The coding regions from five avirulence (avr) genes from Xanthomonas campestris pv. malvacearum (Xcm), the causal agent of bacterial blight of cotton, were joined to the cauliflower mosaic virus 35S promoter and expressed transiently in cotton leaves after Agrobacterium tumefaciens-mediated gene transfer. A genotype-specific necrosis was observed 3 to 6 days post inoculation for constructs derived from avrB4, avrb7, and avrBIn on cotton lines containing the resistance (R) genes B4, b7, and BIn, respectively, but not on susceptible cotton plants. No necrosis was obtained with plasmids that direct expression of avr genes within A. tumefaciens, showing that the cotton response required transfer of the genes into the plant cells. Addition of a signal peptide sequence into the avr constructs to target expressed Avr protein to the apoplast significantly reduced the responses. The results indicate that intracellular expression of Xcm Avr proteins in cotton having the corresponding R gene causes specific host cell death.
