Abstract
Salicylic acid (SA) and its methyl ester, methyl salicylate (MeSA) are well known inducers of local and systemic plant defense responses, respectively. MeSA is a major mediator of systemic acquired resistance (SAR) and its conversion back into SA is thought to be required for SAR. In many plant species, conversion of MeSA into SA is mediated by MeSA esterases of the SABP2 family. Here we show that the Citrus sinensis SABP2 homologue protein CsMES1 catalyzes the hydrolysis of MeSA into SA. Molecular modeling studies suggest that CsMES1 shares the same structure and SA-binding mode with tobacco SABP2. However, an amino acid polymorphism in the active site of CsMES1-related proteins suggested an important role in enzyme regulation. We present evidence that the side chain of this polymorphic residue directly influences enzyme activity and SA binding affinity in CsMES proteins. We also show that SA and CsMES1 transcripts preferentially accumulate during the incompatible interaction between Xanthomonas aurantifolii pathotype C and sweet orange plants. Moreover, we demonstrate that SA and MeSA inhibited citrus canker caused by Xanthomonas citri, whereas an inhibitor of CsMES1 enhanced canker formation, suggesting that CsMES1 and SA play a role in the local defense against citrus canker bacteria.
Highlights
Salicylic acid (SA), known as 2-hydroxybenzoic acid, is a plant hormone that has long been recognized as a defense signaling molecule involved in both local and systemic acquired resistance (SAR) against microbial pathogens in many plant species[1,2,3,4]
CsMES1 eluted with an estimated molecular mass of 54 kDa in size exclusion chromatography, indicating that it is a dimer in solution (Fig. 2B)
We confirmed that CsMES1 is mainly induced during the incompatible interaction between Xanthomonas aurantifolii pathotype C (Xa) and sweet orange, and like tobacco SABP2, is a methyl salicylate (MeSA) esterase
Summary
Salicylic acid (SA), known as 2-hydroxybenzoic acid, is a plant hormone that has long been recognized as a defense signaling molecule involved in both local and systemic acquired resistance (SAR) against microbial pathogens in many plant species[1,2,3,4]. Regarded as one of the main mediators of SAR against numerous microbial pathogens, MeSA is thought to be biologically inert and its conversion back into SA appears to be necessary for the activation of plant defenses at distal sites of pathogen attack[2,3,9,13] This is consistent with the role played by the SA-methylesterases that catalyze the hydrolysis of MeSA into SA. We show that SA and CsMES1 transcripts are predominantly found in sweet orange leaves in response to Xa infection and that while SA and MeSA reduced canker pustule formation caused by Xc, an inhibitor of CsMES1 promoted canker These results suggest that CsMES1 and SA play an important role in the defense against citrus canker pathogens
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