Cytosolic Ca2+ in guard cells and blue light

 

Measurement of changes in cytosolic Ca²⁺ in Arabidopsis guard cells and mesophyll cells in response to blue light.

by Harada A., Shimazaki K. (2009)

in Plant Cell Physiol.50, 360–373. doi: 10.1093/pcp/pcn203 –

PubMed Abstract | CrossRef Full Text

http://pcp.oxfordjournals.org/content/50/2/360

Abstract

Phototropins (phot1 and phot2) are blue light (BL) receptors that mediate responses including phototropism, chloroplast movement and stomatal opening, and increased cytosolic Ca²⁺.

BL absorbed by phototropins activates plasma membrane H⁺-ATPase in guard cells, resulting in membrane hyperpolarization, and drives K⁺ uptake and stomatal opening. However, it is unclear whether the phototropin-mediated Ca²⁺ increase activates the H⁺-ATPase.

Here, we determined cytosolic Ca²⁺concentrations in guard cell protoplasts (GCPs) from Arabidopsis transformed with aequorin. Cytosolic Ca²⁺ increased rapidly in response to BL in GCPs from both the wild type and phot1 phot2 double mutants, but was mostly suppressed by an inhibitor of photosynthetic electron flow (DCMU).

With depleted external K⁺, we observed another slower Ca²⁺increase, which was phototropin- dependent. Fusicoccin, a H⁺-ATPase activator, mimicked the effect of BL.

The slow Ca²⁺ increase thus appears to result from membrane hyperpolarization. The slow Ca²⁺ increase was suppressed by external K⁺ and was restored by blockers of inward-rectifying K⁺ channels, CsCl and tetraethylammonium, suggesting the preferential uptake of K⁺ over Ca²⁺.

Such efficient K⁺ uptake in response to BL was not found in mesophyll cells. Both the fast and the slow Ca²⁺increases were inhibited by Ca²⁺ channel blockers (CoCl₂ and LaCl₃) and a chelating agent (EGTA).

These results indicate that the phototropin-mediated Ca²⁺ increase was not observed prior to H⁺-ATPase activation in guard cells and that Ca²⁺ entered guard cells via Ca²⁺ channels through photosynthesis and phototropin-mediated membrane hyperpolarization.