Role of proline under changing environments: a review

Abstract

When exposed to stressful conditions, plants accumulate an array of metabolites, particularly amino acids. Amino acids have traditionally been considered as precursors to and constituents of proteins, and play an important role in plant metabolism and development. A large body of data suggests a positive correlation between proline accumulation and plant stress. Proline, an amino acid, plays a highly beneficial role in plants exposed to various stress conditions. Besides acting as an excellent osmolyte, proline plays three major roles during stress, i.e., as a metal chelator, an antioxidative defense molecule and a signaling molecule. Review of the literature indicates that a stressful environment results in an overproduction of proline in plants which in turn imparts stress tolerance by maintaining cell turgor or osmotic balance; stabilizing membranes thereby preventing electrolyte leakage; and bringing concentrations of reactive oxygen species (ROS) within normal ranges, thus preventing oxidative burst in plants. Reports indicate enhanced stress tolerance when proline is supplied exogenously at low concentrations. However, some reports indicate toxic effects of proline when supplied exogenously at higher concentrations. In this article, we review and discuss the effects of exogenous proline on plants exposed to various abiotic stresses. Numerous examples of successful application of exogenous proline to improve stress tolerance are presented. The roles played by exogenous proline under varying environments have been critically examined and reviewed.

Keywords: abiotic stress; antioxidant system; proline.

Figure 1. Proline metabolism in higher plants. Solid lines represents the biosynthetic pathway while catabolic pathways are shown with dashed lines. BAC, basic amino acid transporter (for arginine and ornithine exchange); Glu, glutamate; G/P, mitochondrial glutamate/proline antiporter; KG, α-ketoglutarate; P, mitochondrial proline transporter; Pi, inorganic phosphate; ProT, proline transporter; ?, unknown transporters. (Figure adapted from Szabados and Savoure [151])

Figure 2. Proline mediated intracellular redox-regulation as a multifaceted convergent strategy of different stresses. A balance of exogenous/endogenous molecules sets the required internal concentration of proline.