Modulation of polyamine metabolism inArabidopsis thalianaby salicylic acid

Polyamines (PAs) play important roles in plant defense against pathogens, but the regulation of PA metabolism by hormone-mediated defense signaling pathways has not been studied in depth. In this study, the modulation of PA metabolism in Arabidopsis by salicylic acid (SA) was analyzed, by combining the exogenous application of this hormone with the use of PA biosynthesis and SA synthesis/signaling mutants. SA induced notable modifications of polyamine metabolism, mainly consisting in putrescine accumulation both in whole-plant extracts and apoplastic fluids. Put was accumulated at the expense of increased biosynthesis by arginine decarboxylase 2 and decreased oxidation by copper amine oxidase. Enhancement of Put levels by SA was independent of the regulatory protein Non-Expressor of Pathogenesis Related 1 (NPR1) and the signaling kinases MKK4 and MPK3, but depended on MPK6. On its part, plant infection by Pseudomonas syringae pv. tomato DC3000 elicited Put accumulation in a SA-dependent way. The present study demonstrates a clear connection between SA signaling and plant PA metabolism in Arabidopsis and contributes to understand the mechanisms by which SA modulates PA levels during plant-pathogen interactions. Highlight Salicylic acid modulates polyamine biosynthesis and catabolism in Arabidopsis. Regulatory effects of salicylic acid are independent of the master regulator NPR1 and are mediated by the MPK6 kinase.