Plant Cell Growth and Cell Wall Enlargement

Abstract

Abstract Irreversible cell enlargement begins with cell wall loosening which induces wall stress relaxation, leading to cell water uptake and cell enlargement. Growing cell walls consist of a cohesive network of cellulose microfibrils embedded in hydrophilic pectins and cellulose‐binding hemicelluloses. Models of the growing cell wall are provisional hypotheses about how these wall polymers interact to make a strong yet extensible wall. Recent results clarify how wall strength, plasticity and elasticity depend mostly on the stretching, straightening and sliding of cellulose microfibril networks in the wall. Passive sliding of cellulose microfibrils is facilitated by nonenzymic proteins named expansins while various enzymes modify pectins and hemicelluloses, altering their interactions with each other and with cellulose. Growth cessation is correlated with reduced expression of genes that promote wall loosening as well as changes in the direction of cellulose deposition and the structure of matrix polysaccharides, leading to a less extensible cell wall. Key Concepts Irreversible cell enlargement is an essential aspect of plant growth and morphogenesis. Surface enlargement of the cell wall may be highly localised, as in tip‐growing cells, or more evenly distributed over the cell wall surface, a ‘diffuse growth’ pattern common to most cells in the plant body. Cells typically enlarge substantially after they leave the meristem and before they differentiate into mature cells. Cell growth entails simultaneous uptake of water into the cell and irreversible expansion of the cell wall. In physical terms, growth begins with molecular loosening of the cell wall which leads to wall stress relaxation ; this creates the water potential difference needed for water uptake by the cell, resulting in physical enlargement of the cell. The growing cell wall is composed of a strong noncovalent network of cellulose microfibrils embedded in highly hydrated pectins and hemicelluloses that comprise the soft wall matrix. The tensile (in‐plane) mechanics of growing walls is largely determined by the behaviour of the cellulose microfibril network while pectins influence the indentation (out‐of‐plane) mechanics. Growing plant cell walls typically enlarge more rapidly at low pH (called ‘acid growth’), a process that is mediated by nonenzymatic proteins named α‐expansins; α‐expansins facilitate the passive sliding of a network of cellulose microfibrils in response to tensile wall stresses originating from cell turgor pressure. Deposition of new polymers to the wall is usually coordinated with surface expansion, but these are separable processes. Several classes of enzymes modify the structure of pectins and hemicelluloses in the cell wall, potentially affecting cell wall structure and mechanics. Cell wall mechanical properties are complex, encompassing a number of different physical behaviours that often are not well correlated with each other or with cell wall growth. Cessation of cell enlargement likely involves multiple processes, including changes in the direction of cellulose deposition, stiffening of cellulose and matrix networks, and reduced expression of wall loosening proteins.