As sessile organisms, plants are unable to escape from the many abiotic and biotic factors that cause a departure from optimal conditions of growth and development. Low temperature represents one of the most harmful abiotic stresses affecting temperate plants. These species have adapted to seasonal variations in temperature by adjusting their metabolism during autumn, increasing their content of a range of cryo-protective compounds to maximise their cold tolerance. Some of these molecules are synthesised de novo. The down-regulation of some gene products represents an additional important regulatory mechanism. Ways in which plants cope with cold stress are described, and the current state of the art with respect to both the model plant Arabidopsis thaliana and crop plants in the area of gene expression and metabolic pathways during low-temperature stress are discussed.

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Aluminum (Al) is a major factor that limits plant growth in acid soils. It causes a cessation of root growth and changes in root morphology suggesting a role of the root cytoskeleton as a target of Al-toxicity. Here we report a rapid effect of Al on the microtubular cytoskeleton of the suspension tobacco cell lines BY-2 and VBI-0. Viability studies showed that the cells were more sensitive to Al during exponential phase as compared to stationary cells. During the first hours of exposure, Al induced the formation of additional bundles of cortical microtubules (cMTs), whereas the thickness of the individual bundles decreased. Prolonged exposure resulted in disorientation of cMTs. These changes of cMTs preceded the decrease of cell viability by several hours and were accompanied by an increase in the levels of alpha-tubulin (in its tyrosinated form) and elements of the tubulin-folding chaperone CCT. These findings suggest that the microtubular cytoskeleton is one of the early targets of Al toxicity.

Concurrently with cold-induced disintegration of microtubular structures in the cytoplasm, gradual tubulin accumulation was observed in a progressively growing proportion of interphase nuclei in tobacco BY-2 cells. This intranuclear tubulin disappeared upon rewarming. Simultaneously, new microtubules rapidly emerged from the nuclear periphery and reconstituted new cortical arrays, as was shown by immunofluorescence. A rapid exclusion of tubulin from the nucleus during rewarming was also observed in vivo in cells expressing GFP-tubulin. Nuclei were purified from cells that expressed GFP fused to an endoplasmic-reticulum retention signal (BY-2-mGFP5-ER), and green-fluorescent protein was used as a diagnostic marker to confirm that the nuclear fraction was not contaminated by nuclear-envelope proteins. These purified, GFP-free nuclei contained tubulin when isolated from cold-treated cells, whereas control nuclei were void of tubulin. Furthermore, highly conserved putative nuclear-export sequences were identified in tubulin sequences. These results led us to interpret the accumulation of tubulin in interphasic nuclei, as well as its rapid nuclear export, in the context of ancient intranuclear tubulin function during the cell cycle progression.

Cytoskeletal proteins assemble into dynamic polymers that play many roles in nuclear and cell division, signal transduction, and determination of cell shape and polarity. The distribution and dynamics of microtubules (MTs) and actin filaments (AFs) are determined, among other factors, by the location of their nucleation sites. Whereas the sites of microtubule nucleation in plants are known to be located under the plasma membrane and on the nuclear envelope during interphase, there is a striking lack of information about nucleation sites of AFs. In the studies reported herein, low temperature (0 ∞ ∞ ∞ ∞ C) was used to de-polymerize AFs and MTs in tobacco BY-2 ( Nicotiana tabacum L.) cells at interphase. The extent of de-polymerization of cytoskeletal filaments in interphase cells during cold treatment and the subcellular distribution of nucleation sites during subsequent recovery at 25 ∞ ∞ ∞ ∞ C were monitored by means of fluorescence microscopy. The results show that AFs re-polymerized rapidly from sites located in the cortical region and on the nuclear envelope, similarly to the initiation sites of MTs. In contrast to MTs, however, complete reconstitution of AFs was preceded by the formation of transient actin structures including actin dots, rods, and filaments with a dotted signal. Immunoblotting of soluble and sedimentable protein fractions showed no changes in the relative amounts of free and membrane-bound actin or tubulin.