Jill S. Parker

Genetic interactions during root hair morphogenesis in Arabidopsis.

Root hairs are a major site for the uptake of water and nutrients into plants and form an increasingly important model system for studies of development of higher plants and cell biology. We have identified loss-of-function mutations in eight new genes required for hair growth in Arabidopsis: SHAVEN1 (SHV1), SHV2, and SHV3; CENTIPEDE1 (CEN1), CEN2, and CEN3; BRISTLED1 (BST1); and SUPERCENTIPEDE1 (SCN1). We combined mutations in 79 pairs of genes to determine the stages at which these and six previously known genes contribute to root hair formation. Double mutant phenotypes revealed roles for several genes that could not have been predicted from the single mutant phenotypes. For example, we show that TIP1 and RHD3 are required much earlier in hair formation than previous studies have suggested. We present a genetic model for root hair morphogenesis that defines the roles of each gene, and we suggest hypotheses about functional relationships between genes.

A role for gibberellic acid in orienting microtubules and regulating cell growth polarity in the maize root cortex

The role of gibberellins and cortical microtubules in determining the polarity of cell growth in the root cortex of maize (Zea mays L.) was examined. Inhibition of gibberellin biosynthesis, either naturally through mutation (d5 mutant) or by means of chemicals such as 2S,3S paclobutrazol, caused thickening of root apices and increased their starch content. Immunofluorescence microscopy of cortical microtubules, coupled with a comparison of cell widhts, lengths and shapes, indicated that the meristem and immediate post-mitotic zone were the targets of gibberellin deficiency. Cortical cells in these regions were impaired in their ability to develop highly ordered transversal arrays of cortical microtubules. Consequently, the cells became wider and shorter. Application of gibberellic acid re-established the arrangements of cortical microtubules and the polarity of cell growth characteristic for roots having normal levels of gibberellins, it also decreased the starch content. These results indicate that gibberellins are morphogenetically active substances, not only in shoots but also in roots of maize.

Arabidopsis genes with roles in root hair development

After a decade of Arabidopsis genetics many genes with important roles in root hair development have been identified. Mutants have been used to discover genes, define their roles, and identify their DNA sequences. Plant growth regulator signaling and transcription factor genes determine which root epidermal cells form hairs. A network of gene activity has been characterized that produces a swelling on the surface of each hair cell and a single tubular outgrowth from each swelling. The molecular functions of some genes in this network have been revealed. We are approaching a molecular understanding of root hair development that will provide exciting opportunities in plant cell biology and physiology.

The microtubular cytoskeleton in cells of cold-treated roots of maize (Zea mays L.) shows tissue-specific responses

SummaryMicrotubules (MTs) in cells of various tissues at different distances from the apex of the maize root exhibited different sensitivities to cold (5 °C), as judged by MT reorientation and tendency to depolymerization. Their responses seem to be related to their initial intracellular arrangements. Generally, MTs in cells which were ceasing elongation were the least sensitive during the early stages (6–24 h) of cold treatment, but during the later stages (5–7 d) MTs in most of these cells eventually depolymerized. Pericycle cells showed a unique cold response. Here the MTs were conspicuously cold-labile and quickly depolymerized near the root-tip. However, after 1 d many pericycle cells in more proximal regions had repolymerized their MTs as dense, randomly organized arrays. These persisted for the remainder of the cold treatment. A similar resistance to longterm chilling, by means of MT repolymerization, was found in cells of the root cap, quiescent centre and cells of the distal part of the former meristem. MT repolymerization in the cold may enable the apex to resume growth when more favourable (warmer) conditions return.

Central root cap cells are depleted of endoplasmic microtubules and actin microfilament bundles: implications for their role as gravity-sensing statocytes

SummaryIndirect immunofluorescence, using monoclonal antibodies to actin and tubulin, applied to sections of root tips ofLepidium, Lycopersicon, Phleum, andZea, revealed features of the cytoskeleton that were unique to the statocytes of their root caps. Although the cortical microtubules (CMTs) lay in dense arrays against the periphery of the statocytes, these same cells showed depleted complements of endoplasmic microtubules (EMTs) and of actin microfilament (AMF) bundles, both of which are characteristic of the cytoskeleton of other post-mitotic cells in the proximal portion of the root apex. The scarcity of the usual cytoskeletal components within the statocytes is considered responsible for the exclusion of the larger organelles (e.g., nucleus, plastids, ER elements) from the interior of the cell and for the absence of cytoplasmic streaming. Furthermore, the depletion of dense EMT networks and AMF bundles in statocyte cytoplasm is suggested as being closely related to the elevated cytoplasmic calcium content of these cells which, in turn, may also favour the formation of the large sedimentable amyloplasts by not permitting plastid divisions. These latter organelles are proposed to act as statoliths due to their dynamic interactions with very fine and highly unstable AMFs which enmesh the statoliths and merge into peripheral AMFs-CMTs-ER-plasma membrane complexes. Rather indirect evidence for these interactions was provided by showing enhanced rates of statolith sedimentation after chemically-induced disintegration of CMTs. All these unique properties of the root cap statocytes are supposed to effectively enhance the gravity-perceptive function of these highly specialized cells.

Oscillations of axial plant organs.

The tips of roots and shoots commonly show lateral movements as they grow forwards. These occur as both circumnutations (with long periods and large amplitudes) and micronutations (with short periods and small amplitudes). Their properties are reviewed, with emphasis on roots, and possible ways in which they could be regulated are discussed. The mechanisms could include long-range controls (for circumnutations) that depend on transmissible signals using steps common to gravitropism, and short-range controls (for micronutations) that operate within the elongation zone. The former are a property of the apex as a whole, while the latter may be confined to localized groups of cells. Simulation of nutations is presented with a view to isolating key physiological processes. However, this approach is limited by the current inadequate understanding of the growth mechanisms involved.

Symmetric reorganizations of radiating microtubules around pre- and post-mitotic nuclei of dividing cells organized within intact root meristems

Summary Using indirect immunofluorescence, the system of radiating e ndoplasmic m icro t ubule s (REMTs) within intact cells of meristematic root tissues of maize has been examined throughout the cell cycle, paying special attention to its relationship with the pre- and post-mitotic nuclei with which it is associated. At early interphase, REMTs are not uniformly disposed around the nucleus but grow out from feint, though easily recognizable, perinuclear foci. During S and G2 phases, REMTs increase in number and have a close association with the assembly of the p re p rophase b and (PPB) MT array. Later, when the cortical part of the PPB disintegrates, the REMTs align along the nuclear surface, predicting the long axis of the future mitotic spindle. In contrast to naturally wall-less cells, or to cells with perturbed cell walls, these pre-mitotic, as well as the subsequently formed post-mitotic cells display symmetrical rearrangements of their REMTs around the nuclear surface. Mitotic cells sectioned in the median plane show a symmetrical quadripolar MT organization which is obvious at all stages of mitosis. The symmetrical redistributions of the REMTs which occur during the cell cycle are perturbed, or even prevented, by treatments with chemical or with physical anti-MT agents. Nuclei of cells so treated accumulate REMTs, but fail to redistribute them symmetrically. As a result, the pre- and post-mitotic nuclei of root cells treated with anti-MT agents resemble, with respect to their REMTs, the corresponding nuclei of wall-less plant cells, or of cells which have perturbed cell walls. The dynamic REMTs which connect the pre- and post-mitotic nuclei with the cell periphery are suggested as being involved in sensing the position of dividing cells within the intact plant organ. This property of REMTs enables them to spatially control the sequential alignment of cell division planes of immobile walled plant cells which underlies the morphogenesis of higher plant organs.

Molecular characterization of cell populations in the maize root apex

Several features make the root apex of maize (Zea mays L.) a good experimental system for studying cell cycle controls in relation to development in higher plants. Actively dividing meristematic cells, slowly-cycling quiescent centre cells, differentiating cap cells and senescent detaching cap cells are distinctively compartmented. In addition, the quiescent centre can be activated into rapid proliferation in response to stresses. Although there is a wealth of cytological and physiological information about the behaviour of cells in the root apex, very little is known about the molecular factors which control the patterns of cell division and differentiation. Differential screening of cDNA libraries obtained from discrete cell populations from the apex may provide a means to identify genes which are expressed in cell cycle- and differentiation-dependent manners.