Biochemical mechanisms of chilling tolerance in maize
In this research project, certain key biochemical parameters that impart chilling tolerance have been compared between a chilling-susceptible (CO 316) and a chilling-tolerant (CO 328) maize inbred. These parameters have also been compared with seedlings derived from paclobutrazol ((2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-yl)pentan-3-ol) -treated CO 316, a treatment that has been shown to induce chilling tolerance. Investigations on chlorophyll and carotenoid levels, leakage of electrolytes and changes in Fv/Fm ratio of seedlings before, during and following chilling stress, clearly demonstrated that increased stabilization as well as recovery from damage occurred in membranes of CO 328 and paclobutrazol-treated CO 316 when compared to that in CO 316. The active oxygen species generated during chilling are catabolized differently in CO 328, paclobutrazol-treated CO 316 and CO 316. Based on these evidences, chilling tolerance of CO 328 has been attributed to increased activities of glutathione reductase in roots and maintenance of high levels of superoxide dismutase activity in leaves. Paclobutrazol-treated CO 316 is characterized by increased activities of superoxide dismutase and ascorbate peroxidase in leaves and roots, as well as high levels of glutathione reductase in leaves. As well, chilling tolerance of CO 328 and paclobutrazol-treated CO 316 appears to be related to lowered catabolism of free fatty acids to oxidation products, thus enabling the system to recycle intermediates of phospholipid breakdown. Analysis of protein profiles revealed that paclobutrazol-treatment of CO 316 resulted in increased expression of several high molecular weight polypeptides, making the profile more similar to that of CO 328. Again, paclobutrazol-treatment appeared to have affected the turnover of several proteins, especially in roots. The expression of several stress proteins belonging to the heat shock cognate 70, and low molecular weight heat-shock protein category was increased in CO 316P and CO 328. Several of these proteins declined drastically in CO 316. The results suggest that chilling tolerance could be attained through several mechanisms that include the modulation of the activity and levels of antioxidant enzymes, the degradation and potential recycling of membrane lipids, and the expression of stress proteins.