A serine carboxypeptidase-like acyltransferase is required for synthesis of antimicrobial compounds and disease resistance in oats.

Sam T Mugford,Richard K Hughes,George P Lomonossoff,Rebecca J.M Goss,Xiaoquan Qi,Abhijeet Deb Roy,Kalliopi Papadopoulou,Eva Wegel,Rachel Melton,Lionel Hill,Saleha Bakht,Frank Sainsbury,Anne Osbourn,Mark Philo

doi:10.1105/tpc.109.065870

Abstract

Serine carboxypeptidase-like (SCPL) proteins have recently emerged as a new group of plant acyltransferases. These enzymes share homology with peptidases but lack protease activity and instead are able to acylate natural products. Several SCPL acyltransferases have been characterized to date from dicots, including an enzyme required for the synthesis of glucose polyesters that may contribute to insect resistance in wild tomato (Solanum pennellii) and enzymes required for the synthesis of sinapate esters associated with UV protection in Arabidopsis thaliana. In our earlier genetic analysis, we identified the Saponin-deficient 7 (Sad7) locus as being required for the synthesis of antimicrobial triterpene glycosides (avenacins) and for broad-spectrum disease resistance in diploid oat (Avena strigosa). Here, we report on the cloning of Sad7 and show that this gene encodes a functional SCPL acyltransferase, SCPL1, that is able to catalyze the synthesis of both N-methyl anthraniloyl- and benzoyl-derivatized forms of avenacin. Sad7 forms part of an operon-like gene cluster for avenacin synthesis. Oat SCPL1 (SAD7) is the founder member of a subfamily of monocot-specific SCPL proteins that includes predicted proteins from rice (Oryza sativa) and other grasses with potential roles in secondary metabolism and plant defense.

Highlights

Plants produce a diverse array of natural products
We reported the identification of A. strigosa mutants #376 and #616, both of which are deficient in avenacin acylation (Papadopoulou et al, 1999; Qi et al, 2004)
MRNA in situ hybridization revealed that expression of SCPL1 within the root tip is restricted to the epidermal cells (Figure 2C), as is the case for Saponin deficient 1 (Sad1) and Sad2 (Haralampidis et al, 2001; Qi et al, 2006)

Summary

Introduction

Plants produce a diverse array of natural products. These compounds serve important ecological functions, for example by providing protection against pathogens and herbivores (Dixon, 2001). Serine carboxypeptidase-like (SCPL) proteins have emerged as a new group of acyltransferase enzymes that are able to modify plant natural products (Lehfeldt et al, 2000; Li and Steffens, 2000; Shirley et al, 2001; Fraser et al, 2007; Weier et al, 2008). These two different classes of acyltransferases operate by distinct mechanisms. While BAHD acyltransferases use CoAthioesters as the acyl donors, SCPL acyltransferase enzymes use O-glucose esters (Shirley and Chapple, 2003; Milkowski and Strack, 2004; Milkowski et al, 2004; Baumert et al, 2005; Stehle et al, 2008)

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Results

Conclusion