Acylation of defence-related natural products in cereals

This proposal is concerned with understanding the synthesis and function of natural products that are produced by plants. Natural products have important ecological roles. They can also have positive or negative effects on crop quality. In addition to their functions in plants they are exploited for commercial use as drugs, dyes, flavourings, perfumes and for a variety of other purposes. Many of these compounds have complex chemical structures and their synthesis involves multi-step pathways. The genetic and biochemical dissection of natural product pathways in plants, many of which are completely uncharacterised, represents a substantial challenge. This proposal is concerned with a specific aspect of natural product synthesis - acylation (the process of adding an acidic group to a compound). Acylation is a common feature of plant-derived natural products and is likely to influence the biological properties of these compounds in ecological interactions. Acyl groups may also act as 'tags' that ensure appropriate trafficking, sequestration and storage of natural products within the plant. The recent discovery of a novel class of plant enzymes that transfer acyl groups to natural products has opened up new opportunities for investigation and manipulation of natural product pathways. These enzymes - serine carboxypeptidase-like (SCPL) proteins - are related to protein-degrading enzymes. However they do not degrade proteins and instead are able to acylate natural products (i.e. have acyltransferase activity). So far SCPL acyltransferases have been shown to catalyse the formation of compounds that confer insect resistance in wild tomato, UV protectants in thalecress and compounds associated with bitterness, astringency and seed oil extraction problems in brassicas. However they are likely to have much wider-ranging importance throughout plant metabolism. Recently we have shown that an SCPL acyltransferase is required for the synthesis of antimicrobial compounds that protect oat against disease. Here we propose to use a powerful combination of chemical and biological approaches to interrogate the mechanism and significance of this acylation process. We will also investigate the broader significance of SCPL acyltransferases for plant defence in other cereals (focussing on rice).

Natural products have important ecological functions and protect plants against biotic and abiotic stresses. Acylation is a common feature of these compounds and has implications for physical, chemical and biological activity and also for subcellular trafficking/sequestration. Investigation of the mechanisms and biological significance of acylation of natural products therefore has clear strategic relevance not only for crop protection but also for food quality, production of plant-derived drugs, medicines and other commercially valuable compounds. A new class of plant enzymes (serine carboxypeptidase-like (SCPL) proteins) that acylate natural products has recently been discovered. To date SCPL acyltransferases have been shown to be involved in the synthesis of compounds that confer insect resistance in tomato and UV stress tolerance in Arabidopsis and brassicas. Recruitment and diversification of SCPL proteins with acyltransferase functions is likely to be widespread and represents a largely unexplored area of plant metabolism. The Osbourn laboratory has recently shown by genetical analysis that an SCPL protein (Sad7) is required for acylation of triterpene glycosides that confer broad spectrum disease resistance in oats. These experiments will provide a comprehensive understanding of the mechanism and significance of this SCPL-mediated acylation process. They will also address the wider significance of SCPL acyltransferases for plant defence through functional analysis of Sad7 homologues in rice.