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Characterization of Resistance Gene-mediated Signaling and Role of Oleic Acid and Glycerol 3-Phosphate in Plant Defense
Department of Plant Pathology
Plant diseases have a devastating impact on agricultural production every year and annual worldwide crop losses due to disease have been estimated in excess of $100 billion. The use of resistant varieties, pesticide use, and cultural practices such as crop rotation, are often unsuccessful in providing sustained protection against microbial pathogens. Strategies involving the induction of the host's intrinsic defenses could provide growers with an efficient and cost-effective alternative to combat plant diseases. Such an approach would require a thorough understanding of the molecular mechanism(s) underlying pathogen perception and resistance.
2010 Project Description
Study of fatty acid and glycerolipid metabolisms and their role in plant defense has resulted in novel findings that have improved our understanding of defense signaling pathways. These research findings were presented in seven oral and two poster presentations at Annual Society (American Phytopathological Society, American Society of Plant Biology). Work related to this project has also resulted in the training of three undergraduate, four graduate and two postdoctoral researchers.
Our research led to following findings:
1. We demonstrate a direct role for blue-light photoreceptors in resistance (R) protein-mediated plant defense against Turnip Crinkle Virus (TCV) in Arabidopsis. The blue-light photoreceptors, cryptochrome (CRY) 2 and phototropin (PHOT) 2, are specifically required for maintaining the stability of the R protein HRT, and thereby resistance to TCV. Exogenous application of the phytohormone salicylic acid elevates HRT levels in phot2 but not in cry2 background. These data indicate that CRY2 and PHOT2 function distinctly in maintaining post-transcriptional stability of HRT. HRT-mediated resistance is also dependent on CRY1 and PHOT1 proteins, but these do not contribute to the stability of HRT. HRT interacts with the CRY2/PHOT2-interacting protein COP1, a E3 ubiquitin ligase. Exogenous application of a proteasome inhibitor prevents blue-light-dependent degradation of HRT, suggesting that HRT is degraded via the 26S proteasome. These and the fact that PHOT2 interacts directly with the R protein RPS2 suggest that blue-light photoreceptors might be involved in regulation and/or signaling mediated by several R proteins.
2. We show that the defective SAR in fad7-1 plants is not associated with a mutation in FAD7, rather a second-site mutation in GLABRA (GL) 1, a gene well-known for its role in trichome formation. The compromised SAR in gl1 plants is associated with impairment in their cuticles. Furthermore, mutations in two other components of trichome development, GL3 and TRANSPARENT TESTA GLABRA 1, also impaired cuticle development and SAR. This suggests an overlap in the biochemical pathways leading to cuticle and trichome development. Interestingly, exogenous application of gibberellic acid (GA) not only enhanced SAR in wt plants, but also restored SAR in gl1 plants. In contrast to GA, the defense phytohoromes salicylic acid or jasmonic acid were unable to restore SAR in gl1 plants. GA application increased levels of cuticular components but not trichome formation on gl1 plants, thus, implicating cuticle, but not trichomes, as an important component of SAR. Our findings question the prudence of using mutant backgrounds for genetic screens and underscore a need to reevaluate phenotypes previously studied in the gl1 background.
3. We have also established a novel link between glycerol metabolism and SAR. These data have been submitted for publication in a peer-review journal.
Jeong R-D, Kachroo A, Kachroo P (2010) Blue-light photoreceptors are required for stability and function of a resistance protein mediating viral defense in Arabidopsis. Plant Signaling and Behavior 5:1504-1509.
Xia Y, Yu K, Navarre D, Seebold K, Kachroo A, Kachroo P (2010) The glabra1 mutation affects cuticle formation and plant responses to microbes. Plant Physiology 154: 833-846. Highlighted paper.
Jeong R-D, Chandra-Shekara AC, Barman S, Navarre D, Klessig D, Kachroo A, Kachroo P (2010) CRYPTOCHROME 2 and PHOTOTROPIN 2 regulate resistance protein mediated viral defense by negatively regulating a E3 ubiquitin ligase. Proc. Natl. Acad. Sci. USA, 107, 13538-13543. Highlighted by Faculty of 1000 Biology.
Kang H-G, Oh, C-K, Sato M, Katagiri F, Glazebrook J, Takahashi H, Kachroo P, Martin G, Klessig D (2010) Endosome-associated CRT1 functions early in Resistance gene-mediated signaling in Arabidopsis and Tobacco. Plant Cell 22, 918-936.