Can symbiotic fungi aid an invasive plant?

Plant invasions cause tremendous environmental and economic costs. Invader success can be due to an escape from co-evolved enemies, but can another reason be encounters with better mutualists in the non-native range? Our latest publication addresses this question.
Can symbiotic fungi aid an invasive plant?
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As with many fun projects, this one started with discussions over a cold brew. Christoph Rosche visited Ray Callaway’s lab at the University of Montana in Missoula in 2018, and Min Sheng and I were looking for a suitable project for her sabbatical with me at MPG Ranch (www.mpgranch.com) later that year. Christoph had worked extensively on Conyza canadensis — an annual plant in the Asteraceae family that is native to North America and invasive in many parts of the world, including China where Min does her research — and had established a global network of researchers interested in this plant. Conyza forms a root symbiosis with arbuscular mycorrhizal (AM) fungi that can aid in nutrient acquisition, drought tolerance and pathogen protection in exchange for carbon. Min, Christoph and I were curious if the greater performance by Conyza  in the non-native range could partly be due to eco-evolutionary shifts in plant-AM fungal associations as predicted by the enhanced mutualism hypothesis(1).

     

We identified four climatic regions to sample from in the native and non-native range. This ensured that findings were not restricted to narrow spatio-environmental contexts and would not be biased by different environmental conditions between ranges. Together with researchers in Jordan, Nepal, Canada and the USA, we designed a sampling protocol that would work for everyone, and sampled roots, soil, and seeds from 17 populations each in native and non-native ranges. This included nine populations in China that Min sampled with students before arriving in Missoula. To try to isolate the effect of Conyza and control for site-specific differences, we also sampled adjacent communities where Conyza was absent across all populations. It was challenging to acquire permits required to ship samples across borders and to coordinate and ensure consistency in methodology and sample quality among populations. But in the end, it was very much worth the effort.

 

Photo 1. Conyza canadensis sampled in its native range in the USA (left) and in the non-native range in China (right). Biomass and fecundity were significantly greater in the non-native range, and it corresponded with a change in arbuscular mycorrhizal (AM) associations. Photo credit: Ylva Lekberg and Min Sheng.

We found that native and non-native Conyza interacted with different AM fungi. We also found that AM fungal taxa appeared mutualistic in the non-native range but parasitic in the native range based on correlations between taxa abundance and plant biomass. Because adjacent communities did not differ in AM fungal community attributes, differences appeared to be uniquely associated with Conyza. That is, this plant finds new and potentially better mutualists in the non-native range, which may at least partly explain their invasive success.

Photo 2. Conyza canadensis sampled in Jordan (left). Despite a shallow root system, Conyza continues to grow and transpire under very dry conditions where most other plants either die or go dormant. Sampling roots in Montana even required a trip to the hardware store to buy a pickax. Can some of this drought tolerance be due to their high colonization by AM fungi (right), here visualized after a trypan-blue staining of roots? Photo credit: Mohammad Al-Gharaibeh and Min Sheng.

To assess if these differences were due to evolutionary shifts or biogeographical distribution patterns of AM fungi and plasticity, we set up a common garden experiment in the greenhouse using seeds from all populations and soil collected from the native range. To our surprise, we found that native and non-native plants associated with different AM fungi when exposed to the same species pool, and that non-native populations promoted fungal richness and diversity. It appeared that the non-native populations also had some sort of “memory” of AM associations in the field, because AM fungal communities associated with populations from arid sites clustered separately from mesic sites in the greenhouse. No such clustering was observed among native populations, suggesting a more haphazard interaction with AM fungi in the native range. Similar to the field samples, AM fungi colonizing non-native plants in the greenhouse appeared mutualistic whereas AM fungi colonizing native plants appeared parasitic.

 We do not know what caused these differences, nor do we know their relative importance for invasive success, as this would require additional experiments. One explanation for the shift in AM associations may be differences in root exudation, because recent studies have shown non-native plants of other species exude more flavonoids than native plants, and this corresponds with higher AM colonization and plant growth (2,3). We are currently exploring this within a large research collaboration led by Christoph (https://conyzaiconnect.wixsite.com/iconnect) that examines ecological as well as evolutionary biogeographical shifts associated with Conyza.

 It is fitting that as I type this blog about a research idea hatched with Christoph in Missoula, Montana, I am sitting at a biological station in the middle of Müritz National Park in Germany, enjoying again a cold brew with Christoph. I also just received an email from Min saying that her proposal was funded for me to visit her in China for our continued work on Conyza. International collaborations are sometimes challenging for logistical and cultural reasons, but they are always interesting and rewarding.

  1. Reinhart KO, Callaway RM. Soil biota and invasive plants. New Phytol. 2006;170:445–57.
  2. Yu H, He Y, Zhang W, Chen L, Zhang J, Zhang X, et al. Greater chemical signaling in root exudates enhances soil mutualistic associations in invasive plants compared to natives. New Phytol. 2022;
  3. Tian B, Pei Y, Huang W, Ding J, Siemann E. Increasing flavonoid concentrations in root exudates enhance associations between arbuscular mycorrhizal fungi and an invasive plant. ISME J. 2021;1919–30.