Editorial: Signaling in the Phytomicrobiome

Over the last decade we have come to appreciate that there are close relationships between all “higher” organisms and communities of microbes. The human microbiome and its role in human metabolism and health, is being widely investigated. In a similar way, plant-associated microbial communities are now coming under scrutiny. Plants have probably had associated microbes since they colonized the land about 0.5 billion years ago. The terrestrial environment presented water and nutrient acquisition challenges resulting in the evolution of sophisticated plant root systems. However, associatedmicrobes also help address these hurdles, and at lesser energetic costs (Smith et al.). Because most energy enters the terrestrial biosphere at the green leaves of plants, organisms associated with plants have advantageous access to reduced carbon from photosynthesis. So, when plants prosper, associated microorganisms benefit. Microbes are associated with all plant structures, but roots are in constant contact with generally humid, microbe-laden soil, and so are associated with the greatest number and range of microbes. The earliest evidence we have of plant-microbe interactions are fossils showing mycorrhizal relationships from almost 400 million years ago (Smith et al.). We now realize that a plant growing under field conditions is community, not just an individual. While the circumstances of associated microbes are improved when the plants are doing well, the plants must at the same time control their associated microbes, to minimize the presence of those that are potentially detrimental. The microorganisms that colonize plants are collectively termed “the phytomicrobiome”. The genomes of the phytomicrobiome expand the genetic repertoire of the plant. This association has led to the redefinition of Karl August Möbius’ biocenosis (metaorganisms comprising the macroscopic host and its synergistic interdependence with microbes) concept into the holobiont (an individual host and its microbial community) concept (Theis et al., 2016). The holobiont collective genome is the hologenome, the evolutionary unit; the phytomicrobiome is muchmore flexible than the plant genome andmore readily modified than the hologenome (Nogales et al., 2016).