Minisymposium 9: Plant Pathogen/Symbiont Interactions

Abs # 21002: Knockout of the Arabidopsis NPR1-related gene, NPR4, compromises defense responses

Presenter:	Liu, Guosheng , guosheng.liu@nrc-cnrc.gc.ca
Authors	Liu, Guosheng  (A)   Alonso, Jose M (B) (C)  Ecker, Joseph R (C)   Fobert, Pierre  (A)
Affiliations:	(A): National Research Council of Canada/Plant Biotechnology Institute (B): Department of Genetics, North Carolina State University (C): The Salk Institute

Sequencing of the Arabidopsis thaliana genome revealed that there are six members to the NPR1/NIM1 gene family. Given the pivotal role played by NPR1/NIM1 in controlling systemic acquired disease resistance, functional characterization of each member of this family appeared to be justified. In this study, reverse genetics were utilized to analyze the role of one NPR1/NIM1-like gene, which we called NPR4. Conceptual translation of NPR4 suggests that it encodes a protein of 65.1 kDal with 36% identity to NPR1. A T-DNA insertion was identified in the first exon of NPR4, 268 nucleotides into the coding region. No NPR4 transcript was detected in northern blot hybridization of lines homozygous for this insertion. These plants displayed no obvious developmental phenotypes, but were more susceptible to both virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae pv tomato DC3000 as well as to the fungal pathogen Erysiphe cichoracearum. The enhanced disease susceptibility phenotype of the npr4 mutant was consistently milder than that of the npr1-3 mutant and could be complemented by over-expression of a wild-type NPR4 transgene. Whereas npr1 mutants failed to accumulate transcripts of pathogenesis-related genes such as PR1, PR2, and PR5 following treatment with salicylic acid (SA), expression of these genes was only marginally reduced in the npr4 mutant. However, npr4 mutants infected with P. syringae after treatment with SA were compromized in PR gene expression. Transient expression of GFP:NPR4 fusion proteins in onion epidermal cells revealed that they were predominantly localized in nucleus. Furthermore, NPR4 interacted with TGA bZIP transcription factors in the yeast two-hybrid system. In wild-type plants, NPR4 was expressed predominantly in young leaves. It was rapidly induced by pathogen challenge and SA treatment, and was repressed by the jasmonic acid (JA) treatment, suggesting that NPR4 could be implicated in the cross-talk between the SA and JA pathways. Consistent with the above hypothesis, expression of PDF1.2, a marker gene in JA pathway, was rapidly inhibited in the npr4 mutant after treatment with methyl-JA. Together our data implicate NPR4 as a regulator of disease resistance in Arabidopsis, probably as a modulator of PR gene expression. Based on gene expression patterns and mutant phenotypes, both common and unique aspects of NPR1 and NPR4 function could be resolved.