Analyses of barley spike mutant waxes identify alkenes, cyclopropanes and internally branched alkanes with dominating isomers at carbon 9


  • Penny von Wettstein-Knowles

    1. Department of Genetics, Institute of Molecular Biology and Physiology, University of Copenhagen, Øster Farimagsgade 2A, DK-1353 Copenhagen K, Denmark
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About 15% of the epidermal wax on Hordeum vulgare cv. Bonus barley spikes is n-alkanes. Longer homologues are greatly reduced in the eceriferum mutants, cer-a6, cer-e8, cer-n26, cer-n53, cer-n985, cer-x60, cer-yc135 and cer-yl187. Simultaneously hydrocarbons accounting for only traces in the wild-type become prominent in the mutants, although their chain-length distributions remain unchanged. Accordingly several new hydrocarbon series were identified. The two major ones were C23–C35cis monoenoic alkenes (the major 9-ene isomer was part of a homologous series including 11, 13 and 15-enes), and the novel C27–C31 cyclopropanes (the ring carbons of major isomers were 9,10 and 11,12 with lesser amounts of 13,14). Three minor series included 2- and 3-methylalkanes plus C25–C33 internally branched alkanes (methyls on carbons 9, 11, 13, 15 or 17; shorter homologues dominated by the 9 isomer, longer homologues by 11, 13 or 15 isomers). Acyl chains destined for spike waxes are synthesized via acyl and polyketide elongase systems plus associated reductive and decarbonylative/decarboxylative enzyme systems. Both elongation systems are defective in synthesizing C32 acyl chains in all nine mutants. The similarities in the position of the chemical groups (primarily on carbon 9, secondarily on carbon 11) of the alkenes, cyclopropanes and internally branched methyl alkanes imply an origin from a common, hitherto unrecognized associated pathway in barley, designated the enoic pathway. The elongation system leading to the enoic derived hydrocarbons differs from the known elongation systems by inclusion of a mechanism for introducing a double bond.