Proline, Glycinebetaine, and Trehalose Uptake and Inter-Organ Transport in Plants Under Stress

Proline, glycinebetaine, and trehalose function as compatible solutes and are upregulated in plants under abiotic stress. The uptake and inter-organ transport in plants are largely unknown. We review the current information on the transport of these osmoprotectants under abiotic stress. Proline metabolism involves several subcellular compartments. Proline concentrations are regulated by the interplay of biosynthesis, degradation, and transport processes. Among the proline transporter proteins, both general amino acid permeases and selective compatible solute transporters were identified. The review summarized our current knowledge on proline transport under abiotic stress conditions. Trehalose is a nonreducing disaccharide formed by two glucose molecules. Sugar transporters have essential roles in the appropriate distribution of carbohydrates throughout the plants. Trehalose transporter has been poorly characterized because it is difficult to predict the characteristics of sugar transporters based solely on the amino acid sequences. Transport properties of exogenous applied trehalose for abiotic stress tolerance have been discussed. Glycinebetaine is synthesized by two-step oxidations of choline with enzymes choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH). Different biosynthetic pathways among monocot and dicot plants were discussed. Expression and substrate specificity of betaine/proline transporters from various plants were compared. Exogenous application of glycinebetaine to plants under stress conditions improved abiotic stress tolerance and gained some attentions. Further application using the important plants both in laboratory and field will contribute to increase the crop production under stress environments.