Microinjected fluorescent phalloidin in vivo reveals the F-actin dynamics and assembly in higher plant mitotic cells

Microtubule-organizing centers in higher plants

Compelling evidence has recently been obtained for the idea that the nuclear surface of higher plant cells has a microtubule-nucleating function under cell cycle dependent control. Whatever the final organization of microtubules, they all seem to originate in the nuclear periphery. The spatial distribution of microtubules is a secondary process involving actin and microtubule-associated proteins, and other morphogenetic controls. How microtubules are nucleated and/or reorganized at the spindle poles during mitosis remains unknown.

Microfilament Distribution in Maize Meiotic Mutants Correlates with Microtubule Organization

Microtubules and microfilaments often codistribute in plants; their presumed interaction can be tested with drugs although it is not always clear that these are without side effects. In this study, we exploited mutants defective in meiotic cell division to investigate in a noninvasive way the relationship between the two cytoskeletal elements. By staining unfixed, permeabilized cells with rhodamine-phalloidin, spatial and temporal changes in microfilament distribution during maize meiosis were examined. In wild-type microsporocytes, a microtubule array that radiates from the nucleus disappeared during spindle formation and returned at late telophase. This result differed from the complex cytoplasmic microfilament array that is present at all stages, including karyokinesis and cytokinesis. During division, a second class of microfilaments also was observed in the spindle and phragmoplast. To analyze this apparent association of microtubules and microfilaments, we examined several meiotic mutants known to have stage-specific disruptions in their microtubule arrays. Two mutations that altered the number or form of meiotic spindles also led to a dramatic reorganization of F-actin. In contrast, rearrangement of nonspindle, cytoplasmic microtubules did not lead to concomitant changes in F-actin distribution. These results suggested that microtubules and microfilaments interact in a cell cycle-specific and site-specific fashion during higher plant meiosis.

Spatial aspects of cytokinesis in plant cells

Plant cytokinesis in a particular orientation and location can be viewed as having several component stages, often beginning with the establishment of division polarity before karyokinesis occurs. Improved methods for preserving the in situ distribution of actin microfilaments and observations of individual live cells during treatment with cytoskeleton-disrupting drugs are making it possible to elucidate the roles of microtubules and microfilaments in cytokinesis. Current evidence points to involvement of the cytoskeleton throughout the stages of preparation for, and execution of, cytokinesis in many types of plant cell division.

The plant cytoskeleton

Significant progress has been made in four areas: in appreciating the speed with which cortical microtubules reorient in response to environmental signals; in a consolidated understanding of the cytoskeletal nature of the phragmosome--the device that predicts and structures the division plane in vacuolated cells; in the description of new cytoskeletal proteins; and in reports that herald an attack on the cell cycle control of cytoskeletal organization.

Incorporation of Paramecium axonemal tubulin into higher plant cells reveals functional sites of microtubule assembly

Incorporation of Paramecium axonemal tubulin into lysed endosperm cells of the higher plant Haemanthus enabled us to identify sites of microtubule assembly. This exogenous Paramecium tubulin could be traced by specific antibodies that do not stain endogenous plant microtubules. Intracellular copolymerization of protozoan and higher plant tubulins gave rise to hybrid polymers that were visualized by immunofluorescence and by immunoelectron microscopy. The addition of exogenous tubulin revealed many free ends of endogenous microtubules that were competent to assemble ciliate tubulin. The functional roles of the nuclear surface and the equatorial region of the phragmoplast as plant microtubule-organizing centers, which were revealed by the intense incorporation of exogenous tubulin, are discussed. These data shed light on the present debate on higher plant microtubule organizing centers.