| 20015065 |
Chen ZH, Hills A, Lim CK, Blatt MR: Dynamic regulation of guard cell anion channels by cytosolic free Ca (2+) concentration and protein phosphorylation. Plant J. 2009 Dec 15.
Summary In guard cells, activation of anion channels (I (anion)) is an early event leading to stomatal closure. Activation of I (anion) has been associated with abscisic acid (ABA) and its elevation of the cytosolic free Ca (2+) concentration ([Ca (2+)](i)). However, the dynamics of the action of [Ca (2+)](i) on I (anion) has never been established, despite its importance for understanding the mechanics of stomatal adaptation to stress. We have quantified the [Ca (2+)](i) dynamics of I (anion) in Vicia faba guard cells, measuring channel current under a voltage clamp while manipulating and recording [Ca (2+)](i) using Fura-2 fluorescence imaging. We found that I (anion) rises with [Ca (2+)](i) only at concentrations substantially above the mean resting value of 125 +/- 13 nm, yielding an apparent K (d) of 720 +/- 65 nm and a Hill coefficient consistent with the binding of three to four Ca (2+) ions to activate the channels. Approximately 30% of guard cells exhibited a baseline of I (anion) activity, but without a dependence of the current on [Ca (2+)](i). The protein phosphatase antagonist okadaic acid increased this current baseline over twofold. Additionally, okadaic acid altered the [Ca (2+)](i) sensitivity of I (anion), displacing the apparent K (d) for [Ca (2+)](i) to 573 +/- 38 nm. These findings support previous evidence for different modes of regulation for I (anion), only one of which depends on [Ca (2+)](i), and they underscore an independence of [Ca (2+)](i) from protein (de-) phosphorylation in controlling I (anion). Most importantly, our results demonstrate a significant displacement of I (anion) sensitivity to higher [Ca (2+)](i) compared with that of the guard cell K (+) channels, implying a capacity for variable dynamics between net osmotic solute uptake and loss. |
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