Molecular Genetic Analysis of the Xanthomonas oryzae pv. oryzae Avirulence Determinants

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In the establishment of infection, many mammalian and plant bacterial pathogens produce effector molecules that are delivered to host cells. Transport of these effectors is accomplished through several different mechanisms. For example, bacterial Type III secretion systems channel effectors directly into the host cell whereas bacterial Type I systems secrete effectors into the extracellular environment. In many plant/pathogen interactions, host resistance responses are initiated via intracellular receptors encoded by plant disease resistance genes that recognize Type III secreted effector molecules encoded by bacterial avirulence (avr) genes. In contrast, the mechanisms by which plant cell surface receptors respond to extracellular effectors are unknown. Xa21 is a rice resistance gene encoding a presumed receptor kinase that confers resistance to Xanthomonas oryzae pv. oryzae (Xoo) Philippine race 6, the causal agent of bacterial leaf blight, the most destructive bacterial disease of rice in Asia and Africa. Dr. Ronald has shown that the presumed extracellular leucine rich repeat domain of XA21 is responsible for specific recognition of the avirulent strain Xoo Philippine race 6. Dr. Ronald has identified 6 Xoo genes from this strain that are required for the avirulence function. Three of these genes (xooA, xooB, xooC) encode proteins that share similarity with components of Type I secretion systems of gram negative bacteria. The long-term goal of this proposal is to elucidate the relationship and function of these six Xoo avirulence determinants, identify and characterize the effector molecule and analyze the interaction between the effector and rice receptor kinase XA21. The Xa21/Xoo interaction is particularly advantageous for these studies for several reasons. Xa21 is the only disease resistance gene encoding a receptor kinase so far identified and likely recognizes an extracellular effector. Xoo is genetically tractable and the genes encoding Type I secretion systems are characterized. Finally, rice is a model for studies of all cereals and therefore the expected results will have a broad impact on understanding and controlling bacterial diseases of cereals. Accomplishment of the proposed objectives is expected to lead to the first detailed understanding of the roles of Type 1 secretion systems and sulfation in determining host specificity of phytopathogenic bacteria.