A Feedback Regulatory Loop between G3P and Lipid Transfer Proteins DIR1 and AZI1 Mediates Azelaic-Acid-Induced Systemic Immunity

Systemic acquired resistance (SAR), a highly desirable form of plant defense, provides broad-spectrum immunity against diverse pathogens. The recent identification of seemingly unrelated chemical inducers of SAR warrants an investigation of their mutual interrelationships. We show that SAR induced by the dicarboxylic acid azelaic acid (AA) requires the phosphorylated sugar derivative glycerol-3-phosphate (G3P). Pathogen inoculation induced the release of free unsaturated fatty acids (FAs) and thereby triggered AA accumulation, because these FAs serve as precursors for AA. AA accumulation in turn increased the levels of G3P, which is required for AA-conferred SAR. The lipid transfer proteins DIR1 and AZI1, both of which are required for G3P- and AA-induced SAR, were essential for G3P accumulation. Conversely, reduced G3P resulted in decreased AZI1 and DIR1 transcription. Our results demonstrate that an intricate feedback regulatory loop among G3P, DIR1, and AZI1 regulates SAR and that AA functions upstream of G3P in this pathway.