Guard cell cation channels are involved in Na+-induced stomatal closure in a halophyte.
Véry A. A., Robinson M. F., Mansfield T. A., Sanders D. (1998)
Michael F. Robinson,
Terry A. Mansfield,
in Plant J. 14, 509–521. doi: 10.1046/j.1365-313X.1998.00147.x –
The halophyte Aster tripolium, unlike well-studied non-halophytic species, partially closes its stomata in response to high Na+ concentrations. Since A. tripolium possesses no specific morphological adaptation to salinity, this stomatal response, preventing excessive accumulation of Na+ within the shoot via control of the transpiration rate, is probably a principal feature of its salt tolerance within the shoot.
The ionic basis of the stomatal response to Na+ was studied in guard cell protoplasts from A. tripolium and from a non-halophytic relative, Aster amellus, which exhibits classical stomatal opening on Na+.
Patch-clamp studies revealed that plasma membrane K+ channels (inward and outward rectifiers) of the halophytic and the non-halophytic species are highly selective for K+ against Na+, and are very similar with respect to unitary conductance and direct sensitivity to Na+.
On the other hand, both species possess a significant permeability to Na+ through non-rectifying cation channels activated by low (physiological) external Ca2+ concentrations. Finally, it appeared that the differential stomatal response between the two species is achieved, at least in part, by a Na+-sensing system in the halophyte which downregulates K+ uptake.
Thus, increases in guard cell cytosolic Na+ concentration in A. tripolium but not in A. amellus, lead to a delayed (20–30 min) and dramatic deactivation of the K+ inward rectifier. This deactivation is probably mediated by an increase in cytosolic Ca2+ since buffering it abolishes the response.
The possible role of K+ inward rectifiers in the response of A. tripolium’s stomata to Na+, suggested by patch-clamp studies, was confirmed by experiments demonstrating that specific blockade of inward rectifying channels mimics Na+ effects on stomatal aperture, and renders aperture refractory to Na+.