Superoxide radical and auxin are implicated in redistribution of root growth and the expression of auxin and cell-cycle genes in cadmium-stressed rice

Changes of root growth and the expression of auxin and cell-cycle genes were analyzed in rice (Oryza sativa L. cv. Zhonghua No. 11) treated with Cd alone, Cd plus TIBA (2,3,5-triiodobenzoic acid, an inhibitor of polar auxin transport) or IBA (indole-3-butytric acid) and Cd plus Tiron (a scavenger of O2·− or DDC (sodium diethyldithiocarbamate, a superoxide dismutase inhibitor). It was shown that Cd-stimulated rice root growth can be further enhanced by Cd plus IBA or DDC treatments but can be significantly suppressed following Cd together with Tiron or TIBA application. A comprehensive expression analysis of auxin genes, such as OsPID, OsPINs (Pin-Formeds), OsARFs (auxin response factors), and core cell-cycle genes, such as CDKs and CYCs in differently treated roots was conducted by semi-quantitative RT-PCR. The results showed that the expressions of 21 out of 35 tested auxin genes and 51 out of 63 tested cell-cycle genes were regulated by various treatments. Among the modified expression profiles of 72 genes, 53 and 59 genes were up- or down-regulated by auxin (including Cd plus TIBA or IBA treatments) and O2·− (including Cd plus Tiron or DDC treatments), respectively. In the roots treated with Cd plus IBA or TIBA, the expressions of 39 genes changed differently from those under single Cd treated roots. Following Cd plus DDC or Tiron treatment, the transcripts of 36 genes were also altered differently from those under single Cd treatment. Obviously, auxin and O2·− affected the expression of auxin and core cell-cycle genes under Cd stress. Taken together, our results indicated that in Cd-stressed rice plants, O2·− and auxin were implicated in the redistribution of root growth and the expression of auxin and cell-cycle genes.