Here, we review plant–plant communications that improve plant defense against pathogenic microbes. From multiple layers of plant–plant communications, we distinguished two distinct types: wired and wireless communications. Wired communication involves one plant sending a signal to another plant though direct contact via microbial structures and hyper-parasitic plant organs. This can be considered as an information highway mediating plant–plant communication. Wireless communication involves signal transfer across the space separating two plants. We investigated how wired and wireless communications affect plant defense responses. We determined that these signal transduction pathways proceeded via the following three steps: signal input (extracellular signal perception generates an endogenous signal cascade); transferring signal (direct connection from signal producer to receiver through mycorrhizal network and parasitic plants, and indirect signal translocation via plant volatile compounds and exudates); and signal output (receiver plant responses to biotic and abiotic stresses).