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 microtubules; however, it is not known if microtubules are also required by mature guard cells for stomatal function. To study the role of microtubules 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 inhibitors, microtubules in appropriately treated epidermal peels were localized 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 with 1 mM colchicine for 1 h depolymerized nearly all cortical microtubules. Measurements of stomatal aperture showed that neither 1 mM colchicine nor 20 M taxol affected any of the responses tested: remaining closed in the dark, opening in response to light or fusicoccin, and closing in response to calcium and darkness. We conclude that intact microtubule arrays are not invariably required for guard cell function.