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Published on September 28, 2007; 10.1104/pp.107.107300

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Received August 13, 2007
Accepted September 20, 2007

The Cytochrome P450 Enzyme CYP96A15 Is the Mid-Chain Alkane Hydroxylase Responsible for Formation of Secondary Alcohols and Ketones in Stem Cuticular Wax of Arabidopsis

Stephen Greer , Miao Wen , David Bird , Xuemin Wu , Lacey Samuels , Ljerka Kunst , and Reinhard Jetter *

Department of Botany, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada; Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada, V6T 1Z1, Canada

* Corresponding author; email: jetter{at}interchange.ubc.ca.

Most aerial surfaces of plants are covered by cuticular wax that is synthesized in epidermal cells. The wax mixture on the inflorescence stems of Arabidopsis is dominated by alkanes, secondary alcohols and ketones, all thought to be formed sequentially in the decarbonylation pathway of wax biosynthesis. Here, we used a reverse genetic approach to identify a cytochrome P450 enzyme (CYP96A15) involved in wax biosynthesis, and characterized it as a mid-chain alkane hydroxylase (MAH1). Stem wax of T-DNA insertional mutant alleles was found to be devoid of secondary alcohols and ketones (mah1-1), or contain much lower levels of these components (mah1-2 and mah1-3) than wild type. All mutant lines also had increased alkane amounts, partially or fully compensating the loss of other compound classes. In spite of the chemical variation between mutant and wild-type waxes, there were no discernable differences in the epicuticular wax crystals on the stem surfaces. Mutant stem wax phenotypes could be partially rescued by expression of wild-type MAH1 under control of the native promoter as well as the CaMV35S (35S) promoter. 35S-driven overexpression of MAH1 led to ectopic accumulation of secondary alcohols and ketones in Arabidopsis leaf wax, where only traces of these compounds are found in the wild type. The newly formed leaf alcohols and ketones had mid-chain functional groups on or next to the central carbon, thus matching those compounds in the wild-type stem wax. Taken together, the mutant analyses and ectopic expression of MAH1 in leaves suggest that this enzyme can catalyze the hydroxylation reaction leading from alkanes to secondary alcohols, and possibly also a second hydroxylation leading on to the corresponding ketones. MAH1 expression was largely restricted to the expanding regions of the inflorescence stems, specifically to the epidermal pavement cells but not in trichomes and guard cells. MAH1-GFP fusion proteins localized to the ER, providing evidence that both intermediate and final products of the decarbonylation pathway are generated in this sub-cellular compartment, and must subsequently be delivered to the plasma membrane for export towards the cuticle.




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