Rho of plant GTPase MxROP1 regulates Fe deficiency responses by targeting Zinc Ribbon 3 in apple rootstock

ABSTRACT Small G protein is a multifunctional molecular switch that can regulate plant growth, development and responses to the environment. However, how Rho-related GTPase of plants (ROPs) regulates the response to Fe deficiency has not been well clarified. Here, we found that Fe deficiency induced MxROP1 in Malus xiaojinensis at both the transcriptional and translational levels. The overexpression of MxROP1, MxROP1 DN (inactive form) and MxROP1 CA (active form) in apple roots increased the activity of ferric chelate reductase and the ability to acidify the rhizosphere, and lines that overexpressed MxROP1 DN exhibited the strongest reaction to enhance Fe uptake. Yeast two-hybrid library screening indicated that MxROP1 interacted with ZR3.1, a DNL zinc finger protein that negatively regulates Fe deficiency responses. We further identified their interaction in vitro and in vivo using pull-down and bimolecular fluorescence complementation assays, respectively, and MxROP1 DN -MxZR3.1 interacted the most strongly. Furthermore, MxROP1 negatively affected the stability of MxZR3.1 protein in vitro as shown by a cell semi-degradation assay, and the application of MG132 inhibited the degradation of MxZR3.1-HIS proteins. This indicated that MxROP1 caused the degradation of MxZR3.1 protein through the 26S proteasome pathway. Similar results were found in OE-MxROP1+OE-MxZR3.1 transgenic apple callus compared with those in the OE-MxZR3.1 callus. We also demonstrated that MxZR3.1 interacted with MxbHLH39, a known positive transcription factor and core component of Fe deficiency, and MxROP1 affected the interaction of MxZR3.1-MxbHLH39 using a competitive binding assay. This illuminated one MxROP1-MxZR3.1-MxbHLH39 pathway that maintains Fe homeostasis in M. xiaojinensis .