ASSIGNMENT OF AMINO-ACID-RESIDUES OF THE AVR9 PEPTIDE OF CLADOSPORIUM-FULVUM THAT DETERMINE ELICITOR ACTIVITY

The AVR9 peptide of Cladosporium fulvum is an elicitor of the hypersen sitive response in tomato plants carrying the Cf-9 resistance gene (MM -Cf9), To determine the structure-activity relationship of the AVR9 pe ptide, amino acids important for AVR9 elicitor activity were identifie d by independently substituting each amino acid of AVR9 by alanine, In addition, surface-exposed amino acid residues of AVR9 were substitute d by other amino acids, Activity of the mutant Avr9 constructs was stu died by expressing the constructs in MM-Cf9 tomato plants, using the p otato virus X (PVX) expression system and assessing the severity of ne crosis induced by each PVX::Avr9 construct, This allowed direct identi fication of amino acid residues of AVR9 that are essential for elicito r activity, We identified amino acid substitutions that resulted in AV R9 mutants with higher, similar or lower elicitor activity compared to the wild-type AVR9 peptide, Some mutants had completely lost elicitor activity, A selection of peptides, representing different categories, was isolated and injected into leaves of MM-Cf9 plants, The necrosis- inducing activity of the isolated peptides correlated well with the ne crosis induced by the corresponding PVX::Avr9 derivatives, Based on th e necrosis-inducing activity of the mutant AVR9 peptides and the globa l structure of AVR9, we assigned sites in AVR9 that are important for its necrosis-inducing activity, We postulate that the ''hydrophobic be ta-loop'' region of the AVR9 peptide is crucial for necrosis-inducing activity in tomato plants that carry the Cf-9 resistance gene.