Progress and Prospects in Engineering Crops for Osmoprotectant Synthesis

Abstract

Many plants accumulate organic solutes such as quaternary ammonium and tertiary sulfonium compounds, sugar alcohols and the amino acid proline in response to salinity and drought. There is strong experimental support for the hypothesis that accumulation of organic solutes is of adaptive significance. This is based on their distribution in species that evolved under stress habitats, their protective effects in bioassays, positive correlation between their levels and sap osmotic pressures under stress, their cytoplasmic location and more directly, stress protection of model organisms engineered to overproduce the solute. Since only some taxa accumulate osmoprotectants and many do not, attempts have been made to engineer non-accumulating plant species for osmoprotectant synthesis with the ultimate aim of improving stress tolerance. Synthetic pathways to sugar alcohols, amino acid proline and the quaternary ammonium compound glycine betaine have so far been manipulated in transgenic systems. Progress in this area is summarized with emphasis on engineering synthesis of glycine betaine and other quaternary ammonium compounds. Some stress protection in transgenic plants accumulating even low levels of osmoprotectants suggests that there is potential for achieving better level of stress-tolerance by engineering stress-inducible over-production of the osmoprotectant. Analyses of transgenic plants indicate current metabolic constraints in achieving this and strategies for further rounds of engineering to overcome them. In general, the diversity for osmoprotectants in plants has been under-utilized compared to microbial genes. Diverse osmoprotectants in stress-tolerant plants present unique future opportunities and challenges for metabolic engineering both to improve stress tolerance in crops and to understand plant metabolism.