Plastid ω3‐fatty acid desaturase‐dependent accumulation of a systemic acquired resistance inducing activity in petiole exudates of Arabidopsis thaliana is independent of jasmonic acid

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Abstract
Summary: Systemic acquired resistance (SAR) is an inducible defense mechanism that is activated throughout the plant, subsequent to localized inoculation with a pathogen. The establishment of SAR requires translocation of an unknown signal from the pathogen‐inoculated leaf to the distal organs, where salicylic acid‐dependent defenses are activated. We demonstrate here that petiole exudates (PeXs) collected from Arabidopsis leaves inoculated with an avirulent (Avr) Pseudomonas syringae strain promote resistance when applied to Arabidopsis, tomato (Lycopersicum esculentum) and wheat (Triticum aestivum). Arabidopsis FATTY ACID DESATURASE7 (FAD7), SUPPRESSOR OF FATTY ACID DESATURASE DEFICIENCY1 (SFD1) and SFD2 genes are required for accumulation of the SAR‐inducing activity. In contrast to Avr PeX from wild‐type plants, Avr PeXs from fad7, sfd1 and sfd2 mutants were unable to activate SAR when applied to wild‐type plants. However, the SAR‐inducing activity was reconstituted by mixing Avr PeXs collected from fad7 and sfd1 with Avr PeX from the SAR‐deficient dir1 mutant. Since FAD7, SFD1 and SFD2 are involved in plastid glycerolipid biosynthesis and SAR is also compromised in the Arabidopsis monogalactosyldiacylglycerol synthase1 mutant we suggest that a plastid glycerolipid‐dependent factor is required in Avr PeX along with the DIR1‐encoded lipid transfer protein for long‐distance signaling in SAR. FAD7‐synthesized lipids provide fatty acids for synthesis of jasmonic acid (JA). However, co‐infiltration of JA and methylJA with Avr PeX from fad7 and sfd1 did not reconstitute the SAR‐inducing activity. In addition, JA did not co‐purify with the SAR‐inducing activity confirming that JA is not the mobile signal in SAR.

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