Modified branched‐chain amino acid pathways give rise to acyl acids of sucrose esters exuded from tobacco leaf trichomes

Abstract

A major diversion of carbon from branched-chain amino acid biosynthesis/catabolism to form acyl moieties of sucrose esters (6-~-acety~-~,~,~-tr~-~-acy~-a-~-g~ucopyranosy~-~-~-fructofuranos~des) was observed to be associated with specialized trichome head cells which secrete large amounts of sucrose esters. Surface chemistry and acetyl and acyl substituent groups of tobacco (T.I. 1068) sucrose esters were identified and quantified by gas chromatography/mass spectrometry. Sucrose esters were prominent surface constituents and 3-methylvaleric acid, 2-and 3-methylbutyric acid, and methylpropionic acid accounted for 60%, 25% and 9%, respectively, of total C3 -C7 acyl substituents. Radiolabeled Thr, Ile, Val, Leu, pyruvate and Asp, metabolites of branched-chain amino acid pathways, were compared with radioactively labeled acetate and sucrose as donors of carbon to sucrose, acetyl and acyl components of sucrose esters using epidermal peels with undisturbed trichomes. Preparations of biosynthetically competent trichome heads (site of sucrose ester formation) were also examined. Results indicate that 3-methylvaleryl and 2-methylbutyryl groups are derived from the Thr pathway of branched-chain amino acid metabolism, 3-methylbutyryl and methylpropionyl groups are formed via the pyruvate pathway, and that acetyl groups are principally formed directly via acetyl-CoA. Arguments are presented which rule out participation of fatty acid synthase in the formation of prominent acyl acids.Results suggest that the shunting of carbon away from the biosynthesis of Val, Leu and Ile may be due to a low level of amino acid utilization in protein synthesis in specialized glandular head cells of trichomes. This would result in the availability of corresponding 0x0 acids for CoA activation and esterification to form sucrose esters. Preliminary evidence was found for the involvement of cycling reactions in 0x0-acid-chain lengthening and for utilization of pyruvate-derived 2-oxobutyrate to form straight-chain acyl substituents.

Photosynthetic Trichomes Contain a Specific Rubisco with a Modified pH-Dependent Activity

“…Pyruvate might be generated together with CO 2 from malate by the chloroplast NADP+-malic enzyme, which has been identified in proteomic analyses of tobacco trichomes (Van Cutsem et al, 2011;Sallets et al, 2014). Acyl acids of sugar esters are derived from Thr and the pyruvate pathways of branched-chain amino acid metabolism (Kandra et al, 1990). CO 2 is produced by two enzymes: acetolactate synthase, which intervenes in the synthesis of branched-chain keto acid precursors, and 3-isopropylmalate dehydrogenase, which catalyzes the chemical reaction of 3-isopropylmalate into 4-methyl-2-oxopentanoate.…”

Isolation, Fractionation, and Identification of Sucrose Esters from Various Oriental Tobaccos Employing Supercritical Fluids

“…However, the analysis of mainstream particulate phase of cigarette smoke reveals the presence of significant amounts of relatively low molecular weight carboxylic acids possessing branched acyl side chains. Most notable among these acids was 3-methyl pentanoic, 3-methyl butanoic, and 2-methyl propanoic acids (22,23). Branched chain carboxylic acids have also been detected in other Nicotiana species (21).…”

Characterization of the Branched-Chain Amino Acid Aminotransferase Enzyme Family in Tomato

“…Catabolism is believed to be initiated in mitochondria, where the branched-chain keto acid (BCKA) dehydrogenase complex is located (Taylor et al, 2004). The primary fates of BCAAs in plant cells are peptide elongation, glutamate recycling, Glc-and Suc-linked branched-chain esters, branched-chain fatty acid synthesis, and respiration through the synthesis of tricarboxylic acid cycle intermediates (Kandra et al, 1990;Walters and Steffens, 1990;Kroumova et al, 1994;Daschner et al, 1999;Li et al, 2003;Beck et al, 2004;Taylor et al, 2004;Engqvist et al, 2009). BCAA catabolism likely has other functions in plant metabolism.…”