Rhizobium-legume Symbiosis Research Articles

Mechanisms underlying legume-rhizobium symbioses.

Legumes, unlike most land plants, can form symbiotic root nodules with nitrogen-fixing bacteria to secure nitrogen for growth. The formation of nitrogen-fixing nodules on legume roots requires the coordination of rhizobial infection at the root epidermis with cell division in the cortex. The nodules house the nitrogen-fixing rhizobia in organelle-like structures known as symbiosomes, which enable nitrogen fixation and facilitate the exchange of metabolites between the host and symbionts. In addition to this beneficial interaction, legumes are continuously exposed to would-be pathogenic microbes; therefore the ability to discriminate pathogens from symbionts is a major determinant of plant survival under natural conditions. Here, we summarize recent advances in the understanding of root nodule symbiosis signaling, transcriptional regulation, and regulation of plant immunity during legume-rhizobium symbiosis. In addition, we propose several important questions to be addressed and provide insights into the potential for engineering the capacity to fix nitrogen in legume and non-legume plants.

Microscopic and Transcriptomic Analyses of Dalbergoid Legume Peanut Reveal a Divergent Evolution Leading to Nod-Factor-Dependent Epidermal Crack-Entry and Terminal Bacteroid Differentiation.

Root nodule symbiosis (RNS) is the pillar behind sustainable agriculture and plays a pivotal role in the environmental nitrogen cycle. Most of the genetic, molecular, and cell-biological knowledge on RNS comes from model legumes that exhibit a root-hair mode of bacterial infection, in contrast to the Dalbergoid legumes exhibiting crack-entry of rhizobia. As a step toward understanding this important group of legumes, we have combined microscopic analysis and temporal transcriptome to obtain a dynamic view of plant gene expression during Arachis hypogaea (peanut) nodule development. We generated comprehensive transcriptome data by mapping the reads to A. hypogaea, and two diploid progenitor genomes. Additionally, we performed BLAST searches to identify nodule-induced yet-to-be annotated peanut genes. Comparison between peanut, Medicago truncatula, Lotus japonicus, and Glycine max showed upregulation of 61 peanut orthologs among 111 tested known RNS-related genes, indicating conservation in mechanisms of nodule development among members of the Papilionoid family. Unlike model legumes, recruitment of class 1 phytoglobin-derived symbiotic hemoglobin (SymH) in peanut indicates diversification of oxygen-scavenging mechanisms in the Papilionoid family. Finally, the absence of cysteine-rich motif-1-containing nodule-specific cysteine-rich peptide (NCR) genes but the recruitment of defensin-like NCRs suggest a diverse molecular mechanism of terminal bacteroid differentiation. In summary, our work describes genetic conservation and diversification in legume-rhizobia symbiosis in the Papilionoid family, as well as among members of the Dalbergoid legumes.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Microbial consortia inoculation of woody legume Erythrina brucei increases nodulation and shoot nitrogen and phosphorus under greenhouse conditions

The legume-rhizobium symbiosis provides Nitrogen (N), while Legume-AMF symbiosis improves Phosphorus (P) supply to plants. This research was conducted to evaluate the symbiotic effectiveness of the Bradyrhizobium spp. and consortial inoculation of plant growth promoting bacteria -Bradyrhizobium shewense (AU27) and Acinetobacter soli (AU4), and arbuscular mycorrhizhal fungi Glomus sp.1 (AMF1) and Acaulospora sp.1 (AMF2), on growth, production and shoot N and P content of Erythrina brucei.The bacterial and mycorrhizal species were evaluated for phyto-beneficial properties in the greenhouse as individual as well as consortial inoculation.. All Bradyrhizobium species were effective for symbiotic nitrogen fixation. Consortial inoculations comprising of B. shewense (AU27) + A. soli (AU4) + Glomus sp.1 (AMF1) + Acaulospora sp.1 (AMF2) (T7) increased shoot length and shoot dry weight by 140% and 268%, respectively compared to un-inoculated control. Inoculations that involved B. shewense (AU27) + A. soli (AU4) increased shoot nitrogen by 260%, and 1200% increment of shoot P was recorded with inoculations of B. shewense (AU27) + Glomus sp.1 (AMF1) compared to un-inoculated control. These microbial inputs could be candidates for growth enhancement and shoot nitrogen and phosphorus improvement in Erythrina brucei and also as sustainable and eco-friendly agriculture input.

PECULIARITIES OF FORMATION AND FUNCTIONING OF THE SOYBEAN–BRADYRHIZOBIUM JAPONICUM SYMBIOTIC APPARATUS IN RELATION TO PHOTOSYNTHETIC ACTIVITY UNDER THE INFLUENCE OF SEED PROTECTANTS

The aim of work was to investigate the influence of seed treatment with two preparations of fungicidal action, differing in the active substances composition, on the formation and functioning of legume-rhizobial symbiosis in interaction with the photosynthetic apparatus of soybean plants. It was established that in plants treated with preparation Fever, the nitrogen-fixing activity (NFA) difference with the control was insignificant, but under the impact of the preparation Standak Top NFA was by 63% less at the stage of three true leaves, by 48% – at the stage of budding, and by 34% – at the stage of budding–flowering. The net photosynthetic rate, calculated per leaf area unit, was lower than control at all investigated stages of soybean plants development in the variants with the seed protectants applying. A fairly close positive correlations (R2 = 0.79) were observed between the average weight of one nodule and its NFA, and between the last and total NFA of the whole plant (R2 = 0.78). Close correlations between plant NFA, chlorophyll content in leaves, and net photosynthetic rate were revealed. It was concluded that seed protectants significantly influenced on nitrogen fixation and photosynthesis. The strength of manifestation, and the sign of this influence in comparison with the relevant parameters of the control plants depended on the preparation, the stage of plant development, as well as on the index (nodules formation, nitrogen-fixing activity, photosynthetic, chlorophyll content, chlorophyll index, biological productivity of soybean plants) under consideration.

Bridging the Gap: Type III Secretion Systems in Plant-Beneficial Bacteria.

Type III secretion systems (T3SSs) are bacterial membrane-embedded nanomachines translocating effector proteins into the cytoplasm of eukaryotic cells. They have been intensively studied for their important roles in animal and plant bacterial diseases. Over the past two decades, genome sequencing has unveiled their ubiquitous distribution in many taxa of Gram-negative bacteria, including plant-beneficial ones. Here, we discuss the distribution and functions of the T3SS in two agronomically important bacterial groups: the symbiotic nodule-forming nitrogen-fixing rhizobia and the free-living plant-beneficial Pseudomonas spp. In legume-rhizobia symbiosis, T3SSs and their cognate effectors play important roles, including the modulation of the plant immune response and the initiation of the nodulation process in some cases. In plant-beneficial Pseudomonas spp., the roles of T3SSs are not fully understood, but pertain to plant immunity suppression, biocontrol against eukaryotic plant pathogens, mycorrhization facilitation, and possibly resistance against protist predation. The diversity of T3SSs in plant-beneficial bacteria points to their important roles in multifarious interkingdom interactions in the rhizosphere. We argue that the gap in research on T3SSs in plant-beneficial bacteria must be bridged to better understand bacteria/eukaryotes rhizosphere interactions and to support the development of efficient plant-growth promoting microbial inoculants.

Genome-Scale Metabolic Modelling of Lifestyle Changes in Rhizobium leguminosarum.

ABSTRACTBiological nitrogen fixation in rhizobium-legume symbioses is of major importance for sustainable agricultural practices. To establish a mutualistic relationship with their plant host, rhizobia transition from free-living bacteria in soil to growth down infection threads inside plant roots and finally differentiate into nitrogen-fixing bacteroids. We reconstructed a genome-scale metabolic model for Rhizobium leguminosarum and integrated the model with transcriptome, proteome, metabolome, and gene essentiality data to investigate nutrient uptake and metabolic fluxes characteristic of these different lifestyles. Synthesis of leucine, polyphosphate, and AICAR is predicted to be important in the rhizosphere, while myo-inositol catabolism is active in undifferentiated nodule bacteria in agreement with experimental evidence. The model indicates that bacteroids utilize xylose and glycolate in addition to dicarboxylates, which could explain previously described gene expression patterns. Histidine is predicted to be actively synthesized in bacteroids, consistent with transcriptome and proteome data for several rhizobial species. These results provide the basis for targeted experimental investigation of metabolic processes specific to the different stages of the rhizobium-legume symbioses.IMPORTANCE Rhizobia are soil bacteria that induce nodule formation on plant roots and differentiate into nitrogen-fixing bacteroids. A detailed understanding of this complex symbiosis is essential for advancing ongoing efforts to engineer novel symbioses with cereal crops for sustainable agriculture. Here, we reconstruct and validate a genome-scale metabolic model for Rhizobium leguminosarum bv. viciae 3841. By integrating the model with various experimental data sets specific to different stages of symbiosis formation, we elucidate the metabolic characteristics of rhizosphere bacteria, undifferentiated bacteria inside root nodules, and nitrogen-fixing bacteroids. Our model predicts metabolic flux patterns for these three distinct lifestyles, thus providing a framework for the interpretation of genome-scale experimental data sets and identifying targets for future experimental studies.

Симбиотические гены бобовых растений, выявляемые путем транскриптомного анализа

Legumes form symbioses with nodule bacteria (NB) and arbuscular mycorrhiza fungi(AM). The formation of symbioses increases the resistance of plants to stress and contributes to obtaining a stable harvest even in conditions of global climate change. The study of the molecular basis of symbioses formed by legumes is necessary to increase the effectiveness of these symbioses when they are used in modern agriculture. Previously, individual regulatory genes (so-called Sym genes) controlling some stages of symbiosis development were studied for model legumes using mutation analysis. Currently, high throughput sequencing technologies allow us to study a complete set of genes encoding the "molecular symbiosis machine" and necessary for ensuring the metabolic integration of symbionts. Among them, there are genes encoding nodulins (from the “nodule”), proteins specific for legume-rhizobial symbiosis (LRS), and mycorrhizins specific for arbuscular-mycorrhizal (AM) symbiosis. The products of genes whose expression is induced during the development of both LRS and AMS are called symbiosins. The genes encoding symbiosins, nodulins and mycorrhizins have been studied on the model legumes Medicago truncatula Gaertn., Lotus japonicus (Regel.) K. Larsen and Glycine max (L.) Merr., while the symbiosis-specific genes of non-model legumes, as well as the features of their expression, have not been sufficiently studied to date. The use of transcriptomics approaches (i.e., the study of the entire set of expressed genes, the so-called transcriptome) makes it possible to fill this knowledge gap. In this paper, we have summarized the results of research of the expression of the above groups of genes in legumesin recent years.

Phenolic Acids Induce Nod Factor Production in Lotus japonicus-Mesorhizobium Symbiosis.

In legume–rhizobia symbiosis, partner recognition and the initiation of symbiosis processes require the mutual exchange of chemical signals. Chemicals, generally (iso)flavonoids, in the root exudates of the host plant induce the expression of nod genes in rhizobia, and, thus, are called nod gene inducers. The expression of nod genes leads to the production of lipochitooligosaccharides (LCOs) called Nod factors. Natural nod gene inducer(s) in Lotus japonicus–Mesorhizobium symbiosis remain unknown. Therefore, we developed an LCO detection method based on ultra-high-performance liquid chromatography–tandem-quadrupole mass spectrometry (UPLC-TQMS) to identify these inducers and used it herein to screen 40 phenolic compounds and aldonic acids for their ability to induce LCOs in Mesorhizobium japonicum MAFF303099. We identified five phenolic acids with LCO-inducing activities, including p-coumaric, caffeic, and ferulic acids. The induced LCOs caused root hair deformation, and nodule numbers in L. japonicus inoculated with M. japonicum were increased by these phenolic acids. The three phenolic acids listed above induced the expression of the nodA, nodB, and ttsI genes in a strain harboring a multicopy plasmid encoding NodD1, but not that encoding NodD2. The presence of p-coumaric and ferulic acids in the root exudates of L. japonicus was confirmed by UPLC-TQMS, and the induction of ttsI::lacZ in the strain harboring the nodD1 plasmid was detected in the rhizosphere of L. japonicus. Based on these results, we propose that phenolic acids are a novel type of nod gene inducer in L. japonicus–Mesorhizobium symbiosis.

The influence of seed-dressings on the formation and functioning of symbiotic soybean systems with fungicide-resistant rhizobia

. The application of bacterial fertilizers, prepared on the basis of active, competitive strains of nitrogen-fixing microorganisms and fungicidal dressings for pre-sowing seed treatment in soybean cultivation technologies is characterized by high biological and economic efficiency. Therefore, it is important to find such combination of chemicals and microbial preparations, for the complex use of which will be maintained their main purpose. The aim of the work was to investigate the formation and functioning of symbiotic soybean systems with fungicide-resistant nodule bacteria Bradyrhizobium japonicum PC09 and B144 under the influence of Standak Top and Fever seed dressings. Differences in the effect of chemical plant protection products on the formation of symbiotic systems of soybean variety Almaz and nitrogen-fixing microorganisms have been established. A decrease by 9.2‒24.1 % in the number of nodules formed on the roots during the soybean growing season with the combined use of Fever with rhizobia bacterization of PC09 strain was revealed. At inoculation the seed with Tn5- mutant B144, use of the same preparation, the decrease of this indicator compared to control plants was 6.5‒32.8 %. An increase in the weight of root nodules by 10.3‒36 % was found under the effect of Standak Top and bacterization by both strains of rhizobia.There was a decrease in the intensity of N2 assimilation compared to control plants by 24.2 and 42.3 % in the stage of three true leaves with the combined use of Fever with inoculation and a gradual decrease of the toxic effect of pesticides on the functioning of symbiotic systems in the budding-flowering stage. At shared treatment of seeds with rhizobia strains B144 and PC09 and Standak Top revealed an increase in nitrogen fixation intensity by 6.2‒25 % in the stage of three true leaves, 12.5‒58 % in the stage of budding-beginning of flowering and by 28.1‒42.3 % in the stage of bean formation. Maintaining high level of nitrogenase activity of legume-rhizobial symbiosis, formed with the participation of pesticide-resistant nodule bacteria with using of seed dressings, will provide plants with available forms of nitrogen, protect sown seeds from soil and seed infections and create conditions for grain production.

Effects of Bradyrhizobium Japonicum on Nitrogen Concentration in Soybean Leaves and Seeds Cultivated on Acidic Soils

The legume-rhizobium symbiosis plays an important role in Nitrogen (N) uptake by plants, more particularly in the humid tropical region where soils are deeply weathered and have poor electrochemical properties. However, the influence of Bradyrhizobium on N allocation in soybean organs is not clearly understood. Here, we assessed the effect of Bradyrhizobium japonicum on N concentration of soybean leaves and seeds cultivated on acidic soils. We conducted two experiments using a randomized full device with three replicates in two sites (i.e., Kasapa and Kanyameshi). The main plots included three strains of Bradyrhizobium and the uninoculated control while the subplots included soybean variety. We found that different strains of Bradyrhizobium did not induce significant effects on total N concentration in soybean leaves and seeds in the Kasapa site. In contrast, Bradyrhizobium strains affected significantly N concentration in soybean leaves in the Kanyameshi site. We demonstrated that soil N concentration, which varies between investigated sites, positively influences yield and nodulation. We conclude that the efficiency of the soybean-Bradyrhizobium symbiosis and its influence on the allocation of N through soybean organs strongly depend on the chemical characteristics of soil and particularly on the initial N concentrations in soils.

Rhizobia inhabiting <em>Clitoria ternatea</em> L. in Anuradhapura District, Sri Lanka: An assessment of stress tolerance and genetic diversity

Legume-Rhizobial symbiosis plays a greater agronomical and ecological significance as it provides fixed nitrogen through Biological Nitrogen Fixation. <em>Clitoria ternatea </em>is a leguminous plant that hosts a wide range of rhizobial strains. Even though it is a widely distributed plant, comprehensive information and studies conducted on the rhizobial - <em>C.</em> <em>ternatea </em>symbiosis is lacking in Sri Lanka. This study aimed to identify different stress-tolerant rhizobial strains inhabiting root nodules of <em>C.</em> <em>ternatea </em>growing in seven selected locations of Anuradhapura district of Sri Lanka<em>. </em>Twenty-eight pure rhizobial colonies were isolated and they were separately grown in ½ Lupin broths and were subjected to four different physiological conditions, pH, temperature, salinity, and drought. Most of the isolates were well-grown within the pH range of 5.0-8.0, the temperature range of 30-35°C and at 0.2% Polyethylene glycol-8000 (PEG) concentration. There was no observable pattern in the growth of rhizobial strains in different physiological conditions. The twelve rhizobial strains which showed high tolerance to extreme physiological conditions were subjected to a combination of physiological stress conditions of pH 8.0, temperature 36°C, 3.0 % NaCl and 0.4 % PEG. The maximum growth in combination physiological study was observed in a strain collected at the Palugaswewa site. According to the dendrogram prepared by the Enterobacterial Repetitive Intergenic Consensus (ERIC) profile, twelve strains are genetically diverse, as they belonged to11 clusters at 69.89 % of similarity level. These stress-tolerant rhizobial strains could be used for further studies on cross-inoculation of crop legumes as an alternative to the nitrogenous fertilizers.

A proteomic approach to understand the impact of nodulation on salinity stress response in alfalfa (Medicago sativa L.).

Symbiotic nitrogen fixation in legumes is an important source of nitrogen supply in sustainable agriculture. Salinity is a key abiotic stress that negatively affects host plant growth, rhizobium-legume symbiosis and nitrogen fixation. This work investigates how the symbiotic relationship impacts plant response to salinity stress. We assayed the physiological changes and the proteome profile of alfalfa plants with active nodules (NA), inactive nodules (NI) or without nodules (NN) when plants were subjected to salinity stress. Our data suggest that NA plants respond to salinity stress through some unique signalling regulations. NA plants showed upregulation of proteins related to cell wall remodelling and reactive oxygen species scavenging, and downregulation of proteins involved in protein synthesis and degradation. The data also show that NA plants, together with NI plants, upregulated proteins involved in photosynthesis, carbon fixation and respiration, anion transport and plant defence against pathogens. The study suggests that the symbiotic relationship gave the host plant a better capacity to adjust key processes, probably to more efficiently use energy and resources, deal with oxidative stress, and maintain ion homeostasis and health during salinity stress.

Why are rhizobial symbiosis genes mobile?

Rhizobia are one of the most important and best studied groups of bacterial symbionts. They are defined by their ability to establish nitrogen-fixing intracellular infections within plant hosts. One surprising feature of this symbiosis is that the bacterial genes required for this complex trait are not fixed within the chromosome, but are encoded on mobile genetic elements (MGEs), namely plasmids or integrative and conjugative elements. Evidence suggests that many of these elements are actively mobilizing within rhizobial populations, suggesting that regular symbiosis gene transfer is part of the ecology of rhizobial symbionts. At first glance, this is counterintuitive. The symbiosis trait is highly complex, multipartite and tightly coevolved with the legume hosts, while transfer of genes can be costly and disrupt coadaptation between the chromosome and the symbiosis genes. However, horizontal gene transfer is a process driven not only by the interests of the host bacterium, but also, and perhaps predominantly, by the interests of the MGEs that facilitate it. Thus understanding the role of horizontal gene transfer in the rhizobium–legume symbiosis requires a ‘mobile genetic element's-eye view' on the ecology and evolution of this important symbiosis.This article is part of the theme issue ‘The secret lives of microbial mobile genetic elements’.

FORMING SOYBEAN PRODUCTIVITY UPON SEED PRE-TREATMENT WITH FUNGICIDES STANDAK TOP AND FEVER AND INOCULATION WITH RHIZOBIA ON THE DAY OF SOWING

Soybean grain productivity is largely due to a complex of factors, in particular, the efficiency of symbiotic systems formed with nodule bacteria, the activity of growth processes and the formation of vegetative and generative organs by plants, as well as their adaptive plasticity under various environmental factors. The article presents the results of studying the peculiarities of the formation of soybean plant productivity upon seed pre-treatment with fungicides of different classes and inoculation with nodule bacteria on the day of sowing. Objective. To study the dynamics of seed germination, formation of vegetative and generative organs and soybean harvest, functioning of soybean-rhizobial symbiosis, as well as the state of photosynthetic pigment system — content of chlorophyll a and b and carotenoids in the leaves upon seed pre-treatment with fungicides Fever and Standak Top and bacterization with Bradyrhizobium japonicum 634b on the day of sowing. Methods. Microbiological, physiological, biochemical, statistical. Results. Fungicides did not exert toxicity in terms of seed germination, and soybean seed germination under the action of Fever exceeded parameters in the control plants by 20 % (at Day 5 after sowing) and by 7 % (at Day 8). It has been shown that fungicides increased plant height, aboveground mass and accelerated the formation of generative organs (flowers) by plants but did not significantly affect root mass. It was found that fungicides had a pronounced toxic (inhibitory) effect on soybean-rhizobial symbiosis upon seed pre-treatment: the process of nodule formation was suppressed in the initial stages of symbiosis, nitrogen fixation activity was lower than in the control plants by 80–48 % depending on soybean development phase. Thus, the effect of fungicides Fever and Standak Top upon seed pretreatment and inoculation with nodule bacteria on the day of sowing was characterized by significant suppression of functional activity of the symbiotic apparatus in the first half of soybean vegetation and its gradual recovery during formation of generative organs. Although fungicides inhibit the formation and functioning of legume-rhizobial symbiosis but their positive effect on the plant itself (seed germination, plant height, aboveground mass accumulation, dynamics of generative organ formation), as well as the ability of plants to adapt to stress at the biochemical level (stabilization of the content of photosynthetic pigments in the leaves) allowed to form a crop of soybean seeds at or slightly above the level of control. Conclusion. Physiological and biochemical features of the reaction of soybean-rhizobial symbiosis to the action of different fungicides that we had established must be taken into account in developing new strategies to protect plants from pathogens of various aetiologies with the involvement of physiologically active substances having fungicidal activity in combination with inoculation.

The potency of Cassia siamea as phytostabilization in post-mining land reclamation

Revegetation for reclamation is a strategic effort to overcome the negative impacts of coal mining. The growth of reclaimed vegetation has a positive impact in reducing soil density so that it is suitable for the growth of food plants, and also can increase the content of soil organic matter. One of the efforts to restore the function of post-mining land is by planting legumes, one of the species is Cassia siamea (C. siamea). The method used is a literature study. Literature searches were using the Google Scholar, Research Gate, and Science Direct databases. From the results of a literature search, C. siamea can accumulate heavy metals as phytostabilization by reduces the mobility and presence of pollutants in the soil by accumulating the metallic trace elements (MTE) into the roots. These plants produce proline, a substance that is produced when drought occurs so that the inside of the plant does not suffer from differences in water concentrations. C. siamea can restore and improve the physical and chemical properties of the soil to become more fertile and rich in nutrients with Legume-Rhizobia Symbiosis and also has a high RMSI (> 0.500), this value is preferable for reclamation of post-mining land.

Technology of effective microorganisms for the growth of agricultural plants of the Fabaceae family

This article presents the results of studies reflecting the influence of the technology of effective microorganisms (TEM) on the development of plants of the Fabaceae family (Pisum sativum L.). The assessment of the real effectiveness of EM agents in the mild climate of the Central Black Earth region and their influence on field germination and morphophysiological parameters of plants is given. The theoretical foundations of the use of EM agents, their role in the formation of soil microbiological communities and the assessment of their impact as a factor influencing the efficiency of legume-rhizobium symbiosis are presented. The exogenous treatment with biological products significantly increased the number and mass of active nodules on experimental plants, which contributed to the increase in nitrogen-fixing activity, which was an important component for legumes. The authors pay due attention to the assessment of the parameters of photosynthetic activity of plants under the conditions of increased inoculation with microorganisms. Not only an increase in leaf area was noted, but also a longer functioning of the photosynthetic apparatus, which led to an increase in the intensity of the work of the green surface of plants during the entire growing season. The combination of stimulating factors provided a better increase in dry biomass, which was reflected in a significant increase in the value of plant yield. The most intense accumulation of dry matter was observed in the flowering phase, which was also characterized by more intensive inoculation of plants. At the end of the growing season, this was reflected in an increase in the number of beans per plant and seeds in one bean, which had a positive effect on the crop yield indicators. The research results indicate the prospects of the use of the technology of effective microorganisms in plant growing and agriculture for the production of environmentally friendly food products and soil fertility recovery.

Rhizobia induction mechanisms to reduce drought stress in legumes

Drought is one of the main limitations of agricultural productivity and food security, in Andean mountain. The use of atmospheric nitrogen-fixing rhizobia in symbiosis with legumes, and tolerant to a wide range of adverse conditions, such as drought, is a great potential in sustainable agriculture. The aim of this review is to compile studies about drought stress effect on the legume-rhizobia symbiosis and rhizobia mechanisms to induce drought tolerance in legumes. The search for information was conducted from August to December 2020, using key terms. The drought effect on the nodulation and atmospheric nitrogen fixation process is made known, as well as the rhizobia ability to synthesize exopolysaccharides, enzymes, phytohormones, siderophores, osmolytes and solubilize phosphates as induction mechanisms to mitigate drought stress in legumes. This review will serve to propose future research using rhizobia to mitigate the drought effect on the legumes cultivation in environments such as the Andean mountains.

Identification of an Isoflavonoid Transporter Required for the Nodule Establishment of the Rhizobium-Fabaceae Symbiotic Interaction.

Nitrogen (N) as well as Phosphorus (P) are key nutrients determining crop productivity. Legumes have developed strategies to overcome nutrient limitation by, for example, forming a symbiotic relationship with N-fixing rhizobia and the release of P-mobilizing exudates and are thus able to grow without supply of N or P fertilizers. The legume-rhizobial symbiosis starts with root release of isoflavonoids that act as signaling molecules perceived by compatible bacteria. Subsequently, bacteria release nod factors, which induce signaling cascades allowing the formation of functional N-fixing nodules. We report here the identification and functional characterization of a plasma membrane-localized MATE-type transporter (LaMATE2) involved in the release of genistein from white lupin roots. The LaMATE2 expression in the root is upregulated under N deficiency as well as low phosphate availability, two nutritional deficiencies that induce the release of this isoflavonoid. LaMATE2 silencing reduced genistein efflux and even more the formation of symbiotic nodules, supporting the crucial role of LaMATE2 in isoflavonoid release and nodulation. Furthermore, silencing of LaMATE2 limited the P-solubilization activity of lupin root exudates. Transport assays in yeast vesicles demonstrated that LaMATE2 acts as a proton-driven isoflavonoid transporter.

Influence of nitrogen fertilizers on protein productivity of vetch-wheat grain under different water supply conditions

The peculiarity of nitrogen nutrition for legume-cereal mixtures is that due to the intravital transfer of mineral nutrition elements from the legume component of the mixture to the cereal component and vice versa, the cereal component receives nitrogen from the air symbiotically fixed by the legume component. This explains its better development in the mixture than in single-species sowing without the use of nitrogen fertilizers. In unfavorable conditions for legume-rhizobial symbiosis, when the assimilation of air nitrogen is weakened, and the legume and cereal components of the mixture are deficient in nitrogen, therefore, the issue of nitrogen fertilizer use expediency for legume-cereal mixtures is relevant. We have studied the effects of low and medium doses of nitrogen fertilizers including 30, 60 and 90 kg ha-1 (as treatments N30, N60 and N90) on the content and yield of crude protein with the grain yield of the vetch-wheat mixture under the conditions of sod-podzolic sandy loam soil in the central region of the Non-Chernozem zone with insufficient and optimal moisture. The protein content (% of dry matter) was determined by the calculation method according to the nitrogen content (N% × 6.25). On the control without nitrogen fertilizers, the protein content in vetch seeds and in wheat grain under the conditions of optimal moisture (2019) was higher than under the conditions of insufficient moisture (2018), by 0.4% and 0.3%, respectively. Nitrogen fertilizers increased the protein content in wheat grain by 0.2 - 0.4% during both years of study. The protein content in vetch seeds under the influence of nitrogen fertilizers increased only under the conditions of insufficient moisture by 0.2-0.7%. The protein yield with the wheat grain yield during both years of study upraised, as the dose of nitrogen fertilizers elevated by 1.6 - 2.7 times, and by the vetch harvest and insufficient moisture it remained unchanged, and by optimal moisture it droped by 13.7-34.7%. The protein yield with the grain yield of the mixture elevated by 44.4-91.1% under the influence of nitrogen fertilizers in arid conditions completely due to the wheat protein increase, while in conditions of sufficient moisture it droped by 5.2-17.8% due to the vetch protein decrease. Nitrogen fertilizers increased the competitiveness of wheat mixed with vetch under the conditions of insufficient moisture by 1.7-3.0 times, and under the conditions of optimal moisture by 1.9-4.1 times.

From Microns to Meters: Exploring Advances in Legume Microbiome Diversity for Agroecosystem Benefits

Legumes are of primary importance for agroecosystems because they provide protein-rich foods and enhance soil fertility through fixed atmospheric nitrogen. The legume-rhizobia symbiosis that makes this possible has been extensively studied, from basic research on biochemical signaling to practical applications in cropping systems. While rhizobia are the most-studied group of associated microorganisms, the functional benefit they confer to their legume hosts by fixing nitrogen is not performed in isolation. Indeed, non-rhizobia members of the rhizosphere and nodule microbiome are now understood to contribute in multiple ways to nodule formation, legume fitness, and other agroecosystem services. In this review, we summarize advances contributing to our understanding of the diversity and composition of bacterial members of the belowground legume microbiome. We also highlight applied work in legume food and forage crops that link microbial community composition with plant functional benefits. Ultimately, further research will assist in the development of multi-species microbial inoculants and cropping systems that maximize plant nutrient benefits, while reducing sources of agricultural pollution.