Research Accomplishment Reports 2007

Characterization of R-Gene-Mediated Signaling and Cross Talk Between Defense Signaling Pathways

P. Kachroo
Department of Plant Pathology

 

Project Description

Our studies with ssi2 and its suppressors indicate that reduced 18:1 levels in the ssi2 plants are responsible for altered defense signaling. Furthermore, replenishing 18:1 levels results in restoration of wild-type-like signaling in the mutant. This suggests that 18:1 or an 18:1-derived molecule(s) acts as the signal for restoring altered defense signaling in ssi2 plants.

In plants, changes in the levels of oleic acid (18:1), a major monounsaturated fatty acid (FA), results in the alteration of salicylic acid (SA)- and jasmonic acid (JA)-mediated defense responses. In addition to SSI2, the Arabidopsis genome encodes six S-ACP-DES-like enzymes, the native expression levels of which are unable to compensate for a loss-of-function mutation in ssi2. The presence of low levels of 18:1 in the fab2 null mutant indicates that one or more S-ACP-DES isozymes contribute to the 18:1 pool. Biochemical assays show that in addition to SSI2, four other isozymes are capable of desaturating 18:0-ACP but with greatly reduced specific activities, which likely explains the inability of these SSI2 isozymes to substitute for a defective ssi2. However, overexpression of the S-ACP-DES1 isoform in ssi2 plants results in restoration of 18:1 levels and thereby rescues all ssi2-associated phenotypes. Transcript level of S-ACP-DES isoforms is reduced in high 18:1-containing plants. Enzyme activities of the desaturase isoforms in a 5-fold excess of 18:1-ACP show product inhibition of up to 73%. Together these data indicate that 18:1 levels are regulated at both transcriptional and post-translational levels.

Interestingly, reduction of 18:1 can also induce HRT gene expression and confer resistance to Turnip Crinkle Virus (TCV). Resistance to TCV is dependent upon the resistance (R) gene, HRT, and a recessive locus rrt. Resistance is also dependent on salicylic acid (SA), EDS1 and PAD4. Exogenous application of SA confers resistance in RRT-containing plants by increasing HRT transcript levels in a PAD4-dependent manner. However, the 18:1-regulated pathway is independent of SA, rrt, EDS1 and PAD4. Reducing the levels of 18:1, via a mutation in the SSI2-encoded S-ACP-DES, or by exogenous application of glycerol, increased transcript levels of HRT as well as several other R genes. Second site mutations in the ACT1-encoded glycerol-3-phosphate (G3P) acyltransferase or GLY1-encoded G3P dehydrogenase restored 18:1 levels in HRT ssi2 plants and reestablished a dependence on rrt. Resistance to TCV and HRT gene expression in HRT act1 plants was inducible by SA but not by glycerol, while that in HRT pad4 plants was inducible by glycerol but not SA. The low 18:1-mediated induction of R gene expression was also dependent on ACT1 but independent of EDS1, PAD4 and RAR1. Intriguingly, TCV inoculation did not activate this 18:1-regulated pathway in HRT plants, but instead resulted in the induction of several genes that encode 18:1-synthesizing isozymes. These results suggest that the 18:1-regulated pathway may be specifically targeted during pathogen infection and that altering 18:1 levels may serve as a novel strategy for promoting disease resistance.

Impact

Presently, control of pathogens relies primarily on chemical applications but in the long run, this strategy is likely to be untenable. Host resistance provides the grower a cost-effective and environmentally-sound method to combat plant diseases. The outcome of the interaction of plants with a given pathogen is governed by several factors including specific interactions between various defense signaling pathways. Among several signaling molecules proposed to modulate defense responses, salicylic acid and jasmonic acid elicit distinct responses and undergo extensive cross-talk. This project will allow us to decipher molecular mechanisms regulating plant defense responses to pathogens. By elucidating the signaling mechanisms through which plant defense responses are activated, new strategies for developing environmentally-sound and cost effective disease resistant crops can be developed.

Publications

Chandra-Shekara, A.C, Venugopal, S.C., Barman, S.R., Kachroo, A., Kachroo, P. (2007) Plastidial fatty acid levels regulate resistance gene-dependent defense signaling in Arabidopsis. Proceedings of the National Academy of Sciences USA, 104:7277-7282.

Kachroo, A., Shanklin, J., Lapchyk, L., Whittle, E., Hildebrand, D., Kachroo, P. (2007) The Arabidopsis stearoyl-acyl carrier protein-desaturase family and the contribution of leaf isoforms to oleic acid synthesis. Plant Molecular Biology 63: 257-271.

Kachroo A., Kachroo P. (2007) Salicylic acid-, jasmonic acid- and ethylene-mediated regulation of plant defense signaling. In: Genetic Engineering, Principles and Methods. Ed. J. Setlow. 28: 55-83.