Overexpression of TpGSDMT in Rice Seedlings Promotes High Levels of Glycine Betaine and Enhances Tolerance to Salt and Low Temperature

Salt and low temperature are serious abiotic stresses and important constraints to agricultural productivity across the globe. These abiotic stresses negatively affect plant growth and physiological, biochemical, and molecular processes. Glycine betaine (GB) is an important osmoprotectant that enables plants to resist salinity, low temperature, and drought. GB can be synthesized in many organisms, including animals, plants, and bacteria. In higher plants, GB is synthesized through two-step oxidation of choline. However, rice, an important food crop, cannot synthesize GB. Thus, conferring the ability to synthesize GB to rice through genetic engineering is of great significance for enhancing its tolerance to abiotic stress. Recently, an enzyme, GSDMT (glycine, sarcosine, and dimethylglycine methyltransferase) was found in a diatom, <i>Talassiosira pseudonana</i>, and found able to catalyze the three successive methylation steps of glycine to form GB. This biosynthetic pathway for GB synthesis is also the simplest in living organisms. Here, the optimized codon of the <i>TpGSDMT</i> gene sequence was synthesized and cloned into an overexpression vector, pBWA(V)HS, which contains a CaMV 35S promoter, and then, the constructed vector was transferred into rice (<i>Oryza sativa</i> L. ssp. <i>Japonica</i>). The GB content in transgenic rice showing overexpression of <i>TpGSDMT</i> was significantly increased, and these transformants exhibited markedly enhanced tolerance to salt and low temperature. These results indicate that the <i>TpGSDMT</i> gene can be used for the genetic improvement in crop plants' resistance to salinity and low temperature.