Plants in Extreme Environments. Importance of Protective Compounds in Stress Tolerance

Extreme environmental conditions such as drought, cold or high soil salinity impede plant growth and require specific adaptation capacity. In response to environmental stresses, a number of low-molecular-weight compounds can accumulate in plants: protective amino acids, sugar alcohols, sugars and betaine-type quaternary amines. The function of these compounds includes the stabilisation of cellular structures, photosynthetic complexes, specific enzymes and other macromolecules, the scavenging of reactive oxygen species or acting as metabolic signals in stress conditions. Although a correlation between the accumulation of certain osmoprotective compounds and stress tolerance certainly exists, a causal relationship between osmolyte accumulation and enhanced tolerance could not always be confirmed. Nevertheless, the importance of osmoprotective compounds for the adaptation to extreme environmental conditions is supported by numerous studies obtained with natural variants, mutants or transgenic plants with different capabilities to accumulate these metabolites. Combining genetic analysis with metabolic profiling approaches could considerably increase our understanding of plant stress responses and the importance of the protective metabolites in the adaptation to stress conditions.