The detergent-resistant cytoskeleton of higher plant protoplasts contains nucleus-associated fibrillar bundles in addition to microtubules

Cell cycle-dependent dynamics of a plant intermediate filament motif protein with intracellular localization related to microtubules

Although intermediate filaments (IFs) are biochemically and immunologically suggested to exist in plant cells, there are few molecular genetic studies related to the proteins that form these structures. In this study, Arabidopsis AT3G05270 was selected as a candidate gene for a protein constituting IF in plant cells. The protein encoded by AT3G05270 has a large α-helix as well as the IF protein motif indispensable for maintaining the structures of IF. Moreover, fluorescence signals of this protein fused with GFP exhibited cytoskeleton-like filamentous structures in plant cells. Thus, we named the protein encoded by AT3G05270 as Intermediate Filament Motif Protein 1 (IFMoP1). The structures composed of IFMoP1 and their localizations were examined in IFMoP1-GFP-expressing tobacco BY-2 cells whose cell cycle was synchronized using aphidicolin, a DNA synthesis inhibitor, and propyzamide, a microtubule-disrupting agent. The IFMoP1-GFP signals were present at the spindles and phragmoplasts in the mitotic phase. In addition, the frequency of cells with cytoskeleton-like filamentous structures composed of IFMoP1-GFP increased with the increase in cells that completed cell division, and then decreased after several hours. In terms of the relationship in intracellular localization between IFMoP1 and microtubules, the filamentous structures composed of IFMoP1 were present independently of microtubules during interphase. In living cells, these filamentous structures moved along with the nucleus. IFMoP1 co-localized with spindle and phragmoplast microtubules during mitosis, as well as with a part of the cortical microtubules in interphase.

Evidence for nuclear interaction of a cytoskeleton protein (OsIFL) with metallothionein and its role in salinity stress tolerance

Soil salinity is being perceived as a major threat to agriculture. Plant breeders and molecular biologist are putting their best efforts to raise salt-tolerant crops. The discovery of the Saltol QTL, a major QTL localized on chromosome I, responsible for salt tolerance at seedling stage in rice has given new hopes for raising salinity tolerant rice genotypes. In the present study, we have functionally characterized a Saltol QTL localized cytoskeletal protein, intermediate filament like protein (OsIFL), of rice. Studies related to intermediate filaments are emerging in plants, especially with respect to their involvement in abiotic stress response. Our investigations clearly establish that the heterologous expression of OsIFL in three diverse organisms (bacteria, yeast and tobacco) provides survival advantage towards diverse abiotic stresses. Screening of rice cDNA library revealed OsIFL to be strongly interacting with metallothionein protein. Bimolecular fluorescence complementation assay further confirmed this interaction to be occurring inside the nucleus. Overexpression of OsIFL in transgenic tobacco plants conferred salinity stress tolerance by maintaining favourable K⁺/Na⁺ ratio and thus showed protection from salinity stress induced ion toxicity. This study provides the first evidence for the involvement of a cytoskeletal protein in salinity stress tolerance in diverse organisms.

Sequence analysis of keratin-like proteins and cloning of intermediate filament-like cDNA from higher plant cells

Two keratin-like proteins of 64 and 55 ku were purified from suspension cells ofDaucus carota L., and their partial amino acid sequences were determined. The homological analysis showed that the sequence from the 64 ku protein was highly homological to beta-glucosidase, and that from the 55 ku protein had no significant homologue in GenBank. Using conservative sequence of animal IF proteins as primer, we cloned a cDNA fragment fromDaucus carota L. Southern blot and Northern blot results indicated that this cDNA fragment was a single copy gene and expressed both in suspension cells and leaves. Homological analysis revealed that it had moderate homology to a variety of alpha-helical proteins. Our results might shed more light on molecular characterization of IF existence in higher plant.

How I learned to love carrots: the role of the cytoskeleton in shaping plant cells

The carrot cell suspension was originally used because it provided a model system for studying directional cell expansion - a key process in plant morphogenesis. Early immunofluorescence studies of plant microtubules, using these cells, provided hints that the cortical array of microtubules was dynamic and this was later confirmed by microinjection studies on plant epidermal cells. A nonfixation approach for detecting F-actin was then developed on these cells and showed that, unlike animal cells, actin filaments remained associated with the nucleus throughout division and could have a role in aligning the plane of cell division. Currently, we are using detergent-extracted carrot cytoskeletons for isolating microtubule-associated proteins (MAPs). I discuss how MAPs may be involved in the oriented deposition of cellulose in the cell wall.

The 59-kDa polypeptide constituent of 8-10-nm cytoplasmic filaments in Neurospora crassa is a pyruvate decarboxylase

The fungus Neurospora crassa harbors large amounts of cytoplasmic filaments which are homopolymers of a 59-kDa polypeptide (P59Nc). We have used molecular cloning, sequencing and enzyme activity measurement strategies to demonstrate that these filaments are made of pyruvate decarboxylase (PDC, EC 4.1.1.1), which is the key enzyme in the glycolytic-fermentative pathway of ethanol production in fungi, and in certain plants and bacteria. Immunofluorescence analyses of 8-10-nm filaments, as well as quantitative Northern blot studies of P59Nc mRNA and measurements of PDC activity, showed that the presence and abundance of PDC filaments depends on the metabolic growth conditions of the cells. These findings may be of relevance to the biology of ethanol production by fungi, and may shed light on the nature and variable presence of filament bundles described in fungal cells.

Monoclonal antibodies against plant proteins recognise animal intermediate filaments

Four monoclonal antibodies were raised against polypeptides present in a high-salt detergent-insoluble fraction from cells of Chlamydomonas reinhardtii. Indirect immunofluorescence microscopy of fibroblasts and epithelial cells grown in culture using these plant antibodies revealed staining arrays identical to those obtained with well characterised antibodies to animal intermediate filaments. Immunofluorescence microscopy of Chlamydomonas with these monoclonal antibodies and a monoclonal antibody that recognises all animal intermediate filaments (anti-IFA) gave a diffuse, patchy cytoplasmic staining pattern. Both the plant antibodies and anti-IFA stained interphase onion root tip cells in a diffuse perinuclear pattern. In metaphase through to telophase, the labelling patterns colocalised with those of microtubules. Labelling of the phragmoplast was also detected but not staining of the preprophase band. On Western blots of various animal cell lines and tissues, all the antibodies labelled known intermediate filament proteins. On Western blots of whole Chlamydomonas proteins, all the antibodies labelled a broad band in the 57,000 Mr range, and three antibodies labelled bands around 66,000 and 140,000 Mr but with variable intensities. On Western blots of whole onion root tip proteins, all the antibodies labelled 50,000 Mr (two to three bands) polypeptides and a diffuse band around 60,000 Mr and three of the antibodies also labelled several polypeptides in the 90,000-200,000 Mr range. The consistent labelling of these different bands by several different monoclonal antibodies recognising animal intermediate filaments makes these polypeptides putative plant intermediate filament proteins.

Cytochalasin B affects the structural polarity of statocytes from cress roots (Lepidium sativum L.)

The effect of cytochalasin B (CB; 25 micrograms ml-1 in 1% dimethylsulfoxide, DMSO) upon the structural polarity of statocytes in cress roots is demonstrated. If normal, vertically grown roots are incubated in CB, the structural polarity of the statocytes is altered according to the developmental stage of the root. Statocytes from young roots (13 or 17 hours, additionally 7 hours CB) are characterized by proximal ER cisternae and a sparsely developed distal ER-complex. Statocytes from older roots (24 hours, additionally 7 hours CB) still accumulate distal ER, as in control roots, but at the proximal cell pole in the vicinity of the nucleus additional ER is found. These effects are reversed by washing out the drug in DMSO. Growth of the roots under a continuous supply of CB yields statocytes with sedimented nuclei, proximal ER and almost no distal ER. Together with quantitative data from morphometric studies, a dynamic model of the expression of inherent cell polarity in structural polarity is proposed.

A radial system of microtubules extending between the nuclear envelope and the plasma membrane during early male haplophase in flowering plants

The angiosperm male meiocyte is unusual among plant cells in that during early development ordered cellulosic microfibrils are not deposited at the protoplast surface. Instead, a complex series of events takes place which leads to the formation of the pollen wall 'primexine'. The use of immuno-cytochemistry, electron microscopy and experiments with an inhibitor have revealed this cell to contain no cortical microtubules, and its cytoskeleton to be radially organised with microtubules extending from the nucleus to the plasma membrane. It is proposed that these microtubules, perhaps organised from the nuclear envelope, play a part in orientation of the nucleus and in the transport of materials to the cell surface.

Organelles associated with the plasma membrane of tobacco leaf protoplasts

Leaf protoplasts of tobacco (Nicotiana tabacum) heterozygous (Su/su) and normal (su/su) for the sulfur mutation exhibit a characteristic lag period before initiating cell wall formation. The early wall is composed of a network of Calcofluor positive fibrils. Large fragments of plasma membrane from freshly isolated protoplasts were examined by electron microscopy to determine the distribution of associated organelles. Coated vesicles and patches of coat material were present on the inner surface of the plasma membrane. The frequency of coated vesicles observed was considerably less than reported for protoplasts from suspension cultured tobacco cells. Very few microtubules were associated with the plasma membrane but fine filaments were frequently observed.