Identification of Polyketides in the Cuticular Waxes of Triticum aestivum cv. Bethlehem

Cuticular waxes are complex mixtures consisting mostly of very-long-chain aliphatics with single, primary functional groups. However, the waxes of many plant species also include aliphatics with one or more functional groups residing on subterminal or mid-chain carbons. In the present work, the cuticular wax mixtures from flag leaf blades and peduncles of Triticum aestivum cv. Bethlehem were analyzed in a search for novel wax constituents with in-chain functionalities, potentially of polyketide origin. The structures of compounds belonging to six different compound classes were elucidated using gas chromatography-mass spectrometry of various derivatives. Among them, a series of 2,4-ketols was identified, with odd carbon numbers ranging from C-25 to C-37 and peaking at C-33. The analogous C-33 2,4-diketone was identified as well, together with a pair of co-eluting C-31 mid-chain beta-ketol isomers (16-hydroxyhentriacontan-14-one and 14-hydroxyhentriacontan-16-one), a pair of co-eluting C-30 mid-chain alpha-ketol isomers (15-hydroxytriacontan-14-one and 14-hydroxytriacontan-15-one), the corresponding C-30 14,15-diketone, and a pair of co-eluting C-31 ketones (hentriacontan-14-one and hentriacontan-16-one). All newly discovered structures contain ketone functional groups, with similar C13H27 and C15H31 alkyl chains on either side of the functionalities, thus resembling the previously reported very-long-chain beta-diketones dominating the wheat wax mixtures. Based on these structural characteristics, possible biosynthetic pathways leading to the newly identified polyketide-like compounds are proposed.