Abstract
Regulation of proline accumulation in plants under chilling stress remains unclear. In this paper, we treated Jatropha curcas seedlings under chilling stress with exogenous calcium chloride (CaCl2), the plasma membrane Ca2+-channel blocker lanthanum chloride (LaCl3), calmodulin antagonists, chlorpromazine (CPZ), and trifluoperazine (TFP) and investigated the effects of calcium and calmodulin (CaM) on proline accumulation and chilling tolerance. The results showed that CaCl2 treatment significantly enhanced chilling stress-induced proline accumulation. CaCl2 also induced an almost immediate and rapid increase of Δ1-pyrroline-5-carboxylate synthetase (P5CS) and glutamate dehydrogenase activities, the key enzymes in the glutamate pathway of proline biosynthesis, and up-regulated P5CS expression, but it decreased the activity of proline dehydrogenase (ProDH), a key enzyme of proline degradation, and inhibited ProDH expression. Treatment with LaCl3, CPZ, and TFP exhibited the opposite effects to those by CaCl2 treatment. Moreover, CaCl2, LaCl3, CPZ, and TFP had little effect on the activities of ornithine aminotransferase and arginase, the key enzymes in the ornithine pathway of proline biosynthesis. These results indicated that Ca2+-CaM might be involved in signal transduction events, leading to proline accumulation in J. curcas seedlings under chilling stress, and that Ca2+-induced proline accumulation is a combined result of the activation of the glutamate pathways of proline biosynthesis and the simultaneous inhibition of the proline degradation pathway. In addition, CaCl2 treatment increased tissue vitality, decreased the content of the lipid peroxidation product malondialdehyde (MDA), and alleviated electrolyte leakage in J. curcas seedlings under chilling stress, indicating that exogenous Ca2+ can enhance chilling tolerance, and proline might be a key factor in this increased chilling tolerance.
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