Arabidopsis ketoacyl-CoA synthase 16 forms C36/C38 acyl precursors for leaf trichome and pavement surface wax

The aliphatic waxes sealing plant surfaces against environmental stress are generated by fatty acid elongase complexes, each containing a β-ketoacyl-CoA synthase (KCS) enzyme that catalyzes a crucial condensation forming a new C-C bond to extend the carbon backbone. The relatively high abundance of C35 and C37 alkanes derived from C36 and C38 acyl-CoAs in Arabidopsis leaf trichomes (relative to other epidermis cells) suggests differences in the elongation machineries of different epidermis cell types, possibly involving KCS16, a condensing enzyme expressed preferentially in trichomes (Marks et al., 2009; Hegebarth et al., 2016). Here, KCS16 was found expressed primarily in Arabidopsis rosette leaves, but also in flowers and siliques, and the corresponding protein was localized to the ER. The cuticular waxes on young leaves and isolated trichomes of ksc16 loss-of-function mutants were depleted of C35 and C37 alkanes and alkenes, whereas expression of Arabidopsis KCS16 in yeast and ectopic overexpression in Arabidopsis resulted in accumulation of C36 and C38 fatty acid products. Taken together, our results show that KCS16 is the sole enzyme catalyzing the elongation of C34 to C38 acyl-CoAs in Arabidopsis leaf trichomes, and that it contributes to the formation of extra-long compounds in adjacent pavement cells.