Metapangenomics reveals host-driven adaptations of Methylobacterium to the phyllosphere

Bacteria inhabiting leaf surfaces - the phyllosphere - are crucial to plant health and ecosystem functioning. Methylobacterium is a taxonomically diverse, growth-promoting genus, ubiquitous on leaves. Different plant species host distinct Methylobacterium communities, but the genomic and functional basis of Methylobacterium symbiotic associations with particular host species remains poorly understood. Here, we used a metapangenomic approach to quantify the influence of host species on Methylobacterium assemblages, to identify genes potentially involved in Methylobacterium adaptations to host species, and to evaluate the contribution of Methylobacteriums accessory pangenome to these adaptations. We sequenced the metagenomes of 25 phyllosphere communities spanning five host species in a temperate forest in Quebec, Canada, and mapped these metagenomes onto Methylobacteriums pangenome to obtain nucleotide-level coverage and composition for each population on each individual host. We revealed strong divergences in the species- and gene-level community structure of Methylobacterium, driven by host phylogeny and plant form. Conifer communities were notably enriched in genes involved in amino acid, lipid, and carbohydrate metabolism; broadleaves, in genes involved in cell membrane, signalling, defense, and chemotaxis; and trees, in genes related to photosynthesis, oxidative phosphorylation, and translation. The shrub Corylus cornuta was a reservoir of Methylobacterium taxonomic diversity, and harboured numerous accessory genes under positive selection. Methylobacteriums accessory pangenome, evolving under weaker purifying selection, contributed importantly to gene-host associations, supporting its adaptive role. By linking genes to phyllosphere niches, our study shed light on the genetic basis of host adaptation and highlighted the crucial role of forest biodiversity in shaping microbial ecology and evolution.