Biotin as a Growth Factor for Rhizobia

The spent culture supernatant of Pseudomonas syringae contains azelaic acid

Background

Pseudomonas syringae pv. actinidiae (PSA) is an emerging kiwifruit bacterial pathogen which since 2008 has caused considerable losses. No quorum sensing (QS) signaling molecule has yet been reported from PSA and the aim of this study was to identify possible intercellular signals produced by PSA.

Results

A secreted metabolome analysis resulted in the identification of 83 putative compounds, one of them was the nine carbon saturated dicarboxylic acid called azelaic acid. Azelaic acid, which is a nine-carbon (C9) saturated dicarboxylic acid, has been reported in plants as a mobile signal that primes systemic defenses. In addition, its structure,(which is associated with fatty acid biosynthesis) is similar to other known bacterial QS signals like the Diffusible Signal Facor (DSF). For these reason it could be acting as s signal molecule. Analytical and structural studies by NMR spectroscopy confirmed that in PSA spent supernatants azelaic acid was present. Quantification studies further revealed that 20 μg/L of were present and was also found in the spent supernatants of several other P. syringae pathovars. The RNAseq transcriptome study however did not determine whether azelaic acid could behave as a QS molecule.

Conclusions

This study reports of the possible natural biosynthesis of azelaic acid by bacteria. The production of azelaic acid by P. syringae pathovars can be associated with plant-bacteria signaling.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1352-z) contains supplementary material, which is available to authorized users.

Functional analysis of Sinorhizobium meliloti genes involved in biotin synthesis and transport

External biotin greatly stimulates bacterial growth and alfalfa root colonization by Sinorhizobium meliloti strain 1021. Several genes involved in responses to plant-derived biotin have been identified in this bacterium, but no genes required for biotin transport are known, and not all loci required for biotin synthesis have been assigned. Searches of the S. meliloti genome database in combination with complementation tests of Escherichia coli biotin auxotrophs indicate that biotin synthesis probably is limited in S. meliloti 1021 by the poor functioning or complete absence of several key genes. Although several open reading frames with significant similarities to genes required for synthesis of biotin in gram-positive and gram-negative bacteria were found, only bioB, bioF, and bioH were demonstrably functional in complementation tests with known E. coli mutants. No sequence or complementation evidence was found for bioA, bioC, bioD, or bioZ. In contrast to other microorganisms, the S. meliloti bioB and bioF genes are not localized in a biotin synthesis operon, but bioB is cotranscribed with two genes coding for ABC transporter-like proteins, designated here bioM and bioN. Mutations in bioM and bioN eliminated growth on alfalfa roots and reduced bacterial capacity to maintain normal intracellular levels of biotin. Taken together, these data suggest that S. meliloti normally grows on exogenous biotin using bioM and bioN to conserve biotin assimilated from external sources.

Sinorhizobium meliloti cells require biotin and either cobalt or methionine for growth

Sinorhizobium meliloti is usually cultured in rich media containing yeast extract. It has been suggested that some components of yeast extract are also required for growth in minimal medium. We tested 27 strains of this bacterium and found that none were able to grow in minimal medium when methods to limit carryover of yeast extract were used during inoculation. By fractionation of yeast extract, two required growth factors were identified. Biotin was found to be absolutely required for growth, whereas previously the need for this vitamin was considered to be strain specific. All strains also required supplementation with cobalt or methionine, consistent with the requirement for a vitamin B(12)-dependent homocysteine methyltransferase for methionine biosynthesis.

The bio operon on the acquired symbiosis island of Mesorhizobium sp. strain R7A includes a novel gene involved in pimeloyl-CoA synthesis

The symbiosis island of Mesorhizobium sp. strain R7A is a 500 kb chromosomal genetic element that upon transfer converts nonsymbiotic mesorhizobia to symbionts able to nodulate and fix nitrogen with Lotus corniculatus. Four genomic species of nonsymbiotic mesorhizobia have been isolated. All were auxotrophic for thiamin and biotin and three were auxotrophic for nicotinate, whereas derivatives of the strains containing the symbiosis island were prototrophic for all three vitamins. In this work, a 13.2 kb region of the island that converts the nonsymbionts to nicotinate and biotin prototrophy was characterized. The region contained orthologues of the Escherichia coli bioBFD and A genes arranged in an operon with a novel gene, bioZ, a nadABC operon, the nitrogen-fixation regulatory gene nifA, and a homologue of the pantothenate biosynthesis gene panD. The bioZ gene product was similar to beta-ketoacyl-acyl carrier protein synthase III (FabH). bioZ::Tn5 mutants grew poorly in the absence of biotin and the bioZ gene complemented an E. coli bioH mutant, suggesting that its product is involved in the synthesis of pimeloyl-COA: The bio operon was not required for symbiosis, as only mutants in the nifA gene were impaired in symbiosis, and a bioA::Tn5 mutant was not impaired in rhizosphere colonization. The rationale for the vitamin biosynthetic loci being located on an acquired genetic element that is absent from nonsymbiotic mesorhizobia remains to be determined.

Role of lectins in plant-microorganism interactions: I. Binding of soybean lectin to rhizobia

Highly purified soybean lectin (SBL) was labeled with fluorescein isothiocyanate (FITC-SBL) or tritium ((3)H-SBL) and repurified by affinity chromatography. FITC-SBL was found to bind to living cells of 15 of the 22 Rhizobium japonicum strains tested. The lectin did not bind to cells of the other seven R. japonicum strains, or to cells of any of the nine Rhizobium strains tested which do not nodulate soybean. The binding of the lectin to the SBL-positive strains of R. japonicum was shown to be specific and reversible by hapten inhibition with d-galactose or N-acetyl-d-galactosamine.The lectin-binding properties of the SBL-positive R. japonicum strains were found to change substantially with culture age. The percentage of cells in a population exhibiting fluorescence after exposure to FITC-SBL varied between 0 and 70%. The average number of SBL molecules bound per cell varied between 0 and 2 x 10(6). While most strains had their highest percentage of SBL-positive cells and maximum number of SBL-binding sites per cell in the early and midlog phases of growth, one strain had a distinctly different pattern. The SBL-negative strains did not bind lectin at any stage of growth.Quantitative binding studies with (3)H-SBL indicated that the affinity constant for binding of SBL to its receptor sites on R. japonicum is approximately 4 x 10(7)m(-1). Many of the binding curves were biphasic. An inhibitor of SBL binding was found to be present in R. japonicum culture filtrates.

On the cytology and synthetic capacities of natural and artificially produced bacteroids of Rhizobium leguminosarum

Bacteroids of Rhizobium leguminosarum were formed rapidly in media containing 0.35% yeast extract, but not in media containing 0.1% yeast extract. Bacteroid production in the former media could be prevented by increasing the oxygen tension, suggesting that the limited oxygen supply within root nodules may be the principal factor initiating the natural formation of bacteroids. The artificially produced bacteroids were shorter and possessed a more irregular external morphology than bacteroids extracted from red root nodules of field pea, although the internal organization of both types of bacteroids was similar.During the development of bacteroids in laboratory media there was a rapid decrease in viable count and a concurrent increase in nephelos, although this increase was at a reduced rate compared to that exhibited by the normal small rod forms of rhizobia. These data indicate an inhibition of cellular division and a continued synthesis of at least certain protoplasmic constituents. In this connection the capacity to synthesize nucleic acids was similar in both artificially produced bacteroids and the small, rod types of the test organism, although some minor differences were found with bacteroids in situ within the nodules when nodule age varied.All types of bacteroids, whether they were within the nodule, had been extracted from the nodule, or were produced in laboratory media, suffered an impairment in the synthesis of protein and cell wall. Therefore, in several important respects, artificially produced and naturally produced bacteroids are physiologically similar. The alteration in wall-synthesizing capacity and the possible formation of limited amounts of abnormal wall material incapable of employing certain ions, such as Mg2+, for maintenance of rigidity is a likely cause of both the inhibition of cellular division and of the abnormal external morphology of these rather unusual cells.

The utilization of peptides and L- and D-amino acids by effective and ineffective strains of Rhizobium meliloti

Washed cells of three strains of Rhizobium meliloti, effective and ineffective in the fixation of atmospheric nitrogen, were found to utilize certain di- and tri-peptides and the L-, D-, and DL-forms of several amino acids as sole sources of nitrogen. The maximum growth attained on any one amino acid isomer was a function of pH and temperature. Inhibitory effects found in certain racemic mixtures were explained by mutual stereochemical interference of the enzymes attacking the L- and D-isomers. Histidine was notable in that it yielded the greatest amount of growth at all pH values (6.0, 7.0, 7.5) with all three organisms, including the ineffective strain which did not grow at pH 6.0 or 7.5 on any other acid. All strains were relatively acid sensitive, variable degrees of growth inhibition occurring at pH 6.0. Partial hydrolysis of the peptides occurred, but stimulatory effects were observed in many instances and explained on the basis of transpeptidation. Inhibition in media containing peptides of glycine plus D-leucine was traced to an antagonistic effect between the two constituent amino acids after liberation. Although the ineffective and effective strains of rhizobia were found to differ in several ways, the most interesting feature was the stimulation of the growth of the ineffective organism by D-leucylglycine.