Actin Filaments Modulate Both Stomatal Opening and Inward K+-Channel Activities in Guard Cells of Vicia faba L

Abstract

Actin antagonists have previously been shown to alter responses of Commelina communis stomata to physiological stimuli, implicating actin filaments i n the control of guard cell volume changes (M. Kim, P.K. Hepler, 5-0. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077-1084). Since K+ channels i n the guard cell play an important role i n stomatal movements, we examined the possible regulation of K+-channel activities by the state of actin polymerization. Agents affecting actin polymerization altered light-induced stomatal opening and inward K+-channel activities measured by patch clamping in Vicia faba. Cytochalasin D, which induces depolymerization of actin filaments, promoted light-induced 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. lnward K+-channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current levei, suggesting that cytochalasin D facilitates and phalloidin inhibits K+ influx in intact guard cells, thus resulting in enhancement and inhibition of stomatal opening, respectively. To our knowledge, this is the first report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion channels i n plant cells.The regulation of stomatal aperture is critica1 to a plant's ability to balance the need for a C source while avoiding the deleterious effects of water loss. The size of the stomatal opening is established through volume changes of stomatal guard cells under the concerted influence of light, temperature, CO,, and phytohormones (Assmann, 1993). Recently, Kim et al. (1995) showed that cortical actin filaments are distributed radially, fanning out from the stomatal pore site in mature guard cells of Commelinu communis. Moreover, fungal toxins that interfere with the polymerization or depolymerization of actin filaments brought about altered stomatal responses to physiological stimuli. Thus, dynamic changes in the actin cytoskeleton

Signalling drought in guard cells

“…Under a hypertonic condition, actin filaments in the protoplasts are organized into a network configuration that corresponds to a small I Kin and a small stomatal aperture. The correlation of actin filament organization in guard cells and stomatal aperture under normal conditions was described by Lee and colleagues (Kim et al 1995;Hwang et al 1997) and is consistent with the osmosensing mechanism observed by Liu & Luan (1998). Although actin filament organization is linked to the ion channel regulation in both animal and plant cells, little is understood on the molecular mechanism underlying actin-ion channel interaction.…”

Actin Filaments of Guard Cells Are Reorganized in Response to Light and Abscisic Acid

“…The physical state of actin in animal cells affects activities of lipidhydrolyzing enzymes, receptor-and nonreceptor-protein kinases, G-proteins, and their modulators (Carraway and Carraway, 1995), as well as ion channels (Cantiello et al, 1991;Schwiebert et al, 1994;Prat et al, 1996). Our patchclamping data showed that activities of K + channels in guard cells are certainly influenced by application of actin antagonists (Hwang et al, 1997). We are currently investigating other possible target molecules of actin in guard cells.…”

Roles of ion channels and transporters in guard cell signal transduction

“…Recent elegant studies with transiently expressed GFP-tagged KAT1 and dominant negative SNARE fragments show that KAT1 is localized at the PM in punctuate structures and that syntaxins regulate the trafficking and clustering of these channels [41]. Actin cytoskeleton stability also plays a role in K + channel regulation: drugs that induce actin cytoskeleton depolymerization (cytochalasin D) increase inward K + currents in Vicia faba guard cells, whereas phalloidin, which stabilizes the actin cytoskeleton, inhibits these currents [42,43], and perhaps these effects reflect alterations in K + channel trafficking (Fig. 3) cyt .…”