Phyllosphere C1-microorganisms: Their interaction with plants and contribution to the global carbon cycle

Plants emit a variety of volatile organic compounds (VOCs), including C1-compounds such as methane and methanol, which are major components of plant-derived VOCs. A group of microorganisms called methylotrophs or C1-microorganisms utilize these C1-compounds as a single source of carbon and energy and contribute to driving the global carbon cycle between two major greenhouse gases, CO₂ and methane. C1-microorganisms inhabit the surface of the above-ground part of plants (phyllosphere), and utilize methane and methanol before they are released into the atmosphere. Among C1-microorganisms, Methylobacterium spp., the representative of methanol-utilizing bacteria and dominant colonizers in the phyllosphere, are known to exhibit positive effects on plants. Thus, the interactions between C1-microorganisms and plants affect not only the consumption of C1-compounds generated by plants but also CO₂ fixation by plants. This review describes our recent understanding of the ecology and physiology of C1-microorganisms living in the phyllosphere and their application in plant biotechnology. Specifically, the ways in which these phyllosphere C1-microorganisms can be used for mitigating methane emissions as well as their application as biostimulants for increasing crop yield are discussed.