Arbuscular mycorrhizal fungi modulate the growth-defense strategies in an invasive plant and its native congener

The evolutionary success of invasive plants is often interpreted through two prominent yet contrasting frameworks: the Evolution of Increased Competitive Ability (EICA) hypothesis, which posits a genetic re-allocation from defense traits toward growth, and the Enhanced Mutualism Hypothesis (EMH), which emphasizes the contribution of beneficial soil microorganisms. Nevertheless, it remains uncertain whether arbuscular mycorrhizal fungi (AMF) facilitate the expression of the characteristic “low-defense, high-growth” strategy in invasive plant species. We conducted a full-factorial common garden experiment to compare the growth and defense responses of the invasive species Solidago canadensis and its native congener S. decurrens under conditions with and without AMF inoculation. We measured plant growth, leaf chemistry, and herbivore performance. Our results revealed that AMF disproportionately increased shoot and total biomass in the invasive plant compared to its native plant. Crucially, AMF enhanced leaf nitrogen and soluble protein content in the invader without increasing the accumulation of defensive compounds (tannin, phenolics, lignin, cellulose), creating a pronounced “high-nutrition, low-defense” phenotype. In contrast, AMF significantly boosted these same defensive traits in the native plant. This divergent resource allocation had direct ecological consequences: AMF increased Spodoptera litura growth on the invader, while suppressing it on the native plant. Our results suggest that AMF symbiosis may contribute to the expression of a growth-defense trade-off strategy in invasive plants. By linking the EICA hypothesis with the EMH, this study offers insights into the potential role of AMF in shaping growth-defense strategies in invasive plants, and may help explain part of the reasons behind their invasion success.