Loss of Mildew Resistance Locus O (MLO) in barley confers durable resistance to powdery mildew fungi, which has led to its wide deployment in agriculture. Although MLO is a susceptibility factor, it has become nearly synonymous with powdery mildew resistance. However, MLO has been recently implicated in colonization by arbuscular mycorrhizal fungi and a fungal endophyte, confirming its importance for biotrophic interactions and in promoting symbiosis. Other MLO proteins are involved in essential sensory processes, particularly fertilization and thigmotropism. We propose external stimulus perception as a common theme in these interactions and consider a unified biochemical role, potentially relating to reactive oxygen species (ROS) and calcium regulation, for MLOs across tissues and processes.