Physiological roles of lignins – tuning cell wall hygroscopy and biomechanics

Lignins constitute the second most abundant carbon-storing biopolymers in the biosphere. These phenolic polymers accumulate in different concentrations, compositions, and localisations within and between cell wall layers and cell types. Lignins were acquired during plant terrestrialisation 450 million years ago, and the diversification of their chemistries and structures during plant evolution and speciation allowed plant cells to adjust and/or gain new functions for facing developmental and environmental challenges. The main property conferred by any lignin polymer is to modify the hygroscopic capacities of plant cell walls to set their responsiveness to changes in water content. To do so, lignin accumulation increases the impermeable, antioxidant, recalcitrant and/or mechanical properties of cell walls to modify their water responsiveness. Adjusting these diverse properties depends on the chemistry, structure and distribution pattern of the lignin polymers, collectively named topochemistry. Lignin topochemistries are differently regulated spatially and temporally for each cell type. In this review, we provide a unifying description of lignins as regulators of cell wall hygroscopy and biomechanics for plant physiology as well as describe the molecular and cellular processes, enabling each cell wall layer to specifically adjust lignin properties.