Xanthomonas oryzae pv. oryzae AvrXA21 Activity Is Dependent on a Type One Secretion System, Is Regulated by a Two-Component Regulatory System that Responds to Cell Population Density, and Is Conserved in Other Xanthomonas spp.
The rice pathogen recognition receptor, XA21, confers resistance to Xanthomonas oryzae pv. oryzae (Xoo) strains expressing the pathogen-associated molecule, AvrXA21. XA21 codes for a receptor-like kinase consisting of an extracellular leucine rich repeat (LRR) domain, a transmembrane domain, and a cytoplasmic kinase domain (Ronald et al., 1992; Song et al., 1995). We show that AvrXA21 activity requires the presence of rax (required for AvrXA21) A, raxB, and raxC genes that encode components of a type one secretion system (TOSS). In contrast, an hrpC- strain deficient in type three secretion maintains AvrXA21 activity. Xanthomonas campestris pv. campestris (Xcc) can express AvrXA21 activity if raxST, encoding a putative sulfotransferase, and raxA are provided in trans. Expression of rax genes is dependent on population density and other functioning rax genes, suggesting that AvrXA21 is involved in quorum sensing and that the AvrXA21 pathogen-associated molecule represents an entirely new class of Gramnegative bacterial signaling molecules. We discuss the implications of these results for models of plant innate immunity.
Here, we provide a brief overview of some of the major concepts and molecular features of plant and animal innate immune system perception. We then describe new results from our studies of the XA21–AvrXA21 interaction and discuss how these results call for some modifications in the way we think about plant innate immunity strategies.
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