Metabolic Engineering for Stress Tolerance: Installing Osmoprotectant Synthesis Pathways

Abstract

Abiotic environmental stresses such as drought, salinity and low temperature are major limitations for plant growth and crop productivity. Certain plants, marine algae and bacteria have evolved a number of adaptations to such abiotic stresses: some of these adaptations are metabolic and others structural. Accumulation of certain organic solutes (known as osmoprotectants) is a common metabolic adaptation found in diverse taxa. These solutes protect proteins and membranes against damage by high concentrations of inorganic ions. Some osmoprotectants also protect the metabolic machinery against oxidative damage. Many major crops lack the ability to synthesize the special osmoprotectants that are naturally accumulated by stress-tolerant organisms. Therefore, it was hypothesized that installing osmoprotectant synthesis pathways is a potential route to breed stress-tolerant crops. Proving this, recent engineering efforts in model species led to modest but significant improvements in stress tolerance of transgenic plants. Synthetic pathways to two kinds of osmoprotectants—polyols and quaternary ammonium compounds—are discussed here. Results from the metabolic engineering experiments emphasize the need for a greater understanding of primary metabolic pathways from which osmoprotectant synthesis pathways branch. Future research avenues include the identification and exploitation of diverse osmoprotectants in naturally stress-tolerant organisms, and the use of multiple genes and reiterative engineering to increase osmoprotectant flux in response to stress. High-throughput genomic technologies offer a number of tools to refine this by rapidly identifying genes, pathways, and regulatory controls.