Expression of antioxidant enzymes and heat shock proteins in relation to combined stress of cadmium and heat in Vigna mungo seedlings

Abstract

We report the individual and combined effects of heat shock (HS) and cadmium on growth, activities and isoenzyme patterns of superoxide dismutase (SOD), peroxidase (POD) and their gene expression in Vigna mungo seedlings. Expression of low and high molecular mass heat shock proteins (HSPs), under these conditions, has also been investigated. Our results suggest that HS stimulated germination over control in the first 12 h. HS also stimulated growth of seedlings and the effect was visible even up to 96 h. Cd 2+ treatment inhibited both germination and length of seedlings but HS prior to cadmium treatment provided significant cross protection to Cd 2+ effect up to 48 h. Both SOD and POD activities responded differently to HS or Cd 2+ stress. Cd 2+ treatment slightly stimulated SOD activity by 5–10% over control, whereas HS insignificantly inhibited SOD activity. Three SOD isoenzymes (45, 25, 17.8 kDa) were characterized at 0 h as well as immediately after heating as against a single band (160 kDa) of POD at this stage. Based on inhibition studies, the uppermost 45 kDa SOD band was characterized as Mn SOD. Of the lower two SOD bands, one was characterized as Cu/Zn SOD and the other one as a novel trinuclear Cu/Zn/Mn SOD. POD activity increased significantly with HS, both in the presence or absence of Cd 2+. Several POD bands appeared with time in all the treatments, some of which were specific to Cd 2+ and HS treatments. One to two transcripts of SOD and POD were detected and the signals were stronger both in HS and Cd 2+ treatments. The intensity of the signals increased further in a combined stress of HS and Cd 2+. The data also suggest the involvement of both constitutive and inducible cytosolic HSPs to the combined stress response in vivo. The accumulation and induction of these high and small molecular mass HSPs truly reflect the result of a cross talk between response pathways of individual stresses.