An ancestral role for 3-KETOACYL-COA SYNTHASE3 as a negative regulator of plant cuticular wax synthesis

3-KETOACYL-COA SYNTHASE3 (KCS3) lacks canonical catalytic activity and negatively regulates cuticular wax synthesis by reducing the activities of KCS6s across diverse plant taxa from Arabidopsis to moss. The plant cuticle, a structure primarily composed of wax and cutin, forms a continuous coating over most aerial plant surfaces. The cuticle plays important roles in plant tolerance to environmental stress, including stress imposed by drought. Some members of the 3-KETOACYL-COA SYNTHASE (KCS) family are known to act as metabolic enzymes involved in cuticular wax production. Here we report that Arabidopsis (Arabidopsis thaliana) KCS3, which was previously shown to lack canonical catalytic activity, instead functions as a negative regulator of wax metabolism by reducing the enzymatic activity of KCS6, a key KCS involved in wax production. We demonstrate that the role of KCS3 in regulating KCS6 activity involves physical interactions between specific subunits of the fatty acid elongation complex and is essential for maintaining wax homeostasis. We also show that the role of the KCS3-KCS6 module in regulating wax synthesis is highly conserved across diverse plant taxa from Arabidopsis to the moss Physcomitrium patens, pointing to a critical ancient and basal function of this module in finely regulating wax synthesis.