Harnessing Phyllosphere Microbiome for Improving Soil Fertility, Crop Production, and Environmental Sustainability

Various microorganisms colonize plant tissues either through epiphytic (surface), endophytic (inside), or rhizospheric association. The diverse phyllosphere microbiomes interact with plant host either through mutualism, commensalism, and/or pathogenesis, and affect the functioning of various biological processes in plants. Among these microbes, beneficial phyllospheric microorganisms have been demonstrated to positively affect plant growth through multiple mechanisms including enhanced availability of nutrients through nitrogen fixation; solubilization of phosphorous, potassium, and zinc; and production of siderophores and growth-promoting hormones. The indirect mode of plant growth stimulation includes inhibition of pathogens by antagonistic phyllospheric microbes, production of ACC deaminase enzyme, exopolysaccharide secretion, and mitigation of abiotic stresses. Application of beneficial phyllospheric microorganisms as biofertilizers and biocontrol agents has been found to suppress plant diseases and resulted in promotion of plant biomass and development, and increases in crop yield in majority of field trials. In addition, these microbes have been demonstrated to preserve soil fertility and microbial biodiversity along with reduced use of chemical fertilizers and pesticides. However, knowledge on the molecular responses modulated in host plants due to application of phyllospheric microbes is still incomplete. This article provides an up-to-date overview on the prevalence and diversity of the phyllospheric microbes, their growth-promoting traits, and different mechanisms of action employed to increase plant health and crop yield after foliar spray or soil inoculation. Furthermore, bioengineering of phyllospheric microbes is discussed to enhance their biological functioning with a better ability to benefit crop plants, and resulting in improved food production to feed the world's ever-increasing population through an eco-friendly and sustainable approach.