Carbon translocation from a plant to an insect-pathogenic endophytic fungus

Abstract

Metarhizium robertsii is a common soil fungus that occupies a specialized ecological niche as an endophyte and an insect pathogen. Previously, we showed that the endophytic capability and insect pathogenicity of Metarhizium are coupled to provide an active method of insect-derived nitrogen transfer to a host plant via fungal mycelia. We speculated that in exchange for this insect-derived nitrogen, the plant would provide photosynthate to the fungus. By using ¹³CO₂, we show the incorporation of ¹³C into photosynthate and the subsequent translocation of ¹³C into fungal-specific carbohydrates (trehalose and chitin) in the root/endophyte complex. We determined the amount of ¹³C present in root-associated fungal biomass over a 21-day period by extracting fungal carbohydrates and analysing their composition using nuclear magnetic resonance (NMR) spectroscopy. These findings are evidence that the host plant is providing photosynthate to the fungus, likely in exchange for insect-derived nitrogen in a tripartite, and symbiotic, interaction.

Figure 1. Relative per cent ¹³C in Metarhizium and plant-derived sugars.

Resonances present in the ¹³C NMR spectra were integrated relative to an internal standard (100 μl MeOH, added to all samples) and then calculated as a percentage of integration value for the internal standard. (a) Closed circles represent ¹³C in trehalose extracted from root-colonized Metarhizium without an insect present. Closed squares represent ¹³C in trehalose extracted from root-colonized Metarhizium with insect present. Open circles represent ¹³C in trehalose extracted from roots without Metarhizium. (b) Closed circles represent ¹³C in GlcNAc extracted from root-colonized Metarhizium without an insect present. Closed squares represent ¹³C in GlcNAc extracted from root colonized Metarhizium with an insect present. Open circles represent ¹³C in GlcNAc extracted from roots without Metarhizium. (c) Closed circles represent ¹³C GlcNAc extracted from soil with Metarhizium, but without a plant. Closed squares represent ¹³C trehalose extracted from soil with Metarhizium, but without a plant. Open circles represent ¹³C in all sugars extracted from soil without Metarhizium and plant (n=20), bars represent s.e.

Figure 2. Internal cross section of M. robertsii colonized haricot bean roots harvested after 10 days and stained with Chlorazol black.

(a) Metarhizium colonized plant roots, sectioned with a microtome and subsequently stained with Chlorazol black. (b) Plant roots grown without Metarhizium stained with Chlorazol black. Images were taken at × 400 magnification. scale bar, 50 μm.

Figure 3. Exchange of insect-derived nitrogen for plant derived photosynthate.

Representation of the exchange of insect-derived nitrogen for plant derived photosynthate between endophytic Metarhizium and its plant host. Metarhizium is able to colonize a plant root, and provide useable, insect-nitrogen in exchange for plant produced carbon.