Abstract
The interaction between tomato and the leaf mold pathogen Cladosporium fulvum is controlled in a gene-for-gene manner by plant Cf genes that encode membrane-anchored extracytoplasmic leucine-rich repeat (LRR) glycoproteins, which confer recognition of their cognate fungal avirulence (Avr) proteins. Cf-9 and Cf-4 are two such proteins that are 91% identical yet recognize the sequence-unrelated fungal avirulence determinants Avr9 and Avr4, respectively. As shown previously, Cf-4 specificity is determined by three putative solvent-exposed residues in the central LRR and a deletion of two LRR relative to Cf-9. In this study, we focused on identifying the specificity determinants of Cf-9. We generated chimeras between Cf-9 and its close homologue Cf-9B and identified five amino acid residues that constitute major specificity determinants of Cf-9. Introduction of these residues into Cf-9B allowed recognition of Avr9. Consistent with a role in recognition specificity, the identified residues are putatively solvent exposed in the central LRR and occupy hypervariable positions in the global Cf alignment. One of the specificity residues is not found in any other known Cf protein, suggesting the importance of diversifying selection rather than sequence exchange between homologues. Interestingly, there is an overlap between the Cf-4 and Cf-9 specificity-determining residues, precluding a protein with dual specificity.
Highlights
IntroductionHcr Receptor-like proteins (RLP) recognize the presence of cognate proteins of the fungus that are secreted into the apoplast during the infection process where, most likely, they serve as effectors to promote pathogen fitness (Thomma et al 2005; Van Esse et al 2007)
Within these leucine-rich repeat (LRR), it was found that substitution of the residue A433 in LRR of Cf-9 with the corresponding T in Cf-4 caused significantly reduced Avr9-dependent hypersensitive response (HR), and substitution of L457 in LRR of Cf-9 with the corresponding F in Cf-4 abolished function (Van der Hoorn et al 2001b; Wulff et al 2001)
The chimeras were tested for their ability to generate an Avr9-dependent HR when transiently expressed in N. tabacum
Summary
Hcr RLP recognize the presence of cognate proteins of the fungus that are secreted into the apoplast during the infection process where, most likely, they serve as effectors to promote pathogen fitness (Thomma et al 2005; Van Esse et al 2007). Their recognition is highly specific and activates plant defenses to effectively curb fungal growth. These fungal elicitors are characterized by being small, secreted, cysteine-rich proteins but, unlike the Hcr resistance proteins, the fungal effector proteins are sequence unrelated
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