Heat exposure alters the expression of SOD, POD, APX and CAT isozymes and mitigates low cadmium toxicity in seedlings of sensitive and tolerant rice cultivars

A 0–500 μM Cd2+ alone and/or heat stress in rice cv. DR-92 (sensitive) and cv. Bh-1 (tolerant), altered the banding patterns of SOD, CAT, POD and APX enzymes in roots/shoots. In controls, six/seven activity bands for POD in roots/shoots were observed. The band intensities of some decreased under combination of Cd2+ + heat stress. Six SOD isoforms in shoots of cv. Bh-1 and three in cv. DR-92 appeared. In sensitive cv. DR-92 a trinuclear Cu/Zn/Mn-SOD 1 isozyme was upregulated in shoots under Cd/HS/Cd + HS treatments whereas a suppression in the same was noticed in roots. Under Cd alone the Cu/Zn/Mn-SOD 2 was strongly induced in roots which was otherwise absent in all HS treatments. POD R7 band was absent in HS alone but was induced under Cd2+ + HS treatments in rice cv. DR-92. In tolerant cv. Bh-1, isozyme Mn-SOD 3 was induced under Cd alone but was absent under HS. Cu/Zn/Mn-SOD 2 and Cu/Zn/Mn-SOD 3 were strongly induced in roots and shoots under low or moderate Cd2+ + HS treatments. APX R4/CATR2 isozymes which were absent under Cd2+ alone or HS alone were induced under combination of Cd2+ + HS indicating de novo synthesis of enzyme proteins under combined stressors. Decreased band intensities under Cd2+ + HS suggest a cross-talk between response pathways of Cd2+ and heat stress in rice. Results suggest Cd2+ specific, heat-specific, tissue specific and differential expression of SOD/POD/APX/CAT and that Mn-SOD 3/APXR4/CATR2 seem to form important components of antioxidant defense in rice roots under combination of Cd2+ + HS which helps to mitigate the effect of low Cd2+ toxicity in tolerant rice cv. Bh-1.

► HS + Cd2+ induce differential expression of SOD, CAT, and APX isozymes.
► SOD/APX isozymes and POD activity groups form important component of plant defense under Cd2+ + HS.
► Existence of trinuclear SOD isoforms in rice regulated by Cd2+ + HS is indicated.
► Both Cd2+ specific and heat-specific responses to isoform banding patterns were noted.
► A cross-protection of low Cd2+ toxicity by heat stress in rice is indicated.