Rhizosphere microbial activities are fueled largely by carbon from plant roots, but that flux – as rhizodeposition – is dynamic and largely invisible in soils of natural ecosystems. We are exploring a novel tool through which we can manipulate belowground function through natural means: systemic plant virus infection. Phloem flow can be increased by virus infection, potentially spurring increased delivery of phloem contents to growing root tips, and the surrounding rhizosphere, making them “sticky.” Such infection often also decreases root-to-shoot ratio, meaning that shoot demands for nutrients must be met by more intensive mining of soil per unit root, driving further potential change in rhizodeposition. Virus infection thus offers means to manipulate root properties and patterns of rhizodeposition, and to examine their effects on cation-exchange, microbial activity, and the stability of protective mineral–organic matter (OM) associations within the rhizosphere.
In initial work, we use experimental infection of Avena sativa (oats) with the Poaceae-generalist Barley yellow dwarf virus (BYDV) to alter root:shoot ratios and root system size. To evaluate virus-induced changes in rhizodeposits, we use Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and GC-MS to characterize metabolite profiles of phloem sap (obtained via aphid stylectomy) and exudates represented in hydroponics solution.
Results/Conclusions
As expected, we find that virus infection of A. sativa decreases transpiration and photosynthesis, root system length, root:shoot ratios, and overall plant biomass. In addition, FT-ICR-MS reveals previously undocumented virus-associated differences in solution around roots of young plants, with lipids, acids, and phenols more dominant around uninfected roots, and sugars and unsaturated hydrocarbons more dominant around roots of infected plants. These differences are concordant with initial FT-ICR-MS indication of shifts in phloem sap composition and support the hypothesis of virus-induced shifts in rhizodeposition. Experiments are underway to analyze the implications of these infection-associated shifts for release and decomposition of OM bound to soil minerals. Mechanistic understanding derived from this work will help guide future development and application of DOE’s newly developed E3SM Land Model (ELM), which includes a representation of soil carbon preservation through mineral-OM associations, but not the potential vulnerability of those associations to change in the quality, quantity, and dynamics of rhizodeposition. In marine systems, the influence of viruses on biogeochemical cycling is recognized. Despite high infection prevalence in wild and natural vegetation, however, the influence of virus infection on root traits and soil carbon dynamics in terrestrial ecosystems remains largely unexplored and merits attention.