AvrB is phosphorylated on serine and threonine residues by a plant kinase of ca 50 kDa. (ii) In the yeast two-hybrid system, a MAP kinase, a myb-related transcription factor, and Rubisco bind to AvrB-AvrC chimaeras [2] that exhibit AvrB specificity. These proteins do not bind to inactive chimaeras, to a chimaera exhibiting AvrC specificity, or to a non-specific bait protein. (iii) An Arabidopsis kinase is activated by P. syringae pv. tomato harbouring AvrB when the plant harbours the corresponding R gene, RPM1. This kinase has some biochemical properties and a molecular mass expected of MAP kinases.

The physiological relevance of these observations during the AvrB-elicited plant disease resistance will be presented.

References
1. Gopalan S, Bauer DW, Alfano JR et al., 1996. Plant Cell 8, 1095-1105.
2. Tamaki SJ, Kobayashi DY, Keen NT, 1991. Journal of Bacteriology 173, 301-307.