Fig. 4

Plant–microbe and putative microbe-microbe interactions on the leaf surface. Shown is a stylised, simplified leaf surface with resident fungal (e.g. Zymoseptoria) hyphae and bacterial (e.g. Pseudomonas) microcolonies. Leaf surfaces are heterogeneous and contain diverse microhabitats. Areas such as those over anticlinal cell walls where neighbouring cells meet, for example, often have higher nutrient and water availability compared to the leaf as a whole and support greater numbers of microbes (1). Epiphytic microbes have many adaptations to life on the leaf, including secretion of cuticle and cell wall degrading enzymes, expression of transporters, production of plant hormones and biosurfactants and formation of stress-resistant biofilms. In low iron conditions, for example, many microbes produce siderophores, which bind iron with high affinity; the complex is then taken up by either the original producer or other nearby microbes (2). Many bacteria (purple) produce surfactants which increase the wettability of the cuticle (3), increasing nutrient exudation from the plant and increasing nutrient diffusion across the surface. Others produce plant hormones such as the auxin indole acetic acid (IAA) which loosen plant cell walls and increase nutrient efflux onto the leaf surface (4). Increases in nutrient exudation (4) and exudate diffusion (3) benefit nearby microbes, as well as those responsible for the effect. Both bacteria and fungi can secrete and become embedded in a protective extracellular matrix, forming biofilms (5) which may be single species, mixed, or even cross-kingdom. Many fungi also secrete cell wall or cuticle degrading enzymes (6) which, like auxins, increase the permeability of the leaf and the rate of nutrient exudation, as well as directly liberating metabolites. This may attract motile, chemotactic microbes and may play a role in recruiting bacteria to cross-kingdom biofilms (5)