Why does Populus trichocarpa need cuticular waxes? Looking for the answer in natural accessions

Plants communicate with the environment via a chemical language of specialized metabolites. My primary objective is to understand this biochemical response by deciphering the biological role of specialized metabolites, and how that drives the appearance of new genes, new enzymes and eventually, new metabolites. My current research explores natural variation in cuticular waxes in Populus spp. and integrates the metabolite information with a functional genomics approach to identify genes involved in the biosynthesis and regulation of cuticular waxes. The cuticle is the extracellular hydrophobic barrier that protects the aerial surfaces of land plants; its outermost layer consists of cuticular waxes whose composition varies among species, tissues and developmental stages. A screen of natural accessions of Populus trichocarpa revealed that the leaf cuticular waxes are predominantly composed of alkenes. Alkenes are linear hydrocarbons with one or more double bonds, and despite their many industrial applications, including advanced biofuels, their biosynthetic origin in plants was completely unknown. Interestingly, in Populus, the accumulation of alkenes is controlled by leaf development and is limited to the abaxial side of the leaf. By mining the leaf transcriptome of P. trichocarpa, a -ketoacyl CoA synthase (PotriKCS1) was subsequently found to be downregulated in leaves originating from accessions lacking alkenes. We demonstrate biochemically that PotriKCS1 elongates unsaturated fatty acids and is part of an eight-gene cluster of recent evolution in the Salicaceae; one that diversified the complement of cuticular lipids found in Populus trichocarpa leaves. Moreover, the accumulation of alkenes in the leaf cuticle is associated with increased growth and resistance to the leaf spot pathogen Septoria musiva. This knowledge can be used to breed trees more resistant to diseases and better growth, or engineer plant oils in other species.