Xanthomonas oryzae pv. oryzae rax genes required for AvrXa21 activity

Components of innate immune systems in both plants and animals share many conserved features. Most notably, they sense the presence of pathogen-associated molecules (PAMs), which represent conserved molecular struc-tures, and avirulence (Avr) factors that are strain specific molecules produced by phytopathogens. Little is known about how plant hosts sense and respond to PAMs or Avr factors at the cell surface. One of the best-characterized examples is the rice Xa21 receptor kinase that mediates recognition of Xanthomonas oryzae pv. oryzae (Xoo) strains expressing AvrXa21 activity. We have identified eight Xoo genes, falling into three classes, which are required for AvrXa21 activity. raxA, raxB and raxC encode proteins with similarity to components of bacterial type I secretion systems. The raxQ and raxP encoded proteins function in concert to produce phosphoadenosine phosphosulfate (PAPS), an active form of sulfate. The raxSt encoded protein shows similarity with mammalian and bacterial sulfotransferases that use PAPs as the sulfuryl donor. Finally, two genes, RaxH and raxR, encode proteins with similarity to two-components regulatory systems and regulate raxSt expression. Based on our results, we hypothesize that upon sensing of the plant environment, the AvrXa21 molecule is sulfated and then secreted by the RaxABC Type I secretion system making it available for race specific interactions with the rice receptor kinase XA21