Engineering of betaine biosynthesis and transport for abiotic stress tolerance in plants

Drought and salinity are the major factors that decrease crop yield. Organisms thriving in osmotic stress environments need adaptive mechanisms for adjusting their intracellular environment to external osmotic stress conditions. One such mechanism, to prevent water loss from the cells is to accumulate large amounts of low molecular weight organic compatible solutes such as proline, betaine and polyols to balance internal osmolarity of the cells. Accumulation of compatible solutes can be achieved by enhanced synthesis and/or reduced catabolism. Certain plants synthesize betaine in chloroplasts via a two-step oxidation of choline and betaine accumulation is associated with enhanced stress tolerance. Many important crop plants have low levels of betaine or none at all. Hence, betaine biosynthetic pathway is a target for metabolic engineering to enhance stress tolerance in crops. Introduction of betaine synthesis pathway into betaine non-accumulating plants has often improved stress tolerance. However, betaine levels of the engineered plants were generally low. To further enhance the betaine accumulation levels, we need to diagnose factors limitng betaine accumulation in engineered plants. Here we discuss recent progress on metabolic engineering of choline precursors for abiotic stress tolerance in plants.