Fig. 1.

Microbial colonization of the above-ground part of the plant (phyllosphere), as well as the below-ground part (rhizosphere). (A) The microbial colonization on the leaf takes place on the leaf surface (epiphytes) from air-borne and soil-borne inocula and the inner leaf part (endophytes). Microbial colonization can lead to exogenous intraspecies biofilm formation on the leaf surface. (B) Microbe–microbe interactions occur between interspecies and interkingdoms, referred to as quorum sensing. Quorum-sensing molecules impacting microbial recognition and biofilm formation on leaves. (C) Pathogenic microbes colonize host plants by means of their virulence. The genetic make-up of both the host and pathogen contributes to disease progression. However, other microbes in the host phyllosphere can influence this plant–pathogen interaction by either facilitation or antagonism. (D) Plant immune responses are of specific interest as host–microbe interactions shaping the phyllosphere microbiome. Non-host-adapted pathogens are involved in PAMP-triggered immunity (PTI) and recognized via pattern-recognition receptors (PRRs). Host-adapted microbes are recognized via nucleotide-binding leucine-rich repeat receptors (NLRs), summarized in effector-triggered immunity (ETI).