, Volume 29, Issue 1-2, pp 47-76

Molecular analysis of acclimation to cold

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Abstract

Temperature is expected to affect all plant processes. Consistent with this, expression of a large number of specific mRNAs and proteins is up-regulated during cold-acclimation. Their possible functions are outlined, encompassing a wide range of processes, some possibly related to other winter stresses besides cold. Some of the cold-responsive sequences are associated with constitutive or stress-related metabolism, others are probably protective, some may influence freezing, and many are of as yet unknown function. While many of the sequences code for intracellular proteins, a significant number code for apoplastic proteins with a variety of possible functions. Transient influx of calcium into the cytosol appears to be a key step in the response to cold, and also to many other stresses and signals. Similarly, the cold-responsive promoter element identified so far is also responsive to drought and salt. However, how cold is sensed, and any cold-specific aspects of the cold signal transduction pathway, are, so far, unknown. What is clear is that, at least in the model plant Arabidopsis thaliana, cold-, desiccation-, salt- and ABA-triggered signal transduction pathways, and possibly others, run partly in parallel and partly intersect. This may be partly explained by a need for an integrated winter-response. The total number of genes which are cold-responsive and the quantity of resources which this implies are used in this way, indicate that many must have a positive role in acclimation. Experiments which modify membrane lipid unsaturation or solute accumulation, achieved by transformation of plants to express exotic or heterologous genes or by other means, confirm that these factors affect chill- or freezing-tolerance. A transgenic test has shown that one cold-up-regulated gene of previously unknown function contributes to freezing-tolerance, but the small effect re-emphasises the probably cumulative nature of the contributions of many cold-up-regulated sequences to acclimation. On the other hand, mutant analysis indicates some genes may make a comparatively larger contribution. Transformation of alfalfa to overexpress a superoxide dismutase gene increased cold-tolerance and drought-resistance and demonstrated that improvements in field-survival of stresses is possible by transgenic means. Over-expression of a transcription factor, CBF1, conferred freezing-tolerance on Arabidopsis, showing that manipulation of the signal transduction pathway could be an important method for modifying cold-tolerance.