xoo genomics and Identification of the avrXa21 molecule
Most R genes encode presumed intracellular proteins that are predicted to bind intracellular ligands encoded by bacterial avr genes. In contrast, XA21 appears to bind its ligand, called avrXa21, extracellularly. We have identified three Xanthomonas oryze pv. oryzae (Xoo) genes required for AvrXa21 activity (raxA, raxB and raxC). These genes encode the three components of a bacterial type1 secretion system. Type I secretion systems secrete molecules from the bacterial cytoplasm directly to the bacterial cell surface or the extracellular environment.
These results suggest that avrXa21 may be a type1-secreted molecule. This is a significant finding as all other phytobacterial pathogens characterized to date use a type III system for secretion of avr molecules.
We have not yet determined the nature of avrXa21 and are actively seeking this molecule. Several Xanthomonas genomes have been fully sequenced including a draft sequence of Xoo. Bioinformatic analysis indicates that the raxABC subclass of transporters secrete peptides. Although such bacterial peptides have not been shown to interact with plant hosts, we are particularly interested in this possibility because Fls2, systemin and phytosulfokine (PSK) are all small peptides recognized by LRR-RK proteins. In Gram positive bacteria, such peptides are involved in quorum-sensing. Indeed we have shown that expression of rax genes is dependent on population density and other functioning rax genes, suggesting that AvrXA21 is involved in quorum sensing. We have identified 2 two-component regulatory systems that are required for avrXa21 activity. We are using a mutant that overexpresses the response regulator RaxR to screen for genes that are up-regulated in this strain using a Xoo whole genome microarray that we have constructed. Finally, we have identified three rax genes involved in sulfation suggesting that, like PSK, the avrXa21 molecule may be sulfated.
Lee SW, Han SW, Bartely L, Ronald PR
. 2006. The unique characteristics of Xanthomonas oryzae pv. oryzae AvrXa21 and implications for plant innate immunity. PNAS. 103. (49) :18395
Goes da Silva F, Shen Y, Dardick C, Burdman S, Yadav R, Sharma P, and Ronald PC
. 2004. Components of a type I secretion system and a sulfotransferase-like protein are required for the Xa21 receptor kinase mediated defense response. MPMI. 17. 593
Burdman S, Shen Y, Lee S, Xue Q and Ronald PC
. 2004. RaxH/RaxR: a two-component regulatory system in Xanthomonas oryzae pv. oryzae required for avrXa21 activity. MPMI. 17. 602
Shen Y, Sharma P, Silva F, Ronald PC. 2002. The Xanthomonas oryzae pv. oryzae raxP and raxQ genes encode an ATP sulfurylase and APS kinase that are required for AvrXa21 avirulence activity. Mol. Microbio. 44. 37-48
