Final Report Summary - PLANTCOLDMEMORY (Loss of freezing tolerance under warm conditions and low temperature memory in the closely related species Arabidopsis thaliana and Thellungiella salsuginea – metabolomic and transcriptomic approaches) Cold has a major influence on plant growth and survival. Together with drought it is considered the most important factor limiting the distribution of plants on Earth. Freezing damage to native vegetation and crop plants is additionally a ubiquitous problem of major economic significance. Global climate models predict an increase in the mean surface air temperature and frequency and severity of erratic temperature events during this century. Hence, temperate and boreal winters are becoming milder with an associated increased risk of unseasonable warm spells. Warm spells may cause premature loss of plant freezing tolerance (de-acclimation), thereby increasing the risk of subsequent freezing injury. Additionally, shifting phenological patterns such as an earlier start of the growth season and earlier flowering, consistent with climate warming, may enhance the risk of frost injuries caused by increasing temperature variation. Therefore, understanding the regulation and mechanisms of loss of plant freezing tolerance during warm spells is important to ensure the sustainability of food and plant resources under changing climatic conditions. Plants native to temperate and cold climates show natural low temperature acclimation during fall in preparation for winter frost (Figure 1). The natural acclimation response can be elicited experimentally by exposing plants such as Arabidopsis to a short period (several days) of low, non-freezing temperatures. The low temperature acclimation response involves massive changes in gene expression and metabolite content. Ultimately, these changes lead to an altered phenotype, i.e. an increase in freezing tolerance, although the exact contribution of any single factor is still unclear. In spring, overwintering plants lose acclimated freezing tolerance by de-acclimation to resume growth. Experimentally, de-acclimation can be elicited by exposing plants to a period of warm temperatures after cold acclimation. While the kinetics of low temperature acclimation have been rather well studied, de-acclimation and the persistence of the acclimated state under warm conditions (memory phase) has not attracted much attention. Also, how the duration of a memory phase and the extent to which the memory of the acclimated state persists influence a subsequent low temperature response has only rarely been investigated on the phenotypic level (i.e. as freezing tolerance) and not at all on the molecular level. Figure 1: Principal figure showing seasonal changes in the lowest temperature overwintering temperate plants were able to survive during low temperature acclimation in autumn, mid-winter and during spring de-acclimation.The aim of this project was to study regulation and mechanisms of loss of plant freezing tolerance and the potential of plants to “remember” stressful conditions and modify their reactions to a subsequent stress. In addition, through the comparison of different accessions within a species and between two closely related species differing in their stress responses, the project was aiming at identifying conserved responses and molecular responses that are quantitatively related to freezing tolerance and memory function. The persistence of the acclimated state over time under warm conditions (memory phase) and how the duration of such a memory phase influences a subsequent low temperature response were investigated in a number of experiments including the two species Arabidopsis thaliana and Thellungiella salsigunea. The major part of the molecular studies on cold acclimation and freezing tolerance has been performed with Arabidopsis, due to the availability of genetic and genomic resources for this species. Thellungiella is a relative of Arabidopsis that shows a higher abiotic stress tolerance. In these experiments the freezing tolerance of selected accessions was determined after cold acclimation and different durations of memory and re-acclimation treatments. The experiments revealed that the physiological effects of different temperature treatments vary depending on a number of factors, such as the duration of the memory phase, the duration of the re-acclimation phase, as well as the temperature of this second low temperature treatment. From these data, a suitable experimental set-up covering the crucial time points and temperatures has been established for Arabidopsis. This set-up will form the central basis for future investigations of the potential of Arabidopsis to “remember” stressful low-temperature conditions and modify its reactions to a subsequent low-temperature event.To investigate the physiological and molecular mechanisms underlying de-acclimation and determining the extent of the memory in Arabidopsis, metabolic reprogramming and the role of gene expression regulation were investigated after cold acclimation and different durations of de-acclimation treatment. Changes in the content of primary metabolites were investigated by GC-MS, the expression of 1880 genes encoding transcription factors by qRT-PCR and global changes in gene expression by microarray hybridization. Statistical and bioinformatic analysis of these results is currently being undertaken. It is expected that the ongoing analyses will further our knowledge of the mechanisms of loss of plant freezing tolerance and the molecular basis of a plant stress memory. In the research field of plant abiotic stress tolerance, mechanisms of de-acclimation and the potential of plants to “remember” stressful conditions and modify their reactions to a subsequent stress have only rarely been considered. Especially investigations at the molecular genomics level are almost entirely missing. Hence, the results will aid to fill this gap in our knowledge in the area of cold acclimation and freezing toleranceDetailed knowledge of the molecular mechanisms and the regulation of de-acclimation in plants will be of increasing importance in a world challenged by global climate change. Winters in the Northern hemisphere will likely get warmer, thereby decreasing the incidence of extremely severe frost. However, unpredictable warm-spells in early spring followed by freezing events will lead to increased danger of frost damage to both wild and crop plants. In crop plants such as winter cereals and fruit trees and shrubs this could lead to catastrophic yield losses. The results of this project will contribute to an increased understanding of how global climate change will affect plant production in climate-sensitive industries such as agriculture, horticulture and forestry. Increased knowledge in this area enables strategies to be implemented to mitigate risks resulting from climate change. Also, understanding plant de-acclimation mechanisms will help to improve the efficiency of selecting superior cultivars and breed crop plants that are more resistant to these particular climatic challenges to ensure stable yields for European farmers under climate change conditions and thereby contribute to a sustainable agriculture.For further information about this project please contact the host scientist Dirk Hincha (Hincha@mpimp-golm.mpg.de) or the fellow Majken Pagter (mp@bio.aau.dk).
