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Over recent decades, plants have been confronted with deforestation, ecosystem degradation and climate change due to the anthropogenic impact on the environment. In a bid to survive, they have been forced to adapt to an impressive array of ecological niches; indeed, the power of plants should not be underestimated. A plant’s ability to adapt efficiently is in part due to the large variety of specialized metabolites it produces.



An important element of plant survival is the microbiotic composition of the soil surrounding its roots; so important, in fact, that scientists estimate plants use ~ 20 percent of their photosynthetic energy to create root-derived molecules. These molecules promote the formation of select root microbiota from the surrounding soil – the "recipe" of microbiota within the soil is thus unique to each individual plant.



"Collectively, plants make a diverse array of chemicals. These molecules often have complex structures and so plants must use up a lot of energy in making them. This implies that specialized plant metabolites have important functions that enable plant survival, and that the distinct chemical profiles of different types of plants may reflect adaptation for survival in different environmental niches," says Anne Osbourn, Professor and Director of the Norwich Research Park Industrial Biotechnology Alliance at the John Innes Centre.





Creating microbiotic communities



The underlying mechanisms by which plants create specific microbiotic communities within the soil has remained elusive. A particularly interesting group of plant metabolites are the triterpenes, implicated in defense and cell signaling. These specialized metabolites have been shown to possess antifungal and antibacterial functions, thus suggesting a potential role for them in engineering the specific root microbiotic communities.





Ancheng Huang, a postdoctoral scientist also from the John Innes Centre, and colleagues have adopted a collection of techniques including metabolomic analysis and whole-genome sequencing (WGS) to uncover a triterpene biosynthetic network in the model plant Arabidopsis thaliana (common name, thalecress). Their results are published in the journal Science.



"This plant, which was the first to have its genome sequenced decades ago, has been studied intensively by biologists. Despite this we found signs that there were numerous genes with predicted functions in specialized metabolism that were expressed in the roots of A. thaliana, many of which were organized in clusters in the genome like beads on a string," notes Osbourn, "This led us to hypothesize that these genes may encode pathways for the biosynthesis of new metabolites that may influence interactions between A. thaliana roots and soil microorganisms."



The study results showed that the biosynthetic pathways of the triterpene network can synthesize a variety of triterpene compounds that were shown to impact the A. thaliana root microbiota.



"We generated mutant lines of A. thaliana that were unable to make these chemicals and showed that these lines differed markedly from the wild type line in the types of microbes that they recruited from the soil," adds Osbourn. The researchers then used purified preparations of the root-derived chemicals to test their effects on the growth of microbes cultured from the soil. "We found that the chemicals variously inhibited or promoted the growth of these representative cultured soil microbes in ways that mirrored the effects that we observed in soil."



Engineering sustainable agriculture



From the results of the study, the researchers assume that the plant is shaping it's root microbiota for its own benefit, although they note that further work is needed to understand what the benefits of attracting or deterring particular types of microbes are for plant fitness. At a time when sustainability is the word on everybody's lips, this knowledge may be pertinent. "A better understanding of these chemical communication processes may enable us in the future to design improved crops with better yields and to move away from fertilizers and pesticides," Osbourn concludes.



Reference: Huang et al. 2019. A specialized metabolic network selectively modulates Arabidopsis root microbiota. Science. DOI: https://science.sciencemag.org/cgi/doi/10.1126/science.aau6389.