Roles of endogenous glycinebetaine in plant abiotic stress responses

Studies of plant adaptation to saline or dry environments have for several years recognized the role of quaternary ammonium and tertiary sulfonium compounds as nontoxic compatible solutes. One of the most studied compounds is glycinebetaine (GB), earlier known as lycine or oxyneurine. GB is a nontoxic, odorless, tasteless, and colorless amino acid derivative. Several plant species accumulate GB naturally as a response to abiotic stresses. However, many crop species are not able to synthesize GB in physiologically meaningful concentrations. On the other hand, halophytes, plants that can tolerate a high concentration of salt in the soil, are known to synthesize GB in low concentrations even when they are not experiencing any stress. GB is synthesized in chloroplasts from choline via a two-step oxidation reaction. This synthesis pathway has been the target in many attempts to either introduce or increase GB synthesis in plants by a transgenesis approach. However, in many cases the availability of choline obtained through photorespiration has restricted GB synthesis. Furthermore, availability of nitrogen can also restrict GB synthesis, since GB contains approximately 12% nitrogen. GB has so far been considered a relatively inert compound in plants. Since GB does not interact with metabolic pathways in the plant, it remains available at the cellular level for its use. Thus, it can be hypothesized that plants could be able to utilize nitrogen through GB metabolism, for example, during stress recovery. On the other hand, GB synthesis is energetically expensive for plants. Synthesis of 1 GB mole costs approximately 50 ATP moles, whereas synthesis of 1 sucrose mole costs approximately 30 ATP moles. In comparison, osmotic adjustment with inorganic ions is less costly, e.g., 1 NaCl mole equals 3.5 ATP moles. The aim of this chapter is to provide an overview of endogenous GB in plants and the transgenesis approach for introducing and improving GB synthesis in plants.