B1: Function of cuticular waxes on the plant surface in response to wounding
Ivo Feussner & Ljerka Kunst
PhD students: Milena Lewandowska & Lewis Kurschner
The plant surface is covered with a waxy cuticle that serves as a first structural and chemical barrier against pathogens (Samuels et al., 2008). It is synthesized by epidermal cells and consists of two different types of lipophilic materials, cutin and cuticular wax. Cuticular wax is a mixture of highly hydrophobic monomers derived from saturated very long chain fatty acids (VLCFAs) with predominant chain lengths from C26 to C34. VLCFAs made in the epidermis are used for the production of wax components (aldehydes, alcohols, alkanes, ketones, and alkyl esters) via two wax biosynthetic pathways: an alcohol-forming pathway and an alkane-forming pathway. Similar cuticular wax compositions in a wide variety of plant species suggest that cuticle structure and formation are highly conserved across the plant kingdom.
Even though all of the steps involved in the formation of cuticular wax components are known, and representative genes encoding the enzymes have been cloned, the need for the detected complexity of cuticular wax mixtures is not clear. In particular, knowledge on the specific roles and the relative importance of individual molecular classes in plant defence against insects is scarce. So far, only their involvement in forming a mechanical barrier or in insect adhesion or recognition has been suggested. Similarly, our knowledge of the significance of these unusual, extremely long aliphatic molecules in plant resistance against pathogens is incomplete. However, for powdery mildews several lines of evidence suggest that metabolites from the alkane-forming pathway are important for their development and pathogenicity, while the physical structure of epicuticular waxes on the plant surface does not seem to affect the penetration process. Therefore, we will examine and compare the role of cuticular waxes in plant defence against insects with that against biotrophic fungi.