The MxMPK4-1-MxME3-7 Module Enhances ROS-Mediated Iron Deficiency Responses via Malate Metabolism in Apple

Iron is an essential micronutrient required for plant growth. However, its limited bioavailability in soils often triggers Fe-deficiency stress in plants, which induces complex adaptive responses, including signaling activation and metabolic adjustment. MAPK cascade and malate metabolism are both associated with reactive oxygen species signaling. Nevertheless, their interrelationship in the plant Fe-deficiency response remains unclear. Here, metabolomic analysis revealed that the malate metabolic pathway is significantly more active in the Fe-efficient apple rootstock Malus xiaojinensis than in the Fe-inefficient rootstock Malus baccata. We identified MxME3-7, a NADP-malic enzyme, as a responsive factor to Fe deficiency stress that positively regulates Fe uptake in apple rootstocks. Additionally, MxME3-7 physically interacts with and is phosphorylated by MxMPK4-1, a kinase previously characterized as a positive modulator of the Fe-deficiency response. Notably, phosphorylation at the S201 residue of MxME3-7 significantly enhanced ROS signaling levels, thereby improving plant adaptation to Fe deficiency stress. This study elucidates the molecular mechanism of the MxMPK4-1-MxME3-7 module in ROS-mediated plant responses to Fe deficiency. These findings not only provide new insights into the mechanisms underlying plant nutritional stress responses but also offer a potential theoretical basis and genetic resources for enhancing crop tolerance to Fe deficiency stress.