suberin

Suberin is a hydrophobic biopolymer deposited in the cell walls of certain plant tissues, most notably the cork (phellem) of bark and the endodermis of roots. It forms a continuous barrier that helps regulate water and solute movement, protects against pathogens, and contributes to the mechanical stability of tissues.

Suberin consists of two major structural domains. The aliphatic domain forms a polyesters rich in long-chain fatty acids, including ω-hydroxy acids and α,ω-dicarboxylic acids, often linked by ester bonds. Glycerol may be present as a cross-linking component. The polyphenolic domain, integrated with the aliphatic portion in some tissues, contains phenolic compounds such as ferulic and p-hydroxybenzoic acids. The precise composition and the relative amounts of these domains vary by species, tissue, and developmental stage.

Localization and function are best described in two contexts. In roots, suberin forms the Casparian strip in the endodermis and suberin lamellae in the outer cell walls, restricting apoplastic flow and helping regulate uptake of water and minerals. In aerial tissues, suberin is a key component of the cork layer, providing a waterproof barrier that reduces water loss and protects underlying tissues from pathogens and physical damage. Overall, suberin contributes to drought tolerance, soil salinity resistance, and defense against biotic and abiotic stress.

Biosynthesis begins in specialized suberin-forming cells and involves elongation and modification of fatty acid chains to produce ω-hydroxy and dicarboxylic acids, followed by polymerization into an ester-linked polyester. Enzymes such as cytochrome P450 ω-hydroxylases, acyltransferases, and related transporters facilitate monomer production, transport to the wall, and cross-linking.