Unique characteristics of Xanthomonas oryzae pv. oryzae AvrXa21 and implications for plant innate immunity

Department of Plant Pathology, University of California, One Shields Avenue, Davis, CA 95616

Edited by Diter von Wettstein, Washington State University, Pullman, WA, and approved August 28, 2006

Abstract

This article provides a brief overview of some of the major concepts and molecular features of plant and animal innate immune systems. The rice pathogen recognition receptor, XA21, confers resistance to Xanthomonas oryzae pv. oryzae strains producing the AvrXa21 elicitor. Xa21 codes for a receptor-like kinase consisting of an extracellular leucine-rich repeat domain, a transmembrane domain, and a cytoplasmic kinase domain. 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. In contrast, an hrpC ⁻ strain deficient in type three secretion maintains AvrXa21 activity. Xanthomonas campestris pv. campestris can express AvrXa21 activity if raxST, encoding a putative sulfotransferase, and raxA are provided in trans. Expression of rax genes depends on population density and other functioning rax genes. This and other data suggest that the AvrXa21 pathogen-associated molecule is involved in quorum sensing. Together these data suggest that AvrXa21 represents a previously uncharacterized class of Gram-negative bacterial signaling molecules. These results from our studies of the XA21/AvrXa21 interaction call for some modifications in the way we think about innate immunity strategies.

Footnotes

*To whom correspondence should be addressed. E-mail: pcronald{at}ucdavis.edu
Author contributions: S.-W.L. and S.-W.H. designed research; S.-W.L. and S.-W.H. performed research; S.-W.L. contributed new reagents/analytical tools; S.-W.L. analyzed data; and S.-W.L., L.E.B., and P.C.R. wrote the paper.
This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “From Functional Genomics of Model Organisms to Crop Plants for Global Health,” held April 3–5, 2006, at The National Academy of Sciences in Washington, DC. Papers from this Colloquium will be available as a collection on the PNAS web site. The complete program is available on the NAS web site at www.nasonline.org/functional_genomics.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Abbreviations:

LRR,

leucine-rich repeat;

PAMP,

pathogen-associated molecular pattern;

PRR,

pathogen recognition receptor;

RLK,

receptor-like kinase;

Xoo,

Xanthomonas oryzae pv. oryzae;

NBS,

nucleotide-binding site;

TTSS,

type three secretion system;

TOSS,

type one secretion system;

R,

arginine;

D,

asparatate;

IRAK,

interleukin 1 receptor-associated kinase;

Xcc,

X. campestris pv. campestris;

QS,

quorum sensing.