Osmo-sensitive and stretch-activated calcium-permeable channels in Vicia faba guard cells are regulated by actin dynamics

In responses to a number of environmental stimuli, changes of cytoplasmic [Ca2+](cyt) in stomatal guard cells play important roles in regulation of stomatal movements. In this study, the osmo-sensitive and stretch-activated (SA) Ca2+ channels in the plasma membrane of Vicia faba guard cells are identified, and their regulation by osmotic changes and actin dynamics are characterized. The identified Ca2+ channels were activated under hypotonic conditions at both whole-cell and single-channel levels. The channels were also activated by a stretch force directly applied to the membrane patches. The channel-mediated inward currents observed under hypotonic conditions or in the presence of a stretch force were blocked by the Ca2+ channel inhibitor Gd3+. Disruption of actin filaments activated SA Ca2+ channels, whereas stabilization of actin filaments blocked the channel activation induced by stretch or hypotonic treatment, indicating that actin dynamics may mediate the stretch activation of these channels. In addition, [Ca2+] cyt imaging demonstrated that both the hypotonic treatment and disruption of actin filaments induced significant Ca2+ elevation in guard cell protoplasts, which is consistent with our electrophysiological results. It is concluded that stomatal guard cells may utilize SA Ca2+ channels as osmo sensors, by which swelling of guard cells causes elevation of [Ca2+] cyt and consequently inhibits overswelling of guard cells. This SA Ca2+ channel-mediated negative feedback mechanism may coordinate with previously hypothesized positive feedback mechanisms and regulate stomatal movement in response to environmental changes.