Fig. 2: Mechanisms for plant-microbe interactions.Plant/microbe interactions occur in spermoshere, rhizosphere, and phyllosphere. Microbes in the spermosphere aid seedling establishment by controlling germination and the secretion of compounds that inhibit competing pathogenic microbes. The plant/microbe chemical communication network involves primary and secondary metabolites that attract beneficial microbes and restrict pathogenic microbes. Beneficial microbes secret allelochemicals with biocontrol activity. Phytohormones including indole acetic acid (IAA), cytokinin (CK), gibberellic acid (GA), strigolactone (SL), and salicylic acid (SA) are secreted by both plants and microbes. Soil microbes not only enhance plant immunity by triggering systemic resistance responses via MAMP-triggered immunity or effector-triggered immunity, but they also directly inhibit invading pathogens. Beneficial microbes can successfully evade plant immune surveillance by deploying effector proteins to suppress plant immune response (A). The microbe-derived PAMPs and MAMPs are perceived by plasma-membrane-localized pattern-recognition receptors (PRRs) that trigger signaling events including Ca influx, activation of plasma membrane-bound NADPH oxidases (RBOH) that produce reactive oxygen species (ROS burst). Oscillations in calcium concentrations (calcium spiking) and ROS waves act together with a network of signalling proteins as well as Ca-dependent protein kinases (CDPKs, CCaMKs and CIPKs) to control transcriptional reprogramming, activation of defense-related genes, secondary metabolite production, and defence hormone synthesis. This system triggers immune response throughout the plant resulting in systemic acquired resistance (B). Current concepts concerning this signalling cascade are described in detail in the accompanying text.