MAPKs as a cross point in H2O2 and auxin signaling under combined cadmium and zinc stress in rice roots

Abstract

Previously, we have reported the role of MAPKs (mitogen-activated protein kinases) under cadmium stress. This work continue to explore the relationship between MAPKs, H2O2, auxin signaling, and OsHMA and OsZIP gene expression in rice (Oryza sativa L.) roots under combined cadmium (Cd) and zinc (Zn) stress. Compared with Cd, Cd+Zn reduced Cd levels but increased Zn accumulation in the roots. Three OsMAPK genes were negatively regulated, while two OsHMA and two OsZIP genes were positively regulated by MAPK pathways under Cd+Zn stress. Transgenic rice expressing DR5-GUS exhibited enhanced GUS activity in H2O2-, PD (MAPKK inhibitor PD98059)-, or (Cd+Zn)-treated roots, which also exhibited increased H2O2 concentrations, whereas GUS staining decreased in roots in response to Cd+Zn+PD, DMTU (N,N′-dimethylthiourea, a H2O2 scavenger), or Cd+Zn+DMTU treatment, with reduced H2O2 levels. GUS levels were consistent with H2O2 levels, suggesting that MAPK pathway-mediated auxin redistribution occurs via H2O2, and H2O2 functions downstream of MAPK but upstream of auxin signaling pathways. Furthermore, MAPK pathways serve specific functions in regulating the expression of some key genes of auxin signaling (OsYUCCA, OsPIN, OsARF, and OsIAA) under Cd+Zn stress. Overall, MAPK cascades function in the integration of metal transport, H2O2 generation, and auxin signaling in rice seedlings grown under Cd+Zn stress.