Improving Endogenous Nitric Oxide Enhances Cadmium Tolerance in Rice through Modulation of Cadmium Accumulation and Antioxidant Capacity

Nitric oxide (NO) plays an important role in plant stress responses. However, the mechanisms underlying NO-induced stress resistance to cadmium (Cd) stress in rice remain elusive. In this study, rat neuron NO synthase (nNOS)-overexpressing rice plants with higher endogenous NO level showed higher cadmium stress tolerance than the wild-type plants. The results showed that nNOS-overexpressing rice plants accumulated less cadmium in the roots and shoots by downregulating the expression of Cd uptake and transport related genes including OsCAL1, OsIRT2, OsNramp5, and OsCd1. Moreover, nNOS-overexpressing rice plants accumulated less hydrogen peroxide (H2O2), accompanying with higher expression of antioxidant enzyme genes (OsCATA, OsCATB, and OsPOX1) and corresponding higher enzyme activities under cadmium stress. Furthermore, the transcription of melatonin biosynthetic genes, including OsASMT1, OsTDC1, OsTDC3, and OsSNAT2, was also upregulated in nNOS-overexpressing plants, resulting in increased content of melatonin under cadmium treatment compared with the wild-type controls. Taken together, this study indicates that nNOS overexpression improves Cd tolerance of rice seedlings through decreasing cadmium accumulation and enhancing the antioxidant capacity and melatonin biosynthesis of the plants.

Automated summary

This study shows that rice plants overexpressing rat neuron nitric oxide synthase (nNOS) have higher endogenous NO levels and exhibit higher tolerance to cadmium stress. The overexpression of nNOS leads to reduced cadmium accumulation in the roots and shoots and upregulation of antioxidant enzyme genes, resulting in decreased accumulation of hydrogen peroxide (H2O2) under cadmium stress. Furthermore, the overexpression of nNOS also induces the upregulation of melatonin biosynthetic genes, resulting in increased melatonin content under cadmium treatment. Overall, this study suggests that nNOS overexpression improves the cadmium tolerance of rice seedlings through reducing cadmium accumulation, enhancing antioxidant capacity, and increasing melatonin biosynthesis.