Ca2+ signalling in stomatal guard cells

Ca2+ is a ubiquitous second messenger in the signal transduction pathway(s) by which stomatal guard cells respond to external stimuli. Increases in gu ard-cell cytosolic free Ca2+ concentration ([Ca2+](eyt)) hare been observed in response to stimuli that cause both stomatal opening and closure. In ad dition, several important components of Ca2+-based signalling pathways have been identified in guard cells, including the cADP-ribose and phospholipas e C/Ins(1,4,5)P-3-mediated Ca2+-mobilizing pathways. The central role of st imulus-induced increases in [Ca2+](eyt) in guard-cell signal transduction h as been clearly demonstrated in experiments examining the effects of modula ting increases in [Ca2+](eyt) on alterations in guard-cell turgor or the ac tivity of ion channels that act as effecters in the guard-cell turgor respo nse. In addition, the paradox that Ca2+ is involved in the transduction of signals that result in opposite end responses (stomatal opening and closure ) might be accounted for by the generation of stimulus-specific Ca2+ signat ures, such that increases in [Ca2+](eyt) exhibit unique spatial and tempora l characteristics.