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Microtubules participate as morphogenetic tools in two basic processes by which plants develop their characteristic forms : (a) production of new cells in specific sites and with specific initial shapes by partitioning of parental cells, and (b) further shaping of the progeny during their expansion and differentiation (8,18) . In respect of (a), microtubules are present in the mitotic spindle, where they develop in the absence of centrioles (14). Immediately before the division cycle they are deposited as a transitory "pre-prophase band" (PPB) (8,26,27), which in its positioning predicts the site and plane of the future cytokinesis. At telophase another microtubule system contributes to the organization of the phragmoplast, which contains the new partitioning wall, or cell plate. In respect of(b), there are many instances of congruence between the orientation of microtubules in the cell cortex during interphase ("interphase cortical arrays") and the orientation of currently deposited microfibrils of cell wall material (see 12 and 15 for recent summaries) . The inference is that the cell exerts geometrical control over its expansion by setting up specifically oriented microtubule arrays. These in turn guide wall deposition, thereby regulating the mechanical properties of the wall and determining its spatial reaction to the turgor forces that drive cell expansion.Until recently, the only available method for studying the various categories of microtubule array was electron microscopy. However, to gain detailed information on three-dimensional relationships from ultrathin sections, recourse to timeconsuming serial sectioning (12,13) ABSTRACT Cells were prepared for indirect immunofluorescence microscopy after paraformaldehyde fixation of multicellular root apices and brief incubation in cell wall-digesting enzymes. This allowed subsequent separation of the tissue into individual cells or short files of cells which were put onto coverslips coated with polylysine . Unlike spherical protoplasts made from living tissues, these preparations retain the same polyhedral shape as the cells from which they are derived. Cellular contents, including organized arrays of microtubules, are likewise structurally stabilized . Antibodies to porcine brain tubulin react with all types of microtubule array known to occur in plant meristematic cells, namely, interphase cortical microtubules, pre-prophase bands, the mitotic spindle, and phragmoplast microtubules .The retention of antigenicity in permeabilized, isolated, stabilized cells from typical, wall-enclosed plant cells has much potential for plant immunocytochemistry, and in particular should facilitate work on the role of microtubules in the morphogenesis of organized plant tissues.

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Abstract. Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not Abbreviations: CMF(s)=cellulose microfibril(s); GA=gibberellin; GA3 =gibberellic acid; MT(s)=microtubule(s)Correspondence to: H. Kende; FAX: 1 (517)3539168 promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.

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