Molecular analysis of virulence and avirulence activities of the bacterial effector protein XopH and related proteins

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Microbial plant pathogens pose a permanent thread to global food security. Phytopathogenic microbes inject effector proteins into host cells via type III (T3) secretion systems, which are crucial to virulence since they manipulate plant cell physiology to the pathogen’s benefit. Some plant genotypes of otherwise susceptible host species contain resistance (R) proteins that recognize microbial effectors and in turn activate the plant’s defense system. Effectors that are recognized by the plant immune system have been coined as avirulence (Avr) proteins, since their presence converts virulent into avirulent strains. Molecular analysis of Avr-R interactions is imperative to elucidate the mechanistic basis of plant immunity, and provides the key for knowledge-based breeding of pest-resistant crops.We intend to study virulence and avirulence activities of XopH, an effector from the pepper and tomato pathogen Xanthomonas euvesicatoria (Xe). Recent studies revealed that XopH from Xe dephosphorylates phytate (myo-inositol-hexakisphosphate, InsP6), the major P storage compound in plants. XopH is the first T3-effector for which phytase activity has been demonstrated and is the first 1-phytase described in nature (dephosphorylation at position C1). Plants contain numerous inositol polyphosphates, and it remains to be clarified which of the XopH-dependent inositol polyphosphates promotes disease. XopH also triggers an immune reaction in pepper plants containing the cognate R gene Bs7. Preliminary work suggests that Bs7 detects the biological activity, rather than the structure of XopH.The following proposal has two distinct but interconnected aims. 1.) To study XopH and related microbial effectors at the biochemical, cellular and structural level. One major aim here is the identification of XopH-dependent inositol polyphosphates that promote disease and/or trigger plant defense. 2.) To isolate the pepper R gene Bs7 via positional cloning. Availability of a microbial T3 effector with inositol polyphosphate hydrolase activity and its matching host R protein provides the unique opportunity to elucidate for the first time how such an activity can translate into a plant immune reaction.The proposed research will be carried out in a collaborative effort of the Lahaye lab (Tübingen) and the Schaaf lab (Bonn). The Schaaf lab has comprehensive expertise in the biochemical analysis of myo-inositol-derived metabolites. The Lahaye lab has long-term experience in the functional analysis of microbial effectors and the positional-cloning of plant R genes from crop species. We expect that the complementary expertise of both labs will generate strong synergistic effects that will allow a deep mechanistic understanding of virulence and avirulence activities of XopH and related proteins that would not be possible without this concerted collaborative effort.