New Insights into Biosynthesis of Cuticular Wax

Abstract

The outer epidermal cell walls of plant shoots are covered with a cuticle, a continuous lipid structure that protects plants from desiccation, UV‐light, pathogens, and insects. The cuticle is composed of cutin polymer matrix embedded and coated with waxes. Major wax components are derived from very‐long‐chain fatty acid (VLCFA) precursors with chain lengths between C24 and C34. We are taking advantage of wax‐deficient eceriferum (cer) mutants in Arabidopsis to identify and functionally characterize gene products involved in wax biosynthesis, wax export to the cuticle, and regulation of these processes. I will describe our work on the cer2 mutant, which resulted in the identification of a novel component of fatty acid elongation machinery required for the extension of acyl chains beyond C28. This process is unique to plants where C30‐C34 VLCFAs are used for the production of cuticular wax and the pollen coat, critical for pollen hydration. The core VLCFA elongation complex involves four proteins. Condensing enzymes catalyze the first elongation reaction and determine the chain length of fatty acid products. While necessary for the production of all VLCFAs, known condensing enzymes cannot produce VLCFAs longer than C28. Heterologous expression in yeast revealed that CER2 protein could modify the chain‐length specificity of a condensing enzyme, CER6, from C28 to C30. We identified two CER2 homologs in Arabidopsis. Results of our investigation of the specificities and physiological roles of this protein family will be discussed.