A pulse of blue light induces a transient increase in activity of apoplastic K+ in laminar pulvinus of Phaseolus vulgaris L

Mechanism of the Pulvinus-Driven Leaf Movement: An Overview

Leaf movement is a manifestation of plant response to the changing internal and external environment, aiming to optimize plant growth and development. Leaf movement is usually driven by a specialized motor organ, the pulvinus, and this movement is associated with different changes in volume and expansion on the two sides of the pulvinus. Blue light, auxin, GA, H+-ATPase, K+, Cl-, Ca2+, actin, and aquaporin collectively influence the changes in water flux in the tissue of the extensor and flexor of the pulvinus to establish a turgor pressure difference, thereby controlling leaf movement. However, how these factors regulate the multicellular motility of the pulvinus tissues in a species remains obscure. In addition, model plants such as Medicago truncatula, Mimosa pudica, and Samanea saman have been used to study pulvinus-driven leaf movement, showing a similarity in their pulvinus movement mechanisms. In this review, we summarize past research findings from the three model plants, and using Medicago truncatula as an example, suggest that genes regulating pulvinus movement are also involved in regulating plant growth and development. We also propose a model in which the variation of ion flux and water flux are critical steps to pulvinus movement and highlight questions for future research.

Phototropins and blue light-dependent calcium signaling in higher plants

Plants have several kinds of photoreceptors, which regulate growth and development. Recent investigations using Arabidopsis thaliana revealed that the newly found blue light receptor phototropins mediate phototropism, chloroplast relocation, stomatal opening, rapid inhibition of hypocotyl elongation and leaf expansion. Several physiological studies suggest that one of the intermediates in phototropin signaling is cytosolic Ca2+. Studies using phototropin mutants have demonstrated that phototropins induce an increase in cytosolic Ca2+ concentration. However, the function of Ca2+ in the phototropin-mediated signaling process remains largely unknown. This review presents findings about phototropin-mediated calcium mobilization and the involvement of calcium in blue light-dependent plant responses.

Possible involvement of phototropins in leaf movement of kidney bean in response to blue light

The leaf of kidney bean (Phaseolus vulgaris) moves in response to blue light. The movement is induced by a decrease in the turgor pressure of pulvinar motor cells on the irradiated side. In this study, we investigated the initial event of the movement with respect to function of phototropin and the plasma membrane H+-ATPase in the motor cells. The results indicated that, in dark conditions, phototropin existed in a dephosphorylated state and the H+-ATPase existed in a phosphorylated state. A pulse of blue light (30 s) induced the phosphorylation of phototropin and the dephosphorylation of the H+-ATPase as determined by the binding behavior of 14-3-3 protein. Phototropin phosphorylation occurred rapidly, followed by the transient gradual dephosphorylation of the H+-ATPase. When the specific flavoprotein inhibitor diphenyleneiodonium and the protein kinase inhibitors K-252a and staurosporine were administered to pulvinar cells, both phototropin phosphorylation and H+-ATPase dephosphorylation were inhibited. The phosphorylation and dephosphorylation exhibited similar fluence rate dependencies to blue light. These results indicated that phototropin may function upstream of the plasma membrane H+-ATPase and decrease the activity of H+-ATPase by dephosphorylation. We provide evidence for the existence of three kinds of phototropins in pulvinar motor cells.

Blue light inactivates plasma membrane H(+)-ATPase in pulvinar motor cells of Phaseolus vulgaris L

Unilateral blue light irradiation induces bending of pulvini of Phaseolus vulgaris towards the source of light. The pulvinar bending is caused by a decrease in turgor pressure of motor cells that are irradiated with blue light. Decrease in the turgor pressure is caused by the net efflux of K(+) and counter anions, accompanying membrane depolarization. In the present study the effect of blue light on the activity of plasma membrane H(+)-ATPase was studied in relation to the membrane depolarization. The activity of the plasma membrane H(+)-ATPase was measured using protoplast suspensions prepared from laminar pulvini from primary leaves. A pulse of blue light under continuous red light irradiation induced both a transient increase in the external pH and transient inhibition of the vanadate-sensitive ATPase. Continuous blue light irradiation under continuous red light irradiation induced both a sustained increase in the external pH and sustained inhibition of the vanadate-sensitive ATPase. These results show that blue light inhibits the activity of the plasma membrane H(+)-ATPase. Inactivation of the plasma membrane H(+)-ATPase supports the membrane depolarization induced by the blue light irradiation.

Blue-light-dependent osmoregulation in protoplasts of Phaseolus vulgaris Pulvini

Blue light was found to induce shrinkage of the protoplasts isolated from first-leaf lamina pulvini of 18-day-old Phaseolus vulgaris. The response was transient following pulse stimulation, while it was sustainable during continuous stimulation. No apparent difference was found between flexor and extensor protoplasts. Protoplasts of the petiolar segment located close to the pulvinus showed no detectable response. In the plants used, the pulvinus was fully matured and the petiole was ceasing its elongation growth. When younger, 12-day-old, plants were used, however, the petiolar protoplasts did respond to blue light. The pulse-induced response was similar to that in pulvinar protoplasts, although the response to continuous stimulation was transient and differed from that in pulvinar protoplasts. No shrinkage was induced in pulvinar protoplasts when the far-red-light-absorbing form of phytochrome was absent for a period before blue-light stimulation, indicating that the blue-light responsiveness is strictly controlled by phytochrome. Inhibitors of anion channels and H(+)-ATPase abolished the shrinking response, supporting the view that protoplasts shrink by extruding ions. The response of pulvinar protoplasts is probably involved in the blue-light-induced, turgor-based movement of pulvini. The blue-light responding system in pulvini is suggested to have evolved from that functioning in other growing organs.