Plants on the move: towards common mechanisms governing mechanically-induced plant movements

Abstract

One may think that plants seem relatively immobile. Nevertheless, plants not only produce movement but these movements can be quite rapid such as the closing traps of carnivorous plants, the folding up of leaflets in some Leguminosae species and the movement of floral organs in order to increase cross pollination. We focus this review on thigmotropic and thigmonastic movements, both in vegetative and reproductive parts of higher plants. Ultrastructural studies revealed that most thigmotropic and thigmonastic movements are caused by differentially changing cell turgor within a given tissue. Auxin has emerged as a key molecule that modulates proton extrusion and thus causing changes in cell turgor by enhancing the activity of H(+)ATPase in cell membranes. Finding conserved molecules and/or operational molecular modules among diverse types of movements would help us to find universal mechanisms controlling movements in plants and thus improve our understanding about the evolution of such phenomena.

Figure 1.

Thigmonastic movement of leaflets in Mimosa pudica. A: Leaflets open; B: leaflets closing due to touch-induced changes in cell turgor of cells within the pulvinus, a structure located at the base of each leaflet. C: Leaflets closed. The time-lapse between each photograph is about 1 sec.

Figure 2.

Schematic representation of the variation in motor cell shape. The cell remains swollen if pulvini movement is inactive (A), whereas it becomes shrunken after pulvini response (B). Motor cells contain two vacuole types, one tannin-rich (TnV) located near the nucleus (N), the other aqueous and central (V). During shrinkage both vacuoles change their shape. K⁺ and Cl^- fluxes mediate movement by triggering osmotic movement of water. In a cell gaining volume the energy-dependent pumping of protons out of the cell drives K⁺ uptake through specific inward-directed K⁺ channels. In a cell losing volume the flux of Cl^- out of the cell down its concentration gradient drives K⁺ efflux through specific outward-directed K⁺ channels. The electrochemical gradient that enables rapid ion transport through plasma membranes is generated by H⁺-ATPase.

Figure 3.

Dionaea sp, the Venus flytrap (A-B) and Drosera sp (C-E). A: Dionaea modified leaves are divided in two parts, the upper (ul) and the lower leaf (ll). B: The upper leaf has two lobes which center is brightly colored red and contains three sensitive trigger hairs (arrows). The free edge of each lobe is lined with spine-like projections or cilia. C: Drosera have sticky, highly modified colorful leaves that are capable to move acting like small insect traps. D-E: The leaves have several glandular trichomes with mucilage droplets on their apex, commonly called hairs or tentacles. When an insect gets stuck to the tentacles, it acts as a mechanical stimulation which causes the leaf to curl toward and around the prey.

Figure 4.

Tendril coiling movement in Passiflora edulis (passionfruit vine). A: The terminal portion of the tendril touches the stem of a neighboring branch. B: About 2h after the photograph shown in A was taken, the same tendril presents touch-induced coiling (contact coiling) movement around the branch. About 4h after the photograph shown in A was taken, the touch-induced coiling process is irreversible and a spiral begins to form due to tendril differential growth.

Figure 5.

Androgynophore movement in Passiflora sanguinolenta. The image is the result of the superposition of two photographs of flower partially dissected to show the entire androgynophore column. The photographs were taken before (t = 0) and after (t = 1.7 sec) the tip of the androgynophore was touched. The androgynophore column moves toward the stimulus regaining its original position after about 3min if no further stimulus is applied.