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ABSTRACT Plant roots shape the rhizosphere community by secreting compounds that recruit diverse bacteria. Colonization of various plant roots by the motile alphaproteobacterium Azospirillum brasilens e causes increased plant growth, root volume, and crop yield. Bacterial chemotaxis in this and other motile soil bacteria is critical for competitive colonization of the root surfaces. The role of chemotaxis in root surface colonization has previously been established by endpoint analyses of bacterial colonization levels detected a few hours to days after inoculation. More recently, microfluidic devices have been used to study plant-microbe interactions, but these devices are size limited. Here, we use a novel slide-in chamber that allows real-time monitoring of plant-microbe interactions using agriculturally relevant seedlings to characterize how bacterial chemotaxis mediates plant root surface colonization during the association of A. brasilens e with Triticum aestivum (wheat) and Medicago sativa (alfalfa) seedlings. We track A. brasilense accumulation in the rhizosphere and on the root surfaces of wheat and alfalfa. A. brasilense motile cells display distinct chemotaxis behaviors in different regions of the roots, including attractant and repellent responses that ultimately drive surface colonization patterns. We also combine these observations with real-time analyses of behaviors of wild-type and mutant strains to link chemotaxis responses to distinct chemicals identified in root exudates to specific chemoreceptors that together explain the chemotactic response of motile cells in different regions of the roots. Furthermore, the bacterial second messenger c-di-GMP modulates these chemotaxis responses. Together, these findings illustrate dynamic bacterial chemotaxis responses to rhizosphere gradients that guide root surface colonization. IMPORTANCE Plant root exudates play critical roles in shaping rhizosphere microbial communities, and the ability of motile bacteria to respond to these gradients mediates competitive colonization of root surfaces. Root exudates are complex chemical mixtures that are spatially and temporally dynamic. Identifying the exact chemical(s) that mediates the recruitment of soil bacteria to specific regions of the roots is thus challenging. Here, we connect patterns of bacterial chemotaxis responses and sensing by chemoreceptors to chemicals found in root exudate gradients and identify key chemical signals that shape root surface colonization in different plants and regions of the roots.
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A Streptomyces Consortium Contributes Distinct Microbial Interactions During Arabidopsis thaliana Microbiome Assembly
Although plant microbiome assembly involves a series of both plant–microbe and microbe–microbe interactions, the latter is less often directly tested. Here, we investigate a role for Streptomyces strains to influence assembly of other bacteria into root microbiomes through the use of two synthetic communities (SynComs): a 21-member community including four Streptomyces strains and a 17-member community lacking those Streptomyces strains. Following inoculation with these SynComs on wild-type Arabidopsis thaliana Col-0, differential abundance modeling on root endosphere 16S ribosomal RNA gene amplicon sequencing data revealed altered abundance of four diverse SynCom members: Arthrobacter sp. 131, Agrobacterium sp. 33, Burkholderia sp. CL11, and Ralstonia sp. CL21. Modeling results were tested by seedling coinoculation experiments with the four Streptomyces strains and differentially abundant members, which confirmed the predicted decreased abundance for Arthrobacter sp. 131, Agrobacterium sp. 33, and Ralstonia sp. CL21 when Streptomyces strains were present. We further characterized how the phytohormone salicylic acid (SA) mediates Streptomyces strains’ influence over Agrobacterium sp. 33 and Burkholderia sp. CL11 seedling colonization. Although decreased colonization of Ralstonia sp. CL21 and Arthrobacter sp. 131 when Streptomyces spp. are present were not influenced by SA, direct antibiosis of Arthrobacter sp. 131 by Streptomyces was observed. These results highlight a role for Streptomyces-mediated microbial interactions during plant root microbiome assembly as well as distinct mechanisms that mediate them. Understanding the role of microbial interactions during microbiome assembly will inform the production of beneficial microbial treatments for use in agricultural fields.
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- PAR ID:
- 10539364
- Publisher / Repository:
- 10.1094/PBIOMES-11-22-0081-R
- Date Published:
- Journal Name:
- Phytobiomes Journal
- Volume:
- 7
- Issue:
- 4
- ISSN:
- 2471-2906
- Page Range / eLocation ID:
- 515 to 525
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1094/PBIOMES-11-22-0081-R
