Arabidopsis 3-ketoacyl-CoA synthase 17 produces tetracosanoic acids required for synthesizing seed coat suberin.

Very long-chain fatty acids (VLCFAs) are precursors for the synthesis of membrane lipids, cuticular waxes, suberins, and storage oils in plants. The 3-ketoacyl CoA synthase (KCS) catalyzes the condensation of C2 units from malonyl-CoA to acyl-CoA, the first rate-limiting step in VLCFA synthesis. In this study, we revealed that Arabidopsis KCS17 catalyzes the elongation of C22 to C24 VLCFAs required for synthesizing seed coat suberin. Histochemical analysis of Arabidopsis plants expressing GUS under the control of the KCS17 promoter revealed predominant GUS expression in seed coats, petals, stigma, and developing pollens. The expression of KCS17:eYFP driven by the KCS17 promoter was observed in the outer integument1 of Arabidopsis seed coats. The KCS17:eYFP signal was detected in the endoplasmic reticulum of tobacco epidermal cells. The levels of C22 VLCFAs and their derivatives, primary alcohols, α,ω-alkane diols, ω-hydroxy fatty acids, and α,ω-dicarboxylic acids increased by approximately 2-fold, but C24 VLCFAs, ω-hydroxy fatty acids, and α,ω-dicarboxylic acid levels were reduced by half in kcs17-1 and kcs17-2 seed coats relative to the wild type. The seed coat of kcs17 displayed decreased autofluorescence under UV and increased permeability to tetrazolium salt compared to the WT. Seed germination and seedling establishment of kcs17 were more delayed by salt and osmotic stress treatments than WT. KCS17 formed homo- and hetero-interactions with KCR1, PAS2, and ECR, but not with PAS1. Therefore, KCS17-mediated VLCFA synthesis is required for suberin layer formation in Arabidopsis seed coats.