Glycerol-3-phosphate and systemic immunity.

Glycerol-3-phosphate (G3P), a conserved three-carbon sugar, is an obligatory component of energy-producing reactions including glycolysis and glycerolipid biosynthesis. G3P can be derived via the glycerol kinase-mediated phosphorylation of glycerol or G3P dehydrogenase (G3Pdh)-mediated reduction of dihydroxyacetone phosphate. 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 systemic acquired resistance (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.