Evolutionary Development of Salt and Cold Tolerance Traits in Thellungiella halophila, a new Arabidopsis Related Model Species (ARMS).

In view of the world-wide change in environmental conditions, there is an increasing interest in understanding the evolutionary processes that have enabled certain plants to live in adverse conditions like cold, salinity or drought. Arabidopsis thaliana has evolved as the plant model species and it has been successfully used to address a variety of questions in plant biology. However, Arabidopis has also limitations as it is for instance not stress tolerant. There is definitively a need for model-plants with more ?extremophile? characteristics and here we propose to use Thellungiella halophila; an Arabidopsis related plant species that has developed mechanisms that enables it to survive treatment with sea water and freezing as low as -17 degrees celcius. At a recently held Workshop (Paris, Sept. 2004), attended by 20 scientist from the US, Canada, Japan, Israel and Europe (including De Boer and Souer), it was decided to join efforts to develop Thellungiella as the model for extromophile research. This includes the sequencing of the Thellungiella genome, but also the collection of ecotypes from different habitats. The availability of a wide range of ecotypes will be important to fully explore the genetic potential that is present within Thellungiella species by revealing any natural variability in salt and/or cold stress (Koornneef et al., 2004). Thus far, two Thellungiella ecotypes have been collected from China (Shandong) and Canada (Yukon). However, Thellungiella is common within the Russian area as well. Therefore, the first goal of our project will be to collect Thellungiella ecotypes in the European and Asian parts of Russia, differing in climate (temperature) and soils (salinity). Environmental conditions of each habitat will be characterized in detail: temperature, rainfall, soil properties, salinity levels etc. Phenotyping of the new and two known ecotypes with respect to salt and cold treatment will be done under controlled greenhouse or climate chamber conditions. This should reveal variation in specific traits and will provide the basis for segregation analysis, followed by QTL analysis (Koornneef et al., 2004). The final aim is to identify stress tolerance loci and compare these with the loci identified in Arabidopsis. The second goal of the project is to understand how specific mechanisms that are important for the adaptation to high salinity, have evolved in the different species and ecotypes. Examples of such mechanisms are the capacity to extrude Na+ ions back to the soil (SOS1 antiporter protein), to accumulate Na+ ions in the vacuoles (NHX1 antiporter) or to produce high levels of the compatible solute proline (key-enzyme is P5CS). Interestingly, these mechanisms came also to the foreground in a recent study comparing the Arabidopsis and Thellungiella transcriptome (Taji et al., 2004). This study showed that basically these species have the same genes, but the expression and function of some of the genes seems to have adapted to new environments during evolution. E.g. the SOS1 and P5CS genes were salt induced in Arabidopsis, but constitutively active in Thellungiella. We will compare the structure, expression and function of a number of selected genes with functions in ion homeostasis, signaling and detoxification of Arabidopsis, Thellungiella (Shandong) and a number of newly defined ecotypes under control and salt conditions. Questions like differences in promoter and/or coding sequence, mRNA stability, protein degradation will be addressed by swapping the promoter and/or coding sequences. Both the analysis of the Thellungiella ecotypes and the comparison of selected adaptation mechanisms in these ecotypes and Arabidopsis, will provide new insights into the evolution of abiotic stress adaptation strategies in plants.