When young plants of Hordeum vulgare. Bryonia dioica. Cucumis sativus. Phaseolus vulgaris. Mimosa pudica. and Ricinus communis. were given a gentle mechanical stimulus by rubbing the internodes for about 10 s once or twice daily, elongation was significantly retarded. Plants of Cucurbita pepo Pisum sativum and Triticum aestivum did not exhibit any such response. The initial response to rubbing was very rapid, elongation stopping less than 3 min after application of the stimulus. When the stimulus was discontinued after 7 days, elongation accelerated, reaching a normal or supernormal rate within 3 or 4 days. Mechanical stimulation also affected aspects of growth and development other than stem elongation. In Mimosa pudica, flower bud production was retarded, as was the growth of the tendrils, leaves, and petioles in Bryonia dioica. It is suggested that this response be called thigmomorphogenesis, and that it represents an adaptation designed to protect plants from the stresses produced by high winds and moving animals. Some evidence indicates that thigmomorphogenesis may be mediated by ethylene.

No abstract

Thigmomorphogenetic responses occur in many environmental settings. The most pronounced effects are found under conditions of extremely high rates of turbulent wind or water flow. However, it is an ubiquitous phenomenon, since mechanical perturbations are to be encountered under all but the most stringent laboratory conditions. Our present understanding of these phenomena is the result of studies at the ecological, anatomical, physiological, biochemical, biophysical and molecular biological levels.

Plant roots grow in the direction of increasing soil moisture, but studies of hydrotropism have always been difficult to interpret because of the effect of gravity. In this study it was found that roots of the mutant pea ;Ageotropum' are neither gravitropic nor phototropic, but do respond tropically to a moisture gradient, making them an ideal subject for the study of hydrotropism. When the root caps were removed, elongation was not affected but hydrotropism was blocked, suggesting that the site of sensory perception resides in the root cap.

Thigmo mechanisms are adaptations that permit a plant to alter growth rates, change morphology, produce tropisms, avoid barriers, control germination, cling to supporting structures, infect a host plant, facilitate pollination, expedite the movement of pollen, spores, or seeds, and capture prey. Through these varied functions, plant thigmo systems have evolved impressive controls of cell differentiation, localized growth rates, regulated synthesis of novel products, and some elegant traps and projectile systems. For most thigmo events, there will be a dependence upon transmission of a signal from the cell wall through the plasmalemma and into the cytoplasm. We propose the possible involvement of integrin-like proteins, Hechtian strands, and cytoskeletal structures as possible transduction components. Many thigmo mechanisms may use some modification of the calcium/calmodulin signal transduction system, though the details of transduction systems are still poorly understood. While transmission of thigmo signals to remote parts of a plant is associated with the development of action potentials, hormones may also play a role. Thigmo mechanisms have facilitated an enormous array of plant and fungal adaptations that make major contributions to their success despite their relatively sessile or immobile states.