Synchronization of cell cultures of higher plants

Cell cycle stage-specific differential expression of topoisomerase I in tobacco BY-2 cells and its ectopic overexpression and knockdown unravels its crucial role in plant morphogenesis and development

DNA topoisomerases catalyze the inter-conversion of different topological forms of DNA. Cell cycle coupled differential accumulation of topoisomerase I (Topo I) revealed biphasic expression maximum at S-phase and M/G1-phase of cultured synchronized tobacco BY-2 cells. This suggested its active role in resolving topological constrains during DNA replication (S-phase) and chromosome decondensation (M/G1 phase). Immuno-localization revealed high concentrations of Topo I in nucleolus. Propidium iodide staining and Br-UTP incorporation patterns revealed direct correlation between immunofluorescence intensity and rRNA transcription activity within nucleolus. Immuno-stained chromosomes during metaphase and anaphase suggested possible role of Topo I in resolving topological constrains during mitotic chromosome condensation. Inhibitor studies showed that in comparison to Topo I, Topo II was essential in resolving topological constrains during chromosome condensation. Probably, Topo II substituted Topo I functioning to certain extent during chromosome condensation, but not vice-versa. Transgenic Topo I tobacco lines revealed morphological abnormalities and highlighted its crucial role in plant morphogenesis and development.

Assessment of cell viability

Cell viability may be judged by morphological changes or by changes in membrane permeability and/or physiological state inferred from the exclusion of certain dyes or the uptake and retention of others. This unit presents methods based on dye exclusion, esterase activity, and mitochondrial membrane potential, as well as protocols for determining the pre-fixation viability of fixed cells either before or after fixation with amine-reactive dyes suitable for a range of excitation wavelengths. Membrane-impermeable dead cell and live cell dyes as well as dye-exclusion procedures for microscopy are also included.

BY-2 cells: culture and transformation for live cell imaging

Tobacco Bright Yellow-2 (BY-2) suspension cells are a widely used biological material for studying plant cell morphology and physiology. These cells are easy to transform and maintain in culture and tolerate transformation with fluorescent proteins such as the green fluorescent protein and its derivatives. These, by the addition of plant or mammalian targeting sequences, can be directed to specific subcellular locations for the study of cell dynamics in vivo. This unit describes the production of BY-2 cell stable transformants via an Agrobacterium based method to permit the visualisation of cellular components in vivo by epifluorescence or confocal microscopy.

Uptake of endocytic markers by rice cells: variations related to the growth phase

Endocytosis is now considered a basic cellular process common to plant cells. Although both non-specific and receptor-mediated endocytosis appear to take place in plant cells, the physiological role of the latter remains unclear. We have investigated the endocytic process in rice cell suspensions using two biotinylated proteins, peroxidase and bovine serum albumin (bHRP and bBSA), as markers. First, we show that markers are internalized by rice cells and appear in intracellular membranes. The uptake of the two markers is temperature dependent, saturable with time and markers dose and it is competed by free biotin. Thus, it shows the properties of a receptor-mediated process. We also show that uptake of markers is strongly influenced by growth phase as optimal uptake occurs during the lag phase, but the initiation of the exponential growth phase decreases uptake drastically. Arrest of the cell cycle by starvation of either a nutrient (phosphate) or a growth regulator (2,4-dichlorophenoxyacetic acid), both components of the culture medium, does not modify the rate of bBSA uptake. Subsequent readdition of these components results in growth recovery and a dramatic decrease in bBSA uptake. On the other hand, nocodazole treatment, a method to arrest the cell cycle by microtubule depolymerization, inhibited bBSA uptake. The possible causes for this arrest of endocytosis are discussed.

Technical advance: spatio-temporal analysis of mitotic activity with a labile cyclin-GUS fusion protein

Plant growth responds rapidly to developmental and environmental signals, but the underlying changes in cell division activity are poorly understood. A labile cyclin-GUS reporter was developed to facilitate the spatio-temporal analysis of cell division patterns. The chimeric reporter protein is turned over every cell cycle and hence its histochemical activity accurately reports individual mitotic cells. Using Arabidopsis plants transformed with cyclin-GUS, we visualized patterns of mitotic activity in wounded leaves which suggest a role for cell division in structural reinforcement.