Sequestration and transport of lignin monomeric precursors

Lignin is the second most abundant terrestrial biopolymer after cellulose. It is essential for the viability of vascular plants. Lignin precursors, the monolignols, are synthesized within the cytosol of the cell. Thereafter, these monomeric precursors are exported into the cell wall, where they are polymerized and integrated into the wall matrix. Accordingly, transport of monolignols across cell membranes is a critical step affecting deposition of lignin in the secondarily thickened cell wall. While the biosynthesis of monolignols is relatively well understood, our knowledge of sequestration and transport of these monomers is sketchy. In this article, we review different hypotheses on monolignol transport and summarize the recent progresses toward the understanding of the molecular mechanisms underlying monolignol sequestration and transport across membranes. Deciphering molecular mechanisms for lignin precursor transport will support a better biotechnological solution to manipulate plant lignification for more efficient agricultural and industrial applications of cell wall biomass.