Stomatal movements, which regulate gas exchange in plants, involve pronounced changes in the shape and volume of the guard cell. To test whether the changes are regulated by actin filaments, we visualized microfilaments in mature guard cells and examined the effects of actin antagonists on stomatal movements. Immunolocalization on fixed cells and microinjection of fluorescein isothiocyanate-phalloidin into living guard cells of Commelina communis L. showed that cortical microfilaments were radially distributed, fanning out from the stomatal pore site, resembling the known pattern of microtubules. Treatment of epidermal peels with phalloidin prior to stabilizing microfilaments with m-maleimidobenzoyl N-hydroxysuccimimide caused dense packing of radial microfilaments and an accumulation of actin around many organelles. 60th stomatal closing induced by abscisic acid and opening under light were inhibited. Treatment of guard cells with cytochalasin D abolished the radial pattern of microfilaments; generated sparse, poorly oriented arrays; and caused partia1 opening of dark-closed stomata. These results suggest that microfilaments participate in stomatal aperture regulation.

A set of guard cells surrounding stomata of terrestrial plants function much like sliding doors in a building, opening to allow the CO, uptake required for photosynthesis and closing to reduce water loss during periods of water deficit. Such regulation is initiated by sensing environmental and interna1 stimuli such as light, humidity, CO,, and the plant-stress hormone ABA, and is accomplished by osmotic volume changes of the cells. Previous studies have implicated heterotrimeric G-proteins, the H+ pump, and the movement of various ions regulated by ion channels in these processes (for review, see Assmann, 1993). Thus, guard cells provide an ideal system in which to examine whether other molecules, including cytoskeletal elements, take part in plant signaling and, if so, how they interact with better-characterized ones.Actin filaments and microtubules are dynamic cellular components; they disassemble into their building units,

effects of lead on microtubules in the root meristem. LeadLead is an environmental pollutant that interferes with plant growth. Unfortunately, the mechanisms of lead toxicity in treatment perturbed the alignment of microtubules in a concentration-dependent manner beginning at 10 mM. Microtubules of plants are still poorly understood. In this study, we have investigated both the deposition sites and sources of cellular different regions of the root meristem and in different stages of the cell cycle showed differential susceptibility to lead. These toxicity of lead in maize seedlings (Zea mays L. cv. Golden Cross Bantam). Using atomic absorption spectroscopy and effects do not appear to be general phenomena common to toxic metals, since aluminum and copper, at concentrations that X-ray fluorescence microprobing, we show that lead accumulation is highest in the root meristem, and that the accumulation decreased root growth to a comparable level, did not have the same detrimental effects on microtubules. Based on these occurs both in the apoplast and symplast. Since cells are results, we suggest that the damage to microtubules is partly dividing vigorously in this region and because microtubules play responsible for lead-associated toxicity in plants. an important role in cell division, we have further examined the various pollutants, including heavy metals (Ma et al. 1995, Panda et al. 1997, Kovalchuk et al. 1998, Steinkellner et al. 1998. Because formation of multiple micronuclei results from improper nuclear separation and cytokinetic disorder, we suspect that the components of the processes may be directly susceptible to toxic metals. Microtubules play key roles in both nuclear division and cytokinesis in plants. Thus, in the present work, we examined lead deposition sites in seedlings of maize and the effects of lead on the microtubule organization in the region of root where lead deposition was the highest. Materials and methods Plant materialMaize (Zea mays L. cv. Golden Cross Bantam) seeds were sterilized for 15 min with 0.5% calcium hypochlorite, washed several times with tap water and soaked overnight in

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Cortical actin filaments in guard cells of Commelina communis L. show signal-specific organization during stomatal movements [S.-O. Eun and Y. Lee (1997) Plant Physiol 115: 1491-1498; S.-O. Eun and Y. Lee (2000) Planta 210: 1014-1017]. To study the roles of actin in signal transduction, it is advantageous to use Arabidopsis thaliana (L.) Heynh., an excellent model plant with numerous well-characterized mutants. Using an immunolocalization technique, we found that actin deployments in guard cells of A. thaliana were basically identical to those in C. communis: actin proteins were assembled into radial filaments under illumination, and were disassembled by ABA. In addition, we examined actin organization in an ABA-insensitive mutant (abi1-1) to test the involvement of protein phosphatase 2C (PP2C) in the control of actin structure. A clear difference was observed after ABA treatment, namely, neither stomatal closing nor depolymerization of actin filaments was observed in guard cells of the mutant. Our results indicate that PP2C participates in ABA-induced actin changes in guard cells.