Agroinfiltration is a versatile tool that facilitates comparative analysesof Avr9/Cf-9-induced and Avr4/Cf-4-induced necrosis

The avirulence genes Avr9 and Avr4 from the fungal tomato pathogen Cladospo rium fulvum encode extracellular proteins that elicit a hypersensitive resp onse when injected into leaves of tomato plants carrying the matching resis tance genes, Cf-9 and Cf-4, respectively, We successfully expressed both Av r9 and Avr4 genes in tobacco with the Agrobacterium tumefaciens transient t ransformation assay (agroinfiltration), In addition, we expressed the match ing resistance genes, Cf-9 and Cf-4, through agroinfiltration, By combining transient Cf gene expression with either transgenic plants expressing one of the gene partners, Potato virus X (PVX)-mediated Avr gene expression, or elicitor injections, we demonstrated that agroinfiltration is a reliable a nd versatile tool to study Avr/Cf-mediated recognition. Significantly, agro infiltration can be used to quantify and compare Avr/Cf-induced responses, Comparison of different Avr/Cf-interactions within one tobacco leaf showed that Avr9/Cf-9-induced necrosis developed slower than necrosis induced by A vr3/Cf-4, Quantitative analysis demonstrated that this temporal difference was due to a difference in Avr gene activities. Transient expression of mat ching Avr/Cf gene pairs in a number of plant families indicated that the si gnal transduction pathway required for Avr/Cf-induced responses is conserve d within solanaceous species. Most non-solanaceous species did not develop specific Avr/Cf-induced responses. However, coexpression of the Avr4/Cf-4 g ene pair in lettuce resulted in necrosis, providing the first proof that a resistance (R) gene can function in a different plant family.