Regulatory Mechanism Of Rice Root Growth By O₂^·- And MAPK Signaling Under The Interaction Of Cadmium And Zinc

The growth of rice roots is regulated by genetic and environmental factors. Plants employ significant developmental plasticity to respond to environmental changes. O2·-and MAPK play important roles in plant development, cell division、and stress response. Plant growth is inhibited by Cd stress. A proper amount of zinc can protect plant from the damage of Cd. The aim of this research was to investigate the regulatory mechanisms ofO2·- and MAPK signaling in rice roots treated with combined Cd and Zn. This study include two parts:(1) the regulatory mechanism of root growth mediated by O2·- in rice (Oryza sativa L. cv Zhonghua No.11) roots treated with combined Cd and Zn; (2) the regulatory mechanism of root growth mediated by MAPK signaling in rice roots upon Cd alone and Cd together with Zn treatments. The main results are as follows:1. The regulatory mechanism of root growth mediated byO2·- in rice roots treated with combined Cd and Zn.1) The changes of root growth and the expression of OsMAPKs and auxin-related genes as well as cell-cycle core genes were analyzed in rice treated with Cd, Cd+Zn, Cd+Zn+DDC (sodium diethyldithiocarbamate, a superoxide dismutase inhibitor) and Cd+Zn+Tiron (a scavenger of O2·-). It was shown that the addition of Zn resulted in less accumulation of Cd in the root tip than that of single Cd treated one. The growth of roots especially the length and the number of lateral roots was markedly stimulated both by Cd+Zn+DDC and Cd+Zn treatments. In contrast, the growth of the root system was significantly suppressed by Cd+Zn+Tiron application.2) A comprehensive expression analysis of OsMAPKs, auxin genes such as OsPID, OsPINs (Pin-Formeds), OsARFs (auxin response factors), OsIAAs and core cell-cycle genes such as CDKs and CYCs (total 147 tested) in differently treated roots was conducted by using semi-quantitative RT-PCR. The results indicated that a total of 115 out of the 147 genes was up/down-regulated by different treatments in the 9-day-old seedlings, in which there were 64,66 and 87 genes mediated by Cd、Cd+Zn and O2·- (including Cd+Zn+DDC or Tiron) respectively; in the 13-day-old seedlings, a total of 117 out of the 147 genes was up/down-regulated by different treatments, in which there were 66,67 and 74 genes regulated by Cd、Cd+Zn and O2·-(including Cd+Zn+DDC or Tiron) respectively. It was noticed that genes regulated by Cd、Cd+Zn and O2·- largely overlapped. For example, in the 9-and 13 day-old seedlings, there were 28 and 24 genes regulated by Cd、Cd+Zn and O2·-respectively. These results suggested that the regulation of root growth mediated by Zn in Cd-stressed rice was involved in O2·-.O2·-may take part in the regulation of root growth by modified the expression of OsMAPKs, auxin and cell-cycle genes in combined Cd and Zn treated seedlings.2 The regulatory mechanism of root growth mediated by MAPK signaling in rice roots treated with single Cd and Cd plus Zn.1) The changes of root growth and the expression of OsMAPKs and auxin-related genes and cell-cycle core genes were analyzed in rice under six treatments, including CK, CK+PD (MAPKK inhibitor, PD98059), Cd, Cd+PD, Cd+Zn and Cd+Zn+PD. The results showed that the growth of the roots was changed to some extent after the addition of PD as compared to that of none-PD treatments.2) A comprehensive expression analysis of OsMAPK, auxin, and core cell-cycle genes (total 129 tested) in rice roots treated with the MAPK inhibitor on the background of CK, Cd and Cd plus Zn treatments was conducted by using semi-quantitative RT-PCR. The results indicated that in the 129 genes there were 78,71 and 64 genes up/down-regulated by CK+PD, Cd+PD, and Cd+Zn+PD, respectively, as compared to that of none-PD treated ones. Our data implied that MAPK signal path way may regulate the root growth through modified the expression of MAPK, auxin and cell-cycle genes under the three treatment conditions.Taken together, in the Cd and combined Cd and Zn treated seedlings, the regulation of root growth was a complex process, which involved in the interactions between MAPK, auxin, and O2·- signaling as well as the cell cycle regulating factors. These interactions differed not only in the different treatments but also in the different period of root growth. Moreover, different regulations of the root growth and the expression of the tested genes mediated by MAPK signal were observed in the Cd and combined Cd and Zn treated seedlings, suggesting diversity of MAPK signal in regulating rice root growth.