The function of guard cells does not require an intact array of cortical microtubules

Abstract

cellulose microfibrils. These studies have found that cortical microtubules are needed to produce a pair of kidney The development of stomatal guard cells is known to shaped cells surrounding an aperture, as well as to direct require cortical microtubules; however, it is not known the asymmetric deposition of cellulose microfibrils in the if microtubules are also required by mature guard cells cell wall, which enables the stoma to open and close in for stomatal function. To study the role of microtubules response to changes in osmotic potential (Palevitz, 1981; in guard cell function, epidermal peels of Vicia faba Sack, 1987). Once they mature, guard cells change shape were subjected to conditions known to open or close to regulate the stomatal aperture. It is reasonable to ask stomata in the presence or absence of microtubule whether these changes in shape require, or aVect, the inhibitors. To verify the action of the inhibitors, micro- cortical microtubules. But even though the role of microtubules in appropriately treated epidermal peels were tubules in guard cell development was recognized as localized by cryofixation followed by freeze substitu- important long ago, there has been little assessment of tion and embedding in butyl-methyl methacrylate. the role of cortical microtubules in guard cell function. Mature guard cells had a radial array of microtubules, The possible reasons why the role of microtubules in focused toward the thick cell wall of the pore, and the guard cell function has been little studied are both pracappearance of this array was the same for stomata tical and conceptual. Practically, attempts to localize remaining closed in darkness or induced to open by microtubules in mature stomata may have been frustrated light. Treatment of epidermal peels with 1 mM colchic- by the thick cell wall, which hinders chemical fixation. ine for 1 h depolymerized nearly all cortical microtub- Conceptually, microtubules are considered to orient micules. Measurements of stomatal aperture showed that rofibrils and specify cell shape; once guard cells reach neither 1 mM colchicine nor 20 mM taxol affected any maturity, their structure is plausibly suYcient to allow of the responses tested: remaining closed in the dark, many rounds of opening and closing. However, microtubopening in response to light or fusicoccin, and closing ules have roles beyond shaping cells: the cytoskeleton, in response to calcium and darkness. We conclude including microtubules, participates in signal transducthat intact microtubule arrays are not invariably tion. For example, in animal cells, actin filaments terminrequired for guard cell function. ate in special regions of the plasma membrane enriched in tyrosine receptor-kinases (Mochly-Rosen, 1995). For