Melatonin reduces cadmium accumulation via mediating the nitric oxide accumulation and increasing the cell wall fixation capacity of cadmium in rice

Melatonin (MT) is participated in plants' response to cadmium (Cd) tolerance, although its work model remains elusive. Here, the function of MT in adjusting Cd accumulation in rice was investigated. 'Nipponbare' (Nip) was cultured in the-Cd (1/2 Kimura B),-Cd + MT (1/2 Kimura B with 1 mu M MT), +Cd (1/2 Kimura B plus 1 mu M Cd) and +Cd + MT (1/2 Kimura B with 1 mu M Cd and 1 mu M MT) nutrient solutions for 7 d. Cd markedly induced the endogenous MT accumulation in rice roots and shoots, even within 1 h. MT applied exogenously elevated the hemicelluloses level, which in turn increased the cell wall's binding capacity to Cd. Furthermore, MT applied exogenously down-regulated the transcription level of Natural Resistance-Associated Macrophage Protein 1 (OsN-RAMP1), OsNRAMP5, a major facilitator superfamily gene (OsCd1), and IRON-REGULATED TRANSPORTER 1 (OsIRT1), all of which were responsible for Cd intake, thus less Cd was entered into roots. Moreover, MT applied exogenously also up-regulated transcription level of Cadmium accumulation in Leaf 1 (OsCAL1) and Heavy Metal ATPase 3 (OsHMA3), two genes both attributed to the decreased Cd accumulation in shoots through expelling Cd out of cells and chelating Cd in the vacuoles, respectively. In addition, MT applied exogenously further aggra-vated the production of nitric oxide (NO) that induced by Cd, while application of a NO donor-SNP mimicked this alleviatory effect of the MT, indicating MT decreased rice Cd accumulation relied on the accumulation of NO.

Automated summary

Melatonin (MT) is involved in rice's response to cadmium (Cd) tolerance by regulating Cd accumulation. Exogenous application of MT increased the binding capacity of the cell wall to Cd and down-regulated the transcript level of Cd intake genes, leading to less Cd entering the roots. Moreover, MT up-regulated the transcript level of genes responsible for decreasing Cd accumulation in shoots.