Mechanism of Cu- and Cd-Induced Proline Hyperaccumulation in Triticum aestivum (Wheat)

Proline hyperaccumulation, a frequently reported phenomenon in the plant kingdom, is considered an adaptive strategy of plants to cope with a variety of stresses. The present study demonstrated the mechanism of proline hyperaccumulation in Triticum aestivum exposed to excess Cu and Cd. The intracellular level of proline was increased significantly in the plants treated with test metals. Enhanced proline accumulation due to Cu and Cd was negatively correlated with the growth of the treated plants, indicating that plants divert their resources toward induction of survival strategies rather than improving growth and development. The activities of pyrroline-5-carboxylate synthetase (P5CS) and pyrroline-5-carboxylate reductase (P5CR) were increased immediately after metal treatment and remained higher through the end of the experiment, whereas ornithine amino transferase (OAT) activity of the treated plants was lower than P5CS and P5CR enzymes. The activity of proline dehydrogenase (PDH) was decreased sharply in the early phase of metal exposure (up to 12 h) but remained unchanged thereafter until the end of the experiment. Expression analysis of the transcripts of P5CS, OAT, and PDH also supports the above findings and indicates the tissue-specific expression of these genes. Based on these results, increased proline synthesis through the glutamate pathway involving the P5CS and P5CR enzymes is a major contributor to proline hyperaccumulation followed by decreased catabolism of proline.