Holobiomes: Structure and Functions

Holobiomes are functional ecological units composed of various species of microbes that are associated with plants. The relationship between plants and microbes ranges from the existence of microbes on rhizoplane/rhizosphere to endophytes (microbes live between plant cells) to endosymbiosis of microbes in plants. In short, some microbes are associated with plant roots, while others are free living, and they directly or indirectly support plant development. Root exudates act as chemical signals that help the microbes to colonize and grow in the rhizosphere. Plant roots release various organic compounds (carbohydrates, fatty acids, amino acids, sterols, amides, etc.) that act as a source of energy for microbes, and in turn, they convert the nutrient into readily available forms for plants. They fix the atmospheric nitrogen and improve soil fertility. They synthesize phytohormones that act as growth promoters and proliferate plants above and below ground parts. They also modulate the synthesis of phytohormones in plants as they have an impact on a wide array of signaling and metabolic network in plants. So, they not only control the growth but also activate stress-related mechanisms. They perform the function of biocontrol by restricting the energy source for pathogens or by secreting antibiotics in the soil.