Cutin and
suberin are the two major
lipid-based
polymers of plants.
Cutin is the structural
polymer of the epidermal cuticle, the waterproof layer covering primary aerial organs and which is often the structure first encountered by phytopathogens.
Suberin contributes to the control of diffusion of water and solutes across internal root tissues and in periderms. The
enzymes responsible for assembly of the
cutin polymer are largely unknown. We have identified two Arabidopsis
acyltransferases essential for
cutin biosynthesis, glycerol-3-phosphate
acyltransferase (GPAT) 4 and GPAT8. Double knockouts gpat4/gpat8 were strongly reduced in
cutin and were less resistant to desiccation and to
infection by the fungus Alternaria brassicicola. They also showed striking defects in stomata structure including a lack of cuticular ledges between guard cells, highlighting the importance of
cutin in stomatal biology. Overexpression of GPAT4 or GPAT8 in Arabidopsis increased the content of C16 and C18
cutin monomers in leaves and stems by 80%. In order to modify
cutin composition, the
acyltransferase GPAT5 and the
cytochrome P450-dependent fatty acyl
oxidase CYP86A1, two
enzymes associated with
suberin biosynthesis, were overexpressed. When both
enzymes were overexpressed together the epidermal
polyesters accumulated new C20 and C22 omega-hydroxyacids and alpha,omega-diacids typical of
suberin, and the fine structure and water-barrier function of the cuticle were altered. These results identify GPATs as partners of fatty acyl
oxidases in
lipid polyester synthesis and indicate that their cooverexpression provides a strategy to probe the role of
cutin composition and quantity in the function of plant cuticles.