Plant freezing tolerance from phenotypes to molecules

Abstract

Considerable effort has been directed towards understanding how plants adapt to low temperature. In common with many plants, the model plant Arabidopsis thaliana is able to increase its freezing tolerance when exposed to low, nonfreezing temperatures. Additional improvements in freezing tolerance can be achieved by exposing cold acclimated plants to mild freezing temperatures (sub-zero acclimation). The freezing tolerance of Arabidopsis in all three states (nonacclimated, cold acclimated, sub-zero acclimated) is strongly influenced by the geographical origin of the investigated genotype (ecotype). In general, freezing tolerance increases with increasing latitude (from 16 to 66° Northern latitude) and with decreasing habitat temperature during the growth season. Additional genotypic and phenotypic variability can be created by crossing different ecotypes. Plant freezing tolerance is a multigenic trait. Recently, gene expression studies with microarrays and metabolite profiling experiments using gas chromatography-mass spectrometry have revealed thousands of changes in gene expression and hundreds of changes in metabolite levels in response to cold acclimation and sub-zero acclimation. These changes show significant differences in different Arabidopsis ecotypes, opening the possibility of characterizing the functional significance of such changes through correlation with the freezing tolerance phenotype. Through such analyses we are able to identify candidate molecules with a high probability of being functionally important for plant freezing tolerance. We are interested in two types of molecular changes: those responsible for low temperature signal transduction and regulation of gene expression (mainly transcription factors that regulate the expression of many other genes) and molecules that directly protect cellular structures during freezing and/or severe dehydration. To better understand the regulation of gene expression we are currently investigating the interplay of low temperature and circadian clock regulation of gene expression during cold acclimation and the regulation and function of transcription factors during both cold acclimation and sub-zero acclimation. To identify molecules that may directly affect cellular stability, we use metabolite profiling by mass spectrometry based techniques. This allows us to search for correlations between the cellular content of many compounds and the freezing tolerance of the tissues. The function of compounds of specific interest (e.g. oligosaccharides, LEA proteins) is investigated in detail using biophysical approaches such as fluorescence spectroscopy and infrared spectroscopy to determine their exact mechanisms of action.