Symbiotic N2 Fixation Research Articles

English

It has been demonstrated that symbiotic N2 fixation enhances soil fertility and productivity as well as increases carbon sequestration and nutrient conservation. Green house experiments were conducted to ascertain the effect of garlic extract on the nodulation of cowpea and groundnut plants. Different concentrations of the extract (20, 40, 60 and 80%), were applied to the plant at 2 weeks after planting. Both legumes were adversely affected by the garlic extract, but the effect was more pronounced on the groundnut plants. Before application of the garlic extract to the soil, the soil pH was 6.5, phosphorus 8.3 mol/kg, calcium 2.51 mol/kg, potassium 0.45 mol/kg, and nitrogen 0.295 mol/kg, and these make the soil fertile enough for nodulation. However, after garlic extract application, there was reduced nodulation in the roots and a marked drop in plant height, leaf area and root development in comparison to the control. This adverse effect was more with increasing concentration of the extract.  It is recommended, that land for cowpea or groundnut product remains free of garlic and members of the families Allium and Cyperus plants. They should not be left in the soil or turned over in it, during land clearing and should be grown in inter-crop.  This is because their exudates during decomposition or growth could have inhibitory effects on the growth and nodulation of legumes.    Keywords: Garlic extract, nodulation, legumes, nitrogen fixation.

Demography of Symbiotic Nitrogen-Fixing Trees Explains Their Rarity and Successional Decline in Temperate Forests in the United States.

Symbiotic nitrogen (N) fixation is the major N input to many ecosystems. Although temperate forests are commonly N limited, symbiotic N-fixing trees (“N fixers”) are rare and decline in abundance as succession proceeds–a challenging paradox that remains unexplained. Understanding demographic processes that underlie N fixers’ rarity and successional decline would provide a proximate answer to the paradox. Do N fixers grow slower, die more frequently, or recruit less in temperate forests? We quantified demographic rates of N-fixing and non-fixing trees across succession using U.S. forest inventory data. We used an individual-based model to evaluate the relative contribution of each demographic process to community dynamics. Compared to non-fixers, N fixers had lower growth rates, higher mortality rates, and lower recruitment rates throughout succession. The mortality effect contributed more than the growth effect to N fixers’ successional decline. Canopy and understory N fixers experienced these demographic disadvantages, indicating that factors in addition to light limitation likely contribute to N fixers’ successional decline. We show that the rarity and successional decline of N-fixing trees in temperate forests is due more to their survival disadvantage than their growth disadvantage, and a recruitment disadvantage might also play a large role.

NITROGEN FIXATION OF GRAIN LEGUMES DIFFERS IN RESPONSE TO NITROGEN FERTILISATION

SUMMARYLegume crops are not usually fertilised with mineral N. However, there are at least two agronomic cases when it would be advantageous to distribute N fertiliser to legume crops: at sowing, before the onset of nodule functioning, and when a legume is intercropped with a cereal. We highlight the impact of various levels of fertiliser nitrogen on grain yield, nodulation capacity and biological nitrogen fixation in the four most common grain legume crops grown in central Italy. Chickpea (Cicer arietinum L.), field bean (Vicia faba L. var. minor), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) were grown in soil inside growth boxes for two cropping seasons with five nitrogen fertilisation rates: 0, 40, 80, 120 and 160 kg ha−1. In both years, experimental treatments (five crops and five levels of N) were arranged in a randomised block design. We found that unfertilised plants overall yielded grain, total biomass and nitrogen at a similar level to plants supplied with 80–120 kg ha−1 of mineral nitrogen. However, above those N rates, the production of chickpea, pea and white lupin decreased, thus indicating that the high supply of N fertiliser decreased the level of N2 fixed to such an extent that the full N2-fixing potential might not be achieved. In all four grain legumes, the amount of N2 fixed was positively related to nodule biomass, which was inversely related to the rate of the N fertiliser applied. The four grain legumes studied responded differently to N fertilisation: in white lupin and chickpea, the amount of nitrogen derived from N2 fixation linearly decreased with increasing N supply as a result of a reduction in nodulation and N2 fixed per unit mass of nodules. Conversely, in field bean and pea, the decrease in N2 fixation was only due to a reduction in nodule biomass since nodule fixation activity increased with N supply. Our results suggest that the legume species and the N rate are critical factors in determining symbiotic N2-fixation responses to N fertilisation.

Symbiotic nitrogen fixation and interspecific transfer by Caragana microphylla in a temperate grassland with 15N dilution technique

Quantification of symbiotic nitrogen (N) fixation by legume and its N transfer to the neighboring plants is an important prerequisite to understand the N cycling in the N-limited grasslands. The objective of this study is to explore the symbiotic N2-fixation by a legume shrub (Caragana microphylla) and its N transfer to coexisting non-legumes with the 15N dilution technique. A paired experimental design (legume vs. non-legume quadrats) was used. A small amount (0.03gNm−2) of 15N-labeled KNO3 (99.5% 15N enrichment) was evenly distributed in these paired labeled quadrats, with only water additions as the control. One month after the 15N addition, aboveground parts of C. microphylla and four non-legumes of reference plants (i.e. Cleistogenes squarrosa, Artemisia frigida, Artemisia capillaris and Leymus chinensis) in both labeled and controlled quadrats were harvested. Symbiotic N2-fixation and transfer to non-legumes were estimated based on the aboveground biomass and its 15N in these plant species. The N derived from the atmosphere (%Ndfa) of C. microphylla was estimated to range from 84 to 95%. According to the aboveground biomass of C. microphylla, symbiotic N2-fixation was estimated to be 4.3gm−2yr−1 in the mixed C. microphylla community in the temperate grassland of northern China. The amount of N transfer from C. microphylla to their coexisting plants varied with plant species. About half of above N in C. squarrosa derived from C. microphylla, while L. chinensis showed almost no N derived from C. microphylla. Both A. frigida and A. capillaris showed about 36% and 37% of N derived from C. microphylla, respectively. Totally, about 17% of N fixed in the aboveground part of C. microphylla was transferred to its associated species. Considering the amount of N fixed and transferred by C. microphylla, we conclude that C. microphylla plays an important role in the N cycling of temperate grasslands in northern China.

Isoscapes resolve species-specific spatial patterns in plant-plant interactions in an invaded Mediterranean dune ecosystem.

Environmental heterogeneity and plant-plant interactions are key factors shaping plant communities. However, the spatial dimension of plant-plant interactions has seldom been addressed in field studies. This is at least partially rooted in a lack of methods that can accurately resolve functional processes in a spatially explicit manner. Isoscapes, that is, spatially explicit representations of stable isotope data, provide a versatile means to trace functional changes on spatial scales, for example, related to N-cycling (foliar δ15N) and water use efficiency (WUEi, foliar δ13C). In a case study in a nutrient-depleted Mediterranean dune ecosystem, we analysed the spatial impact of the invasive N2-fixing Acacia longifolia on three native species of different functional types using δ15N and δ13C isoscapes and spatial autocorrelation analyses. Isoscapes revealed strong spatial patterns in δ15N and δ13C with pronounced species-specific differences, demonstrating distinct spatial ranges of plant-plant interactions. A coniferous tree and an ericaceous dwarf shrub showed significant enrichment in δ15N within a range of 5-8 m surrounding the canopy of A. longifolia, indicating input of N originating from symbiotic N2-fixation by the invader. In the dwarf shrub, which was most responsive to invader influence, enrichment in δ13C additionally demonstrated spatially explicit changes to WUEi, while a native N2-fixer was unresponsive to the presence of the invader. Furthermore, δ15N and δ13C isoscapes yielded different patterns, indicating that plant-plant interactions can have distinct spatial distributions and ranges based on the process measured. Additionally, the magnitude of the effect differed between field situations with high and low invasion pressure. This study highlights that the spatial scale must be accounted for when assessing the effects and outcome of species interactions. Functional tracers such as stable isotopes enable us to quantify spatial ranges of plant-plant interactions, providing empirical data that can help to better understand and predict complex species interactions in multifaceted natural environments.

Increasing Nitrogen Fixation and Seed Development in Soybean Requires Complex Adjustments of Nodule Nitrogen Metabolism and Partitioning Processes

Legumes are able to access atmospheric di-nitrogen (N2) through a symbiotic relationship with rhizobia that reside within root nodules. In soybean, following N2 fixation by the bacteroids, ammonia is finally reduced in uninfected cells to allantoin and allantoic acid [1]. These ureides present the primary long-distance transport forms of nitrogen (N), and are exported from nodules via the xylem for shoot N supply. Transport of allantoin and allantoic acid out of nodules requires the function of ureide permeases (UPS1) located in cells adjacent to the vasculature [2, 3]. We expressed a common bean UPS1 transporter in cortex and endodermis cells of soybean nodules and found that delivery of N from nodules to shoot, as well as seed set, was significantly increased. In addition, the number of transgenic nodules was increased and symbiotic N2 fixation per nodule was elevated, indicating that transporter function in nodule N export is a limiting step in bacterial N acquisition. Further, the transgenic nodules showed considerable increases in nodule N assimilation, ureide synthesis, and metabolite levels. This suggests complex adjustments of nodule N metabolism and partitioning processes in support of symbiotic N2 fixation. We propose that the transgenic UPS1 plants display metabolic and allocation plasticity to overcome N2 fixation and seed yield limitations. Overall, it is demonstrated that transporter function in N export from nodules is a key step for enhancing atmospheric N2 fixation and nodule function and for improving shoot N nutrition and seed development in legumes.

Soil organic carbon stocks in estuarine and marine mangrove ecosystems are driven by nutrient colimitation of P and N.

Mangroves play an important role in carbon sequestration, but soil organic carbon (SOC) stocks differ between marine and estuarine mangroves, suggesting differing processes and drivers of SOC accumulation. Here, we compared undegraded and degraded marine and estuarine mangroves in a regional approach across the Indonesian archipelago for their SOC stocks and evaluated possible drivers imposed by nutrient limitations along the land‐to‐sea gradients. SOC stocks in natural marine mangroves (271–572 Mg ha−1 m−1) were much higher than under estuarine mangroves (100–315 Mg ha−1 m−1) with a further decrease caused by degradation to 80–132 Mg ha−1 m−1. Soils differed in C/N ratio (marine: 29–64; estuarine: 9–28), δ 15N (marine: −0.6 to 0.7‰; estuarine: 2.5 to 7.2‰), and plant‐available P (marine: 2.3–6.3 mg kg−1; estuarine: 0.16–1.8 mg kg−1). We found N and P supply of sea‐oriented mangroves primarily met by dominating symbiotic N2 fixation from air and P import from sea, while mangroves on the landward gradient increasingly covered their demand in N and P from allochthonous sources and SOM recycling. Pioneer plants favored by degradation further increased nutrient recycling from soil resulting in smaller SOC stocks in the topsoil. These processes explained the differences in SOC stocks along the land‐to‐sea gradient in each mangrove type as well as the SOC stock differences observed between estuarine and marine mangrove ecosystems. This first large‐scale evaluation of drivers of SOC stocks under mangroves thus suggests a continuum in mangrove functioning across scales and ecotypes and additionally provides viable proxies for carbon stock estimations in PES or REDD schemes.

Response of determinate and indeterminate soybean cultivars to basal and topdressing N fertilization compared to sole inoculation with Bradyrhizobium

Soybean (Glycine max (L.) Merrill) new cultivars have high N demands to sustain increasing grain yields. One may ask whether these cultivars would need basal and/or topdressing N application as perhaps symbiotic N2 fixation would not catch up with the increased N needs. In this study, three field experiments were performed in three different ecoregions of Brazil to compare soybean with determinate (BRS-294-RR and BRS-295-RR) and indeterminate (Vmax-RR, BMX-Potência-RR and BRS 284) growth types. The experiments consisted of four treatments, all inoculated with efficient Bradyrhizobium strains, but with differences in N fertilizer (as urea) application: (T1) control without N fertilizer; (T2) basal application of 30kgNha−1 at sowing; (T3) topdressing of 50kgNha−1 at full flowering; (T4) 30kgNha−1 and 50kgNha−1 as basal and topdressing, respectively. None of the basal or topdressing treatments improved biomass production or grain yield. However, N fertilizer negatively affected nodule number and dry weight, pointing out a down-regulation in the symbiotic performance, without improving crop yields. Similar results were observed in cultivars of both growth types, indicating that biological nitrogen fixation can supply N needs of new soybean genotypes of both growth types.

Root exudates drive interspecific facilitation by enhancing nodulation and N2 fixation

Plant diversity in experimental systems often enhances ecosystem productivity, but the mechanisms causing this overyielding are only partly understood. Intercropping faba beans (Vicia faba L.) and maize (Zea mays L.) result in overyielding and also, enhanced nodulation by faba beans. By using permeable and impermeable root barriers in a 2-y field experiment, we show that root-root interactions between faba bean and maize significantly increase both nodulation and symbiotic N2 fixation in intercropped faba bean. Furthermore, root exudates from maize promote faba bean nodulation, whereas root exudates from wheat and barley do not. Thus, a decline of soil nitrate concentrations caused by intercropped cereals is not the sole mechanism for maize promoting faba bean nodulation. Intercropped maize also caused a twofold increase in exudation of flavonoids (signaling compounds for rhizobia) in the systems. Roots of faba bean treated with maize root exudates exhibited an immediate 11-fold increase in the expression of chalcone-flavanone isomerase (involved in flavonoid synthesis) gene together with a significantly increased expression of genes mediating nodulation and auxin response. After 35 d, faba beans treated with maize root exudate continued to show up-regulation of key nodulation genes, such as early nodulin 93 (ENOD93), and promoted nitrogen fixation. Our results reveal a mechanism for how intercropped maize promotes nitrogen fixation of faba bean, where maize root exudates promote flavonoid synthesis in faba bean, increase nodulation, and stimulate nitrogen fixation after enhanced gene expression. These results indicate facilitative root-root interactions and provide a mechanism for a positive relationship between species diversity and ecosystem productivity.

Facultative nitrogen fixation by legumes in the central Congo basin is downregulated during late successional stages

AbstractThe contribution of the legume community to the nitrogen cycle during natural forest regeneration remains poorly understood. We systematically assessed the changes in abundance and nodulation status of all legumes, across taxa and plant types, in a forest succession gradient in the Equateur Province of the Democratic Republic of the Congo. Our results clearly show that symbiotic N2 fixation is downregulated during late successional stages.

Complementarity among four highly productive grassland species depends on resource availability.

Positive species richness-productivity relationships are common in biodiversity experiments, but how resource availability modifies biodiversity effects in grass-legume mixtures composed of highly productive species is yet to be explicitly tested. We addressed this question by choosing two grasses (Arrhenatherum elatius and Dactylis glomerata) and two legumes (Medicago × varia and Onobrychis viciifolia) which are highly productive in monocultures and dominant in mixtures (the Jena Experiment). We established monocultures, all possible two- and three-species mixtures, and the four-species mixture under three different resource supply conditions (control, fertilization, and shading). Compared to the control, community biomass production decreased under shading (-56%) and increased under fertilization (+12%). Net diversity effects (i.e., mixture minus mean monoculture biomass) were positive in the control and under shading (on average +15 and +72%, respectively) and negative under fertilization (-10%). Positive complementarity effects in the control suggested resource partitioning and facilitation of growth through symbiotic N2 fixation by legumes. Positive complementarity effects under shading indicated that resource partitioning is also possible when growth is carbon-limited. Negative complementarity effects under fertilization suggested that external nutrient supply depressed facilitative grass-legume interactions due to increased competition for light. Selection effects, which quantify the dominance of species with particularly high monoculture biomasses in the mixture, were generally small compared to complementarity effects, and indicated that these species had comparable competitive strengths in the mixture. Our study shows that resource availability has a strong impact on the occurrence of positive diversity effects among tall and highly productive grass and legume species.

Evolution of root nodule bacteria: Reconstruction of the speciation processes resulting from genomic rearrangements in a symbiotic system

The processes of speciation and macroevolution of root nodule bacteria (rhizobia), based on deep rearrangements of their genomes and occurring in the N₂-fixing symbiotic system, are reconstructed. At the first stage of rhizobial evolution, transformation of free-living diazotrophs (related to Rhodopseudomonas) to symbiotic N₂-fixers (Bradyrhizobium) occurred due to the acquisition of the fix gene system, which is responsible for providing nitrogenase with electrons and reducing equivalents, as well as for oxygen-dependent regulation of nitrogenase synthesis in planta, and then of the nod genes responsible for the synthesis of the lipo- chito-oligosaccharide Nod factors, which induce root nodule development. The subsequent rearrangements of bacterial genomes included: (1) increased volume of hereditary information supported by species, genera (pan-genome), and individual strains; (2) transition from the unitary genome to a multicomponent one; and (3) enhanced levels of bacterial genetic plasticity and horizontal gene transfer, resulting in formation of new genera, of which Mesorhizobium, Rhizobium, and Sinorhizobium are the largest, and of over 100 species. Rhizobial evolution caused by development and diversification of the Nod factor synthesizing systems may result in both increased host specificity range (transition of Bradyrhizobium from autotrophic to symbiotrophic carbon metabolism in interaction with a broad spectrum of legumes) and to its contraction (transition of Rhizobium and Sinorhizobium to "altruistic" interaction with legumes of the galegoid clade). Reconstruction of the evolutionary pathway from symbiotic N₂-fixers to their free-living ancestors makes it possible to initiate the studies based on up-to-date genome screening technologies and aimed at the issues of genetic integration of organisms into supracpecies complexes, ratios of the macro- and microevolutionary mechanisms, and developmetn of cooperative adaptations based on altruistic relationship between the symbiotic partners.

Nitrogen and phosphorus availability at early stages of soil development in the Damma glacier forefield, Switzerland; implications for establishment of N2-fixing plants

Di-nitrogen (N2)-fixing plants are absent in the pioneer stages of glacial forefields in the European Alps despite low amounts of nitrogen (N) in the soil. We aimed to evaluate whether symbiotic N2-fixation is needed to meet the N demand of plants during the early stages of soil formation, and how phosphorus (P) availability affects plant establishment. We measured total and available N and P in soil and N and P in the vegetation along the 137 year chronosequence in front of the Damma glacier (Switzerland). Available N as determined by in situ resin N decreased from the pioneer stage (<16 years since deglaciation) to the intermediate (57–80 years), likely resulting from increased plant N uptake and reduced funnelling of N by rocks. N concentrations in the vegetation were positively correlated to in situ resin N, and the N:P-ratio of the vegetation was negatively correlated with P concentrations in the vegetation. The N requirement of plants in the pioneer stage of soil development in the forefield is fulfilled by atmospheric N deposition being funnelled between rocks. N2-fixation is not needed as the vegetation is often limited by P, or co-limited by N and P.

Impacts of the alien trees Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L. on soil nutrients and microbial communities

Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L. are two aggressive invasive trees in riparian areas in Central Spain. We aim to test whether soil properties, nutrient mineralization rates and soil bacterial communities of riparian forest dominated by the native Populus alba L. can be altered by the presence of A. altissima or R. pseudoacacia. In autumn 2011 and spring 2012 we conducted a field soil sampling in three sites where invasive and native trees were paired. In addition, in a 6-month greenhouse experiment (GHE), we grew A. altissima, R. pseudoacacia and P. alba from seeds in a soil collected from a native area. We quantified soil organic matter (OM), nitrogen (N), phosphorous (P), nitrate (NO3−–N), ammonium (NH4+–N), pH, potential net ammonification and nitrification rates, phosphomonoesterase (PME) activity and the composition of soil bacterial community in soils from the field study and from the GHE. Both the field and the GHE results showed the capability of A. altissima to decrease soil total N and of R. pseudoacacia to increase soil mineral N. In the field, all invaded soils had greater NO3−–N than P. alba soils. R. pseudoacacia field soils had greater PME activity, total N and net ammonification rates while A. altissima soils had lower OM, NH4+–N, net nitrification and total N mineralization rates than those of P. alba. Differences in the composition of soil bacterial communities were only found in the field, being more evident between A. altissima and P. alba than between R. pseudoacacia and P. alba field soils. Symbiotic N2 fixation could explain the capability of R. pseudoacacia to increase soil mineral N, while the potential of A. altissima to decrease total soil N may be attributed to changes in the balance between N input and losses from the soil. Although the GHE results indicated that the invasive trees can start changing soil conditions during early stages of establishment, more impacts found in the field study suggests that several soil properties, the composition of soil bacteria communities and microbial activities need longer time since invasion to be altered.

Yield Stability of Dry Bean Genotypes across Nitrogen‐Fixation‐Dependent and Fertilizer‐Dependent Management Systems

ABSTRACTDespite the inherent N2–fixing ability of legumes, the actual symbiotic N2 fixation (SNF) of dry bean (Phaseolus vulgaris L.) compared with other legumes is relatively low. Accordingly, application of inorganic nitrogen in bean fields has often been recommended to maximize economical yield. The genetic diversity for SNF in common bean may provide the opportunity to develop bean genotypes with stable yield across N‐fertilizer‐dependent and SNF‐dependent production practices. A population of 140 recombinant inbred lines (RILs) of a cross between high and low N‐fixing genotypes, ‘Mist’ and ‘Sanilac’, respectively, was evaluated under two different N management conditions, SNF and N fertilizer dependent, across multiple environments. While N management did not significantly affect the overall yield, genotypes responded differentially to SNF‐dependent and N‐fertilizer‐dependent environments. Among the RILs with higher than average yield, the stability analysis identified 6% as generally adapted to all environments, regardless of N management. The study highlights the opportunity to select bean genotypes that maintain their yielding ability under SNF‐dependent management systems.

Competition and growth of a grass–legume mixture fertilised with nitrogen and phosphorus: effect on nutrient acquisition, root morphology and symbiosis with soil microorganisms

Achieving a fast initial growth is crucial for legumes because grasses grow more rapidly and compete much better with forbs. In a pot experiment with a nutrient-deficient soil, we added nitrogen (N), phosphorus (P) and N + P to pure and mixed stands of Lotus tenuis and Festuca arundinacea and investigated the effects of on plant growth, nutrient uptake and symbiotic associations with arbuscular mycorrhizae and rhizobia. Plant yield, N and P acquisition, mycorrhizal colonisation, rhizobial nodulation and root length were measured and root diameter and root surface area were calculated after two harvests. Species responded differently to specific nutrients when grown pure or mixed. Comparing pure with mixed stands in soils fertilised with P and N + P, L. tenuis showed decreased shoot and particularly root biomass, whereas F. arundinacea showed increases in both biomasses. This suggests that the competitiveness of the grass with the legume increased upon P and N + P addition. In mixed stands, F. arundinacea produced 51–64% of the total shoot biomass and 69–74% of the total root biomass with P and N + P, respectively. Root length and root surface area were greater and the roots thinner in F. arundinacea than in L. tenuis. Addition of P and N + P increased rhizobial nodulation in legume roots but decreased mycorrhizal colonisation in both plants. Supply of N does not necessarily favour grasses, whereas P supply favours legumes. Optimisation of P nutrition might help to maximise N inputs into grasslands by symbiotic N-fixation and decrease inputs of inorganic N by fertilisation.

Survey of long-term productivity and nutritional status of Leucaena leucocephala-grass pastures in subtropical Queensland

A survey of the productivity and nutritional status of leucaena (Leucaena leucocephala subsp. glabrata) pastures of different ages was conducted in subtropical Queensland from 2006 to 2008. Four leucaena stands (aged 8, 20, 31 and 38 years) growing on the same Vertosol soil type at Brian Pastures Research Station were surveyed. In the higher rainfall season of 2007–2008, leucaena yields and rainfall-use efficiency were highest in 8-year-old stands [2128 kg total dry matter (DM)/ha or 4.0 kg DM/ha.mm] and lowest in 38-year-old stands (978 kg total DM/ha or 1.9 kg DM/ha.mm). The reduced productivity in the 38-year-old leucaena pasture was associated with a decline in stem number/plant and leucaena plant density due to observed plant mortality. The reduced yield and vigour of aging leucaena was associated with nitrogen deficiency related to declining phosphorus and sulfur availability for adequate symbiotic N2 fixation and leucaena plant growth. Nutrient deficiencies of phosphorus and sulfur in leaf tissue were related to low to medium initial soil fertility (7–27 mg/kg of bicarbonate extractable phosphorus in the top 20 cm), coupled with inherent subsoil constraints (shallow soils, sodicity and high pH), and exacerbated by both long-term removal of nutrients by grazing animals and a reduction in soil phosphorus and sulfur availability over time. To maintain the productivity of leucaena pastures, plant nutritional status needs to be monitored in order to determine strategic fertiliser application.

The Role of Rhizobial ACC Deaminase in the Nodulation Process of Leguminous Plants

Symbiotic rhizobia-legumes associations are extremely important in terms of sustainable agricultural practices. This symbiosis involves a complex interaction between both partners, plant and bacterium, for bacterial infection and the formation of symbiotic N-fixing nodules. In this regard, the phytohormone ethylene plays a significant role in nodule formation, acting as an inhibitor of the nodulation process. Ethylene not only regulates nodule development but also regulates many other plant developmental cues, including various stress responses that inhibit overall plant growth. Some rhizobia produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, thus, being able to decrease ACC and, consequently, decrease deleterious ethylene levels that affect the nodulation process. This occurs because ACC is the immediate precursor of ethylene in all higher plants. Hence, rhizobia that express this enzyme have an increased symbiotic potential. In addition to the direct role that ACC deaminase plays in the nodulation processper se, in a limited number of instances, ACC deaminase can also modulate nodule persistence. This review focuses on the important role of rhizobial ACC deaminase during the nodulation process, emphasizing its significance to legume growth promotion.

Genome-Wide Association Analysis of Diverse Soybean Genotypes Reveals Novel Markers for Nitrogen Traits.

Nitrogen is a primary plant nutrient that plays a major role in achieving maximum economic yield. Insufficient availability most often limits soybean [Glycine max (L.) Merr.] crop growth. Symbiotic N2 fixation in soybean is highly sensitive to limited water availability, and breeding for reduced N2 fixation sensitivity to drought is considered an important objective to improve yields under drought. The objective of this study was to identify single nucleotide polymorphism (SNP) markers associated with N traits. A collection of 373 diverse soybean genotypes were grown in four field environments (2 yr and two locations) and characterized for N derived from atmosphere (Ndfa), N concentration ([N]), and C/N ratio. The population structure of 373 soybean genotypes was assessed based on 31,145 SNPs and genome-wide association analysis using a unified mixed model identified SNPs associated with Ndfa, [N], and C/N ratio. Although the Ndfa, [N], and C/N ratio values were significantly different between the two locations in both years, results were consistent among genotypes across years and locations. While numerous SNPs were identified by association analysis for each trait in only one of the four environments, 17, 19, and 24 SNPs showed a significant association with Ndfa, [N], and C/N ratio, respectively, in at least two environments as well as with the average across all four environments. These markers represent an important resource for pyramiding favorable alleles for drought tolerance and for identifying extremes for comparative physiological studies.

Responses of symbiotic N2 fixation in Alnus species to the projected elevated CO2 environment

Nitrogen fixation in Alnus species in response to elevated CO 2 may depend on the presence of non-N 2 -fixing tree species in addition to soil conditions. Alnus is a major genus of actinorhizal plants. Symbiosis with Frankia allows the Alnus species to fix nitrogen (N) at the rate of several to 320 kg N ha−1 year−1 with a nodule biomass of 16–480 kg ha−1. Alnus species ensures an effective supply of N to soils because of the high N content of leaf litter, rapid decomposition rate, and the influx of herbivorous insects. In addition, the association between regenerated endozoochorous species and Alnus hirsuta suggests that N2 fixation in Alnus species influences the distribution patterns of regenerated plants as well as improve soil fertility. N2 fixation by the Alnus–Frankia symbiotic relationship may be positively associated with elevated carbon dioxide (CO2) levels. Nodule biomass increased under elevated CO2 due to enhanced plant growth, rather than changes in biomass allocation. The inhibitory effect of high soil N on nodulation was retained under elevated CO2, and the effects of elevated CO2 on N2 fixation depended on soil P availability, drought, and many other abiotic and biotic factors. Recent free-air CO2 enrichment experiments have demonstrated increased N2 fixation in A. glutinosa exposed to elevated CO2 in mixed-species stands containing non-N2-fixers but not in monocultures, suggesting that N2 fixation depends on an association with non-N2-fixing tree species. Because elevated CO2 can alter the N and P contents and stoichiometry of plants, it will be necessary to evaluate N allocation and accumulation of biomass when investigating the response of Alnus species to future global climate change.