Availability of carbon and nitrogen in soil affects Metarhizium robertsii root colonization and transfer of insect-derived nitrogen.

The endophytic, insect pathogenic fungus, Metarhizium, exchanges insect-derived nitrogen for photosynthate as part of a symbiotic association similar to well-known mycorrhizal relationships. However, little is known about this nitrogen transfer in soils where there is an abundance of nitrogen and/or carbon. Here, we applied D-glucose and ammonium nitrate to soil to examine the effect on root colonization and transfer of labelled nitrogen (15N) from an insect (injected with 15N-ammonium sulfate) to Metarhizium robertsii, into leaves of the common bean, Phaseolus vulgaris, over the course of 28 days. Application of exogenous carbon and/or nitrogen to soils significantly reduced detectable 15N in plant leaves. Metarhizium root colonization, quantified with real-time PCR, revealed colonization persisted under all conditions but was significantly greater on roots in soil supplemented with glucose and significantly lower in soil supplemented with ammonium nitrate. Fungal gene expression analysis revealed differential expression of sugar and nitrogen transporters (mrt, st3, nrr1, nit1, mep2) when Metarhizium was grown in pure broth culture or in co-culture with plant roots under various carbon and nitrogen conditions. The observation that Metarhizium maintained root colonization in the absence of nitrogen transfer, and without evidence of plant harm, is intriguing and indicates additional benefits with ecological importance.