<i>Metarhizium</i> fungi as plant symbionts

Abstract Metarhizium fungi are pathogens of arthropods, with several early‐diverged species infecting lizards or mushrooms. Most Metarhizium species are also beneficial plant symbionts, and they mainly colonize rhizosphere and rhizoplane with some strains colonizing leaves and stems. The development of Metarhizium and plant association commence either by the attachment of spores to roots or rhizosphere colonization. Rhizosphere hyphae are further attracted to roots by nutrient‐rich root exudates, facilitating rhizoplane colonization. Both undifferentiated and differentiated (characterized by appressorial formation) hyphae can penetrate between root epidermal cells, with sporadical colonization. Plant‐derived reactive oxygen species play a crucial role in limiting root cell colonization by Metarhizium . Metarhizium utilizes plant‐derived sugars and fatty acids for the growth while transferring insect‐derived nitrogen to the plants. Additionally, Metarhizium facilitates plant’s utilization of environmental phosphorus, zinc, and iron via increasing the bioavailability of these nutrients. Metarhizium colonization can improve plant resistance to herbivores, microbial pathogens, and abiotic stresses. Given their versatile benefits to plants, Metarhizium has been developed as insecticides, fertilizers, and plant immunity promoters. In addition to terrestrial ecosystems, recent studies report that Metarhizium also inhabits aquatic and wetland ecosystems, which could further enhance the understanding of their biology and ecology and widen their applications.