When Dodder Attacks Soybeans, It Invites its Friends

A new study published in Agronomy by Wen Luo and colleagues reveals how the parasitic plant dodder (Cuscuta chinensis) impacts the root microbiome and metabolites of soybean plants. They found that when Dodder attacks a plant, the plant doesn’t just suffer from the parasitism. The parasite opens the door to pathogenic fungi to attack the plant. The research provides novel insights into the complex interplay between parasitic plants, crop hosts, microbes and soil nutrients.

The study results from an analysis of DNA to see what colonises a plant’s roots when dodder attacks. Targeted sequencing of root fungi showed dodder parasitism enriches for Alternaria, a potentially harmful fungal genus. According to the findings, the relative abundance of Alternaria increased in dodder-parasitised soybean roots. This microbial disruption may handicap soybean plants, allowing the parasitic Dodder to siphon off more resources.

The research also reveals how dodder parasitism alters the relationships between soybean root fungi and soil nutrients. The study found dodder parasitism strengthened the connections between root fungal communities and soil nitrogen levels. Specifically, the abundance of the fungal genus Alternaria in soybean roots showed a positive correlation with the soil’s nitrogen content. This suggests that while soybean’s ability to fix nitrogen may be impaired under dodder parasitism, Alternaria and other pathogenic fungi could use the excess nitrogen to invade the roots.

The researchers also analysed how dodder parasitism affects soybean root metabolites. Metabolomic profiling revealed that dodder parasitism induced the accumulation of flavonoids, a class of plant secondary metabolites, in soybean roots. Specifically, levels of the flavonoid kaempferol and its derivatives increased substantially under dodder parasitism. Flavonoids are known to play defensive roles in plant-microbe interactions.

The ramped-up production of flavonoids like kaempferol may help soybean roots resist invasion by pathogenic microbes like Alternaria. It seems soybean mounts a biochemical counterattack when parasitised by Dodder, alongside the microbial shift favouring Alternaria. This metabolomic glimpse reveals how plants and microbes battle chemically below ground. Understanding these complex molecular interplays could lead to new bio-based strategies to protect crops from parasitic weeds.

Boosting nutrients like nitrogen may inadvertently favour parasitic weed growth by promoting root pathogens. And Dodder appears to disrupt soybean’s control over both symbiotic microbes and defensive chemicals. Unravelling these complex underground battles could be pivotal to protecting soybean and other susceptible crops. With Dodder causing major soybean losses worldwide, these glimpses into Dodder’s subterranean sabotage highlight an important battleground.

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Luo, W., Li, Y., Luo, R., Wei, G., Liu, Y. and Chen, W. (2023) “Dodder parasitism leads to the enrichment of pathogen Alternaria and flavonoid metabolites in soybean root,” Agronomy (Basel, Switzerland), 13(6), p. 1571. Available at: https://doi.org/10.3390/agronomy13061571.