Arabidopsis KCS5 and KCS6 Play Redundant Roles in Wax Synthesis

<i>3-ketoacyl-CoA synthases (KCSs),</i> as components of a fatty acid elongase (FAE) complex, play key roles in determining the chain length of very-long-chain fatty acids (VLCFAs). <i>KCS6,</i> taking a predominate role during the elongation from C26 to C28, is well known to play an important role in wax synthesis. <i>KCS5</i> is one paralog of <i>KCS6</i> and its role in wax synthesis remains unknown. Wax phenotype analysis showed that in <i>kcs5</i> mutants, the total amounts of wax components derived from carbon 32 (C32) and C34 were apparently decreased in leaves, and those of C26 to C32 derivatives were obviously decreased in flowers. Heterologous yeast expression analysis showed that <i>KCS5</i> alone displayed specificity towards C24 to C28 acids, and its coordination with <i>CER2</i> and <i>CER26</i> catalyzed the elongation of acids exceeding C28, especially displaying higher activity towards C28 acids than <i>KCS6</i>. BiLC experiments identified that <i>KCS5</i> physically interacts with <i>CER2</i> and <i>CER26</i>. Wax phenotype analysis of different organs in <i>kcs5</i> and <i>kcs6</i> single or double mutants showed that <i>KCS6</i> mutation causes greater effects on the wax synthesis than <i>KCS5</i> mutation in the tested organs, and simultaneous repression of both protein activities caused additive effects, suggesting that during the wax biosynthesis process, <i>KCS5</i> and <i>KCS6</i> play redundant roles, among which <i>KCS6</i> plays a major role. In addition, simultaneous mutations of two genes nearly block drought-induced wax production, indicating that the reactions catalyzed by <i>KCS5</i> and <i>KCS6</i> play a critical role in the wax biosynthesis in response to drought.