Exploring the Functions of ProteinsSecreted by the Hrp Type III SecretionSystem of Pseudomonas syringae

摘要

P syringae pathogenesis is dependent on the Hrp type III secretion system(T3SS), which injects effector (Avr/Hop) proteins into plant cells. To better understandhow the T3SS functions in pathogenesis, we are exploring the delivery and activity ofthese proteins. The model strain P syringae pv. tomato DC3000 appears to stronglyexpress and inject approximately 30 effectors into plant cells. We are taking threeapproaches to identifying informative biological phenotypes. First, we are expressingthese effectors in a galactose-inducible maner in yeast (Saccharomyces cerevisiae)to identify those that inhibit or kill this model eukaryote. Among the several effectorswhose expression is toxic to yeast, we are focusing on HopAA1-1, which appears tolocalize to yeast mitochondria. C-terminal truncations of HopAA1-1 that diminishkilling in yeast also diminish killing of plant cells following Agrobacterium-mediatedtransient expression. Our observations with HopAA1-1 and other effectors supportthe utility of yeast for exploring effector activities. Second, we are using P fluorescensexpressing cloned clusters of P syringae hrp/hrc genes to explore the ability ofindividual effectors to suppress the basal resistance that is otherwise induced by thisnonpathogen. Our focus in this work is on proteins that appear to function in theapoplast, such as HopP1 , a harpin with a lytic transglycosylase domain. Third, we areconstructing DC3000 polymutants in which multiple effector genes are deleted, andwe are testing these mutants for their ability to grow and cause disease in Arabidopsis,tomato, and Nicotiana benthamiana. Our focus in this work is on hopQ1-1, whosedeletion enables DC3000 (which normally produces virtually no symptoms onN. benthamiana) to cause extensive necrosis. Deletion of hopQ1-1 has no apparenteffect on DC3000 interactions with Arabidopsis and tomato, but deletion of othercombinations of effectors results in marked reductions in virulence in these hosts.The overall goal of this work is to understand how the T3SS enables P syringae tosuppress defenses and cause disease in a host-specific manner in plants.