Endophytic coming out: The expressorium as a novel fungal structure specialized in outward-directed penetration of the leaf cuticle

Plants develop beneficial interactions with several microbes, including bacteria and fungi. The diversity of such associations is not restricted to the taxonomical status of the interacting microorganisms, but extends to the features of their accommoda- tion by the host and the consequent type of symbiotic relationship, ranging from optional advantages to full dependency. Within such broad – and often faded – boundaries, the detailed study of single plant–microbe associations may reveal the unexpected. This is the case with the paper by Becker et al. present in this issue of New Phytologist (pp. 240–254), where an elegant approach combining confocal and electron microscopy observations with the use of mutant strains depicts a novel type of specialized hyphal structure – a so-called expressorium – in the grass endophyte Epichlo€e festucae.