Identification of a BAHD acetyltransferase that produces protective acyl sugars in tomato trichomes

Abstract

Glandular secreting trichomes on the surface of tomato plants and many of its relatives in the Solanaceae produce a mixture of O-acyl sugars that contribute to insect resistance. The majority of acyl sucroses produced by the cultivated tomato (Solanum lycopersicum) contain three or four short chain aliphatic acyl esters, and tetra-acyl sucroses have an acetyl group as one of the acyl chains. We previously reported overlapping S. lycopersicum × Solanum pennellii introgression lines (ILs) that fail to accumulate high levels of acetylated tetra-acyl sucroses. A survey of the annotated genes in this region of cultivated tomato chromosome 1 revealed three candidate acyltransferases that were tested for function using virus-induced gene silencing. A member of the BAHD family of acyltransferases (Solyc01g105580, SlAT2) was shown to encode an acetyl-CoA-dependent acyltransferase enzyme capable of acyl sucrose acetylation in vitro. RNAi suppression of SlAT2 in transgenic S. lycopersicum cv. M82 resulted in reduced acyl sugar acetylation, whereas expression of the functional S. lycopersicum allele of SlAT2 in the triacyl sucrose producing IL1-3 restored the ability of the IL to synthesize acetylated tetra-acyl sugars. Transgenic plants with the SlAT2 promoter driving GFP expression showed fluorescence in tips cells of long, slender trichomes that is consistent with acyl sugar acetylation occurring in these cells.

In vitro reconstruction and analysis of evolutionary variation of the tomato acylsucrose metabolic network

“…S1D and S2E). This pattern is identical to that of Sl-ASAT3 (25), Sl-ASAT4 (29), and Sl-IPMS3 (26) and supports the hypothesis that Sl-ASAT1 and Sl-ASAT2 have functions in acylsugar biosynthesis.…”

“…3B). The high-collision-energy negative-ion mode mass spectra of these four compounds revealed fragment ion spectra indistinguishable from the tri-and tetraacylsucroses extracted from M82 that were previously documented by Schilmiller et al (29).…”

“…Cultivated tomato produces primarily tri-and tetraacylated sucrose esters in the tip cell of the long multicellular "type I/IV" trichomes (14,29). Although two trichome apical cell-expressed BAHD acyltransferases that produce tri-and tetraacylsucroses from diacylsucroses were recently described (25,29), the enzymes that convert sucrose to diacylated sucrose have not been previously reported. We used functional genomics approaches to identify candidate enzymes for these earlier steps in the pathway.…”

“…The phylogenetically related ASAT4 enzyme (formerly AT2) is responsible for making tetraacylsucroses in S. lycopersicum and S. habrochaites trichomes by acetylating triacylsucroses using acetyl-CoA (C2-CoA) (29). A variety of loss-of-function alleles of this enzyme are found in populations of S. habrochaites from northern Peru and Ecuador (24), reinforcing the idea that ASAT diversification plays an important role in shaping the strong phenotypic diversity in trichomes of wild tomato.…”

“…Published studies revealed major differences in trichome metabolite accumulation (15,25,26,29) in several introgression lines (ILs), which have regions of the S. pennellii LA0716 genome substituted in place of the S. lycopersicum M82 genome (31). Rescreening the ILs by liquid chromatography-time of flight mass spectrometry (LC-ToF MS) identified a more subtle phenotype: accumulation of two lowabundance species in IL4-1 not seen in the M82 parent (Fig.…”