Biosynthesis and Regulation of Secondary Cell Wall

Abstract

Secondary plant cell walls are produced by specialized plant cell types required for mechanical support and water transport. They are a microfibril-based composite constituted by celluloses, hemicelluloses, and lignins. Cellulose microfibrils are associated with hemicelluloses leading to the formation of a polysaccharide network that is impregnated by lignin. They are thick and un-extensible, unlike primary walls. The formation of secondary cell walls involves a series of overlapping events as changes in the biosynthetic machinery of their components: microtubule reorganization, formation of cellulose synthase complexes that synthesize cellulose microfibrils with different characteristics, a shift from pectin to secondary wall hemicelluloses involving xylans or mannans with different side-chain decoration, and synthesis of monolignols and polymerization for lignin production. All the abovementioned processes require a precise regulation not only of each event on its own but also to achieve the integration and coordination of the whole. The main actors in the current model of secondary wall regulation are the transcription factors that act as master switches. They are organized in a hierarchical way where NAC transcription factors are at the top of the network and MYB and others are downstream in the signaling cascade.