Here, we review the current knowledge on microbe-derived small molecules that can act as signalling compounds to stimulate plant growth and health by beneficial microbes on the one hand, but also as weapons for plant invasion by pathogens on the other. Molecular communication thus forms an integral part of the establishment of both beneficial and pathogenic plant–microbe relations.
Plants signal to their associated microbes via exudation of photosynthetically fixed carbon sources, quorum-sensing mimicry molecules and selective secondary metabolites such as strigolactones and flavonoids. Numerous ingenious mechanisms have been described by which pathogenic and beneficial microbes in the plant microbiome communicate with their host, including the delivery of immune-suppressive effector proteins and the production of phytohormones, toxins and other bioactive molecules. Like pathogens, beneficial microbes can modulate host immunity to efficiently colonize the nutrient-rich niches within and around the roots and aerial tissues of a plant, a phenomenon mirroring the establishment of commensal microbes in the human gut. By contrast, beneficial microbes provide plants with important services, such as enhanced nutrient uptake and protection against pests and diseases.
However, the minority of microbes that are pathogens can severely impact crop quality and yield, thereby endangering food security.
Plants live in close association with a myriad of microbes that are generally harmless.
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