Abstract
In this work, cuticular waxes from flag leaf blades and peduncles of Triticum aestivum cv. Bethlehem were investigated in search for novel wax compounds. Seven wax compound classes were detected that had previously not been reported, and their structures were elucidated using gas chromatography-mass spectrometry of various derivatives. Six of the classes were identified as series of homologs differing by two methylene units, while the seventh was a homologous series with homologs with single methylene unit differences. In the waxes of flag leaf blades, secondary alcohols (predominantly C27 and C33), primary/secondary diols (predominantly C28) and esters of primary/secondary diols (predominantly C50, combining C28 diol with C22 acid) were found, all sharing similar secondary hydroxyl group positions at and around C-12 or ω-12. 7- and 8-hydroxy-2-alkanol esters (predominantly C35), 7- and 8-oxo-2-alkanol esters (predominantly C35), and 4-alkylbutan-4-olides (predominantly C28) were found both in flag leaf and peduncle wax mixtures. Finally, a series of even- and odd-numbered alkane homologs was identified in both leaf and peduncle waxes, with an internal methyl branch preferentially on C-11 and C-13 of homologs with even total carbon number and on C-12 of odd-numbered homologs. Biosynthetic pathways are suggested for all compounds, based on common structural features and matching chain length profiles with other wheat wax compound classes.
Highlights
Most above-ground organs of land plants are covered by a hydrophobic coating, known as the cuticle, which is sealing them against uncontrolled loss of water
Each of these comprised a series of compounds separated into spaced Gas Chromatography (GC) peaks with shared characteristic Mass Spectrometry (MS) fragmentation patterns, and each class was recognized as a homologous series of compounds
Bethlehem revealed the presence of (i) homologous series of secondary alcohols, primary/secondary diols and primary/secondary diol esters, all with secondary hydroxyls on and around identical methylene units 12 carbons away from one chain end; (ii) esterified C13 and C15 2-alkanols with hydroxyl- or keto-functions on C-7 or C-8; (iii) a homologous series of γlactones, as such derived from fatty acids with hydroxyl functions on C-4; and (iv) alkanes with carbon numbers ranging from C28 to C33 and methyl branches on C-11 or C-12
Summary
Most above-ground organs of land plants are covered by a hydrophobic coating, known as the cuticle, which is sealing them against uncontrolled loss of water. The cuticle is composed of the polyester cutin and cuticular wax that can be extracted with organic solvents [1]. Cutin consists of saturated and unsaturated long-chain (LC, C16 and C18) hydroxy or epoxy fatty acids, linked via ester bonds either directly between fatty acids or via glycerol [2,3]. Cuticular waxes are typically very-long-chain (VLC, >C20) saturated aliphatic compounds, bearing no PLOS ONE | DOI:10.1371/journal.pone.0165827. Most commonly encountered are homologous series of even-numbered fatty acids, primary alcohols, alkyl esters and aldehydes, as well as odd-numbered alkanes, secondary alcohols and ketones [1,4]. In the wax mixtures of some plant species, alicyclics (e.g., triterpenoids) and aromatics (e.g., alkylresorcinols) can be quite abundant [5,6,7,8,9]
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