Elicitors of plant defense induce the accumulation of amino acids that suppress both bacterial virulence and growth

Plant immunity relies on the perception of Microbe-Associate Molecular Patterns (MAMPs) from invading microbes to induce defense responses that suppress attempted infections. It has been proposed that MAMP-triggered immunity (MTI) suppresses bacterial infections by suppressing the onset of bacterial virulence. However, the mechanisms by which plants exert this action are poorly understood. Here we uncover that MAMP-perception in Arabidopsis induces the accumulation of free amino acids (AA) in a salicylic acid (SA) dependent manner. When co-infiltrated with Glutamine and Serine, two of the MAMP-induced highly accumulating amino acids, Pseudomonas syringae pv. tomato DC3000 expressed low levels of virulence genes and failed to produce robust infections in otherwise susceptible plants. When applied exogenously, Glutamine and Serine suppressed bacterial virulence and bacterial growth directly without the need for MAMP-perception and SA-signaling, bypassing MAMP-elicited defense. In addition, an increased level of endogenous Glutamine in the leaf apoplast of a gain-of-function mutant of Glutamine Dumper-1 rescued the partially compromised bacterial virulence and growth suppressing phenotype of the SA-induced deficient-2 (sid2) mutant. Our data suggest that MTI suppresses bacterial infections by the direct suppressing effect of accumulating amino acids on the onset of bacterial virulence.