Building blocks toward sustainable biofertilizers: variation in arbuscular mycorrhizal spore germination when immobilized with diazotrophic bacteria in biodegradable hydrogel beads.

AIM: We investigated whether there was interspecies and intraspecies variation in spore germination of 12 strains of arbuscular mycorrhizal fungi when co-entrapped with the diazotrophic plant growth-promoting bacteria, Azospirillum brasilense Sp7 in alginate hydrogel beads. METHODS AND RESULTS: Twelve Rhizophagus irregularis, Rhizophagus intraradices, and Funneliformis mosseae strains were separately combined with a live culture of Azospirillum brasilense Sp7. Each fungal-bacterial consortia was supplemented with sodium alginate to a 2% concentration (v/v) and cross-linked in calcium chloride (2% w/v) to form biodegradable hydrogel beads. One hundred beads from each combination (total of 1200) were fixed in solidified modified Strullu and Romand media. Beads were observed for successful spore germination and bacterial growth over 14 days. In all cases, successful growth of A. brasilense was observed. For arbuscular mycorrhizal fungi, interspecies variation in spore germination was observed, with R. intraradices having the highest germination rate (64.3%), followed by R. irregularis (45.5%) and F. mosseae (40.3%). However, a difference in intraspecies germination was only observed among strains of R. irregularis and F. mosseae. Despite having varying levels of germination, even the strains with the lowest potential were still able to establish with the plant host Brachypodium distachyon in a model system. CONCLUSIONS: Arbuscular mycorrhizal spore germination varied across strains when co-entrapped with a diazotrophic plant growth-promoting bacteria. This demonstrates that hydrogel beads containing a mixed consortium hold potential as a sustainable biofertilizer and that compatibility tests remain an important building block when aiming to create a hydrogel biofertilizer that encases a diversity of bacteria and fungi. Moving forward, further studies should be conducted to test the efficacy of these hydrogel biofertilizers on different crops across varying climatic conditions in order to optimize their potential.

An Azospirillum brasilense chemoreceptor that mediates nitrate chemotaxis has conditional roles in the colonization of plant roots.

Motile plant-associated bacteria use chemotaxis and dedicated chemoreceptors to navigate gradients in their surroundings and to colonize host plant surfaces. Here, we characterize a chemoreceptor that we named Tlp2 in the soil alphaproteobacterium Azospirillum brasilense. We show that the Tlp2 ligand-binding domain is related to the 4-helix bundle family and is conserved in chemoreceptors found in the genomes of many soil- and sediment-dwelling alphaproteobacteria. The promoter of tlp2 is regulated in an NtrC- and RpoN-dependent manner and is most upregulated under conditions of nitrogen fixation or in the presence of nitrate. Using fluorescently tagged Tlp2 (Tlp2-YFP), we show that this chemoreceptor is present in low abundance in chemotaxis-signaling clusters and is prone to degradation. We also obtained evidence that the presence of ammonium rapidly disrupts Tlp2-YFP localization. Behavioral experiments using a strain lacking Tlp2 and variants of Tlp2 lacking conserved arginine residues suggest that Tlp2 mediates chemotaxis in gradients of nitrate and nitrite, with the R159 residue being essential for Tlp2 function. We also provide evidence that Tlp2 is essential for root surface colonization of some plants (teff, red clover, and cowpea) but not others (wheat, sorghum, alfalfa, and pea). These results highlight the selective role of nitrate sensing and chemotaxis in plant root surface colonization and illustrate the relative contribution of chemoreceptors to chemotaxis and root surface colonization.IMPORTANCEBacterial chemotaxis mediates host-microbe associations, including the association of beneficial bacteria with the roots of host plants. Dedicated chemoreceptors specify sensory preferences during chemotaxis. Here, we show that a chemoreceptor mediating chemotaxis to nitrate is important in the beneficial soil bacterium colonization of some but not all plant hosts tested. Nitrate is the preferred nitrogen source for plant nutrition, and plants sense and tightly control nitrate transport, resulting in varying nitrate uptake rates depending on the plant and its physiological state. Nitrate is thus a limiting nutrient in the rhizosphere. Chemotaxis and dedicated chemoreceptors for nitrate likely provide motile bacteria with a competitive advantage to access this nutrient in the rhizosphere.

Biosynthesis of carotenoids in Azospirillum brasilense Cd is mediated via squalene (C30) route.

Azospirillum brasilense is a non-photosynthetic α-Proteobacteria, belongs to the family of Rhodospirillaceae and produces carotenoids to protect itself from photooxidative stress. In this study, we have used Resonance Raman Spectra to show similarity of bacterioruberins of Halobacterium salinarum to that of A. brasilense Cd. To navigate the role of genes involved in carotenoid biosynthesis, we used mutational analysis to inactivate putative genes predicted to be involved in carotenoid biosynthesis in A. brasilense Cd. We have shown that HpnCED enzymes are involved in the biosynthesis of squalene (C30), which is required for the synthesis of carotenoids in A. brasilense Cd. We also found that CrtI and CrtP desaturases were involved in the transformation of colorless squalene into the pink-pigmented carotenoids. This study elucidates role of some genes which constitute very pivotal role in biosynthetic pathway of carotenoid in A. brasilense Cd.

Plant seedlings of peas, tomatoes, and cucumbers exude compounds that are needed for growth and chemoattraction of Rhizobium leguminosarum bv. viciae 3841 and Azospirillum brasilense Sp7.

This study characterizes seedling exudates of peas, tomatoes, and cucumbers at the level of chemical composition and functionality. A plant experiment confirmed that Rhizobium leguminosarum bv. viciae 3841 enhanced growth of pea shoots, while Azospirillum brasilense Sp7 supported growth of pea, tomato, and cucumber roots. Chemical analysis of exudates after 1 day of seedling incubation in water yielded differences between the exudates of the three plants. Most remarkably, cucumber seedling exudate did not contain detectable sugars. All exudates contained amino acids, nucleobases/nucleosides, and organic acids, among other compounds. Cucumber seedling exudate contained reduced glutathione. Migration on semi solid agar plates containing individual exudate compounds as putative chemoattractants revealed that R. leguminosarum bv. viciae was more selective than A. brasilense, which migrated towards any of the compounds tested. Migration on semi solid agar plates containing 1:1 dilutions of seedling exudate was observed for each of the combinations of bacteria and exudates tested. Likewise, R. leguminosarum bv. viciae and A. brasilense grew on each of the three seedling exudates, though at varying growth rates. We conclude that the seedling exudates of peas, tomatoes, and cucumbers contain everything that is needed for their symbiotic bacteria to migrate and grow on.

Maize-Azospirillum brasilense interaction: accessing maize's miRNA expression under the effect of an inhibitor of indole-3-acetic acid production by the plant.

MicroRNA (miRNA) is a class of non-coding RNAs. They play essential roles in plants' physiology, as in the regulation of plant development, response to biotic and abiotic stresses, and symbiotic processes. This work aimed to better understand the importance of maize's miRNA during Azospirillum-plant interaction when the plant indole-3-acetic acid (IAA) production was inhibited with yucasin, an inhibitor of the TAM/YUC pathway. Twelve cDNA libraries from a previous Dual RNA-Seq experiment were used to analyze gene expression using a combined analysis approach. miRNA coding genes (miR) and their predicted mRNA targets were identified among the differentially expressed genes. Statistical differences among the groups indicate that Azospirillum brasilense, yucasin, IAA concentration, or all together could influence the expression of several maize's miRNAs. The miRNA's probable targets were identified, and some of them were observed to be differentially expressed. Dcl4, myb122, myb22, and morf3 mRNAs were probably regulated by their respective miRNAs. Other probable targets were observed responding to the IAA level, the bacterium, or all of them. A. brasilense was able to influence the expression of some maize's miRNA, for example, miR159f, miR164a, miR169j, miR396c, and miR399c. The results allow us to conclude that the bacterium can influence directly or indirectly the expression of some of the identified mRNA targets, probably due to an IAA-independent pathway, and that they are somehow involved in the previously observed physiological effects.

Synergic association of the consortium Arthrospira maxima with the microalga growth-promoting bacterium Azospirillum cultured under the stressful biogas composition.

The present study evaluates the association of the blue-green microalga Arthrospira maxima (Spirulina), which is known for its CO2 fixation, biomass, and high-value metabolite production, with the microalga growth-promoting bacterium Azospirillum brasilense under the stressful composition of biogas. The results demonstrated that A. maxima co-cultured with A. brasilense under the high CO2 (25%) and methane (CH4; 75%) concentrations of biogas recorded a CO2 fixation rate of 0.24 ± 0.03 g L-1 days-1, thereby attaining a biomass production of 1.8 ± 0.03 g L-1. Similarly, the biochemical composition quality of this microalga enhanced the attainment of higher contents of carbohydrates, proteins, and phycocyanin than cultured alone. However, metabolites other than tryptophan (Trp) and indole-3-acetic acid could have supported this beneficial interaction. Overall, the results demonstrate that this prokaryotic consortium of A. maxima-A. brasilense established a synergic association under biogas, which represents a sustainable strategy to improve the bio-refinery capacity of this microalga and increase the usefulness of A. brasilense in multiple economic sectors.

Chromosomal gene of hybrid multisensor histidine kinase is involved in motility regulation in the rhizobacterium Azospirillum baldaniorum Sp245 under mechanical and water stress.

Azospirillum alphaproteobacteria, which live in the rhizosphere of many crops, are used widely as biofertilizers. Two-component signal transduction systems (TCSs) mediate the bacterial perception of signals and the corresponding adjustment of behavior facilitating the adaptation of bacteria to their habitats. In this study, we obtained the A. baldaniorum Sp245 mutant for the AZOBR_150176 gene, which encodes the TCS of the hybrid histidine kinase/response sensory regulator (HSHK-RR). Inactivation of this gene affected bacterial morphology and motility. In mutant Sp245-HSHKΔRR-Km, the cells were still able to synthesize a functioning polar flagellum (Fla), were shorter than those of strain Sp245, and were impaired in aerotaxis, elaboration of inducible lateral flagella (Laf), and motility in semiliquid media. The mutant showed decreased transcription of the genes encoding the proteins of the secretion apparatus, which ensures the assembly of Laf, Laf flagellin, and the repressor protein of translation of the Laf flagellin's mRNA. The study examined the effects of polyethylene glycol 6000 (PEG 6000), an agent used to simulate osmotic stress and drought conditions. Under osmotic stress, the mutant was no longer able to use collective motility in semiliquid media but formed more biofilm biomass than did strain Sp245. Introduction into mutant cells of the AZOBR_150176 gene as part of an expression vector led to recovery of the lost traits, including those mediating bacterial motility under mechanical stress induced by increased medium density. The results suggest that the HSHK-RR under study modulates the response of A. baldaniorum Sp245 to mechanical and osmotic/water stress.

Azospirillum brasilense AerC and Tlp4b Cytoplasmic Chemoreceptors Are Promiscuous and Interact with the Two Membrane-Bound Chemotaxis Signaling Clusters Mediating Chemotaxis Responses.

Chemotaxis in Bacteria and Archaea depends on the presence of hexagonal polar arrays composed of membrane-bound chemoreceptors that interact with rings of baseplate signaling proteins. In the alphaproteobacterium Azospirillum brasilense, chemotaxis is controlled by two chemotaxis signaling systems (Che1 and Che4) that mix at the baseplates of two spatially distinct membrane-bound chemoreceptor arrays. The subcellular localization and organization of transmembrane chemoreceptors in chemotaxis signaling clusters have been well characterized but those of soluble chemoreceptors remain relatively underexplored. By combining mutagenesis, microscopy, and biochemical assays, we show that the cytoplasmic chemoreceptors AerC and Tlp4b function in chemotaxis and localize to and interact with membrane-bound chemoreceptors and chemotaxis signaling proteins from both polar arrays, indicating that soluble chemoreceptors are promiscuous. The interactions of AerC and Tlp4b with polar chemotaxis signaling clusters are not equivalent and suggest distinct functions. Tlp4b, but not AerC, modulates the abundance of chemoreceptors within the signaling clusters through an unknown mechanism. The AerC chemoreceptor, but not Tlp4b, is able to traffic in and out of chemotaxis signaling clusters depending on its level of expression. We also identify a role of the chemoreceptor composition of chemotaxis signaling clusters in regulating their polar subcellular organization. The organization of chemotaxis signaling proteins as large membrane-bound arrays underlies chemotaxis sensitivity. Our findings suggest that the composition of chemoreceptors may fine-tune chemotaxis signaling not only through their chemosensory specificity but also through their role in the organization of polar chemotaxis signaling clusters. IMPORTANCE Cytoplasmic chemoreceptors represent about 14% of all chemoreceptors encoded in bacterial and archaeal genomes, but little is known about how they interact with and function in large polar assemblies of membrane-bound chemotaxis signaling clusters. Here, we show that two soluble chemoreceptors with a role in chemotaxis are promiscuous and interact with two distinct membrane-bound chemotaxis signaling clusters that control all chemotaxis responses in Azospirillum brasilense. We also found that any change in the chemoreceptor composition of chemotaxis signaling clusters alters their polar organization, suggesting a dynamic interplay between the sensory specificity of chemotaxis signaling clusters and their polar membrane organization.

Acetylene Reduction Assay: A Measure of Nitrogenase Activity in Plants and Bacteria.

Nitrogen is one of the most abundant elements in the biosphere, but its gaseous form is not biologically available to many organisms, including plants and animals. Diazotrophic microorganisms can convert atmospheric nitrogen into ammonia, a form that can be absorbed by plants in a process called biological nitrogen fixation (BNF). BNF is catalyzed by the enzyme nitrogenase, which not only reduces N2 to NH3 , but also reduces other substrates such as acetylene. The acetylene reduction assay (ARA) can be used to measure nitrogenase activity in diazotrophic organisms, either in symbiotic associations or in their free-living state. The technique uses gas chromatography to measure the reduction of acetylene to ethylene by nitrogenase in a simple, quick, and inexpensive manner. Here, we demonstrate how to: prepare nodulated soybean plants and culture free-living Azospirillum brasilense for the ARA, use the gas chromatograph to detect the ethylene formed, and calculate the nitrogenase activity based on the peaks generated by the chromatograph. The methods shown here using example organisms can be easily adapted to other nodulating plants and diazotrophic bacteria. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Acetylene reduction assay in root nodules Basic Protocol 2: Acetylene reduction assay using diazotrophic bacteria Basic Protocol 3: Calculation of nitrogenase activity Support Protocol 1: Production of acetylene from calcium carbide Support Protocol 2: Calibration of the gas chromatograph Support Protocol 3: Total protein quantification.

Integrated Metabolomics and Morpho-Biochemical Analyses Reveal a Better Performance of Azospirillum brasilense over Plant-Derived Biostimulants in Counteracting Salt Stress in Tomato.

Increased soil salinity is one of the main concerns in agriculture and food production, and it negatively affects plant growth and crop productivity. In order to mitigate the adverse effects of salinity stress, plant biostimulants (PBs) have been indicated as a promising approach. Indeed, these products have a beneficial effect on plants by acting on primary and secondary metabolism and by inducing the accumulation of protective molecules against oxidative stress. In this context, the present work is aimed at comparatively investigating the effects of microbial (i.e., Azospirillum brasilense) and plant-derived biostimulants in alleviating salt stress in tomato plants by adopting a multidisciplinary approach. To do so, the morphological and biochemical effects were assessed by analyzing the biomass accumulation and root characteristics, the activity of antioxidant enzymes and osmotic stress protection. Furthermore, modifications in the metabolomic profiles of both leaves and root exudates were also investigated by ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/QTOF-MS). According to the results, biomass accumulation decreased under high salinity. However, the treatment with A. brasilense considerably improved root architecture and increased root biomass by 156% and 118% in non-saline and saline conditions, respectively. The antioxidant enzymes and proline production were enhanced in salinity stress at different levels according to the biostimulant applied. Moreover, the metabolomic analyses pointed out a wide set of processes being affected by salinity and biostimulant interactions. Crucial compounds belonging to secondary metabolism (phenylpropanoids, alkaloids and other N-containing metabolites, and membrane lipids) and phytohormones (brassinosteroids, cytokinins and methylsalicylate) showed the most pronounced modulation. Overall, our results suggest a better performance of A. brasilense in alleviating high salinity than the vegetal-derived protein hydrolysates herein evaluated.

Engineering D-glucose utilization in Azospirillum brasilense Sp7 promotes rice root colonization.

Bacteria of the genus Azospirillum include several plant associated bacteria which often promote the growth of their host plants. Although the host range of Azospirillum brasilense Sp7 is much wider than its close relative Azospirillum lipoferum 4B, it lacks the ability to efficiently utilize D-glucose for its growth. By comparing the genomes of both the species, the genes of A. lipoferum 4B responsible for conferring D-glucose utilization ability in A. brasilese Sp7 were identified by cloning individual or a combination of genes in a broad host range expression vector, mobilizing them in A. brasilense Sp7 and examining the ability of exconjugants to use D-glucose as sole carbon source for growth. These genes also included the homologs of genes involved in N-acetyl glucosamine utilization in Pseudomonas aeruginosa PAO1. A transcriptional fusion of the 5 genes encoding glucose-6-phosphate dehydrogenase and 4 components of glucose phosphotransferase system were able to improve D-glucose utilization ability in A. brasilense Sp7. The A. brasilense Sp7 strain engineered with D-glucose utilization ability showed significantly improved root colonization of rice seedling. The improvement in the ability of A. brasilense Sp7 to colonize rice roots is expected to bring benefits to rice by promoting its growth. KEY POINTS: • Genes required for glucose utilization in Azospirillum lipoferum were identified. • A gene cassette encoding glucose utilization was constructed. • Transfer of gene cassette in A. brasilense improves glucose utilization and rice root colonization..

Synergistic effects of Azospirillum brasilense and Bacillus cereus on plant growth, biochemical attributes and molecular genetic regulation of steviol glycosides biosynthetic genes in Stevia rebaudiana.

The current study aimed to scale up the favorable bio-stimulants for enhancing the growth and breeding strategies of Stevia rebaudiana to increase sugar productivity. Inoculation of 45-day-old S. rebaudiana plantlets with Bacillus cereus and Azospirillum brasilense alone or in combination for 30 days allowed comparisons among their effects on enhancement and improvement of plant growth, production of bioactive compounds and expression of steviol glycoside genes. B. cereus SrAM1 isolated from surface-sterilized Stevia rebaudiana leaves was molecularly identified using 16s rRNA and tested for its ability to promote plant growth. Beneficial endophytic B. cereus SrAM1 induced all plant growth-promoting traits, except solubilization of phosphate, therefore it showed high effectiveness in the promotion of growth and production of bioactive compounds. Treatment of plants with B. cereus SrAM1 alone revealed carbohydrates content of 278.99 mg/g, total soluble sugar of 114.17 mg/g, total phenolics content of 34.05 mg gallic acid equivalent (GAE)/g dry weight) and total antioxidants activity of 32.33 mg (A.A)/g dry weight). Thus, plantlets inoculated with B. cereus SrAM1 alone exhibited the greatest responses in physiological and morphological parameters, but plantlets inoculated with B. cereus SrAM1 + A. brasilense showed a maximal upregulation of genes responsible for the biosynthesis of steviol glycosides (Kaurene oxidase, ent-KO; UDP-dependent glycosyl transferases of UGT85C2, UGT74G1, UGT76G1). Taken together, the used bacterial strains, particularly B. cereus SrAM1 could significantly improve the growth of S. rebaudiana via dynamic interactions in plants.

Beneficial Bacterium Azospirillum brasilense Induces Morphological, Physiological and Molecular Adaptation to Phosphorus Deficiency in Arabidopsis.

Although most cultivated soils have high levels of total phosphorus (P), the levels of bioavailable inorganic P (Pi) are insufficient. The application of plant-growth-promoting rhizobacteria (PGPR) is an eco-friendly strategy for P utilization; however, PGPR-mediated plant responses that enhance Pi acquisition remain unexplored. Here, we investigated the effect of Azospirillum brasilense on Arabidopsis adaptation to Pi deficiency. Results showed that A. brasilense inoculation alleviated Pi-deficiency-induced growth inhibition and anthocyanin accumulation and increased the total P content in Arabidopsis plants. A comprehensive analysis of root morphology revealed that A. brasilense increased root hair density and length under Pi-limited conditions. We further demonstrated that A. brasilense enhanced the acid phosphatase activity and upregulated the expression of several Pi transporter genes, such as PHOSPHATE1 (PHO1), PHOSPHATE TRANSPORTER 1:(PHT1:1) and PHT1;4. However, A. brasilense did not enhance the growth o total P content in pht1;1, pht1;4 and pht1;1pht1;4 mutants. Moreover, A. brasilense could not increase the P content and PHT1;1 expression in the root hairless mutant rsl4rsl2, because of the occurrence of low-Pi-induced PHT1;1 and PHT1;4 in root hairs. These results indicate that A. brasilense can promote root hair development and enhance acid phosphatase activity and Pi transporter expression levels, consequently improving the Pi absorption capacity and conferring plant tolerance to Pi deficiency.

Expression, purification and characterization of the transcription termination factor Rho from Azospirillum brasilense.

The Transcription Termination factor Rho is a ring-shaped, homohexameric protein that causes transcript termination by interaction with specific sites on nascent mRNAs. The process of transcription termination is essential for proper expression and regulation of bacterial genes. Although Rho has been extensively studied in the model bacteria Escherichia coli (EcRho), the properties of other Rho orthologues in other bacteria are poorly characterized. Here we present the heterologous expression and purification of untagged Rho protein from the diazotrophic environmental bacterium Azospirillum brasilense (AbRho). The AbRho protein was purified to >99% through a simple, reproducible and efficient purification protocol, a two-step chromatography procedure (affinity/gel filtration). By using analytical gel filtration and dynamic light scattering (DLS), we found that AbRho is arranged as an homohexamer as observed in the EcRho orthologue. Secondary structure and enzyme activity of AbRho was also evaluate indicating a properly folded and active protein after purification. Enzymatic assays indicate that AbRho is a RNA-dependent NTPase enzyme.

Deciphering the role of the two conserved motifs of the ECF41 family σ factor in the autoregulation of its own promoter in Azospirillum brasilense Sp245.

In Azospirillum brasilense, an extra-cytoplasmic function σ factor (RpoE10) shows the characteristic 119 amino acid long C-terminal extension found in ECF41-type σ factors, which possesses three conserved motifs (WLPEP, DGGGR, and NPDKV), one in the linker region between the σ2 and σ4 , and the other two in the SnoaL_2 domain of the C-terminal extension. Here, we have described the role of the two conserved motifs in the SnoaL_2 domain of RpoE10 in the inhibition and activation of its activity, respectively. Truncation of the distal part of the C-terminal sequence of the RpoE10 (including NPDKV but excluding the DGGGR motif) results in its promoter's activation suggesting autoregulation. Further truncation of the C-terminal sequence up to its proximal part, including NPDKV and DGGGR motif, abolished promoter activation. Replacement of NPDKV motif with NAAAV in RpoE10 increased its ability to activate its promoter, whereas replacement of DGGGR motif led to reduced promoter activation. We have explored the dynamic modulation of σ2 -σ4 domains and the relevant molecular interactions mediated by the two conserved motifs of the SnoaL2 domain using molecular dynamics simulation. The analysis enabled us to explain that the NPDKV motif located distally in the C-terminus negatively impacts transcriptional activation. In contrast, the DGGGR motif found proximally of the C-terminal extension is required to activate RpoE10.

Examining effects of rhizobacteria in relieving abiotic crop stresses using carbon-11 radiotracing.

In agriculture, plant growth promoting bacteria (PGPB) are increasingly used for reducing environmental stress-related crop losses through mutualistic actions of these microorganisms, activating physiological and biochemical responses, building tolerances within their hosts. Here we report the use of radioactive carbon-11 (t½ 20.4 min) to examine the metabolic and physiological responses of Zea mays to Azospirillum brasilense (HM053) inoculation while plants were subjected to salinity and low nitrogen stresses. Host metabolism of "new" carbon resources (as 11 C) and physiology including [11 C]-photosynthate translocation were measured in response to imposed growth conditions. Salinity stress caused shortened, dense root growth with a 6-fold increase in foliar [11 C]-raffinose, a potent osmolyte. ICP-MS analyses revealed increased foliar Na+ levels at the expense of K+ . HM053 inoculation relieved these effects, reinstating normal root growth, lowering [11 C]-raffinose levels while increasing [11 C]-sucrose and its translocation to the roots. Na+ levels remained elevated with inoculation, but K+ levels were boosted slightly. Low nitrogen stress yielded longer roots possessing high levels of anthocyanins. Metabolic analysis revealed significant shifts in "new" carbon partitioning into the amino acid pool under low nitrogen stress, with significant increases in foliar [11 C]-glutamate, [11 C]-aspartate, and [11 C]-asparagine, a noted osmoprotectant. 11 CO2 fixation and [11 C]-photosynthate translocation also decreased, limiting carbon supply to roots. However, starch levels in roots were reduced under nitrogen limitation, suggesting that carbon repartitioning could be a compensatory action to support root growth. Finally, inoculation with HM053 re-instated normal root growth, reduced anthocyanin, boosted root starch, and returned 11 C-allocation levels back to those of unstressed plants.

Active indole-3-acetic acid biosynthesis by the bacterium Azospirillum brasilense cultured under a biogas atmosphere enables its beneficial association with microalgae.

AIMS: This study assessed, at the physiological and molecular levels, the effect of biogas on indole-3-acetic acid (IAA) biosynthesis by Azospirillum brasilense as well as the impact of this bacterium during CO2 fixation from biogas by Chlorella vulgaris and Scenedesmus obliquus. METHODS AND RESULTS: IpdC gene expression, IAA production and the growth of A. brasilense cultured under air (control) and biogas (treatment) were evaluated. The results demonstrated that A. brasilense had a better growth capacity and IAA production (105.7 ± 10.3 µg ml-1 ) when cultured under biogas composed of 25% CO2  + 75% methane (CH4 ) with respect to the control (72.4 ± 7.9 µg ml-1 ), although the ipdC gene expression level was low under the stressful condition generated by biogas. Moreover, this bacterium was able to induce a higher cell density and CO2 fixation rate from biogas by C. vulgaris (0.27 ± 0.08 g l-1 d-1 ) and S. obliquus (0.22 ± 0.08 g l-1 d-1 ). CONCLUSIONS: This study demonstrated that A. brasilense has the capacity to grow and actively maintain its main microalgal growth-promoting mechanism when cultured under biogas and positively influence CO2 fixation from the biogas of C. vulgaris and S. obliquus. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings broaden research in the field of Azospirillum-microalga interactions and the prevalence of Azospirillum in environmental and ecological topics in addition to supporting the uses of plant growth-promoting bacteria to enhance biotechnological strategies for biogas upgrading.

Structure Based Protein Engineering of Aldehyde Dehydrogenase from Azospirillum brasilense to Enhance Enzyme Activity against Unnatural 3-Hydroxypropionaldehyde.

3-Hydroxypropionic acid (3HP) is a platform chemical and can be converted into other valuable C3-based chemicals. Because a large amount of glycerol is produced as a by-product in the biodiesel industry, glycerol is an attractive carbon source in the biological production of 3HP. Although eight 3HP-producing aldehyde dehydrogenases (ALDHs) have been reported so far, the low conversion rate from 3-hydroxypropionaldehyde (3HPA) to 3HP using these enzymes is still a bottleneck for the production of 3HP. In this study, we elucidated the substrate binding modes of the eight 3HP-producing ALDHs through bioinformatic and structural analysis of these enzymes and selected protein engineering targets for developing enzymes with enhanced enzymatic activity against 3HPA. Among ten AbKGSADH variants we tested, three variants with replacement at the Arg281 site of AbKGSADH showed enhanced enzymatic activities. In particular, the AbKGSADHR281Y variant exhibited improved catalytic efficiency by 2.5-fold compared with the wild type.

Cometabolism of Ethanol in Azospirillum brasilense Sp7 Is Mediated by Fructose and Glycerol and Regulated Negatively by an Alternative Sigma Factor RpoH2.

Azospirillum brasilense is a plant growth-promoting rhizobacterium that is not known to utilize ethanol as a sole source of carbon for growth. This study shows that A. brasilense can cometabolize ethanol in medium containing fructose or glycerol as a carbon source and contribute to its growth. In minimal medium containing fructose or glycerol as a carbon source, supplementation of ethanol caused enhanced production of an alcohol dehydrogenase (ExaA) and an aldehyde dehydrogenase (AldA) in A. brasilense. However, this was not the case when malate was used as a carbon source. Inactivation of aldA in A. brasilense resulted in the loss of the AldA protein and its ethanol utilizing ability in fructose- or glycerol-supplemented medium. Furthermore, ethanol inhibited the growth of the aldA::Km mutant. The exaA::Km mutant also lost its ability to utilize ethanol in fructose-supplemented medium. However, in glycerol-supplemented medium, A. brasilense utilized ethanol due to the synthesis of a new paralog of alcohol dehydrogenase (ExaA1). The expression of exaA1 was induced by glycerol but not by fructose. Unlike exaA, expression of aldA and exaA1 were not dependent on σ54. Instead, they were negatively regulated by the RpoH2 sigma factor. Inactivation of rpoH2 in A. brasilense conferred the ability to use ethanol as a carbon source without or with malate, overcoming catabolite repression caused by malate. This is the first study showing the role of glycerol and fructose in facilitating cometabolism of ethanol by inducing the expression of ethanol-oxidizing enzymes and the role of RpoH2 in repressing them. IMPORTANCE This study unraveled a hidden ability of Azospirillum brasilense to utilize ethanol as a secondary source of carbon when fructose or glycerol were used as a primary growth substrate. It opens the possibility of studying the regulation of expression of the ethanol oxidation pathway for generating high yielding strains that can efficiently utilize ethanol. Such strains would be useful for economical production of secondary metabolites by A. brasilense in fermenters. The ability of A. brasilense to utilize ethanol might be beneficial to the host plant under the submerged growth conditions.

The Azospirillum brasilense type VI secretion system promotes cell aggregation, biocontrol protection against phytopathogens and attachment to the microalgae Chlorella sorokiniana.

The plant-growth-promoting bacterium Azospirillum brasilense is able to associate with the microalgae Chlorella sorokiniana. Attachment of A. brasilense increases the metabolic performances of the microalgae. Recent genome analyses have revealed that the A. brasilense Az39 genome contains two complete sets of genes encoding type VI secretion systems (T6SS), including the T6SS1 that is induced by the indole-3-acetic acid (IAA) phytohormone. The T6SS is a multiprotein machine, widespread in Gram-negative bacteria, that delivers protein effectors in both prokaryotic and eukaryotic cells. Here we show that the A. brasilense T6SS is required for Chlorella-Azospirillum synthetic mutualism. Our data demonstrate that the T6SS is an important determinant to promote production of lipids, carbohydrates and photosynthetic pigments by the microalgae. We further show that this is likely due to the role of the T6SS during the attachment stage and for the production of IAA phytohormones. Finally, we demonstrate that the A. brasilense T6SS provides antagonistic activities against a number of plant pathogens such as Agrobacterium, Pectobacterium, Dickeya and Ralstonia species in vitro, suggesting that, in addition to promoting growth, A. brasilense might confer T6SS-dependent bio-control protection to microalgae and plants against bacterial pathogens.