Signaling in the phytomicrobiome

Almost all of the energy in the terrestrial biosphere entered biosystems at the green leaves of plants. Organisms that have a reliable first access to this energy are in an advantaged position. Approximately 20% of the photosynthates are released as root exudates that form a highly enriched portion of the rhizosphere which in turn help the plants have a persistent and a relatively constant microbial community structures associated with them. Hence, despite constant changes in the environment, soil constitutes one of the most productive of earth’s ecospheres and is a hub for evolutionary and other adaptive activities. It seems likely that plants have had associated microbes since they first colonized the land almost half a billion years ago. Good fossils of endomycorrhizal associations from the early Devonian period (>400 million years ago) exist, and it seems likely that bacterial associations have been present for at least as long. As plants spread through the terrestrial environment and evolved to a wider range of habitats it is very likely that their associations with bacterial evolved along with them. This community of microbes is the phytomicrobiome. Over the last few decades it has been shown that plants and microbes can use specific signal compounds to communicate during the establishment of beneficial plant-microbe interactions. This is now fairly well described for the legume-rhizobia nitrogen fixing symbiosis, and somewhat elucidated for mycorrhizal associations. It has also been shown that bacterial communicate among themselves, through quorum sensing and other mechanisms, and one would presume that this also occurs in the phytomicrobiome. It is also the case that plants detect materials produced by potential pathogens and respond by activating aspects of their pathogen-response systems. This intercommunication in the rhizosphere also dictates aspects of the above ground plant architecture and the related above ground symbiotic/pathogenic microbial communities. Understanding the signalling responses via biochemical, genomics, proteomics and metabolomic studies of plant-microbe interaction has added valuable knowledge towards developing effective low cost and eco-friendly alternates to reduce fossil fuel based chemical fertilizers. This knowledge is now underpinning the developing concept of rhizosphere engineering, which could cater to various soil and climatic conditions, and help address variability in crop productivity around the world. Further, this understanding will also allow for the design of targeted strategies for engineer the phytomicrobiome for increased plant productivity.