A study on the dynamics of the symbiosis between Metarhizium and plants

Abstract

Members of the Metarhizium genus exist as both insect pathogens and plant endophytes. Agricultural formulations of these fungi are utilized for their biocontrol of insect pests and numerous additional benefits for crop plants (i.e., nutrient transfer, pathogen antagonism, increased biomass, etc.). In order to develop improvements to these formulations, it is vital to understand how specific factors, such as the nutrient availability and microbial community of the soil, as well as production of secondary metabolites, influence the interaction of Metarhizium with target crop species.

Metarhizium is capable of translocating nitrogen obtained through insect parasitism to plant hosts in exchange for carbohydrates. Using wax moth larvae (Galleria mellonella) injected with 15N-ammonium sulfate, it was demonstrated that nutrient-rich soil effectively inhibited transfer of insect-derived 15N from Metarhizium into the leaves of haricot bean, Phaseolus vulgaris. Colonization was maintained under all conditions and was not correlated with nitrogen transfer. Nitrogen application reduced initial colonization, but it recovered at later timepoints.

The persistence of Metarhizium within the rhizosphere is influenced by the microbial community but, reciprocally, the structure of the community may respond to Metarhizium application. Although soil amendment with M. robertsii did not affect overall diversity of the root microbiome of P. vulgaris, Illumina sequencing demonstrated significant effects on particular bacterial and fungal taxa. The relative abundance of several plant growth promoting microorganisms (e.g. Bradyrhizobium) increased after Metarhizium application. When challenged with the specific bean root rot pathogen Fusarium solani f. sp. phaseoli, both the microbiome and M. robertsii were able to suppress disease.

The production of fungal secondary metabolites such as destruxins may dictate interactions with plant hosts. During co-culture with bean or corn, destruxin production varied by species of Metarhizium and plant. Similar to previous reports, M. robertsii and M. acridum generally produced relatively high and low levels of destruxins, respectively. However, numerous destruxins were synthesized by M. acridum during co-culture with corn. Unraveling the metabolomic profile of these fungal-plant interactions may provide insight into mechanisms behind maintaining symbioses and patterns of strain compatibility, as well as aid in strain selection for agriculture and discovery of novel bioactive metabolites.