Metabolism and signal transduction of the plant immune signal pipecolic acid

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Our previous work revealed a critical function for the Lysine-derived plant metabolite pipecolic acid (Pip) in systemic acquired resistance (SAR) and the SAR-associated defense priming phenomenon. In the first period of this project, we have defined the interplay between the immune signals Pip and salicylic acid in the activation of plant defense responses and resistance. Moreover, we showed that the biosynthesis of Pip from L-Lys in plants proceeds via a two-step biochemical process. The alpha-L-Lys-aminotransferase ALD1 catalyzes the conversion of L-Lys to 2,3-dehydropipecolic acid, which is subsequently reduced to Pip. The reductive step involves the action of the plant reductase ORNCD1/SARD4. In addition, we elucidated the biochemical function of the SAR regulator FMO1 as a Pip N-hydroxylase which catalyzes the formation of N hydroxypipecolic acid (Pip-OH) from L-Pip in vitro and in planta. Exogenous Pip-OH complements the immune defects of fmo1 mutant plants and acts as an efficient resistance-enhancing compound in plants towards infection by bacterial and oomycete pathogens. In the next project phase, we aim to further advance the understanding of the Pip metabolic pathway in Arabidopsis. We will biochemically and functionally investigate the reductive step in Pip biosynthesis and the glycosylation of Pip-OH. By using a combination of LC-MS-, NMR-, and GC-MS-based analytical methods, we aim at identifying and characterizing hitherto unknown metabolites involved in Pip metabolic pathway. Both a genetic screen and targeted studies on the role of the immune regulator NPR1 will shed light on the immune signaling processes downstream of Pip-OH accumulation. Analyses of the significance of Pip in SAR long-distance signaling, which had been started in the first project phase, will be continued by using a genetic approach and extended with respect to the function hydroxylated Pip derivatives in this process. Finally, the plant resistance-enhancing action of Pip-OH towards oomycete infection will be investigated at the cytological and molecular levels.