Hg and Cd induced changes in proline content and activities of proline biosynthesizing enzymes in Phaseolus aureus and Triticum aestivum

The effect of mercury and cadmium, in the form of HgCl2 and CdCl2 respectively, on proline accumulation and two key proline biosynthesizing enzymes, Delta(1)-pyrroline-5-carboxylate synthetase (P5CS) and Delta(1)-pyrroline-5-carboxylate reductase (P5CR), was investigated in Phaseolus aureus Roxb. and Triticum aestivum L. The 5-d-old seedlings were exposed to 0.05, 0.1, 0.2 or 0.4 mM concentrations of the metals in Hoagland solution for 12 and 36 h. T. aestivum exhibited considerably greater accumulation of proline than P. aureus in response to the metal treatment. Among the two metals, Hg induced greater accumulation of proline than Cd. The activity of P5CS increased significantly in response to the metal treatment, particularly in T. aestivum in which the activity of the enzyme in the control was much higher than that was in P. aureus. The activity of P5CR on the other hand mostly decreased in response to the metal treatment. The study indicated a strong dependence of the metal induced proline accumulation on the constitutive P5CS content of the plants.