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Experimental evolution-driven identification of Arabidopsis rhizosphere competence genes in Pseudomonas protegens

By Erqin Li, Hao Zhang, Henan Jiang, Corné M. J. Pieterse, Alexandre Jousset, Peter A.H.M. Bakker, Ronnie de Jonge

Posted 02 Dec 2020
bioRxiv DOI: 10.1101/2020.12.01.407551

Beneficial plant root-associated microorganisms carry out a range of functions that are essential for plant performance. Establishment of a bacterium on plant roots, however, requires overcoming several challenges, including competition with neighboring microorganisms and host immunity. Forward and reverse genetics has led to the identification of mechanisms that are used by beneficial microorganisms to overcome these challenges such as the production of iron-chelating compounds, the formation of strong biofilms, or the concealment of characteristic microbial molecular patterns that trigger the host immune system. However, how such mechanisms arose from an evolutionary perspective is much less understood. To study bacterial adaptation in the rhizosphere, we employed experimental evolution to track the physiological and genetic dynamics of root-dwelling Pseudomonas protegens in the Arabidopsis thaliana rhizosphere under axenic conditions. This simplified binary one plant-one bacterium system allows for the amplification of key adaptive mechanisms for bacterial rhizosphere colonization. We identified 35 mutations, including single-nucleotide polymorphisms, insertions, and deletions, distributed over 28 genes. We found that mutations in genes encoding global regulators, and in genes for siderophore production, cell surface decoration, attachment, and motility accumulated in parallel, underlining that bacterial adaptation to the rhizosphere follows multiple strategies. Notably, we observed that motility increased in parallel across multiple independent evolutionary lines. Altogether these results underscore the strength of experimental evolution to identify key genes, pathways, and processes for bacterial rhizosphere colonization, and a methodology for the development of elite beneficial microorganisms with enhanced root-colonizing capacities that can support sustainable agriculture in the future.

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