Root Cap Cuticle: Structure and Functions

The cuticle that covers the surface of primary organs of the shoot consists of a structural polyester of cutin associated with waxes. It plays a key role in protecting the plant against biotic and abiotic stresses. The intriguing expression of genes involved in cutin biosynthesis at the tip of primary and lateral roots of Arabidopsis has led to the investigation of the cell wall ultrastructure at the root cap.

The columella and lateral root cap cells of young primary roots of Arabidopsis, as well as of other species, are covered at the surface of the outer cell wall with an electron-dense layer highly similar to the Arabidopsis leaf cuticle. A comparable, but even more pronounced outer cell wall layer was identified at the emerging lateral root. Further evidence for a root cap cuticle (RCC) was obtained by its removal by cutinases, staining with lipid dyes and modifications in structure and properties in mutants of several cutin biosynthesis genes showing root cap expression.

The aliphatic polyester composition of root tips of 2–day old seedlings revealed that the RCC-polyester is rich in C18:2 dicarboxylic acid, which is the main monomer of the Arabidopsis leaf cuticle, but contains also numerous atypical components, including polyunsaturated fatty acids.

Characterization of biosynthesis, properties and functions of RCC shows that it is a dynamic, semi-permeable structure protecting the young root against osmotic stress. Potential additional roles during plant development will be discussed.