Proline: a key player in plant abiotic stress tolerance

Dramatic accumulation of proline is a common physiological response in plants exposed to various abiotic stresses. Accumulation of proline could be due to de novo synthesis, decreased degradation, lower utilization, or hydrolysis of proteins. Extensive intercellular proline transport occurs between the cytosol, chloroplasts, and mitochondria due to its compartmentalized metabolism. Although all functions of proline in stress tolerance are still a matter of debate, it is suggested that proline contributes to stabilization of sub-cellular structures, scavenging free radicals, and buffering cellular redox potential. It also chelates heavy metals, modulates cellular functions, and even triggers gene expression. Apparently, proline acts as stress-related signal exhibiting cross tolerance to a range of different stresses. Besides these significant roles, its metabolism is found to be coupled to several key pathways such as pentose phosphate, tricarboxylic acid, or urea cycles and contributes to, i.e., purine synthesis and the phenylpropanoid pathway. Although the molecular basis of regulation of proline metabolism is still largely obscure, the genetic engineering of proline content could lead to new opportunities to achieve plant stress tolerance.