Phosphorus availability regulates nitrogen fixation rate through a key diazotrophic assembly: Evidence from a subtropical Moso bamboo forest subjected to nitrogen application.

Biological N fixation (BNF) is an important N input process for terrestrial ecosystems. Long-term N application increases the availability of N, but may also lead to phosphorus (P) deficiency or an imbalance between N and P. Here, we performed a 5-year N application experiment in a subtropical Phyllostachys heterocycla forest in site and a P application experiment in vitro to investigate the effect of N application on the BNF rate and its regulatory factor. The BNF rate, nifH gene, free-living diazotrophic community composition and plant properties were measured. We found that N application suppressed the BNF rate and nifH gene abundance, whereas the BNF rate in soils with added P was significantly higher overall than that in soils without added P. Moreover, we identified a key diazotrophic assembly (Mod#2), primarily comprising Bradyrhizobium, Geobacter, Desulfovibrio, Anaeromyxobacter, and Pseudodesulfovibrio, which explained 77 % of the BNF rate variation. There was a significant positive correlation between the Mod#2 abundance and soil available P, and the random forest results showed that soil available P is the most important factor affecting the Mod#2 abundance. Our findings highlight the importance of soil P availability in regulating the activities of key diazotrophs, and thus increasing P supply may help to promote N accumulation and primary productivity through facilitating the BNF process in forest ecosystems.

Cooperative interactions between nitrogen fixation and phosphorus nutrition in legumes.

Legumes such as soybean are considered important crops as they provide proteins and oils for humans and livestock around the world. Different from other crops, leguminous crops accumulate nitrogen (N) for plant growth through symbiotic nitrogen fixation (SNF) in coordination with rhizobia. A number of studies have shown that efficient SNF requires the cooperation of other nutrients, especially phosphorus (P), a nutrient deficient in most soils. During the last decades, great progress has been made in understanding the molecular mechanisms underlying the interactions between SNF and P nutrition, specifically through the identification of transporters involved in P transport to nodules and bacteroids, signal transduction, and regulation of P homeostasis in nodules. These studies revealed a distinct N-P interaction in leguminous crops, which is characterized by specific signaling cross talk between P and SNF. This review aimed to present an updated picture of the cross talk between N fixation and P nutrition in legumes, focusing on soybean as a model crop, and Medicago truncatula and Lotus japonicus as model plants. We also discuss the possibilities for enhancing SNF through improving P nutrition, which are important for high and sustainable production of leguminous crops.

Chemical Stimulation of Heterocyte Differentiation by the Feather Moss Hylocomium splendens: a Potential New Step in Plant-Cyanobacteria Symbioses.

Cyanobacteria associated with mosses play a key role in the nitrogen (N) cycle in unpolluted ecosystems. Mosses have been found to release molecules that induce morphophysiological changes in epiphytic cyanobionts. Nevertheless, the extent of moss influence on these microorganisms remains unknown. To evaluate how mosses or their metabolites influence N2 fixation rates by cyanobacteria, we assessed the nitrogenase activity, heterocyte frequency and biomass of a cyanobacterial strain isolated from the feather moss Hylocomium splendens and a non-symbiotic strain when they were either growing by themselves, together with H. splendens or exposed to H. splendens water, acetone, ethanol, or isopropanol extracts. The same cyanobacterial strains were added to another moss (Taxiphyllum barbieri) and a liverwort (Monosolenium tenerum) to assess if these bryophytes affect N2 fixation differently. Although no significant increases in nitrogenase activity by the cyanobacteria were observed when in contact with H. splendens shoots, both the symbiotic and non-symbiotic cyanobacteria increased nitrogenase activity as well as heterocyte frequency significantly upon exposure to H. splendens ethanol extracts. Contact with T. barbieri shoots, on the other hand, did lead to increases in nitrogenase activity, indicating low host-specificity to cyanobacterial activity. These findings suggest that H. splendens produces heterocyte-differentiating factors (HDFs) that are capable of stimulating cyanobacterial N2 fixation regardless of symbiotic competency. Based on previous knowledge about the chemical ecology and dynamics of moss-cyanobacteria interactions, we speculate that HDF expression by the host takes place in a hypothetical new step occurring after plant colonization and the repression of hormogonia.

Integrating the impact of global change on the niche and physiology of marine nitrogen-fixing cyanobacteria.

Marine nitrogen fixation is a major source of new nitrogen to the ocean, which interacts with climate driven changes to physical nutrient supply to regulate the response of ocean primary production in the oligotrophic tropical ocean. Warming and changes in nutrient supply may alter the ecological niche of nitrogen-fixing organisms, or 'diazotrophs', however, impacts of warming on diazotroph physiology may also be important. Lab-based studies reveal that warming increases the nitrogen fixation-specific elemental use efficiency (EUE) of two prevalent marine diazotrophs, Crocosphaera and Trichodesmium, thus reducing their requirements for the limiting nutrients iron and phosphorus. Here, we coupled a new diazotroph model based upon observed diazotroph energetics of growth and resource limitation to a state-of-the-art global model of phytoplankton physiology and ocean biogeochemistry. Our model is able to address the integrated response of nitrogen fixation by Trichodesmium and Crocosphaera to warming under the IPCC high emission RCP8.5 scenario for the first time. Our results project a global decline in nitrogen fixation over the coming century. However, the regional response of nitrogen fixation to climate change is modulated by the diazotroph-specific thermal performance curves and EUE, particularly in the Pacific Ocean, which shapes global trends. Spatially, the response of both diazotrophs is similar with expansion towards higher latitudes and reduced rates of nitrogen fixation in the lower latitudes. Overall, 95%-97% of the nitrogen fixation climate signal can be attributed to the combined effect of temperature on the niche and physiology of marine diazotrophs, with decreases being associated with a reduced niche and increases resulting due to a combination of expanding niche and temperature driven changes to EUE. Climate change impacts on both the niche and physiology of marine diazotrophs interact to shape patterns of marine nitrogen fixation, which will have important implications for ocean productivity in the future.

Molecular mechanisms underlying iron and phosphorus co-limitation responses in the nitrogen-fixing cyanobacterium Crocosphaera.

In the nitrogen-limited subtropical gyres, diazotrophic cyanobacteria, including Crocosphaera, provide an essential ecosystem service by converting dinitrogen (N2) gas into ammonia to support primary production in these oligotrophic regimes. Natural gradients of phosphorus (P) and iron (Fe) availability in the low-latitude oceans constrain the biogeography and activity of diazotrophs with important implications for marine biogeochemical cycling. Much remains unknown regarding Crocosphaera's physiological and molecular responses to multiple nutrient limitations. We cultured C. watsonii under Fe, P, and Fe/P (co)-limiting scenarios to link cellular physiology with diel gene expression and observed unique physiological and transcriptional profiles for each treatment. Counterintuitively, reduced growth and N2 fixation resource use efficiencies (RUEs) for Fe or P under P limitation were alleviated under Fe/P co-limitation. Differential gene expression analyses show that Fe/P co-limited cells employ the same responses as single-nutrient limited cells that reduce cellular nutrient requirements and increase responsiveness to environmental change including smaller cell size, protein turnover (Fe-limited), and upregulation of environmental sense-and-respond systems (P-limited). Combined, these mechanisms enhance growth and RUEs in Fe/P co-limited cells. These findings are important to our understanding of nutrient controls on N2 fixation and the implications for primary productivity and microbial dynamics in a changing ocean.

Nodulating white lupins take advantage of the reciprocal interplay between N and P nutritional responses.

The low bioavailability of nutrients, especially nitrogen (N) and phosphorus (P), is one of the most limiting factors for crop production. In this study, under N- and P-free nutrient solution (-N-P), nodulating white lupin plants developed some nodules and analogous cluster root structures characterized by different morphological, physiological, and molecular responses than those observed upon single nutrient deficiency (strong acidification of external media, a better nutritional status than -N+P and +N-P plants). The multi-elemental analysis highlighted that the concentrations of nutrients in white lupin plants were mainly affected by P availability. Gene-expression analyses provided evidence of interconnections between N and P nutritional pathways that are active to promote N and P balance in plants. The root exudome was mainly characterized by N availability in nutrient solution, and, in particular, the absence of N and P in the nutrient solution triggered a high release of phenolic compounds, nucleosides monophosphate and saponines by roots. These morphological, physiological, and molecular responses result from a close interplay between N and P nutritional pathways. They contribute to the good development of nodulating white lupin plants when grown on N- and P-free media. This study provides evidence that limited N and P availability in the nutrient solution can promote white lupin-Bradyrhizobium symbiosis, which is favourable for the sustainability of legume production.

Plant growth promoting rhizobacteria improve growth and yield related attributes of chili under low nitrogen availability.

Nitrogen (N) is a macronutrient desired by crop plants in large quantities. However, hiking fertilizer prices need alternative N sources for reducing its requirements through appropriate management practices. Plant growth promoting rhizobacteria (PGPR) are well-known for their role in lowering N requirements of crop plants. This study assessed the impact of PGPR inoculation on growth, allometry and biochemical traits of chili under different N doses. Two PGPR, i.e., Azospirillum 'Er-20' (nitrogen fixing) and Agrobacterium 'Ca-18' (phosphorous solubilizing) were used for inoculation, while control treatment had no PGPR inoculation. Six N doses, i.e., 100, 80, 75, 70, 60 and 50% of the N required by chili were included in the study. Data relating to growth traits, biochemical attributes and yield related traits were recorded. Interaction among N doses and PGPR inoculation significantly altered all growth traits, biochemical attributes and yield related traits. The highest values of the recorded traits were observed for 100% N with and without PGPR inoculation and 75% N with PGPR inoculation. The lowest values of the recorded traits were noted for 50% N without PGPR inoculation. The PGPR inoculation improved the measured traits compared to the traits recorded noted in same N dose without PGPR inoculation. Results revealed that PGPR had the potential to lower 25% N requirement for chili. Therefore, it is recommended that PGPR must be used in chili cultivation to lower N requirements.

Deciphering Trifolium pratense L. holobiont reveals a microbiome resilient to future climate changes.

The plant microbiome supports plant growth, fitness, and resistance against climate change. Trifolium pratense (red clover), an important forage legume crop, positively contributes to ecosystem sustainability. However, T. pratense is known to have limited adaptive ability toward climate change. Here, the T. pratense microbiomes (including both bacteria and fungi) of the rhizosphere and the root, shoot, and flower endospheres were comparatively examined using metabarcoding in a field located in Central Germany that mimics the climate conditions projected for the next 50-70 years in comparison with the current climate conditions. Additionally, the ecological functions and metabolic genes of the microbial communities colonizing each plant compartment were predicted using FUNGuild, FAPROTAX, and Tax4Fun annotation tools. Our results showed that the individual plant compartments were colonized by specific microbes. The bacterial and fungal community compositions of the belowground plant compartments did not vary under future climate conditions. However, future climate conditions slightly altered the relative abundances of specific fungal classes of the aboveground compartments. We predicted several microbial functional genes of the T. pratense microbiome involved in plant growth processes, such as biofertilization (nitrogen fixation, phosphorus solubilization, and siderophore biosynthesis) and biostimulation (phytohormone and auxin production). Our findings indicated that T. pratense microbiomes show a degree of resilience to future climate changes. Additionally, microbes inhabiting T. pratense may not only contribute to plant growth promotion but also to ecosystem sustainability.

Phosphorus deprivation affects composition and spatial distribution of membrane lipids in legume nodules.

In legumes, symbiotic nitrogen (N) fixation (SNF) occurs in specialized organs called nodules after successful interactions between legume hosts and rhizobia. In a nodule, N-fixing rhizobia are surrounded by symbiosome membranes, through which the exchange of nutrients and ammonium occurs between bacteria and the host legume. Phosphorus (P) is an essential macronutrient, and N2-fixing legumes have a higher requirement for P than legumes grown on mineral N. As in the previous studies, in P deficiency, barrel medic (Medicago truncatula) plants had impaired SNF activity, reduced growth, and accumulated less phosphate in leaves, roots, and nodules compared with the plants grown in P sufficient conditions. Membrane lipids in M. truncatula tissues were assessed using electrospray ionization-mass spectrometry. Galactolipids were found to increase in P deficiency, with declines in phospholipids (PL), especially in leaves. Lower PL losses were found in roots and nodules. Subsequently, matrix-assisted laser desorption/ionization-mass spectrometry imaging was used to spatially map the distribution of the positively charged phosphatidylcholine (PC) species in nodules in both P-replete and P-deficient conditions. Our results reveal heterogeneous distribution of several PC species in nodules, with homogeneous distribution of other PC classes. In P poor conditions, some PC species distributions were observed to change. The results suggest that specific PC species may be differentially important in diverse nodule zones and cell types, and that membrane lipid remodeling during P stress is not uniform across the nodule.

Maximizing growth and productivity of onion (Allium cepa L.) by Spirulina platensis extract and nitrogen-fixing endophyte Pseudomonas stutzeri.

The study focuses on the impact of foliar spraying cyanobacterium Spirulina platensis extract and the inoculation with the endophyte N2-fixing Pseudomonas stutzeri, and their mixture in the presence of different nitrogen doses on growth and yield of onion under field conditions. Bioactive compounds of Spirulina and Pseudomonas were analyzed by GC-MC and amino acid production of Spirulina by the amino acid analyzer. Hydrogen cyanide (HCN), indole acetic acid (IAA), ammonia (NH3), pectinase activity, and N2-fixation of Pseudomonas were measured. Plant height (cm), leaf length (cm), number of green leaves, bulb diameter (cm), fresh and dry weight of plant (g), chlorophyll a, b of leaves, bulb weight (g), marketable bulb yield (t. ha-1), cull bulb weight (t. ha-1), total bulb yield (t. ha-1), bulb diameter (cm), total soluble solids (TSS%), dry matter content (DM%), evaluation of storage behavior, and economic feasibility were estimated. Spirulina extract has several bioactive compounds. Pseudomonas can produce HCN, NH3, IAA, pectinase, and nitrogen fixation. The application of mixture with recommended dose of nitrogen increases the onion plant parameters, marketable yield, total bulb yield, bulb weight, bulb diameter, TSS%, DM%, net return, benefit-cost ratio (B:C), lowest cumulative weight loss% of bulbs during storage, and reduce culls weight compared with other treatments in two seasons. Application of S. platensis extract and inoculation with endophyte nitrogen-fixing P. stutzeri enhance the growth and productivity of the onion under different doses of nitrogen fertilizer.

Endophytic bacteria of garlic roots promote growth of micropropagated meristems.

Garlic (Allium sativum L.) is one of the crops whose economic importance has increased considerably in recent years in Brazil. The use of plant growth-promoting bacteria (PGPB) is a useful alternative for reducing the cost of agricultural inputs and, consequently, for increasing productivity. Therefore, the main objective of this work was to isolate and evaluate potential growth promoters in plants and plant pathogenic fungi growth inhibitors using endophytic bacteria from garlic roots and bacteria from the Agricultural Microbiology Culture Collection at the Federal University of Lavras. Besides verifying improvements in the growth and physiology of garlic meristems grown in vitro under the action of PGPB. Forty-eight endophytic bacteria were identified using matrix-assisted laser desorption/ionization-time of flight mass spectrometry based on the protein profile of each isolate. Four isolates were chosen according to their ability to fix nitrogen, to produce auxin and solubilize phosphate. The cultivation of garlic meristems in tissue culture with these bacteria was established at a population level of 106 CFU/mL. The evaluated criteria were: (1) the colonization capacity of the bacteria inside the garlic plants determined through scanning electron microscopy; (2) the chlorophyll content; and (3) the growth of garlic plants in vitro post-PGPB inoculation. Volatiles emitted by those isolates inhibited fungi growth. The inoculation of garlic meristems with Enterobacter cloacae and Burkholderia cepacia promoted an improvement in the growth and physiological attributes of garlic, indicating the feasibility of their application as plant growth promoters for commercial cultivation.

Diversity and plant growth-promoting functions of diazotrophic/N-scavenging bacteria isolated from the soils and rhizospheres of two species of Solanum.

Studies of the interactions between plants and their microbiome have been conducted worldwide in the search for growth-promoting representative strains for use as biological inputs for agriculture, aiming to achieve more sustainable agriculture practices. With a focus on the isolation of plant growth-promoting (PGP) bacteria with ability to alleviate N stress, representative strains that were found at population densities greater than 104 cells g-1 and that could grow in N-free semisolid media were isolated from soils under different management conditions and from the roots of tomato (Solanum lycopersicum) and lulo (Solanum quitoense) plants that were grown in those soils. A total of 101 bacterial strains were obtained, after which they were phylogenetically categorized and characterized for their basic PGP mechanisms. All strains belonged to the Proteobacteria phylum in the classes Alphaproteobacteria (61% of isolates), Betaproteobacteria (19% of isolates) and Gammaproteobacteria (20% of isolates), with distribution encompassing nine genera, with the predominant genus being Rhizobium (58.4% of isolates). Strains isolated from conventional horticulture (CH) soil composed three bacterial genera, suggesting a lower diversity for the diazotrophs/N scavenger bacterial community than that observed for soils under organic management (ORG) or secondary forest coverture (SF). Conversely, diazotrophs/N scavenger strains from tomato plants grown in CH soil comprised a higher number of bacterial genera than did strains isolated from tomato plants grown in ORG or SF soils. Furthermore, strains isolated from tomato were phylogenetically more diverse than those from lulo. BOX-PCR fingerprinting of all strains revealed a high genetic diversity for several clonal representatives (four Rhizobium species and one Pseudomonas species). Considering the potential PGP mechanisms, 49 strains (48.5% of the total) produced IAA (2.96-193.97 µg IAA mg protein-1), 72 strains (71.3%) solubilized FePO4 (0.40-56.00 mg l-1), 44 strains (43.5%) solubilized AlPO4 (0.62-17.05 mg l-1), and 44 strains produced siderophores (1.06-3.23). Further, 91 isolates (90.1% of total) showed at least one PGP trait, and 68 isolates (67.3%) showed multiple PGP traits. Greenhouse trials using the bacterial collection to inoculate tomato or lulo plants revealed increases in plant biomass (roots, shoots or both plant tissues) elicited by 65 strains (54.5% of the bacterial collection), of which 36 were obtained from the tomato rhizosphere, 15 were obtained from the lulo rhizosphere, and 14 originated from samples of soil that lacked plants. In addition, 18 strains showed positive inoculation effects on both Solanum species, of which 12 were classified as Rhizobium spp. by partial 16S rRNA gene sequencing. Overall, the strategy adopted allowed us to identify the variability in the composition of culturable diazotroph/N-scavenger representatives from soils under different management conditions by using two Solanum species as trap plants. The present results suggest the ability of tomato and lulo plants to enrich their belowground microbiomes with rhizobia representatives and the potential of selected rhizobial strains to promote the growth of Solanum crops under limiting N supply.

N2-fixing black locust intercropping improves ecosystem nutrition at the vulnerable semi-arid Loess Plateau region, China.

The Loess Plateau in northwestern China constitutes one of the most vulnerable semi-arid regions in the world due to long-term decline in forest cover, soil nutrient depletion by agricultural use, and attendant soil erosion. Here, we characterize the significance of N2-fixing Robinia pseudoacacia L. and non-N2-fixing Juglans regia L. for improving nutrient availability and water retention in soil by comparing a range of biological and physicochemical features in monoculture and mixed plantations of both species. We found that N2-fixing Robinia facilitates the nitrogen and phosphorus composition of non-N2-fixing Juglans in the mixed stand as a consequence of improved soil nutrient availability, evident as higher levels of nitrogen and labile carbon compared to mono-specific stands. This demonstrates that intercropping N2-fixing Robinia with non-N2-fixing woody plants can greatly improve soil carbon and nitrogen bioavailability as well as whole-plant nutrition and can potentially mediate water retention with additional sequestration of soil organic carbon in the range of 1 t C ha-1 year-1. Thus, intercropping N2-fixing woody species (e.g. Robinia pseudoacacia or Hippophae rhamnoides L.) with locally important non-N2-fixing tree and shrub species should be considered in afforestation strategies for landscape restoration.

Ascorbic acid formulation for survivability and diazotrophic efficacy of Azotobacter chroococcum Avi2 (MCC 3432) under hydrogen peroxide stress and its role in plant-growth promotion in rice (Oryza sativa L.).

Oxidative stress generates reactive oxygen species which causes cell damage of living organisms and are normally detoxified by antioxidants. Indirect reports signify the damages caused by reactive oxygen species and neutralized by antioxidant, but the direct evidence to confirm this hypothesis is still unclear. To validate our hypothesis, an attempt was made in a diazotrophic bacterium (Azotobacter chroococcum Avi2) as a biological system, and hydrogen peroxide (H2O2) and ascorbic acid were used as oxidative stress and antioxidant supplement, respectively. Additionally, rice plant-growth attributes by Avi2 was also assessed under H2O2 and ascorbic acid. Results indicated that higher concentration of H2O2 (2.5 mM-4.5 mM) showed the complete mortality of Avi2, whereas one ppm ascorbic acid neutralized the effect of H2O2. Turbidity, colony forming unit, DNA quantity, nifH gene abundance, indole acetic acid and ammonia productions were significantly (p < 0.5) increased by 11.93%, 17.29%, 19.80%, 74.77%, 71.89%, and 42.53%, respectively in Avi2-treated with 1.5 mM H2O2 plus ascorbic acid compared to 1.5 mM H2O2 alone. Superoxide dismutase was significantly (p < 0.5) increased by 60.85%, whereas catalase and ascorbate peroxidase activities were significantly (p < 0.05) decreased by 64.28% and 68.88% in Avi2-treated with 1.5 mM H2O2 plus ascorbic acid compared to 1.5 mM H2O2 alone. Germination percentage of three rice cultivars (FR13a, Naveen and Sahbhagi dhan) were significantly (p < 0.5) increased by 20%, 13.33%, and 4%, respectively in Avi2-treated with 0.6 mM H2O2 plus ascorbic acid compared with uninoculated control. Overall, this study indicated that ascorbic acid formulation neutralizes the H2O2-oxidative stress and enhances the survivability and plant growth-promoting efficacy of A. chroococcum Avi2 and therefore, it may be used as an effective formulation of bio-inoculants in rice under oxidative stress.

Nutrient supply controls particulate elemental concentrations and ratios in the low latitude eastern Indian Ocean.

Variation in ocean C:N:P of particulate organic matter (POM) has led to competing hypotheses for the underlying drivers. Each hypothesis predicts C:N:P equally well due to regional co-variance in environmental conditions and biodiversity. The Indian Ocean offers a unique positive temperature and nutrient supply relationship to test these hypotheses. Here we show how elemental concentrations and ratios vary over daily and regional scales. POM concentrations were lowest in the southern gyre, elevated across the equator, and peaked in the Bay of Bengal. Elemental ratios were highest in the gyre, but approached Redfield proportions northwards. As Prochlorococcus dominated the phytoplankton community, biodiversity changes could not explain the elemental variation. Instead, our data supports the nutrient supply hypothesis. Finally, gyre dissolved iron concentrations suggest extensive iron stress, leading to depressed ratios compared to other gyres. We propose a model whereby differences in iron supply and N2-fixation influence C:N:P levels across ocean gyres.

Positive and negative regulation of transferred nif genes mediated by indigenous GlnR in Gram-positive Paenibacillus polymyxa.

Ammonia is a major signal that regulates nitrogen fixation in most diazotrophs. Regulation of nitrogen fixation by ammonia in the Gram-negative diazotrophs is well-characterized. In these bacteria, this regulation occurs mainly at the level of nif (nitrogen fixation) gene transcription, which requires a nif-specific activator, NifA. Although Gram-positive and diazotrophic Paenibacilli have been extensively used as a bacterial fertilizer in agriculture, how nitrogen fixation is regulated in response to nitrogen availability in these bacteria remains unclear. An indigenous GlnR and GlnR/TnrA-binding sites in the promoter region of the nif cluster are conserved in these strains, indicating the role of GlnR as a regulator of nitrogen fixation. In this study, we for the first time reveal that GlnR of Paenibacillus polymyxa WLY78 is essentially required for nif gene transcription under nitrogen limitation, whereas both GlnR and glutamine synthetase (GS) encoded by glnA within glnRA operon are required for repressing nif expression under excess nitrogen. Dimerization of GlnR is necessary for binding of GlnR to DNA. GlnR in P. polymyxa WLY78 exists in a mixture of dimers and monomers. The C-terminal region of GlnR monomer is an autoinhibitory domain that prevents GlnR from binding DNA. Two GlnR-biding sites flank the -35/-10 regions of the nif promoter of the nif operon (nifBHDKENXhesAnifV). The GlnR-binding site Ⅰ (located upstream of -35/-10 regions of the nif promoter) is specially required for activating nif transcription, while GlnR-binding siteⅡ (located downstream of -35/-10 regions of the nif promoter) is for repressing nif expression. Under nitrogen limitation, GlnR dimer binds to GlnR-binding siteⅠ in a weak and transient association way and then activates nif transcription. During excess nitrogen, glutamine binds to and feedback inhibits GS by forming the complex FBI-GS. The FBI-GS interacts with the C-terminal domain of GlnR and stabilizes the binding affinity of GlnR to GlnR-binding site Ⅱ and thus represses nif transcription.

What is the agronomic potential of biofertilizers for maize? A meta-analysis.

Biofertilizers are promoted as a strategy for sustainable intensification of agriculture, but their efficacy varies widely among published studies and it is unclear whether they deliver the promised benefits. Studies are commonly conducted under controlled conditions prior to deployment in the field, yet the predictive value of such studies for field-scale productivity has not been critically examined. A meta-analysis was conducted using a novel host crop-specific approach to evaluate the agronomic potential of bacterial biofertilizers for maize. Yield increases tended to be slightly higher and more variable in greenhouse studies using field soil than in the field, and greenhouse studies poorly predicted the influence of moderating climate, soil and taxonomic variables. We found greater efficacy of Azospirillum spp. and lower efficacy of Bacillus spp. and Enterobacter spp. under field conditions. Surprisingly, biofertilizer strains with confirmed plant-growth-promoting traits such as phosphorus solubilization, nitrogen fixation and phytohormone production in vitro were associated with lower yields in the field than strains not confirmed to possess these traits; only 1-aminocyclopropane-1-carboxylate deaminase synthesis increased yields. These results indicate the need for a novel biofertilizer development framework that integrates information from native soil microbial communities and prioritizes field validation of results.

Seasonal resource conditions favor a summertime increase in North Pacific diatom-diazotroph associations.

In the North Pacific Subtropical Gyre (NPSG), an annual pulse of sinking organic carbon is observed at 4000 m between July and August, driven by large diatoms found in association with nitrogen fixing, heterocystous, cyanobacteria: Diatom-Diazotroph Associations (DDAs). Here we ask what drives the bloom of DDAs and present a simplified trait-based model of subtropical phototroph populations driven by observed, monthly averaged, environmental characteristics. The ratio of resource supply rates favors nitrogen fixation year round. The relative fitness of DDA traits is most competitive in early summer when the mixed layer is shallow, solar irradiance is high, and phosphorus and iron are relatively abundant. Later in the season, as light intensity drops and phosphorus is depleted, the traits of small unicellular diazotrophs become more competitive. The competitive transition happens in August, at the time when the DDA export event occurs. This seasonal dynamic is maintained when embedded in a more complex, global-scale, ecological model, and provides predictions for the extent of the North Pacific DDA bloom. The model provides a parsimonious and testable hypothesis for the stimulation of DDA blooms.

Nitrogen-fixing rhizobial strains isolated from Desmodium incanum DC in Argentina: Phylogeny, biodiversity and symbiotic ability.

Desmodium spp. are leguminous plants belonging to the tribe Desmodieae of the subfamily Papilionoideae. They are widely distributed in temperated and subtropical regions and are used as forage plants, for biological control, and in traditional folk medicine. The genus includes pioneer species that resist the xerothermic environment and grow in arid, barren sites. Desmodium species that form nitrogen-fixing symbiosis with rhizobia play an important role in sustainable agriculture. In Argentina, 23 native species of this genus have been found, including Desmodium incanum. In this study, a total of 64 D. incanum-nodulating rhizobia were obtained from root nodules of four Argentinean plant populations. Rhizobia showed different abiotic-stress tolerances and a remarkable genetic diversity using PCR fingerprinting, with more than 30 different amplification profiles. None of the isolates were found at more than one site, thus indicating a high level of rhizobial diversity associated with D. incanum in Argentinean soils. In selected isolates, 16S rDNA sequencing and whole-cell extract MALDI TOF analysis revealed the presence of isolates related to Bradyrhizobium elkanii, Bradyrhizobium japonicum, Bradyrhizobium yuanmingense, Bradyrhizobium liaoningense, Bradyrhizobium denitrificans and Rhizobium tropici species. In addition, the nodC gene studied in the selected isolates showed different allelic variants. Isolates were phenotypically characterized by assaying their growth under different abiotic stresses. Some of the local isolates were remarkably tolerant to high temperatures, extreme pH and salinity, which are all stressors commonly found in Argentinean soils. One of the isolates showed high tolerance to temperature and extreme pH, and produced higher aerial plant dry weights compared to other inoculated treatments. These results indicated that local isolates could be efficiently used for D. incanum inoculation.