More than words: the chemistry behind the interactions in the plant holobiont

Plants and microbes have evolved sophisticated ways to communicate and coexist. The simplest interactions that occur in plant-associated habitats, i.e., those involved in disease detection, depend on the production of microbial pathogenic and virulence factors and the host's evolved immunological response. In contrast, microbes can also be beneficial for their host plants in a number of ways, including fighting pathogens and promoting plant growth. In order to clarify the mechanisms directly involved in these various plant-microbe interactions, we must still deepen our understanding of how these interkingdom communication systems, which are constantly modulated by resident microbial activity, are established and, most importantly, how their effects can span physically separated plant compartments. Efforts in this direction have revealed a complex and interconnected network of molecules and associated metabolic pathways that modulate plant-microbe and microbe-microbe communication pathways to regulate diverse ecological responses. Once sufficiently understood, these pathways will be biotechnologically exploitable, for example, in the use of beneficial microbes in sustainable agriculture. The aim of this review is to present the latest findings on the dazzlingly diverse arsenal of molecules that efficiently mediate specific microbe-microbe and microbe-plant communication pathways during plant development and on different plant organs.