UDPglucose: fatty acid transglucosylation and transacylation in triacylglucose biosynthesis

Glandular trichomes of the wild tomato Lycopersicon pennellii Corr. (D'Arcy) secrete large amounts of 2,3,4-tri-O-acylglucoses possessing straight- and branched-chain fatty acids of short to medium chain length (C4-C12). Although previous biosynthetic studies suggested that glucose acylation proceeded via acyl CoA intermediates, repeated attempts to demonstrate isobutyryl-CoA-dependent glucose acylation were unsuccessful. When [14C]isobutyrate is administered to detached L. pennellii leaves, the label is readily converted to 1-O-isobutyryl-beta-D-glucose. This is immediately followed by the appearance of di- and triacylated glucose esters. L. pennellii extracts catalyzed the formation of 1-O-isobutyryl-beta-D-glucose from isobutyrate and UDPglucose, and detached L. pennellii trichomes catalyzed transfer of the isobutyryl moiety from synthetic 1-O-isobutyryl-beta-D-glucose to D-glucose. Detached L. pennellii trichomes also catalyzed the formation of diacylglucose and triacylglucose via transfer of the isobutyryl moiety from 1-O-[14C]isobutyryl-beta-D-glucose to mono- or diacylglucoses, respectively. These studies suggest a multistep mechanism in which activation of fatty acids to their respective high-energy 1-O-acyl-beta-D-glucopyranose derivatives is followed by transfer of the 1-O-acyl moiety to non-anomeric positions of other glucose and/or partially acylated glucose molecules. This appears to be the primary mechanism of activation and fatty acid esterification to glucose in L. pennellii trichomes. Cultivated tomato, L. esculentum Mill., also activates free fatty acids to their 1-O-acyl-beta-D-glucose derivatives but lacks the acyl transfer mechanism for synthesizing polyacylated sugars.