Comparative Sequence and Functional Analysis of KCS6 and KCS5 from Arabidopsis thaliana and Brassica juncea Establishes Functional Equivalency and Role in Stress Management

KCS (β-ketoacyl-CoA synthases) proteins are known to play a role in the elongation of very long-chain fatty acid (VLCFA) by iterative addition of 2 carbon from malonyl-CoA and catalyzing multiple elongation steps from C18 to C34 (Blacklock and Jaworski 2006). KCS genes encoding elongase as condensing enzymes constitute a large gene family with individual members having substrate specificity responsible for the elongation of specific chain length; the role of individual condensing enzymes and the basis of their substrate specificity however remain to be investigated. In A. thaliana, of the 21 members of KCS gene family, roles of only a handful (KCS1, 2, 6, 9, 10, 16, 18, 20) have been elucidated, and CER6/CUT1/KCS6 is reported to be the only KCS involved exclusively in cuticular wax biosynthesis through elongation of chain length longer than C24. We have previously reported a paralogous copy of KCS6, KCS5/CER60 present across Brassicaceae sharing an average of 82% similarity at nucleotide level and 88% identity between the proteins. However, the contribution of KCS6 and KCS5 in VLCFA production, cuticular wax biosynthesis, plant development, and adaptation remain to be fully characterized. In the present investigation, comparative analysis indicated a correlation between the sequence and structural similarity. Complementation of Saccharomyces cerevisiae elo3 mutants confirmed the conservation of function across orthologs and paralogs and demonstrated the role in conferring abiotic stress tolerance. Reverse genetic models in Arabidopsis thaliana for KCS5 and KCS6 exhibited altered developmental growth, wax profile, and tolerance to abiotic stress. The present study thus highlights the role of KCS5 and KCS6 in VLCFA and wax biosynthesis and amelioration of abiotic stresses.