Covalent associations between lignin and wall proteins including GRP and HRGP have been proposed from analysis of fractions and immunocytochemical and recombinant DNA studies on secondarily thickened cell walls of mature and developing woods and legumes [reviewed by Lam and coworkers (127)] but the linkages have not been chemically characterized.

4.3.2.2 Suberin: a polyaliphatic-polyphenolic association

Some plant cells such as cottonseed hairs and the cork cells of the periderm, which forms bark, develop a suberin layer between the primary wall and the plasma membrane prior to secondary wall formation. Suberin has a polyaliphatic domain and a unique polyphenolic domain containing hydroxycinnamates (128, 129).

4.3.2.3 Cutin: an aliphatic polyester

The outer wall of the epidermal cells of stems and leaves has a multilayered cuticle, which includes a thick cuticular layer composed of cutin, a high molecular weight polyester of C16 and C18 hydroxy fatty acids which may be covalently bound to polysaccharides that form part of the underlying cell wall (130, 131). A second component of the cuticle, cutan, is a non-hydrolysable, aliphatic biopolymer believed to be composed of polyunsaturated fatty acids joined by ether bonds (132). Waxes overlie the cuticle surface.