Mechanics of Reversible Deformation during Leaf Movement and Regulation of Pulvinus Development in Legumes

Abstract

Plant cell deformation is a mechanical process that is driven by differences in the osmotic pressure inside and outside of the cell and is influenced by cell wall properties. Legume leaf movements result from reversible deformation of pulvinar motor cells. Reversible cell deformation is an elastic process distinct from the irreversible cell growth of developing organs. Here, we begin with a review of the basic mathematics of cell volume changes, cell wall function, and the mechanics of bending deformation at a macro scale. Next, we summarize the findings of recent molecular genetic studies of pulvinar development. We then review the mechanisms of the adaxial/abaxial patterning because pulvinar bending deformation depends on the differences in mechanical properties and physiological responses of motor cells on the adaxial versus abaxial sides of the pulvinus. Intriguingly, pulvini simultaneously encompass morphological symmetry and functional asymmetry along the adaxial/abaxial axis. This review provides an introduction to leaf movement and reversible deformation from the perspective of mechanics and molecular genetics.

Keywords: ELP1/PLP; adaxial/abaxial identity; bending moment; cell wall; cellulose microfibrils; leaf movement; motor cells; pulvinus; reversible deformation; turgor pressure.

Figure 1

Examples of pulvini in three types of leaves. A simple leaf (left) has pulvini at the two ends of a petiole. A trifoliate leaf (middle) has pulvini at the base of a petiole and of leaflets. A pinnate leaf (right) has pulvini at the base of petiolules as well as at that of a petiole and of leaflets. St, a stem; Pe, a petiole; Ptl, a petiolule.

Figure 2

Schematic view of the relationship between water potential and cell volume.

Figure 3

Mechanical models of the bending deformation. (A) A cantilever model of bending deformation. Arrows indicate the degree of expansion or contraction inside an object. (B) Schematic views of the degree of bending modeled as an arc of virtual circles of varying size.

Figure 4

Cross section of a pulvinus observed by scanning electron microscopy (A) and bending deformation of tissue slices of motor cells immediately after (B, left) and after >20 min (B, right) of soaking in hypotonic solution. (A), Desmodium paniculatum; (B), Pueraria montana var. lobata.