Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture

Department

Plant Pathology

First Advisor

Dr. Pradeep Kachroo

Abstract

Plants use several strategies to defend themselves against microbial pathogens. These include basal resistance, which is induced in response to pathogen encoded effector proteins, and resistance (R) protein-mediated resistance that is activated upon direct or indirect recognition of pathogen encoded avirulence protein(s). The activation of Rmediated signaling is often associated with generation of a signal, which, upon its translocation to the distal uninfected parts, confers broad-spectrum immunity against related or unrelated pathogens. This phenomenon known as systemic acquired resistance (SAR) is one of the well-established forms of induced defense response. However, the molecular mechanism underlying SAR remains largely unknown. Induction of plant defense is often associated with a fitness cost, likely because it involves reprogramming of the energy-providing metabolic pathways. Glycerol metabolism is one such pathway that feeds into primary metabolism, including lipid biosynthesis. In this study, I evaluated the role of glycerol-3-phosphate (G3P) in host-pathogen interaction. Inoculation with the hemibiotrophic fungal pathogen Colletotrichum higginsianum led to increased accumulation of G3P in wild-type plants. Mutants impaired in biosynthesis of G3P showed enhanced susceptibility, suggesting a correlation between G3P levels and basal defense. Conversely, increased biosynthesis of G3P correlated with enhanced resistance. The Arabidopsis genome encodes one copy of glycerol kinase (GK), which catalyzes phosphorylation of glycerol to G3P, and five copies of G3P dehydrogenase (G3Pdh), which catalyze reduction of dihydroxyacetone phosphate to G3P. Analysis of plants mutated in various G3Pdh's showed that plastidal lipid biosynthesis was only dependent on the GLY1 isoform but the pathogen induced G3P pool required the function of GLY1 and two other G3Pdh isoforms. Interestingly, compromised G3P biosynthesis in GK and G3Pdh mutants also compromised SAR, which was restored when G3P was provided exogenously. Detailed biochemical analysis showed that G3P was transported to distal tissues and that this process was dependent on a lipid transfer protein, DIR1. Together, these results show that G3P plays an important role in both basal- and induced-defense responses.

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