Elucidating the effects of manganese on the growth and cadmium accumulation of OsNRAMP5 mutant rice

Low-cadmium (Cd)-accumulating rice (Oryza sativa L.) cultivated by OsNRAMP5 mutation is a promising approach for ensuring food safety and human health. However, the impacts of reduced manganese (Mn) uptake caused by OsNRAMP5 mutation on rice growth and Cd accumulation under varying Mn levels remains unknown. Here, hydroponics and pot experiments were used to investigate the growth and Cd accumulation of an OsNRAMP5 mutant rice, Zhong'an 7 (ZA7), and the wild type, Zhongzao 35 (ZZ35), under different Mn supply conditions, and the Mn thresholds for ZA7 were determined. There was lower biomass of ZA7 than ZZ35 under hydroponic conditions with 0-0.05 μM Mn supply, and this was alleviated at Mn concentrations ranging from 1 to 200 μM. In pot Experiment I the grain yields of ZA7 in five soils increased from 5.92 to 15.0 g pot^-1 as the inherent soil available Mn ranged from 3.17 to 400 mg kg^-1. In pot Experiment II the exogenous MnSO₄ significantly increased ZA7 grain yields by 135-173 % and reduced grain Cd concentrations by 50.3-72.9 % (p < 0.05). Furthermore, both hydroponic and pot experiments show that Cd concentrations in ZA7 were significantly lower than those in ZZ35, and decreased further with increasing Mn supply. Finally, the relationship between soil available-Mn (diethylene-triamine-pentaacetic acid-extractable, DTPA-Mn), which is directly related to plant Mn uptake, and ZA7 grain yields fitted well. The optimum DTPA-Mn content for maximum yield of ZA7 occurred at 263 mg kg^-1 and the minimum DTPA-Mn threshold ensuring 90 % of the optimum grain yield was 165 mg kg^-1. Thus, the results demonstrated the greater Mn-deficiency sensitivity and higher Mn requirement of ZA7 compared to ZZ35, shedding new light on the wider application of OsNRAMP5 mutant rice under different Mn conditions.