Early responses to Cd stress by tolerant and susceptible rice cultivars

Cadmium (Cd) pollution poses a rapidly escalating danger to the environment. The comprehensive understanding of variability in intraspecific tolerance among various indica cultivars exposed to Cd stress is obscure. This recent study intended to evaluate the response of 25 µM and 50 µM CdCl2 at 24 h, 48 h, and 72 h in a high Cd accumulating rice genotype (IR64) and a low Cd accumulating rice genotype (Dudheswar), under hydroponic conditions. Under Cd stress, Dudheswar exhibited higher ROS detoxification by an upregulated ROS-scavenging antioxidant system. An array of osmoregulants and Cd chelating compounds was accumulated in high amounts, providing tolerance towards Cd stress. Contradictorily, ROS generation and ROS-induced damage indices were highly escalated in IR64. Genes like OsHMA3, OsPCS1 were upregulated by 10–20 fold in Dudheswar compared to IR64, which might facilitate Cd sequestration and detoxification. OsNRAMP genes were upregulated by 6–8 fold in IR64, suggesting higher xylem and phloem loading of Cd. The partial coding sequences and 3′ UTR sequences of OsNRAMP1 and OsNRAMP5 were compared, revealing no differences. Similarly, no difference was observed in the sequences of the OsNRAMP5 promoter. Our results suggest that differential expression of OsNRAMPs rather than any sequence difference observed in the analyzed regions (partial coding sequences, 3′ UTRs, or OsNRAMP5 promoter) might be responsible for conferring genotypic variability in responses towards Cd exposure, affecting Cd accumulation, activation of defense pathways and maintenance of metabolic homeostasis. Our study, for the first time, explicitly reveals the early and divergent responses activated by tolerant and susceptible rice seedlings when subjected to Cd stress that would facilitate advanced understanding of the variation in stress-coping mechanisms between cultivars.