Proline accumulation, accepted as an indicator of environmental stress, is also considered to have important protective roles.Some authors have suggested that proline acts as an antioxidant in Cd-stressed cells and thereby improves metal tolerance (Sharma and Dietz 2006, Siripornadulsil et al. 2002). Others have reported that many plants have been reported to accumulate proline (Pro) when exposed to heavy metals (Alia and Saradhi 1991, Talanova et al. 2000). However, the precise mechanism and the functional significance of proline accumulation in plants under heavy metal stress have not been elucidated to date. Costa and Morel (1994) have suggested that proline accumulation in plants under Cd stress is induced by a Cd imposed decrease of the plant water potential. However, proline maintains the water balance under Cd stress thereby suggesting that proline- mediated alleviation of water deficit stress could substantially contribute to the Cd tolerance of the plant. But the direct conclusive evidence as to the role for the water potential in heavy metal-induced proline accumulation is still lacking. A recent study has shown that proline alleviates Cd toxicity by detoxifying ROS, and increasing the activity of SOD and CAT and glutathione content (Xu et al. 2009). Rai (2002) suggests that the possible role of proline against heavy metals is by forming chelates with the metals. There are indications that proline forms a proline-metal complex and protects the activity of glucose-6-phosphate dehydrogenase and nitrate reductase against inhibition by Cd and Zn (Sharma et al.1998). Apart from acting as an heavy metal alleviator, proline also acts as an important cytoplasmic osmoticum, a scavenger of free radicals, source of nitrogen and carbon for post stress growth, a stabilizer of membranes, machinery for protein synthesis and a sink for energy to regulate redox potential (Rai et al. 2004). Muneer et al. (2011) reported that proline content increase at all concentrations of cadmium exposure in Vigna radiata and maximum increase was found at 0.50 mM which was about 119 to 120%. In Cajanus cajanand Vigna mungo there was an accumulation of proline under heavy metals (Co, Cd. Zn and Pb) treatment (Alia and Saradhi 1991). Similar results were observed in case of Cajanus cajan when given aluminium treatment (Bhamburdekar and Chavan 2011). 4.2.f. Phenolics
Baca lebih lanjut
Baca lebih lajut