Hydrogen sulfide enhances rice tolerance to nickel through the prevention of chloroplast damage and the improvement of nitrogen metabolism under excessive nickel

Hydrogen sulfide (H2S) modulates plant tolerance to abiotic stresses, but its regulatory effects on nitrogen metabolism and chloroplast protection under nickel (Ni) stress in crop plants remain elusive. Taking this into account, we investigated the potential roles of sodium hydrosulfide (NaHS), a H2S generator, in the improvement of growth performance of rice plants under Ni stress. Results showed that NaHS successfully reversed the adverse effects of Ni, as reflected in plant growth and biomass, and photosynthesis attributes including photosynthetic rates, stomatal conductance, transpiration rate, internal CO2 concentration and photosynthetic pigment contents. NaHS generated H2S plays a crucial role in controlling the photosynthetic machinery of rice as evidenced by the ultrastructure of chloroplast viewed under transmission electron microscope (TEM). The reduced content of Ni in roots and leaves of NaHS-supplemented Ni-stressed plants has revealed the restricted uptake and accumulation of Ni. A rescue of NaHS to the Ni-induced decline in nitrate (NO3-) content and the activities NO3- biosynthesizing enzymes nitrate reductase, nitrite reductase, glutamate synthase, glutamate oxaloacetate transaminase, glutamine synthetase, and glutamate pyruvate transaminase in leaves indicated a positive role of H2S on NO3- metabolism in rice under Ni stress. NaHS application also reverted Ni-mediated increases in ammonium (NH4+) content and glutamate dehydrogenase activity, implying H2S-induced alleviation of NH4+ toxicity. The regulatory effects of H2S on nitrogen metabolism was further confirmed by increased and decreased transcript abundance of NO3- and NH4+ metabolism associated genes, respectively. Our study suggests a decisive role of H2S in controlling Ni toxicity as elucidated by the novel findings such as enhanced gas exchanged parameters, Ni homeostasis and chloroplast protection. Moreover, this article highlights the significance of H2S in controlling chloroplast biogenesis and nitrogen metabolism in rice crop under Ni stress.