Combined Treatment with Cadmium and Zinc Enhances Lateral Root Development by Regulating Auxin Redistribution and Cell-Cycle Gene Expression in Rice Seedlings

Enhanced lateral root (LR) development is of critical importance for rice plants adapting to heavy-metal-stress conditions. LR development is affected by heavy metals, such as aluminium (Al), copper (Cu), lead (Pb), zinc (Zn), chromium (Cr) and cadmium (Cd), or metals in combination, such as Cd and As. However, it has not been reported yet whether the combination of Cd and Zn affect LR growth in rice. Here, we studied the associations between LR growth, auxin signaling, and the cell cycle in the combination of Cd and Zn-treated rice (Oryza sativa L. cv. Zhonghua no. 11). Combined treatment with Cd and Zn significantly enhances LR development in rice seedlings. Cd levels decreased and Zn levels increased in the lateral root development regions (LRDRs) with the treatment of (Cd + Zn) compared to the treatment of Cd alone. Zncounteracted over-accumulation of auxin caused by Cd- and (Cd + Zn)-treatment significantly promoted LR growth by maintaining appropriate auxin distribution in the roots. Experiments using TIBA (2,3,5-triiodobenzoic acid, an inhibitor of polar auxin transport), BFA (brefeldin A, a protein transport inhibitor), IBA (indole-3-butyric acid), MG132 (a protein degradation inhibitor) and DR5-GUS staining revealed that (Cd + Zn)-treatment influences the distribution of auxin through polar auxin transport and protein transport/degradation pathways. By evaluating expression levels of some key auxin-signaling genes and cell-cycle-related genes in roots treated with (Cd + Zn) or Cd alone, we found that (Cd + Zn)-treatment affects specific genes involved in auxin signaling and the cell cycle compared with Cd alone, and the treatment duration of 7 and 9 days showed different regulated manner. Our findings should help to elucidate how the effects of (Cd + Zn)-treatment on auxin signaling and the cell cycle influence LR growth.