White Locoweed Toxicity Is Facilitated by a Fungal Endophyte and Nitrogen-Fixing Bacteria

Abstract

Mutualistic interactions with fungal endophytes and dinitrogen-fixing bacteria are known to exert key biological influences on the host plant. The influence of a fungal endophyte alkaloid on the toxicity of a plant has been documented in Oxytropis sericea. Oxytropis sericea is a perennial legume responsible for livestock poisoning in western North America. Livestock poisoning is attributed to the alkaloid swainsonine, which is synthesized inside the plant by the fungal endophyte Embellisia sp. In this study, the ability of Oxytropis sericea to form a dinitrogen-fixing symbiosis with Rhizobium and the effects of this symbiosis on the production of swainsonine by Embellisia sp. were evaluated in a greenhouse environment. Seeds of O. sericea were grown in plastic containers. Twenty-week-old O. sericea seedlings were inoculated with four strains of Rhizobium. Twenty weeks after inoculation, plant growth and root nodulation by Rhizobium were measured. Dinitrogen fixation was confirmed using an acetylene reduction assay (ARA) on excised root nodules. Dry leaves were analyzed for swainsonine content. A second set of plants was treated with fungicide to evaluate the effect of reduced fungal endophyte infection on plant growth and swainsonine production. All inoculated plants produced indeterminate nodules. The ARA indicated that 98% of the excised nodules were fixing dinitrogen. Rhizobium-treated plants had greater swainsonine concentrations than the non-inoculated controls. Plants that established from seeds treated with fungicide had lower biomass than non-fungicide-treated controls and plants treated with foliar fungicide. Plants treated with foliar fungicide and the controls had greater swainsonine concentrations than the plants that received seed fungicide. This greenhouse study is the first report of nodulation and dinitrogen fixation in O. sericea. It also demonstrates that dinitrogen fixation increases the production of swainsonine in O. sericea plants infected with Embellisia sp. Results from this study suggest that dinitrogen fixation affects swainsonine production and has the potential to support the symbiosis between Embellisia sp. and O. sericea when soil nitrogen is limited. Oxytropis sericea competitiveness appears to be facilitated by an ability to simultaneously associate with Rhizobium and a fungal symbiont.

Localization of endophyticUndifilumfungi in locoweed seed and influence of environmental parameters on a locoweed in vitro culture system

“…The detection of fungus from root tissues was somewhat surprising because we have not isolated Undifilum from the roots of field-collected plants (Braun et al 2003). Locoweed roots are normally nodulated in field conditions (Valdez Barillas et al 2007), and we did not add nodulating bacteria to our embryo culture system. Thus, within the embryo culture-system conditions, the fungus may have been able to infect the roots, while in the field, bacteria may replace or compete with the fungi for space within the roots.…”

Rethinking of the Roles of Endophyte Symbiosis and Mycotoxin in Oxytropis Plants

“…In this context, it is also worth noticing that endophyte levels decrease in underground compared to aboveground tissues [46] (Figure 2). Therefore, it was speculated that locoweed endophytes may prefer to live in tissues with stronger photosynthetic activity and higher lysine levels [34], similar to the evidence demonstrated in grass and other plant species [47,48]. It is possible that the endophyte may preferentially infect certain tissue types or are distributed in a gradient as the plant grows from a seed to maturity plant.…”

Fungal Diversity in the Soil of the Oxytropis glacialis Root System on the Qinghai-Tibet Plateau

“…In previous studies, gramineous plant fungal in grasslands can improve plant stress resistance such as to extreme environments, pests, diseases, cold drought, saline conditions, and alkali conditions ( Nan and Li, 2004 ; Huang and Huang, 2008 ), and related studies have shown that phosphorus stress and osmotic stress stimulate seedling growth of O. punctatus ( Cui et al, 2009 ). Locoweeds can grow better under low pH and drought stress in the field ( Barillas et al, 2007 ). Locoweeds containing SW fungal reduce the number of animals that feeding on them, but the ecological effects of SW fungal on locoweeds have not been studied in depth.…”