THE FUNCTION OF GUARD-CELLS DOES NOT REQUIRE AN INTACT ARRAY OF CORTICAL MICROTUBULES

The development of stomatal guard cells is known to require cortical m icrotubules; however, it is not known if microtubules are also require d by mature guard cells for stomatal function. To study the role of mi crotubules in guard cell function, epidermal peels of Vicia faba were subjected to conditions known to open or close stomata in the presence or absence of microtubule inhibitors. To verify the action of the inh ibitors, microtubules in appropriately treated epidermal peels were lo calized by cryofixation followed by freeze substitution and embedding in butyl-methyl methacrylate. Mature guard cells had a radial array of microtubules, focused toward the thick cell wall of the pore, and the appearance of this array was the same for stomata remaining closed in darkness or induced to open by light. Treatment of epidermal peels wi th 1 mM colchicine for 1 h depolymerized nearly all cortical microtubu les. Measurements of stomatal aperture showed that neither 1 mM colchi cine nor 20 mu M taxol affected any of the responses tested: remaining closed in the dark, opening in response to light or fusicoccin, and c losing in response to calcium and darkness. We conclude that intact mi crotubule arrays are not invariably required for guard cell function.