Identification of likely orthologs of tobacco salicylic acid‐binding protein 2 and their role in systemic acquired resistance in <i>Arabidopsis thaliana</i>

<jats:title>Summary</jats:title><jats:p>Salicylic acid‐binding protein 2 (SABP2) is essential for the establishment of systemic acquired resistance (SAR) in tobacco; SABP2’s methyl salicylate (MeSA) esterase activity is required in healthy systemic tissues of infected plants to release the active defense phytohormone SA from MeSA, which serves as a long‐distance signal for SAR. In the current study, we characterize a new gene family from <jats:italic>Arabidopsis thaliana</jats:italic> encoding 18 potentially active α/β fold hydrolases that share 32–57% identity with SABP2. Of 14 recombinant AtMES (MES for methyl esterase) proteins tested, five showed preference for MeSA as a substrate and displayed SA inhibition of MeSA esterase activity <jats:italic>in vitro</jats:italic> (<jats:italic>AtMES1</jats:italic>, ‐<jats:italic>2</jats:italic>, ‐<jats:italic>4</jats:italic>, ‐<jats:italic>7</jats:italic>, and ‐<jats:italic>9</jats:italic>). The two genes encoding MeSA esterases with the greatest activity, <jats:italic>AtMES1</jats:italic> and ‐<jats:italic>9</jats:italic>, as well as <jats:italic>AtMES7</jats:italic> were transcriptionally upregulated during infection of Arabidopsis with avirulent <jats:italic>Pseudomonas syringae</jats:italic>. In addition, conditional expression of <jats:italic>AtMES1</jats:italic>, ‐<jats:italic>7</jats:italic>, or ‐<jats:italic>9</jats:italic> complemented SAR deficiency in <jats:italic>SABP2</jats:italic>‐silenced tobacco, suggesting that these three members of the <jats:italic>AtMES</jats:italic> family are <jats:italic>SABP2</jats:italic> functional homologs (orthologs). Underexpression by knockout mutation and/or RNAi‐mediated silencing of multiple <jats:italic>AtMES</jats:italic> genes, including <jats:italic>AtMES1</jats:italic>, ‐<jats:italic>2</jats:italic>, ‐<jats:italic>7</jats:italic>, and ‐<jats:italic>9</jats:italic>, compromised SAR in Arabidopsis and correlated with enhanced accumulation of MeSA in the systemic tissue of SAR‐induced plants. Together, the data show that several members of the <jats:italic>AtMES</jats:italic> gene family are functionally homologous to <jats:italic>SABP2</jats:italic> and redundant for MeSA hydrolysis and probably SAR. These data suggest that MeSA is a conserved SAR signal in Arabidopsis and tobacco.</jats:p>