Minisymposium 25: Pathogen virulence mechanisms

Abs # 46005: Activation of Arabidopsis NHO1 during innate immune responses and its suppression by bacterial virulence mechanisms

Presenter:	Jian-Min, Zhou , jzhou@ksu.edu
Authors	Li, Xinyan  (A)   He, Ping  (A)   Satya, Chintamanani  (A)   Douglas, Baker  (A)   Xiaoyan,  Tang  (A)   Jian-Min, Zhou  (A)
Affiliations:	(A): Department of Plant Pathology, Kansas State University
Web Site:	http://www.oznet.ksu.edu/plantpath/

The Arabidopsis NHO1 (NONHOST RESISTANCE 1) gene encodes a glycerol kinase required for resistance to nonhost and incompatible Pseudomonas bacteria, but it has little effect against the virulent P. syringae pv. tamato strain DC3000. Interestingly, the NHO1 expression is induced by nonhost and incompatible Pseudomonas bacteria, but not DC3000. We hypothesize that NHO1 is induced in plants in response to a Pathogen-Associated Molecular Pattern (PAMP). We also hypothesize that DC3000 actively suppresses NHO1 expression, likely through its virulence factors. To dissect signaling mechanisms involved in NHO1 induction, we constructed a NHO1-LUC reporter line. As expected, the transgenic plants displayed a strong induction of NHO1-LUC in response to nonhost and incompatible Pseudomonas strains. This induction does not require an intact type III secretion system (TTSS), as the P. s. pv. phaseolicola mutant lacking the TTSS structural gene hrpA is largely normal in NHO1-LUC induction. This suggests that a surface-associated PAMP instead of type III effectors is responsible for the NHO1 induction. In addition, we isolated a novel Arabidopsis mutant, rrb129, that showed reduced NHO1 induction. Consistent with this finding, rrb129 was compromised in resistance to a nonhost strain, P. s. pv. tabaci. NHO1 contains five W-boxes in its promoter, suggesting a role of WRKY proteins in its regulation. Microarray analysis uncovered three WRKY genes co-expressed with NHO1. Interestingly, the induction of all three WRKY genes also requires RRB129, suggesting that these WRKY genes link RRB129 to NHO1 regulation. RRB129 has been isolated. Its role in plant innate immunity will be discussed. To understand bacterial virulence mechanisms that target NHO1, we tested various DC3000 mutants for their ability to suppress NHO1-LUC expression. While the wild-type DC3000 bacteria showed a complete lack of NHO1-LUC induction, the hrcC and hrpL mutants strongly induced NHO1-LUC, demonstrating that TTSS is required for the suppression. The results are also consistent with the notion that DC3000 carries a PAMP that is capable of inducing NHO1-LUC in the absence of virulence factors. A DC3000 mutant lacking the phytotoxin coronatine was slightly less effective in suppressing NHO1-LUC, suggesting a minor role in NHO1 suppression. Together these results support our hypotheses that NHO1 induction is important for plant innate immunity and that its suppression contributes to DC3000 virulence.