Potassium Channels in Samanea saman Protoplasts Controlled by Phytochrome and the Biological Clock

Abstract

Leaflet movement in legumes depends on rhythmic, light-regulated ion fluxes in opposing regions of the leaf-moving organ. In flexor and extensor protoplasts from Samanea saman Merrill, opening and closing of K(+) channels were rhythmic in constant darkness. When channels were open in flexor protoplasts they were closed in extensor protoplasts, and vice versa. The rhythms were shifted by a delay in the onset of constant darkness, a response typical of endogenous circadian rhythms. During the light period, the channels in flexor protoplasts were sensitive to red light that was followed by premature darkness; phytochrome was implicated as the photoreceptor.

Signal Transduction in Leaf Movement

“…This is consistent with the expected behavior of these cells in the intact plant; during the dark period flexor cells are swollen from the uptake of K + and water and extensor cells are shrunken, having lost K + and water. If the protoplasts were then maintained in constant darkness, at the time the lights would have normally come on, channels in flexor protoplasts spontaneously closed, whereas those in extensor protoplasts spontaneously opened (Kim et al, 1993). Thus, the channels were open in extensor cells and closed in flexor cells during what would have been the normal light period had the lights been turned on, consistent with the expected states of the channels in the cells of the intact plant in the daytime.…”

“…In particular, delaying the transition from light to darkness generally delays the rhythm by the same amount of time, as if light halted the clock at the end of the light period and darkness released it. Kim et al (1993) showed that a 2-h extension of the light period before the protoplasts were transferred to darkness delayed the subsequent channel opening and closing in constant darkness by exactly 2 h, confirming that a circadian clock was controlling the channels.…”

“…Isolated pulvinar protoplasts are also responsive to light signals (Kim et al, 1992(Kim et al, , 1993. In the dark period, the closed inward-directed Kt channels of extensor cells are opened within 3 min by blue light, as demonstrated by a K+-induced depolarization reported by the added fluorescent dye (Kim et al, 1992).…”

Plant Circadian Rhythms

“…Like many plants, Arabidopsis displays rhythmic cotyledon and leaf movement, although this rhythm in Arabidopsis is based on differential growth and thus differs from the rhythmic turgordriven expansion and contraction of the pulvinus that underlies rhythmic leaf movement in legumes, including Tamarindus and Mimosa (Kim et al, 1993). In Arabidopsis, there is a circadian rhythm in the elongation rate of the abaxial and adaxial cells of the petiole that confers an oscillation in position of cotyledons and leaves (Engelmann and Johnsson, 1998).…”

Different circadian oscillators control Ca 2+ fluxes and Lhcb gene expression

“…Because plant tissue is highly pigmented, it is probably true for both the aequorin and luciferase signals that the luminescence measured is most likely to be that emitted from the uppermost layer of cells, because photons emitted from deep within the tissue will mostly be absorbed before they can escape (17) (18). By regulating changes in turgor, [Ca 2ϩ ] c oscillations also could be the molecular linkage between a pacemaker and circadian movements of stems and petioles (19,20).…”