ACTIN-FILAMENTS MODULATE BOTH STOMATAL OPENING AND INWARD K-CHANNEL ACTIVITIES IN GUARD-CELLS OF VICIA-FABA L()

Actin antagonists have previously been shown to alter responses of Com melina communis stomata to physiological stimuli, implicating actin fi laments in the control of guard cell volume changes (M. Kim, P.K. Hepl er, S.-O. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077-1084). S ince K+ channels in the guard cell play an important role in stomatal movements, we examined the possible regulation of K+-channel activitie s by the state of actin polymerization. Agents affecting actin polymer ization altered light-induced stomatal opening and inward K+-channel a ctivities measured by patch clamping in Vicia faba. Cytochalasin D, wh ich induces depolymerization of actin filaments, promoted light-induce d stomatal opening and potentiated the inward K+ current in guard cell protoplasts. Phalloidin, a stabilizer of filamentous actin, inhibited both light-induced stomatal opening and inward K+ current. Inward K+- channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current level, su ggesting that cytochalasin D facilitates and phalloidin inhibits K+ in flux in intact guard cells, thus resulting in enhancement and inhibiti on of stomatal opening, respectively. To our knowledge, this is the fi rst report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion c hannels in plant cells.