Root Cap Cuticle: Structure and Functions

Until now, considered as exclusive to aerial plant organs, the cuticle consists of a cutin-based protective structure. 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 cells. The root cap cells of young primary roots and lateral 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. 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 atypical components, including polyunsaturated fatty acids. The RCC of young primary roots and emerging lateral roots play important roles in root physiology and development. The RCC of young primary roots acts as a well-functional diffusion barrier by protection the root meristem from toxic compounds and has thus functional importance for the plant during seedling establishment. Moreover, the RCC of the lateral primordium is needed to avoid organ adhesion causing a delay in lateral root emergence. Plant cuticles of different aerial organs have been exclusively associated with epidermal tissues of the shoot. Our discovery of a cuticle at the root cap now challenges this dogma and adds a new element to our understanding of root anatomy, development, and physiology