Untitled Essay Research Paper Involvement of K — страница 3

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the membrane potential was below -40mV and open when the membrane potential was depolarized to above -40mV. A voltage-gated K+ channel that is opened upon depolarization has been observed in every patch clamp study of the plasma membranes of higher plants, including Samanea motor cells and Mimosa pulviner cells. It is proposed that electrogenic H+ secretion results in a proton motive force, a gradient in pH and in membrane potential, that facilitates the uptake of K+, Cl-, sucrose, and other anions. External sodium acetate promotes closure and inhibits opening in Albizzia. This effect could be caused by a decrease in transmembrane pH gradients. The promotion of opening and inhibition of closure of leaves by fusicoccin and auxin in Cassia, Mimosa, and Albizzia also implicate H+ in

the solute uptake of motor cells, since both chemicals are H+/ATPase activators, stimulating H+ secretion from the plant cells into the apoplast. Vanadate, an H+/ATPase inhibitor, inhibits rhythmic leaflet closure in Albizzia. Although this conflicts with the movement effected by fusicoccin and auxin, it is believed that vanadate affects different cells, acting upon flexor rather than extensor cells. The model indicates that there are two possible types of H+ pumps. One is the electrogenic pump that creates the pmf mentioned above and opens the K+ channels. The other pump is a H+/K+ exchanger, in which K+ is pumped into the cell as H+ is pumped out of the cell in a type of antiport. The presence of this typ of pump is only hypothetical, however, since at present there is no

evidence to support it. Thus there are two possible ways for K+ to enter the pulvini cells. The buildup of the pH gradient may also promote Cl- entry into the cell via a H+/Cl- cotransporter as the H+ trickles back into the cell. Cl- ions may also be driven by the electrochemical gradient for Cl- via Cl- channels, as with K+. A large Cl- channel was observed in the membrane of Samanea flexor protoplasts. The channel closed at membrane potentials above 50mV and opened at potentials as low as -100mV. Light-driven changes in membrane potential may be involved in the activation of these proton pumps. This may be mediated by effects on cytoplasmic Ca2+. Ca2+-chelators inhibit the nyctinastic folding as well as the photonastic unfolding responses in Cassia. Thus Ca2+ may act as a

second messenger in a calmodulin-dependent reaction. The Ca2+ may be what turns on the electrogenic proton pumps, causing changes in membrane potential. However, there is no direct evidence to support this hypothesis, although chemicals that are known to change calcium levels have been shown to alter the leaf movement of Cassia fasciculata and other nyctinastic plants. One study involving Samanea postulates that Ca2+ channels are also present in the plasma membrane of pulvini cells, and inositol triphoshate, a second messenger in the signal transduction pathway in animals, stimulates the opening of these channels. This insinuates that some light signal binds to a receptor on the outside of the cell and stimulates this transduction pathway. However, whether this hypothesis is true

is unclear. It has also been proposed that an outwardly directed Ca2+ pump functions as a transport mechanism to restore homeostasis after Ca2+ uptake through channels. The changes in Cl- levels in the apoplast are less then that for K+. The Cl- levels are 75% that of K+ in Albizzia, 40-80% in Samanea, and 40% in Phaseolus. Therefore, other negatively charged ions must be used to compensate for the positive charges of the K+. Malate concentrations vary, and it is lower in shrunken cells than in swollen cells. It is believed that malate is synthesized when there is not enough Cl- present to counteract the charges of the K+. An experiment with soybeans (Cronland) examined the role of K+ channels and H+/ATPase in the plasma membrane in paraheliotropic movement. This was done by

treating the pulvini with the K+ channel blocker tetraethylammonium chloride (TEA), the H+/ATPase activator fusicoccin, and the H+/ATPase inhibitors vanadate and erythrosin-B. In all cases the leaf movements of the plant were inhibited, leading to the hypothesis that the directional light results in an influx of K+ into the flexor cells from the apoplast and an efflux of K+ from the extensor cells into the apoplast, and these movements are driven by H+/ATPase pumps. This combined reaction results in the elevation of the leaflet towards the light. In this study, the diheliotropic movements of C. pallida are examined. The purpose of this experiment is to determine which ions, if any, are used by pulvini cells of Crotalaria pallida Aiton to control the uptake of water, thereby