Hydrogen peroxide-mediated growth of the root system occurs via auxin signaling modification and variations in the expression of cell-cycle genes in rice seedlings exposed to cadmium stress.

The link between root growth, H₂O₂, auxin signaling, and the cell cycle in cadmium (Cd)-stressed rice (Oryza sativa L. cv. Zhonghua No. 11) was analyzed in this study. Exposure to Cd induced a significant accumulation of Cd, but caused a decrease in zinc (Zn) content which resulted from the decreased expression of OsHMA9 and OsZIP. Analysis using a Cd-specific probe showed that Cd was mainly localized in the meristematic zone and vascular tissues. Formation and elongation of the root system were significantly promoted by 3-amino-1,2,4-triazole (AT), but were markedly inhibited by N,N'-dimethylthiourea (DMTU) under Cd stress. The effect of H₂O₂ on Cd-stressed root growth was further confirmed by examining a gain-of-function rice mutant (carrying catalase1 and glutathione-S-transferase) in the presence or absence of diphenylene iodonium. DR5-GUS staining revealed close associations between H₂O₂ and the concentration and distribution of auxin. H₂O₂ affected the expression of key genes, including OsYUCCA, OsPIN, OsARF, and OsIAA, in the auxin signaling pathway in Cd-treated plants. These results suggest that H₂O₂ functions upstream of the auxin signaling pathway. Furthermore, H₂O₂ modified the expression of cell-cycle genes in Cd-treated roots. The effects of H₂O₂ on root system growth are therefore linked to auxin signal modification and to variations in the expression of cell-cycle genes in Cd-stressed rice. A working model for the effects of H₂O₂ on Cd-stressed root system growth is thus proposed and discussed in this paper.