Cloning and Function Characterization of a β-Ketoacyl-acyl-ACP Synthase I from Coconut (Cocos nucifera L.) Endosperm

Coconuts contain beneficial medium-chain fatty acids (MCFAs), particularly lauric acid. The mechanism by which coconuts accumulate MCFAs has been a topic of research because of increasing energy demands. beta-Ketoacyl-ACP synthase (KAS) I, a requisite enzyme for chain length regulation in plants, was isolated from coconut endosperm complementary DNA (cDNA) libraries and designated CocoKASI (GenBank accession: JX275887). CocoKASI contained an open reading frame (ORF) of 1485 bp encoding a polypeptide of 494 amino acids (nucleotide positions 134-1618). Expression of CocoKASI determined by fluorescence quantitative real-time PCR was 60 % higher in developing coconut endosperm than in leaves. Fatty acid profiles of endosperm at two different stages showed that the levels of C8:0, C10:0, C12:0, C14:0, and C18:0 increased, while the relative compositions of other fatty acid species, especially C16:0, C18:1, and C18:3, were reduced. Overexpression of CocoKASI cDNA using the seed-specific tobacco (Nicotiana tabacum L.) napin promoter increased short-chain and MCFAs from C6:0 to C16: 0, although CocoKASI transcript levels varied 100-fold in tobacco seeds from different transgenic plants. These data suggested that CocoKASI had a preference for catalyzing the elongation of C4:0-C14:0 on ACP to yield C6:0-C16:0 fatty acids in plants. This result was similar to the findings reported for KASI in other species. This study characterized and determined the function of KASI from a tropical oil crop. The results give insights into the regulation of fatty acid biosynthesis and the accumulation of MCFAs in higher plants. The findings will contribute to the engineering, altering, and accumulation of short-chain and MCFAs to meet energy demands.