Intercalary growth in hyphae of filamentous fungi

Processes governing hyphal extension in filamentous fungi have long been accepted as being confined to the extreme apex, the location of the polarised cellular machinery controlling accumulation and deposition of cell wall material. Well-characterised exceptions to this paradigm are to be found in fruiting bodies of Basidiomycete mushrooms, which grow rapidly through cumulative increases in the volume of intercalary compartments. Recently, further evidence of intercalary hyphal growth has come to light in the biotrophic epichloë endophytes of grasses which, when colonising aerial plant tissues, grow diffusely along the length of vegetative filaments in synchrony with host intercalary development. This unique adaptation differs fundamentally from intercalary extension in mushrooms in that Epichloë hyphal compartments undergo mitosis and septation during growth and compartment volumes do not increase significantly. The endosymbiotic coloniser has apparently evolved this novel mechanism of regulated growth as a result of attachment of hyphae to growing host cells which would otherwise compromise hyphal integrity. Further examples of intercalary extension in vegetative multi-hyphal structures such as rhizomorphs of Armillaria gallica or thalli of lichens such as Usnea longissima suggest that intercalary growth in vegetative filaments may be more common than currently thought. In this review, evidence supporting intercalary extension as a recognised growth modality in vegetative hyphae is discussed, and tentative insights into the evolutionary forces driving intercalary growth processes in fungi are advanced.

Research highlights
► Plasticity in fungal growth strategies is examined.
► Vegetative fungi normally grow exclusively at the hyphal apex.
► Conversely Epichloë grass endophytes grow via synthesis of intercalary compartments.
► Intercalary growth enables the endophyte to grow synchronously with the grass host.
► Intercalary extension in other vegetative fungal structures is considered.