The insect‐pathogenic fungus Metarhizium robertsii (Clavicipitaceae) is also an endophyte that stimulates plant root development

• Premise of the study: The soil ‐ inhabiting insect‐pathogenic fungus Metarhizium robertsii also colonizes plant roots endophytically, thus showing potential as a plant symbiont. Metarhizium robertsii is not randomly distributed in soils but preferentially associates with the plant rhizosphere when applied in agricultural settings. Root surface and endophytic colonization of switchgrass ( Panicum virgatum) and haricot beans ( Phaseolus vulgaris) by M. robertsii were examined after inoculation with fungal conidia. • Methods: We used light and confocal microscopy to ascertain the plant endophytic association with GFP‐expressing M. robertsii . Root lengths, root hair density, and lateral roots emerged were also observed. • Key results: Initially, M. robertsii conidia adhered to, germinated on, and colonized roots. Furthermore, plant roots treated with Metarhizium grew faster and the density of plant root hairs increased when compared with control plants. The onset of plant root hair proliferation was initiated before germination of M. robertsii on the root (within 1–2 d). Plants inoculated with M. robertsii Δ MAD2 (plant adhesin gene) took significantly longer to show root hair proliferation than the wild type. Cell free extracts of M. robertsii did not stimulate root hair proliferation. Longer‐term (60 d) associations showed that M. robertsii endophytically colonized cortical cells within bean roots. Metarhizium appeared as a mycelial aggregate within root cortical cells as well as between the intercellular spaces with no apparent damage to the plant. • Conclusions: These results suggest that M. robertsii is not only rhizosphere competent but also displays a beneficial endophytic association with plant roots that results in the proliferation of root hairs.