Nitrogen Fixation In Soybean Research Articles

A Nodule-Localized Small Heat Shock Protein GmHSP17.1 Confers Nodule Development and Nitrogen Fixation in Soybean

Small heat shock proteins (sHSPs) are ubiquitous proteins present in all organisms. The sHSPs are not only upregulated under heat shock as well as other stresses but also are expressed in unstressed cells, indicating quite diverse functions of sHSPs. However, there is little known about the role of sHSPs in nodulation and nitrogen fixation in soybean. In this study, we cloned a candidate protein of sHSP, GmHSP17.1, from proteome of nodule and analyzed its function in soybean nodulation. We found that GmHSP17.1 was a cytosolic protein and preferentially expressed during nodule development. An overexpression of GmHSP17.1 in composite transgenic plants showed increases in nodule number, fresh weight, nodule size, area of infection cells, and nitrogenase activity, and subsequently promoted the content of nitrogen and growth of soybean plants. While GmHSP17.1 RNA interference (RNAi) lines showed significantly impaired nodule development and nitrogen fixation efficiency. Through liquid chromatography-tandem mass spectrometry (LC-MS/MS), GmRIP1 was identified as the first potential target of GmHSP17.1, and was shown to be specifically expressed in soybean nodules. The interaction between GmHSP17.1 and GmRIP1 was further confirmed by yeast-two hybrid (Y2H), bimolecular fluorescence complementation (BiFC) in vivo and pull-down assay in vitro. Furthermore, peroxidase activity was markedly increased in GmHSP17.1 overexpressed nodules and decreased in RNAi lines. As a result, the reactive oxygen species (ROS) content greatly decreased in GmHSP17.1 overexpression lines and increased in suppression lines. Taken together, we conclude that GmHSP17.1 plays an important role in soybean nodulation through interacting with GmRIP1. Our results provide foundation for studying the mechanism of nitrogen fixation and for the genetics improvement of legume plants.

Integrated Proteomics and Metabolomics Analysis of Nitrogen System Regulation on Soybean Plant Nodulation and Nitrogen Fixation

The specific mechanisms by which nitrogen affects nodulation and nitrogen fixation in leguminous crops are still unclear. To study the relationship between nitrogen, nodulation and nitrogen fixation in soybeans, dual-root soybean plants with unilateral nodulation were prepared by grafting. At the third trifoliate leaf (V3) to fourth trifoliate leaf (V4) growth stages (for 5 days), nitrogen nutrient solution was added to the non-nodulated side, while nitrogen-free nutrient solution was added to the nodulated side. The experiment was designed to study the effects of exogenous nitrogen on proteins and metabolites in root nodules and provide a theoretical reference for analyzing the physiological mechanisms of the interaction between nitrogen application and nitrogen fixation in soybean root nodules. Compared with no nitrogen treatment, exogenous nitrogen regulated the metabolic pathways of starch and sucrose metabolism, organic acid metabolism, nitrogen metabolism, and amino acid metabolism, among others. Additionally, exogenous nitrogen promoted the synthesis of signaling molecules, including putrescine, nitric oxide, and asparagine in root nodules, and inhibited the transformation of sucrose to malic acid; consequently, the rhizobia lacked energy for nitrogen fixation. In addition, exogenous nitrogen reduced cell wall synthesis in the root nodules, thus inhibiting root nodule growth and nitrogen fixation.

Foliar Application with Iron Oxide Nanomaterials Stimulate Nitrogen Fixation, Yield, and Nutritional Quality of Soybean.

Sustainable strategies for the management of iron deficiency in agriculture are warranted because of the low use efficiency of commercial iron fertilizer, which confounds global food security and induces negative environmental consequences. The impact of foliar application of differently sized γ-Fe2O3 nanomaterials (NMs, 4-15, 8-30, and 40-215 nm) on the growth and physiology of soybean seedlings was investigated at different concentrations (10-100 mg/L). Importantly, the beneficial effects on soybean were size- and concentration-dependent. Foliar application with the smallest size γ-Fe2O3 NMs (S-Fe2O3 NMs, 4-15 nm, 30 mg/L) yielded the greatest growth promotion, significantly increasing the shoot and nodule biomass by 55.4 and 99.0%, respectively, which is 2.0- and 2.6-fold greater than the commercially available iron fertilizer (EDTA-Fe) with equivalent molar Fe. In addition, S-Fe2O3 NMs significantly enhanced soybean nitrogen fixation by 13.2% beyond that of EDTA-Fe. Mechanistically, transcriptomic and metabolomic analyses revealed that (1) S-Fe2O3 NMs increased carbon assimilation in nodules to supply more energy for nitrogen fixation; (2) S-Fe2O3 NMs activated the antioxidative system in nodules, with subsequent elimination of excess reactive oxygen species; (3) S-Fe2O3 NMs up-regulated the synthesis of cytokinin and down-regulated ethylene and jasmonic acid content in nodules, promoting nodule development and delaying nodule senescence. S-Fe2O3 NMs also improved 13.7% of the soybean yield and promoted the nutritional quality (e.g., free amino acid content) of the seeds as compared with EDTA-Fe with an equivalent Fe dose. Our findings demonstrate the significant potential of γ-Fe2O3 NMs as a high-efficiency and sustainable crop fertilizer strategy.

SHORT-ROOT paralogs mediate feedforward regulation of D-type cyclin to promote nodule formation in soybean

Nitrogen fixation in soybean takes place in root nodules that arise from de novo cell divisions in the root cortex. Although several early nodulin genes have been identified, the mechanism behind the stimulation of cortical cell division during nodulation has not been fully resolved. Here we provide evidence that two paralogs of soybean SHORT-ROOT (GmSHR) play vital roles in soybean nodulation. Expression of GmSHR4 and GmSHR5 (GmSHR4/5) is induced in cortical cells at the beginning of nodulation, when the first cell divisions occur. The expression level of GmSHR4/5 is positively associated with cortical cell division and nodulation. Knockdown of GmSHR5 inhibits cell division in outer cortical layers during nodulation. Knockdown of both paralogs disrupts the cell division throughout the cortex, resulting in poorly organized nodule primordia with delayed vascular tissue formation. GmSHR4/5 function by enhancing cytokinin signaling and activating early nodulin genes. Interestingly, D-type cyclins act downstream of GmSHR4/5, and GmSHR4/5 form a feedforward loop regulating D-type cyclins. Overexpression of D-type cyclins in soybean roots also enhanced nodulation. Collectively, we conclude that the GmSHR4/5-mediated pathway represents a vital module that triggers cytokinin signaling and activates D-type cyclins during nodulation in soybean.

Biochar-Induced Soil Available Mo Enhancement to Increase Biological Nitrogen Fixation in Soybean in an Ultisol

Biochar-Induced Soil Available Mo Enhancement to Increase Biological Nitrogen Fixation in Soybean in an Ultisol

Critical role for uricase and xanthine dehydrogenase in soybean nitrogen fixation and nodule development.

De novo purine biosynthesis is required for the incorporation of fixed nitrogen in ureide exporting nodules, as formed on soybean [Glycine max (L.) Merr.] roots. However, in many cases, the enzymes involved in this pathway have been deduced strictly from genome annotations with little direct genetic evidence, such as mutant studies, to confirm their biochemical function or importance to nodule development. While efforts to develop large mutant collections of soybean are underway, research on this plant is still hampered by the inability to obtain mutations in any specific gene of interest. Using a forward genetic approach, as well as CRISPR/Cas9 gene editing via Agrobacterium rhizogenes-mediated hairy root transformation, we identified and characterized the role of GmUOX (Uricase) and GmXDH (Xanthine Dehydrogenase) in nitrogen fixation and nodule development in soybean. The gmuox knockout soybean mutants displayed nitrogen deficiency chlorosis and early nodule senescence, as exemplified by the reduced nitrogenase (acetylene reduction) activity in nodules, the internal greenish-white internal appearance of nodules, and diminished leghemoglobin production. In addition, gmuox1 nodules showed collapsed infected cells with degraded cytoplasm, aggregated bacteroids with no discernable symbiosome membranes, and increased formation of poly-β-hydroxybutyrate granules. Similarly, knockout gmxdh mutant nodules, generated with the CRISPR/Cas9 system, also exhibited early nodule senescence. These genetic studies confirm the critical role of the de novo purine metabolisms pathway not only in the incorporation of fixed nitrogen but also in the successful development of a functional, nitrogen-fixing nodule. Furthermore, these studies demonstrate the great utility of the CRISPR/Cas9 system for studying root-associated gene traits when coupled with hairy root transformation.

Regulation of Phosphorus Supply on Nodulation and Nitrogen Fixation in Soybean Plants with Dual-Root Systems

Phosphorus (P) is an important nutrient affecting nodulation and nitrogen fixation in soybeans. To further investigate the relationship of phosphorus with soybean nodulation and nitrogen fixation, the seedling grafting technique was applied in this study to prepare dual-root soybean systems for a sand culture experiment. From the unfolded cotyledon stage to the initial flowering stage, one side of each dual-root soybean system was irrigated with nutrient solution containing 1 mg/L, 31 mg/L, or 61 mg/L of phosphorus (phosphorus-application side), and the other side was irrigated with a phosphorus-free nutrient solution (phosphorus-free side), to study the effect of local phosphorus supply on nodulation and nitrogen fixation in soybean. The results are described as follows: (1) Increasing the phosphorus supply increased the nodules weight, nitrogenase activity, ureide content, number of bacteroids, number of infected cells, and relative expression levels of nodule nitrogen fixation key genes (GmEXPB2, GmSPX5, nifH, nifD, nifK, GmALN1, GmACP1, GmUR5, GmPUR5, and GmHIUH5) in root nodules on the phosphorus-application side. Although the phosphorus-application and phosphorus-free sides demonstrated similar changing trends, the phosphorus-induced increases were more prominent on the phosphorus-application side, which indicated that phosphorus supply systematically regulates nodulation and nitrogen fixation in soybean. (2) When the level of phosphorus supply was increased from 1 mg/L to 31 mg/L, the increase on the P– side root was significant, and nodule phosphorus content increased by 57.14–85.71% and 68.75–75.00%, respectively; ARA and SNA were 218.64–383.33% and 11.41–16.11%, respectively, while ureide content was 118.18–156.44%. When the level of phosphorus supply was increased from 31mg/L to 61mg/L, the increase in the regulation ability of root and nodule phosphorus content, ARA, SNA, and ureide content were low for roots, and the value for nodules was lower than when the phosphorus level increased from 1 mg/L to 31 mg/L. (3) A high-concentration phosphorus supply on one side of a dual-root soybean plant significantly increased the phosphorus content in the aboveground tissues, as well as the roots and nodules on both sides. In the roots on the phosphorus-free side, the nodules were prioritized for receiving the phosphorus transported from the aboveground tissues to maintain their phosphorus content and functionality.

Effects of maize and soybean intercropping on nodule growth, nitrogen fixation of soybean under low phosphorus condition

Effects of maize and soybean intercropping on nodule growth, nitrogen fixation of soybean under low phosphorus condition

Quantifying country-to-global scale nitrogen fixation for grain legumes: I. Reliance on nitrogen fixation of soybean, groundnut and pulses

Quantifying country-to-global scale nitrogen fixation for grain legumes: I. Reliance on nitrogen fixation of soybean, groundnut and pulses

A small heat shock protein, GmHSP17.9, from nodule confers symbiotic nitrogen fixation and seed yield in soybean

SummaryLegume–rhizobia symbiosis enables biological nitrogen fixation to improve crop production for sustainable agriculture. Small heat shock proteins (sHSPs) are involved in multiple environmental stresses and plant development processes. However, the role of sHSPs in nodule development in soybean remains largely unknown. In the present study, we identified a nodule‐localized sHSP, called GmHSP17.9, in soybean, which was markedly up‐regulated during nodule development. GmHSP17.9 was specifically expressed in the infected regions of the nodules. GmHSP17.9 overexpression and RNAi in transgenic composite plants and loss of function in CRISPR‐Cas9 gene‐editing mutant plants in soybean resulted in remarkable alterations in nodule number, nodule fresh weight, nitrogenase activity, contents of poly β‐hydroxybutyrate bodies (PHBs), ureide and total nitrogen content, which caused significant changes in plant growth and seed yield. GmHSP17.9 was also found to act as a chaperone for its interacting partner, GmNOD100, a sucrose synthase in soybean nodules which was also preferentially expressed in the infected zone of nodules, similar to GmHSP17.9. Functional analysis of GmNOD100 in composite transgenic plants revealed that GmNOD100 played an essential role in soybean nodulation. The hsp17.9 lines showed markedly more reduced sucrose synthase activity, lower contents of UDP‐glucose and acetyl coenzyme A (acetyl‐CoA), and decreased activity of succinic dehydrogenase (SDH) in the tricarboxylic acid (TCA) cycle in nodules due to the missing interaction with GmNOD100. Our findings reveal an important role and an unprecedented molecular mechanism of sHSPs in nodule development and nitrogen fixation in soybean.

An Overview of the Morphological, Genetic and Metabolic Mechanisms Regulating Phosphorus Efficiency Via Root Traits in Soybean

Phosphorus (P) is an essential macronutrient for optimum productivity of several crops including soybean, but it is largely limited in most arable lands. The persistent depletion of phosphate rocks is a major constraint for plant growth and development. Improved P efficiency (PE, uptake and use efficiency) drives an enhanced biological nitrogen fixation in soybean, which could consequently lead to an increased grain and biomass production. Thus, to mitigate P limitation in soybean production, a wide range of genetic approaches have been deployed. These approaches have unravelled the morphological, genetic, phytohormonal and metabolic mechanisms involved in soybean adaptations to P-deficient conditions. Most of the approaches are targeted at root modifications due to the importance of roots in the mediation of early responses to P-deficient conditions. PE among soybean genotypes in P-deficient conditions is mostly dependent on the root plasticity, development of shallow root architecture, symbiosis with arbuscular mycorrhizal fungi and exudation of phosphatases and organic acid anions. Genetic manipulation of soybean has revealed a number of important genes (GmPAPs, GmPTs, GmPLDZ2, GmIPS1 and GmExPB2), transcription factors (C2H2 zinc finger protein, WRKY and MYB) and quantitative trait loci (q14-2 and q19-2) modulating P mobilisation, uptake and utilization under P limiting conditions. Through the modification of root traits, the hormonal (biosynthesis, transport and secretion of ethylene, auxin and jasmonic acid) and metabolic (phenylpropanoid biosynthesis and phenylalanine metabolisms) pathways also modulate PE in soybean. Recent advances in genetic manipulations of root traits offer promising ways for enhancing soybean production, particularly in a P limiting environment.

Biochar can improve biological nitrogen fixation by altering the root growth strategy of soybean in Albic soil

Albic soil is a low-yielding soil that is widely distributed in Northeast China. The high viscosity and acidity and the lack of nutrients in the Albic layer limit the growth of crop. In our previous studies, we found that applying biochar as a soil amendment could improve the properties of Albic soil and promote soybean growth. Increases in the nitrogen contents of the soil and the soybeans were key aspects of these improvements. Soybean is a nitrogen-fixing crop, the increase in nitrogen in the Albic soil may have been due to an improvement in biological nitrogen fixation by the soybean with biochar amendment, but the function mechanism was still uncertain. We hypothesized that biochar could improve biological nitrogen fixation of soybean by affecting soybean root growth in the Albic soil. Therefore, we conducted pot experiments with five treatment levels (0, 10, 20, 30, and 40 g·kg−1 biochar) for two years to study how biochar affects the root growth strategy and biological nitrogen fixation of soybean based on its root structure and root nutrient acquisition ability at different stages. The soybean root structure and activity indexes, nodulation ability and nitrogen uptake were measured at different growth stages; in the second year, at the late seed-filling stage, the stable 15N isotope method was used to elucidate the biological nitrogen fixation process. Regarding root structure at the pod-setting stage, biochar resulted in increases in root length density, specific root length, root diameter and specific tip density but a decrease in root tissue mass density at the pod-setting stage. Biochar improved root nutrient acquisition by increasing root activity, root tip number and root-bleeding sap amount. The change in root growth strategy contributed to the promotion of biological nitrogen fixation by the rhizobia that live symbiotically with soybean, thereby increasing crop yield.

Differential RNA Editing and Intron Splicing in Soybean Mitochondria during Nodulation.

Nitrogen fixation in soybean consumes a tremendous amount of energy, leading to substantial differences in energy metabolism and mitochondrial activities between nodules and uninoculated roots. While C-to-U RNA editing and intron splicing of mitochondrial transcripts are common in plant species, their roles in relation to nodule functions are still elusive. In this study, we performed RNA-seq to compare transcript profiles and RNA editing of mitochondrial genes in soybean nodules and roots. A total of 631 RNA editing sites were identified on mitochondrial transcripts, with 12% or 74 sites differentially edited among the transcripts isolated from nodules, stripped roots, and uninoculated roots. Eight out of these 74 differentially edited sites are located on the matR transcript, of which the degrees of RNA editing were the highest in the nodule sample. The degree of mitochondrial intron splicing was also examined. The splicing efficiencies of several introns in nodules and stripped roots were higher than in uninoculated roots. These include nad1 introns 2/3/4, nad4 intron 3, nad5 introns 2/3, cox2 intron 1, and ccmFc intron 1. A greater splicing efficiency of nad4 intron 1, a higher NAD4 protein abundance, and a reduction in supercomplex I + III2 were also observed in nodules, although the causal relationship between these observations requires further investigation.

Nodulation and biological nitrogen fixation in soybean (Glycine max L.) as influenced by phosphorus fertilization and arbuscular mycorrhizal inoculation

Abstract Arbuscular mycorrhizal fungi (AMF) can be used to promote the productivity of legumes on phosphorus- (P) deficient soils. The present study investigates the inoculation effects of three AMF species (Funneliformis mosseae, Rhizophagus intraradices, and Claroideoglomus etunicatum) and the control (uninoculated) on nitrogen fixation efficiency and growth performance of tropical soybean cultivar (TGx1448-2E) under varying P fertilizer rates (0, 20, and 40 kg P2O5 ha−1) in a derived savannah of Nigeria. The results showed that shoot and root dry matter, number of nodules, relative ureide abundance (RUA), nitrogen derived from atmosphere (Ndfa), total N fixed, shoot P uptake, grain, and biomass yield significantly increased with AMF inoculation, with better performance observed in plants inoculated with Rhizophagus intraradices and Funneliformis mosseae compared to Claroideoglomus etunicatum. Similarly, the soybean growth variables, P uptake, and nitrogen fixation activities increased with increasing P application rates. Conversely, AMF root colonization significantly reduced with increasing P rate. Interaction of AMF inoculation and P rates significantly influenced soybean growth and nitrogen fixation. R. intraradices inoculation with 20 kg P2O5 ha−1 resulted in the highest amount of RUA, Ndfa, N fixed, and grain yield. It could be concluded from this study that R. intraradices with moderate P rate could be used to enhance nodulation, nitrogen fixation, and soybean yield in P-deficient soils.

A VIT‐like transporter facilitates iron transport into nodule symbiosomes for nitrogen fixation in soybean

Effective legume-rhizobia symbiosis depends on efficient nutrient exchange. Rhizobia need to synthesize iron-containing proteins for symbiotic nitrogen fixation (SNF) in nodules, which depends on host plant-mediated iron uptake into the symbiosome. We functionally investigated a pair of vacuolar iron transporter like (VTL) genes, GmVTL1a/b, in soybean (Glycine max) and evaluated their contributions to SNF, including investigations of gene expression patterns, subcellular localization, and mutant phenotypes. Though both GmVTL1a/b genes were specifically expressed in the fixation zone of the nodule, GmVTL1a was the lone member to be localized at the tonoplast of tobacco protoplasts, and shown to facilitate ferrous iron transport in yeast. GmVTL1a targets the symbiosome in infected cells, as verified by in situ immunostaining. Two vtl1 knockout mutants had lower iron concentrations in nodule cell sap and peribacteroid units than in wild-type plants, suggesting that GmVTL1 knockout inhibited iron import into symbiosomes. Furthermore, GmVTL1 knockout minimally affected soybean growth under nonsymbiotic conditions, but dramatically impaired nodule development and SNF activity under nitrogen-limited and rhizobia-inoculation conditions, which eventually led to growth retardation. Taken together, these results demonstrate that GmVTL1a is indispensable for SNF in nodules as a transporter of ferrous iron from the infected root cell cytosol to the symbiosome.

The Hemoglobin Bjgb From Bradyrhizobium diazoefficiens Controls NO Homeostasis in Soybean Nodules to Protect Symbiotic Nitrogen Fixation.

Legume-rhizobia symbiotic associations have beneficial effects on food security and nutrition, health and climate change. Hypoxia induced by flooding produces nitric oxide (NO) in nodules from soybean plants cultivated in nitrate-containing soils. As NO is a strong inhibitor of nitrogenase expression and activity, this negatively impacts symbiotic nitrogen fixation in soybean and limits crop production. In Bradyrhizobium diazoefficiens, denitrification is the main process involved in NO formation by soybean flooded nodules. In addition to denitrification, nitrate assimilation is another source of NO in free-living B. diazoefficiens cells and a single domain hemoglobin (Bjgb) has been shown to have a role in NO detoxification during nitrate-dependent growth. However, the involvement of Bjgb in protecting nitrogenase against NO in soybean nodules remains unclear. In this work, we have investigated the effect of inoculation of soybean plants with a bjgb mutant on biological nitrogen fixation. By analyzing the proportion of N in shoots derived from N2-fixation using the 15N isotope dilution technique, we found that plants inoculated with the bjgb mutant strain had higher tolerance to flooding than those inoculated with the parental strain. Similarly, reduction of nitrogenase activity and nifH expression by flooding was less pronounced in bjgb than in WT nodules. These beneficial effects are probably due to the reduction of NO accumulation in bjgb flooded nodules compared to the wild-type nodules. This decrease is caused by an induction of expression and activity of the denitrifying NO reductase enzyme in bjgb bacteroids. As bjgb deficiency promotes NO-tolerance, the negative effect of NO on nitrogenase is partially prevented and thus demonstrates that inoculation of soybean plants with the B. diazoefficiens bjgb mutant confers protection of symbiotic nitrogen fixation during flooding.

Dynamics of symbiotic relationship of soybean with Bradyrhizobium diazoefficiens and involvements of root-secreted daidzein behind the continuous cropping

Symbiosis with a selectively nodulating rhizobium strain, such as Bradyrhizobium diazoefficiens USDA110, improves soybean productivity. In continuous cropping, however, the concentrations of root exudates – of which daidzein is one of the major secretions – might influence nodulating rhizobium communities and cause ineffective nitrogen fixation in soybean. The dynamics of indigenous rhizobia that form nodules with the soybean cultivars Tamahomare, Enrei and Tamabaguro at three cycles of a continuous cropping plot under different experimental conditions were examined. PCR-RFLP analysis targeted to the 16S-23S rRNA gene internal transcribed spacer (ITS) region of a bacterium, confirmed by sequencing analysis, was used to determine the bacterial type for each root nodule. The results showed that as the number of cycles increased, the proportion of B. diazoefficiens USDA110 of all cultivars decreased. Under all experimental conditions, the average differences between the proportion of B. diazoefficiens USDA110 of the first and third cycles in Tamahomare, Enrei and Tamabaguro were 67%, 70% and 44%, respectively. After the HPLC analysis of the root-secreted isoflavonoids, the results showed that the root-secreted daidzein from Tambaguro was excreted in the highest levels among the three cultivars. Increase of the proportion of B. diazoefficiens USDA110 among seedlings treated with daidzein was also noted. Therefore, the present study reports that the symbiotic relationship of soybean with B. diazoefficiens USDA110 decreases in continuous cropping and suggests that root-secreted daidzein helps to maintain populations of this rhizobial strain in rhizosphere of some cultivars.

Nutritional and environmental effects on biological nitrogen fixation in soybean: A meta-analysis

Biological N2 fixation (BNF) and mineral soil absorption are complementary N sources for soybean growth. BNF is sensitive to nutrient and environmental conditions, though a comprehensive synthesis of the studies underlying these conclusions is currently lacking. Our objective was to conduct a meta-analysis of nutrient and environmental effects on BFN. Articles reporting manipulative treatments and BNF estimations were compiled, and data and metadata were extracted. N fertilization reduced BNF ∼70% relative to the unfertilized control in greenhouse experiments and ∼44% in field experiments. This effect was higher for vegetative than for reproductive stage applications. Fertilization with other nutrients stimulated BNF relative to the unfertilized treatments. Water stress reduced BNF 40% relative to the unstressed control. The negative impact of water stress was larger when stress was applied at vegetative (-70%) compared to reproductive (-30%) stages. Flooding reduced BNF by 40% relative to the non-flooded control with highest effect when applied during vegetative stage (-82%). Increased temperature reduced BNF nitrogenase activity. Carbon dioxide enrichment has a stimulatory effect on BNF. With the exception of N fertilization, soybean and Rhizobium fitness showed a high positive correlation across nutrients and environmental factors. This work summarizes for the first time the impact and relative response of BNF to different nutrient and environmental factors. Results showed an initial critical stage (i.e. vegetative stage) for BNF that is common to most environmental factors negatively affecting BNF. This suggests that common strategies might exist to increase BNF that are independent of the specific identity of the stressor. Also, the positive correlation between host and Rhizobium fitness suggest that strategies to increase BNF potential are the same that those required to increase crop performance. We observed publication bias for some variables, suggesting that effect sizes might be inflated compared to true effect sizes. The only way to solve this problem is to find venues for publication of no significant results when sound experimental approaches and reasonable statistical power can be proved.

Interplay between nitrogen fertilizer and biological nitrogen fixation in soybean: implications on seed yield and biomass allocation

Legumes rely on soil mineral nitrogen (N) and biological N fixation (BNF). The interplay between these two sources is biologically interesting and agronomically relevant as the crop can accommodate the cost of BNF by five non-mutually exclusive mechanisms, whereby BNF: reduces shoot growth and seed yield, or maintains shoot growth and seed yield by enhanced photosynthesis, or reduced root:shoot ratio, or maintains shoot growth but reduces seed yield by reducing the fraction of shoot biomass allocated to seed (harvest index), or reducing concentration of oil and protein in seed. We explore the impact of N application on the seasonal dynamics of BNF, and its consequences for seed yield with emphasis on growth and shoot allocation mechanisms. Trials were established in 23 locations across the US Midwest under four N conditions. Fertilizer reduced the peak of BNF up to 16% in applications at the full flowering stage. Seed yield declined 13 kg ha−1 per % increase in RAUR6. Harvest index accounted for the decline in seed yield with increasing BNF. This indicates the cost of BNF was met by a relative change in dry matter allocation against the energetically rich seed, and in favor of energetically cheaper vegetative tissue.

Characterization of Genetic Basis on Synergistic Interactions between Root Architecture and Biological Nitrogen Fixation in Soybean.

Soybean [Glycine max (L.) Merr] is an important legume crop and its yield largely depends on root architecture (RA) and biological nitrogen fixation (BNF). However, the relationship between RA and BNF, and its genetics behind remain unclear. Here, two soybean genotypes contrasting in RA and their 175 F9:11 recombinant inbred lines (RILs) were evaluated in field. The shallow-root parent, JD12, had better nodulation and higher yield than the deep-root parent, NF58. Strong correlations between shoot dry weight (SDW) and RA or BNF traits existed in the RILs, and the shallow-root group had more and heavier nodules, as well as higher SDW. After inoculating with rhizobia, roots became shallower and bigger, showing strong synergistic interactions between RA and BNF. In total, 70 QTLs were identified for the 21 tested traits. Among them, qBNF-RA-C2, qBNF-RA-O, and qBNF-RA-B1, were newly identified QTLs for BNF and/or RA traits in soybean, which co-located with the QTLs for SDW detected presently, and with the QTLs for yield identified previously. The results together suggest that there are synergistic interactions between RA and BNF, and the QTLs identified here could be used for breeding new soybean varieties with higher yields through optimization of RA traits and BNF capacity.