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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.
2011 Project Description
Study of glycerol metabolism and role of defense proteins has resulted in novel findings that have improved our understanding of defense signaling pathways. These research findings were presented in five oral and three poster presentations at International meetings (American Phytopathological Society, American Society of Plant Biology, Brazilan Phytopatholical Society, Polish Society for Experimental Biology). Work related to this project has also resulted in the training of two undergraduate, four graduate and one postdoctoral researchers.
Our research led to following finding:
1. We showed that glycerol-3-phosphate, a conserved three-carbon sugar, plays an important role in systemic acquired resistance (SAR). Previously, we showed G3P levels contribute to basal resistance against the hemibiotrophic pathogen, Colletotrichum higginsianum. Inoculation of Arabidopsis with C. higginsianum correlated with an increase in G3P levels and a concomitant decrease in glycerol levels in the host. Plants impaired in GLY1 encoded G3Pdh accumulated reduced levels of G3P after pathogen inoculation and showed enhanced susceptibility to C. higginsianum.
Recently, we showed that G3P is also a potent inducer of SAR in plants. SAR is initiated after a localized infection and confers whole-plant immunity to secondary infections. SAR involves generation of a signal at the site of primary infection, which travels throughout the plants and alerts the un-infected distal portions of the plant against secondary infections.
Plants unable to synthesize G3P are defective in SAR and exogenous G3P complements this defect. Exogenous G3P also induces SAR in the absence of a primary pathogen. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues. Together, these observations suggest that the cooperative interaction of DIR1 and G3P mediates the induction of SAR in plants.
2. We evaluated defense related functions of EDS1, PAD4, SAG101 proteins, which are thought to act downstream of resistance protein-mediated signaling. We showed that SAG101 interacts with PAD4 via EDS1 and that the SAG101-EDS1-PAD4 ternary complex is present in the nucleus.
EDS1, which is present in the cytoplasm and nucleus, is detected preferentially in the nucleus in the presence of SAG101. The presence of PAD4 restores the cytoplasmic localization of EDS1. Conversely, the SAG101-EDS1-PAD4 ternary complex, which is detected primarily in the nucleus, is redirected to cytoplasm in the presence of an extranuclear form of EDS1. These results show that protein localization changes in relation to the subcellular localization and/or relative levels of their interacting partners.
We further showed that Arabidopsis plants encode two functional isoforms of EDS1. Both isoforms interact with self and each other, as well as form ternary complexes. SAG101, which is thought to serve as a substitute for PAD4, functions independently in defense signaling against turnip crinkle virus. Our results suggest that EDS1, PAD4, SAG101 function independently as well as in a ternary complex to mediate plant defense signaling.
Chanda, B., Xia, Y., Mandal, M., Yu, K., Sekine, K., Gao, Q-M., Selote, D., Hu, Y., Stromberg, A., Navarre, D., Kachroo, A., Kachroo, P. (2011) Glycerol-3-phosphate, a critical mobile inducer of systemic immunity in Plants. Nature Genetics. 43: 421-427.
Mandal, M.K., Chanda, B., Xia, Y., Yu, K., Sekine, K., Gao, Q-M., Selote D., Kachroo, A., Kachroo, P. (2011) Glycerol-3-phosphate and systemic immunity. Plant Signaling and Behavior, 6: 1871-1874.
Zhu, S., Jeong, R-D., Venugopal, S.C., Lapchyk, L., Navarre, D., Kachroo, A., Kachroo, P*. (2011) Regulatory roles of SAG101 and redundatn EDS1 isoforms in resistance protein-mediated signaling against Turnip Crinkle Virus. PLoS Pathogens, 7: e1002318