Mutation in the pssZ Gene Negatively Impacts Exopolysaccharide Synthesis, Surface Properties, and Symbiosis of Rhizobium leguminosarum bv. trifolii with Clover.

Paulina Lipa,Joanna Kopcińska,Monika Janczarek,Agnieszka Zdybicka-Barabas,José-María Vinardell

doi:10.3390/genes9070369

Paulina Lipa, Joanna Kopcińska + Show 3 more

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https://doi.org/10.3390/genes9070369

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Abstract

Rhizobium leguminosarum bv. trifolii is a soil bacterium capable of establishing a nitrogen-fixing symbiosis with clover plants (Trifolium spp.). This bacterium secretes large amounts of acidic exopolysaccharide (EPS), which plays an essential role in the symbiotic interaction with the host plant. This polymer is biosynthesized by a multi-enzymatic complex located in the bacterial inner membrane, whose components are encoded by a large chromosomal gene cluster, called Pss-I. In this study, we characterize R. leguminosarum bv. trifolii strain Rt297 that harbors a Tn5 transposon insertion located in the pssZ gene from the Pss-I region. This gene codes for a protein that shares high identity with bacterial serine/threonine protein phosphatases. We demonstrated that the pssZ mutation causes pleiotropic effects in rhizobial cells. Strain Rt297 exhibited several physiological and symbiotic defects, such as lack of EPS production, reduced growth kinetics and motility, altered cell-surface properties, and failure to infect the host plant. These data indicate that the protein encoded by the pssZ gene is indispensable for EPS synthesis, but also required for proper functioning of R. leguminosarum bv. trifolii cells.

Highlights

Rhizobium leguminosarum bv. trifolii is a Gram-negative bacterium that exists as a free-living organism in the soil or establishes nitrogen-fixing symbiosis with clover plants (Trifolium spp.).This microorganism belongs to a large and diverse group of soil bacteria, collectively called rhizobia, which possess the ability to induce nodules on roots and stems of legumes [1,2]
We have characterized a R. leguminosarum bv. trifolii strain harboring a mutation in the pssZ gene, which is located in the Pss-I region. We demonstrate that this gene plays an essential role in EPS synthesis, and affects different cell-surface properties and symbiosis of
We found that pssZ is an individual open designated the exo44 mutation, in the pssZ gene was identified (between positions 1428 and 1429 nt, reading frame (ORF) that does not form part of any operon; the mutation in this gene would

Summary

Introduction

Trifolii is a Gram-negative bacterium that exists as a free-living organism in the soil or establishes nitrogen-fixing symbiosis with clover plants (Trifolium spp.) This microorganism belongs to a large and diverse group of soil bacteria, collectively called rhizobia, which possess the ability to induce nodules on roots and stems of legumes [1,2]. Recent findings indicate that the function of EPS in the legume–rhizobium symbiosis is more complex than initially anticipated and depends largely on the host plant This polysaccharide is required for effective symbiosis of bacteria with a great majority of legumes, which form indeterminate-type nodules (e.g., clover, vetch, pea, and alfalfa) [1,4,5]. These strains are only able to induce the formation of small, partially infected or even empty, nodule-like structures on the compatible host plants that are ineffective in nitrogen fixation [6,7,8,9]

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