Rice crop productivity is an important area of research under the current climate scenario, with rising temperature and low nutrient availability, particularly phosphorus. The interactive effects of high temperature and P starvation on growth, flowering, yield, and signalling components was studied in two rice genotypes − IR64 (a high yielding genotype) and IR64-Pup1 (IR64 introgressed with the Pup1 QTL associated with tolerance to low P available soil). The experiment was conducted in pots with two P levels, control (41.5 mg P kg-1 soil) and low P (4.2 mg P kg-1 soil). At booting stage, one set of pots with each P level were transferred to a temperature gradient tunnel to impose the high temperature stress, while the other set remained at ambient temperature. After ten days of exposure to high temperature, flag leaves were collected from five main culms for biochemical and expression analysis of microRNAs. The above ground biomass (AGB) and yield attributes were recorded at maturity. Due to the effects of elevated temperature (eT) × low P, a significant reduction was observed in AGB, leaf area, and yield in both rice genotypes, while the reduction was higher in IR64 as compared to IR64-Pup1.
The activity of antioxident enzymes, such as superoxide dismutase, peroxidase, and ascorbate peroxidase, increased by >2-fold under low P as well as its interaction with eT, while catalase and glutathinone reductase activities increased only under P starvation. Among genotypes, IR64-Pup1 showed lower antioxidant enzymes activities and higher ascorbate and reduced glutathione accumulation than IR64, thereby resulting in improved biomass and yield under eT and low P stress. The relative expression of miR399a and miR399b was upregulated by P starvation and its interactions with eT, while the expression of its target, OsPHO2, was down-regulated. An increase in sugar levels under P starvation downregulated the expression of juvenility related miRNA i.e., miR156 but upregulated the expression of miR172, which mediates the transition from juvenility to flowering leading to early flowering in low P-grown plants than the control. Thus, the interactive effect of eT with P starvation results in altered metabolic processes that ultimately influence the physiology of rice plants.
Key words: High temperature, miRNA, Pup-1 QTL, Phosphorus, Rice