Paraburkholderia phymatum Homocitrate Synthase NifV Plays a Key Role for Nitrogenase Activity during Symbiosis with Papilionoids and in Free-Living Growth Conditions.

Paula Bellés-Sancho,Marta Adriana Pinto-Carbó,Sebastian Hug,Nicola Zamboni,Martina Lardi,Yilei Liu,Gabriella Pessi

doi:10.3390/cells10040952

Paula Bellés-Sancho, Marta Adriana Pinto-Carbó + Show 5 more

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https://doi.org/10.3390/cells10040952

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Journal: Cells	Publication Date: Apr 20, 2021
Citations: 10	License type: CC BY 4.0

Affiliation: University of Zurich, ETH Zurich

Abstract

Homocitrate is an essential component of the iron-molybdenum cofactor of nitrogenase, the bacterial enzyme that catalyzes the reduction of dinitrogen (N2) to ammonia. In nitrogen-fixing and nodulating alpha-rhizobia, homocitrate is usually provided to bacteroids in root nodules by their plant host. In contrast, non-nodulating free-living diazotrophs encode the homocitrate synthase (NifV) and reduce N2 in nitrogen-limiting free-living conditions. Paraburkholderia phymatum STM815 is a beta-rhizobial strain, which can enter symbiosis with a broad range of legumes, including papilionoids and mimosoids. In contrast to most alpha-rhizobia, which lack nifV, P. phymatum harbors a copy of nifV on its symbiotic plasmid. We show here that P. phymatum nifV is essential for nitrogenase activity both in root nodules of papilionoid plants and in free-living growth conditions. Notably, nifV was dispensable in nodules of Mimosa pudica despite the fact that the gene was highly expressed during symbiosis with all tested papilionoid and mimosoid plants. A metabolome analysis of papilionoid and mimosoid root nodules infected with the P. phymatum wild-type strain revealed that among the approximately 400 measured metabolites, homocitrate and other metabolites involved in lysine biosynthesis and degradation have accumulated in all plant nodules compared to uninfected roots, suggesting an important role of these metabolites during symbiosis.

Highlights

Nitrogen in the form of dinitrogen gas (N2) comprises approximately 78% of the Earth’s atmosphere
In line with de Meyer et al [18], who had grouped Paraburkolderia according to their nodulation characteristics, we classified those 21 strains in three different groups (Figure 1): free-living Paraburkholderia strains, which do not nodulate legumes (P. aromaticivorans BN5, P. bannensis E25, P. caballeronis LMG26416, P. ferrariae NBRC106233, P. heleia NBRC101817, P. kururiensis JCM10599, P. silvatlantica SRMrh20, P. tropica Ppe8, P. unamae MTI-641, P. xenovorans LB400), Paraburkholderia nodulating mimosoid plants (P. diazotrophica LMG26031, P. mimosarum LMG23256, P. nodosa DSM21604, P. phenoliruptrix BR3459a, P. phymatum STM815, P. piptadeniae STM7183, P. ribeironis STM7296) and Paraburkholderia nodulating papilionoid plants (P. dilworthii WSM3556, P. kirstenboschensis Kb15, P. sprentiae WSM5005, P. tuberum STM678)
In all the three strains used for the reconstruction of the nifV genomic location (P. xenovorans LB400, P. phymatum STM815 and P. tuberum STM678), nifV is predicted to be located at the beginning of an operon together with the nitrogenase-stabilizing nifW gene and the fixABCX genes involved in electron transport (Figure 2, Supplementary Figure S4)

Summary

Introduction

Nitrogen in the form of dinitrogen gas (N2) comprises approximately 78% of the Earth’s atmosphere. The agricultural industry has mainly relied on synthetic fertilizers in order to obtain higher crop yields. Their substantial contribution to environmental pollution has led to an urgent search for more environmentally sustainable alternatives [3,4,5]. Paraburkholderia phymatum STM815 [10] is an exception to this rule This strain is classified as a Paraburkholderia nodulating mimosoids [19] and has been isolated from several different Mimosa spp. P. phymatum is able to establish nitrogen-fixing symbioses with papilionoids and to reduce nitrogen in free-living conditions [22,26,27]

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