Coping with stress: mechanics of the expanding leaf

The precise control of physical properties of growing tissues is crucial for plant morphogenesis. Sahaf and Sharon (pages 5509–5515 in this issue) examined the mechanics of the expanding leaf and showed that plant tissues respond to stress by changing their mechanical properties. A new method is proposed to distinguish reversible and irreversible tissue deformation, an important step in understanding the physics of a growing cell wall. Leaf blades could hold the key to understanding how plants regulate their growth in different directions. In recent years, considerable progress has been made in research on the molecular and genetic basis of plant mor-phogenesis. However, organ growth is governed by physical processes occurring at multiple scales. Plant cells grow due to the irreversible (or plastic) deformation of their stiff cell wall under tension. The cell wall is a complex hydrated gel composed mainly of pectins and hemicellulose reinforced by stiff cellulose microfibrils. Different models of plant cell wall structure agree on the load-bearing role of cellulose, but the mechanical function of other wall components remains unclear (Cosgrove, 2015). Precise measurements of cell wall behavior under various mechanical conditions are needed to further refine and validate structural models. The mechanical force driving expansion of plant cells primarily results from turgor pressure. In an isolated cell, the magnitude and orientation of tensile stresses is determined only by its geometry and internal pressure. In an expanding tissue, local differences in wall properties or stresses would in theory cause individual cells to grow at different rates. Plant cells are, however, glued to their neighbors via cell walls, forcing them to grow as a continuous tissue and creating residual stresses, also termed tissue stresses (Baskin and Jensen, 2013). In modeling terms, this means the specified growth (i.e. the expansion cells would exhibit if they did not have neighbors) differs from the resultant growth, i.e. the only expansion we can actually observe (Kennaway et al., 2011). The discrepancy between specified and resultant growth depends on how tissues deal with residual stresses, e.g. reducing stresses by deforming passively or building stresses up by resisting them. Since neither specified growth nor mechanical stresses can be measured directly, the best way to investigate this reaction is to apply an external force to a growing tissue. Sahaf and Sharon (2016) show that tobacco leaves expand globally at a similar rate in all directions under natural conditions , i.e. tissue growth is isotropic. However, …