The endomicrobiome and weed invasiveness in Mediterranean ecosystems worldwide

Mediterranean ecosystems, one of the most significant global hotspots of biodiversity, are threatened by invasive weeds. Although endomicrobiomes – the vast array of microbes colonising living plant tissues – are known to affect plant fitness, their contribution to weed invasiveness remains virtually unknown. Here, we experimentally assess the role of the endomicrobiome in the invasiveness of Taraxacum officinale (common dandelion), a widespread weed in Mediterranean ecosystems worldwide. In a culling experiment across five generations, we compare the fitness of T. officinale from these ecosystems on five continents grown with intact or reduced native seed-borne endomicrobiomes. Additionally, we report a competition experiment with F₁ and F₅ individuals assessing their impacts on native local Asteraceae species. We found that T. officinale individuals harboring intact endomicrobiomes show faster and more favorable trait development compared with individuals with reduced endomicrobiomes. Enhanced competitiveness of endomicrobiome-colonised T. officinale plants with local Asteraceae species is apparently caused by increased synthesis of allelochemicals in shoots and rhizosphere soil, with gene expression analyses also showing the endomicrobiome to affect the expression by T. officinale of stress response and RNA-directed DNA methylation genes. Our findings provide insights into the mechanisms underlying weed invasiveness in Mediterranean ecosystems.