Not without my microbiome: Legume-rhizobia symbiosis influences bacterialcommunity in plant roots14 November 2016

Light microscopy picture of Lotus japonicus nodulesection infected with its natural symbiont Mesorhizobiumloti. Credit: MPI for Plant Breeding Research

Some plants can meet their nitrogen requirementsby obtaining it from the atmosphere. To do this,they use bacteria in their roots, with which theyestablish a symbiotic relationship. Paul Schulze-Lefert's Research Group at the Max PlanckInstitute for Plant Breeding Research in Cologneand Simona Radutoiu from Aarhus University inDenmark have investigated the microbialcommunities in and around the roots of theseplants. They discovered that the absence oflegume-rhizobia symbiosis causes drastic changesin the composition of the microbial communitiespresent in the root. These changes also remainstable under conditions that prevent the formationof any nodules because the plant obtains sufficientbound nitrogen from the soil. A set of genes in thehost plant is necessary to establish a functioningsymbiosis and the same genes are needed to formextraordinarily stable and characteristic microbial

communities in the nodules, root and area aroundthe root. Thus, these plant genes have a directinfluence on the composition of the rootmicrobiome.

All plants need nitrogen for their growth; however,they are unable to obtain it directly from theatmosphere. They can only use nitrogen in thebound form of nitrate or ammonium ions, whichthey obtain either from the soil or through theaddition of nitrogen fertiliser to agricultural soil. Yetone plant family, the legumes, has developed theability to obtain nitrogen through a symbioticrelationship with naturally occurring soil bacteria.To accommodate these bacteria, the legumescreate specialized root organs known as nodules.The symbionts, which are known as rhizobia,transform atmospheric nitrogen in the nodules intonitrogenous compounds which are then available tothe plant.

Schulze-Lefert, Radutoiu and their colleagues RafalZgadzaj and Ruben Garrido-Oter used a form oflegume native to Japan (Lotus japonicus) as amodel plant and compiled an inventory of the root-associated microbiomes in the wild type and fourmutants. "We were able to show that whensymbiosis is lost, at least six different abundantbacterial orders become almost undetectable in theroot," says Garrido-Oter. "The loss of the symbiosisresults instead in the accumulation of some ofthese bacterial orders in the rhizosphere, that is thearea outside the root. It would appear that thesebacteria no longer hold a molecular ticket thatwould enable them to enter the root," he explains.

The scientists do not currently know which of thesignals associated with symbiosis acts as amolecular ticket for root entry, however thedramatic and stable changes in the microbiomeclearly result from the fact that the symbiosis no

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longer functions. Moreover, the scientists were ableto show that nodules and roots are not populatedsequentially by the bacteria but simultaneously.Thus, the bacteria do not migrate from one area tothe other but deliberately seek out the roots andnodules and without making any detours.

The team of scientists working with Schulze-Lefertand Radutoiu have two possible explanations forthese findings. It is possible that the rhizobia areaccompanied by an entire entourage of helperbacteria when they migrate to the roots andnodules in the course of the symbiosis. However, itis also possible that the signalling molecules, withwhich the roots attract the nodule bacteria from thesoil, also attract other bacteria that have nothing todo with the actual symbiosis but use the samesignal to gain admission to the plant.

The findings are remarkable for two reasons:"Because Lotus japonicus remains smaller andpaler than the wild type in the absence ofsymbiosis, and because this stunted growth cannotbe adequately compensated through nitrogenfertilisation, it is likely that this phenotype is directlylinked with the microbial community typical of themutants," says Garrido-Oter. "Without the genesand genetic products for intact symbiosis, amicrobiome becomes established that fails tosupport maximum plant growth."

As legumes are frequently used in agriculture asintermediate crops for improving the nitrogencontent of the soil, the significance of these findingsexceeds the boundaries of basic research. Up tonow it was assumed that symbiosis was merelymediated by the rhizobia inside legume nodules. Itwould now appear, however, that the symbiosisalso engages the characteristic microbialcommunities found in the root and rhizosphere.Based on this, it would not be sufficient to inoculatelegume seeds with the rhizobacteria alone, as isthe current practice: they should also be inoculatedwith the relevant bacteria from the root microbiome.

More information: Root nodule symbiosis inLotus japonicus drives the establishment ofdistinctive rhizosphere, root, and nodule bacterialcommunities, PNAS, DOI:

10.1073/pnas.1616564113

Provided by Max Planck Society

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APA citation: Not without my microbiome: Legume-rhizobia symbiosis influences bacterial community inplant roots (2016, November 14) retrieved 7 January 2022 from https://phys.org/news/2016-11-microbiome-legume-rhizobia-symbiosis-bacterial-roots.html

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