Phyllosphere represents the largest global interface of the aerial parts of the plant, comprising mainly stem and leaves, which is inhabited by various groups of microorganisms. Analyses of the spatial abundance of microflora, diversity, and distribution of microbial communities and the influence of abiotic and biotic factors have revealed that this niche is unique. This reflects the impact of both evolutionary and ecological factors, leading to sorting of microbial species, delineation of keystone species or microbial hubs, mediated by inter-kingdom connectivity and networking. Production of hormones, pigments, volatiles, extracellular polysaccharides (EPS), cross-kingdom signals, and quorum sensing are characteristic facets, which promote proliferation and survival in the harsh and inhospitable habitat of the phyllosphere, exposed to radiation and environmental extremes. The use of both traditional morphology-culturing-based taxonomy and modern tools of metagenomics, proteomics, and transcriptomics has illustrated that the diversity among bacterial members is mainly restricted to Actinobacteria, Proteobacteria, Firmicutes, Bacteroidetes, and less frequently, Cyanobacteria; oomycetous communities are common inhabitants, besides fungi. Despite scanty published work on foliar disease-related aspects, the phyllosphere can provide a model microenvironment, in which the interactions between the pathogen and biocontrol agent can be visualized and modulated. The major aims of the present review are as follows: (i) to elucidate the mechanisms of microbial colonization and decipher the nature of spatial and temporal changes in the abundance and diversity in this niche; (ii) to illustrate the significance of the different taxonomic groups; and (iii) outline future strategies for research on the phyllosphere microbiome.
