The organization of microtubules in guard cell mother cells of Zea mays

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Abstract
The cortical interphase microtubules of the guard cell mother cells (GMC) of Z. mays form a well-grouped band (interphase microtubule band, IMB) lining the midregion of the lateral and periclinal walls, which are the only expanding walls during interphase. In advanced interphase GMC, another population of microtubules emerge from the cortical cytoplasm of the midregion of the periclinal walls, entering deep into the cytoplasm. Elongated proplastids converge on the above cortical regions, possibly aligned by the microtubules with which they are associated. The IMB depolymerizes prior to mitosis and a preprophase microtubule band (PMB) is organized adjacent to the proximal, distal, and periclinal walls. In transverse sections the preprophase-early prophase nucleus appears slightly elliptical or spindle-shaped, sometimes exhibiting acute angular profiles at its poles. Extranuclear microtubules closely juxtaposed with the nuclear envelope converge on the poles of the nucleus, close to the regions of the PMB adjacent to the periclinal walls. The observations suggest a local interplay between the PMB and (or) the PMB cytoplasmic site on the one hand and the nuclear envelope and (or) the extranuclear microtubules on the other. The microtubules of both bands lining the perclinal walls and the sites of their junctions with the anticlinal ones are more closely grouped than those running along the anticlinal walls, and they exhibit intimate associations with numerous dictyosome vesicles. This perferential gathering of dictyosome vesicles, among others, possibly manifests a mechanism promoting the thickening of the expanding regions of the above walls. The inhibition of the symmetrical divisions of the GMC by a continuous colchicine treatment leads to the formation of epidermal idioblasts possessing some of the characteristics of the guard cells. Furthermore, in the absence of microtubules, local wall thickenings are deposited in the middle of the periclinal walls and at their junctions with the anticlinal ones. From the observations it seems likely that guard cell differentiation commences in GMC, and that the cortical cytoplasm and (or) the plasmalemma of the midregion and the edges of the periclinal walls of the GMC possess some factor(s) favoring their preferential thickening. Cortical microtubule organizing centers (MTOC) appear to be localized in these regions and activated in a programmed fashion.