Guard cell anion channels, Ca2+ and proteins

 

 

Dynamic regulation of guard cell anion channels by cytosolic free Ca2+concentration and protein phosphorylation

by Chen Z.-H.,

Hills A., Lim C. K.,

Blatt M. R.

(2010)

in The Plant Journal, 2010, 61, 5, 816 – DOI: 10.1111/j.1365-313X.2009.04108.x

Wiley Online Library

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-313X.2009.04108.x/full

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 Ca²⁺ concentration ([Ca²⁺]_i). However, the dynamics of the action of [Ca²⁺]_i on I_anion has never been established, despite its importance for understanding the mechanics of stomatal adaptation to stress.

We have quantified the [Ca²⁺]_i dynamics of I_anion in Vicia faba guard cells, measuring channel current under a voltage clamp while manipulating and recording [Ca²⁺]_i using Fura-2 fluorescence imaging.

We found that I_anion rises with [Ca²⁺]_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 Ca²⁺ ions to activate the channels.

Approximately 30% of guard cells exhibited a baseline of I_anionactivity, but without a dependence of the current on [Ca²⁺]_i. The protein phosphatase antagonist okadaic acid increased this current baseline over twofold.

Additionally, okadaic acid altered the [Ca²⁺]_i sensitivity of I_anion, displacing the apparent K_d for [Ca²⁺]_i to 573 ± 38 nm.

These findings support previous evidence for different modes of regulation for I_anion, only one of which depends on [Ca²⁺]_i, and they underscore an independence of [Ca²⁺]_i from protein (de-)phosphorylation in controlling I_anion.

Most importantly, our results demonstrate a significant displacement of I_anion sensitivity to higher [Ca²⁺]_icompared with that of the guard cell K⁺ channels, implying a capacity for variable dynamics between net osmotic solute uptake and loss.