Nitrogen Fixation In Legumes Research Articles

Nitrogen Nutrition of Plants in an Alpine Lichen Heath under the Conditions of Soil Enrichment with Biogenic Elements

The results of a long-term experiment on the enrichment of the soil (Umbric Leptosol) of an alpine lichen heath with biogenic elements have shown that calcium addition and reduction of soil acidity do not change the nitrogen regime of the soil and patterns of nitrogen nutrition of plants. The increase in phosphorus availability contributes to the mobilization of soil organic nitrogen and shifts in the role of mycorrhiza in the nitrogen nutrition of plants, which determines their heavier isotopic nitrogen composition. At the same time, this reduces the intensity of nitrogen fixation by a nitrogen-fixing legume species, Oxytropis kubanensis. The increase in the availability of nitrogen leads to its active uptake by all studied plant species except Oxytropis kubanensis, as well as to the heavier isotopic nitrogen composition of plants. However, changes in the nitrogen status differ between plant species.

Impact of Hydrogen on the Transcriptome of Sinorhizobium meliloti 1021 Using RNA-sequencing Technology.

Hydrogen formed during nitrogen fixation in legumes can enter the surrounding soil and confer multiple benefits to crops. Here, we used Sinorhizobium meliloti 1021, whose genome was sequenced in 2001, as a model bacterium to study the relationship between the bacterium and legume. We investigated the effects of hydrogen on the gene expression in S. meliloti using RNA-sequencing technology. We identified 43 genes whose expression was altered by hydrogen treatment; among these, 39 were downregulated, and 4 were upregulated. These genes accounted for 1.5% of the total 2941 annotated genes of the S. meliloti genome. Gene ontology and pathway analyses revealed that the hydrogen-regulated genes were associated with catalytic activity and binding. Further, these genes were primarily involved in arginine, proline, and β-alanine metabolism. Real-time PCR revealed that the transcription levels of SMc02983, cyoB, cyoC, and cyoD were reduced after hydrogen treatment. These results provide a theoretical framework for exploring new metabolic pathways of S. meliloti.

Current Progress in Nitrogen Fixing Plants and Microbiome Research.

In agroecosystems, nitrogen is one of the major nutrients limiting plant growth. To meet the increased nitrogen demand in agriculture, synthetic fertilizers have been used extensively in the latter part of the twentieth century, which have led to environmental challenges such as nitrate pollution. Biological nitrogen fixation (BNF) in plants is an essential mechanism for sustainable agricultural production and healthy ecosystem functioning. BNF by legumes and associative, endosymbiotic, and endophytic nitrogen fixation in non-legumes play major roles in reducing the use of synthetic nitrogen fertilizer in agriculture, increased plant nutrient content, and soil health reclamation. This review discusses the process of nitrogen-fixation in plants, nodule formation, the genes involved in plant-rhizobia interaction, and nitrogen-fixing legume and non-legume plants. This review also elaborates on current research efforts involved in transferring nitrogen-fixing mechanisms from legumes to non-legumes, especially to economically important crops such as rice, maize, and wheat at the molecular level and relevant other techniques involving the manipulation of soil microbiome for plant benefits in the non-legume root environment.

Advances in the mechanism of cereal/legume intercropping promotion of symbiotic nitrogen fixation

Biological nitrogen fixation by rhizobia in symbiotic association with legumes is a valuable N source for agroecosystems, and thus it is crucial for increased sustainability of agricultural production. Previous studies found that legume and cereal intercropping can promote symbiotic nitrogen fixation mainly due to rhizosphere nitrogen depletion by the cereal and that soil mineral nitrogen is absorbed more by cereals than legumes, which forces the legume to increase its reliance on symbiotic nitrogen fixation. However, in addition to the nitrogen depletion theory in the rhizosphere, stoichiometry (such as nitrogen and phosphorus coupling) and molecular regulation mechanisms are also important in understanding legume nitrogen fixation. The legume usually maintains a stable stoichiometric relationship between nitrogen and phosphorus. Symbiotic nitrogen fixation of legumes tends to have a high demand for phosphorus, mainly because nitrogen fixation by legumes is a high energy-consuming process, and the required energy is directly provided by ATP. Therefore, when the phosphorus obtained by the intercropped legume is insufficient to meet the increase in symbiotic nitrogen fixation, legume growth is limited by phosphorus. Under phosphorus limitation, the phosphorus uptake strategy of the legume can be stimulated, i.e. by increasing the infection of arbuscular mycorrhizal fungi (AMF) and secretion of root exudates. The AMF mycelia of legumes can extend beyond the rhizosphere, linking the roots to the surrounding soil microhabitats and expanding the area of phosphorus absorbed by the roots. On the other hand, mobilizing rhizosphere insoluble phosphorus through root exudates is also an important pathway for improving phosphorus utilization. Increased soil phosphorus availability promotes plant growth and increased allocation of carbon (C) sources to roots and nodules, resulting in a larger root system or greater nodule formation, finally higher symbiotic nitrogen fixation. In addition, the formation of root nodules is influenced by rhizosphere talk between the cereal and legume, which are directly related to the level of nitrogen. A recent study found that under low N concentration conditions, cereal root exudates stimulated flavonoid biosynthesis and secretion partly through upregulating expression of the Chalcone-flavanone isomerase gene ( CFI ) in legumes. Meanwhile, an auxin-responsive GH3 family gene ( GH3.1 ) and the nodulation gene ENODL2 displayed significant increases, suggesting that nodulation continues under the regulation of auxin signaling and the nodulin-like protein. At nodule maturity, cereal root exudates continue to up-regulate key nodulation genes, such as NODL4 , ENOD93 and the N fixation gene FixI3 , finally promoting symbiotic nitrogen fixation. Moreover, a high concentration of soil mineral N significantly suppressed expression of the early nodulin gene NIN in Lotus corniculatus , which inhibited nodulation and symbiotic nitrogen fixation. In the cereal/legume intercropping system, rhizosphere nitrogen depletion can relieve the inhibition of nitrate on the expression of key nodulation and N fixation genes, thus promoting nodulation and symbiotic nitrogen fixation. In conclusion, intercropping can promote nitrogen uptake and optimize nitrogen use efficiency through the coupling effects of rhizosphere nitrogen depletion, phosphorus mobilization and the regulation of nitrogen fixation genes. This review will help improve our understanding of the mechanism of symbiotic nitrogen fixation in the cereal/legume intercropping system.

Role of putrescine (Put) in imparting salt tolerance through modulation of put metabolism, mycorrhizal and rhizobial symbioses in Cajanus cajan (L.) Millsp.

Salt stress is a major environmental constraint that limits growth and nitrogen-fixation in legumes. Role of putrescine (Put) and arbuscular mycorrhiza (AM) in improving functional efficiency of legumes has gained importance in recent years. Present investigations assessed the impact of Put (1 mM) seed priming and/or Rhizophagus irregularis inoculation on growth, mycorrhizal and rhizobial symbioses with an emphasis to correlate the same with nodular Put metabolism in pigeonpea (Cajanus cajan L.) genotypes (Tolerant-Pusa 2001 and Sensitive-Pusa 991) under salt stress. Salinity declined plant biomass, with greater negative effects on roots than shoots which reduced the mycorrhizal colonization as well as nodulation ability of Sinorhizobium fredii AR-4. The decline in nitrogen-fixing efficiency could be correlated with increase in Na+ concentrations in roots as well as nodules. Salinity reduced endogenous Put by increasing diamine oxidase (DAO) as well as decreasing arginine decarboxylase (ADC) as well as ornithine decarboxylase (ODC) activities in nodules. Put priming enhanced per cent mycorrhizal colonization which further increased the rhizobial symbiotic efficiency, more in Pusa 2001 than Pusa 991. Both Put priming and AM inoculations reduced Na+ uptake and improved nutrient status especially P in both underground organs, with AM more effective than Put. The further decline in Na+ uptake was recorded when both amendments were given together which enhanced nitrogen-fixing ability of nodules by modulating anabolic and catabolic enzyme activities responsible for Put biosynthesis. Hence, +Put+AM can be used as an effective strategy to improve symbiotic potential and arrest nodule senescence in pigeonpea under salt stress.

A convenient, soil-free method for the production of root nodules in soybean to study the effects of exogenous additives.

Legumes develop root nodules that harbor endosymbiotic bacteria, rhizobia. These rhizobia convert nitrogen to ammonia by biological nitrogen fixation. A thorough understanding of the biological nitrogen fixation in legumes and its regulation is key to develop sustainable agriculture. It is well known that plant hormones affect nodule formation; however, most studies are limited to model legumes due to their suitability for in vitro, plate‐based assays. Specifically, it is almost impossible to measure the effects of exogenous hormones or other additives during nodule development in crop legumes such as soybean as they have huge root system in soil. To circumvent this issue, the present research develops suitable media and growth conditions for efficient nodule development under in vitro, soil‐free conditions in an important legume crop, soybean. Moreover, we also evaluate the effects of all major phytohormones on soybean nodule development under identical growing conditions. Phytohormones such as abscisic acid (ABA) and jasmonic acid (JA) had an overall inhibitory effect and those such as gibberellic acid (GA) or brassinosteroids (BRs) had an overall positive effect on nodule formation. This versatile, inexpensive, scalable, and simple protocol provides several advantages over previously established methods. It is extremely time‐ and resource‐efficient, does not require special training or equipment, and produces highly reproducible results. The approach is expandable to other large legumes as well as for other exogenous additives.

Pennsylvania Germans, An Interpretive Encyclopedia, by Simon J. Bronner and Joshua R. Brown, editors.

<i>Pennsylvania Germans, An Interpretive Encyclopedia</i>, by Simon J. Bronner and Joshua R. Brown, editors.

Nodule-specific PLAT domain proteins are expanded in the Medicago lineage and required for nodulation.

Symbiotic nitrogen fixation in legumes is mediated by an interplay of signaling processes between plant hosts and rhizobial symbionts. In legumes, several secreted protein families have undergone expansions and play key roles in nodulation. Thus, identifying lineage-specific expansions (LSEs) of nodulation-associated genes can be a strategy to discover candidate gene families. Using bioinformatic tools, we identified 13 LSEs of nodulation-related secreted protein families, each unique to either Glycine, Arachis or Medicago lineages. In the Medicago lineage, nodule-specific Polycystin-1, Lipoxygenase, Alpha Toxin (PLAT) domain proteins (NPDs) expanded to five members. We examined NPD function using CRISPR/Cas9 multiplex genome editing to create Medicago truncatula NPD knockout lines, targeting one to five NPD genes. Mutant lines with differing combinations of NPD gene inactivations had progressively smaller nodules, earlier onset of nodule senescence, or ineffective nodules compared to the wild-type control. Double- and triple-knockout lines showed dissimilar nodulation phenotypes but coincided in upregulation of a DHHC-type zinc finger and an aspartyl protease gene, possible candidates for the observed disturbance of proper nodule function. By postulating that gene family expansions can be used to detect candidate genes, we identified a family of nodule-specific PLAT domain proteins and confirmed that they play a role in successful nodule formation.

Evaluating the Effectiveness of Rhizobium Inoculants and Micronutrients as Technologies for Nepalese Common Bean Smallholder Farmers in the Real-World Context of Highly Variable Hillside Environments and Indigenous Farming Practices

Studies have shown the potential of rhizobia and associated micronutrients to enhance symbiotic nitrogen fixation in legumes. Tens of millions of smallholder farmers, however, farm on mountain hillsides in highly variable soil and microenvironments, with different crop rotations, inputs and cultural practices. Here, on the terraces of the Nepalese Himalayas, we evaluated rhizobium inoculants (local, exotic), micronutrients (molybdenum, boron) and their combinations as technologies for smallholder farmers under highly variable microenvironments and traditional practices. The study was conducted as a series of participatory on-farm trials with 39 terrace farmers in two mid-hill districts of Nepal (Dhading, Kaski) from 2015 to 2017. Plots were measured for relevant agronomic traits. As expected, when comparing treatment plots with adjacent control plots within each farm, the results demonstrated tremendous farm-to-farm variability for nodulation, vegetative biomass, shoot nitrogen content, grain yield, and grain N content. Despite the variation observed, the data showed that the number of farms that showed yield increases from the rhizobium interventions, compared to those that suffered yield losses, was generally 2:1. We discuss potential experimental and socio-agronomic reasons for the variable results, including rainfall, which appeared critical. The results demonstrate the promise of rhizobium interventions for hillside smallholder farmers, even in a highly variable context.

Biochar enhancement of facilitation effects in agroforestry: early growth and physiological responses in a maize-leucaena model system

Biochar, or pyrolyzed organic matter intended for use as a soil amendment, has a variety of properties of interest from the perspective of agroforestry, in particular its potential to increase soil C sequestration and enhance yields by increased retention of soil mineral nutrients. Nitrogen-fixing legumes commonly show large growth responses to biochar, and we hypothesized that combinations of legumes and non-legumes would show increased enhancement of yields in mixture due to enhanced facilitation related to increased nutrient retention. This hypothesis was tested in a glasshouse pot trial involving the leguminous tree leucaena (Leucaena leucocephala), and maize (Zea mays) grown for 120 days in mixture and monoculture in both an unamended soil and soil amended with a slow-pyrolysis coconut husk biochar at 10 t/ha. Responses were quantified in terms of biomass production and partitioning, leaf-level gas-exchange, leaf chlorophyll content, and (for leucaena) root nodule formation. Consistent with predictions, leucaena showed more pronounced growth and physiological responses to biochar than did maize. The system also showed increased total biomass yields relative to monocultures in the two-species mixtures with biochar (relative yield in mixture [RYM] = 1.69; 95% CI 1.41–1.81), but not in the control treatment (RYM = 1.25; 95% CI 0.93–1.76), and positive effects of biochar on yield were generally greater for both species when grown in mixture. Growth responses to biochar were most pronounced for stem and root biomass, and growth responses were consistent with changes in leaf-level photosynthesis and chlorophyll content. In addition, biochar had large effects on biomass partitioning, enhancing stem allocation in both species. Our results suggest that biochar additions may augment facilitative interactions in agroforestry and intercropped systems on acidic, nutrient-deficient soils by enhancing system nutrient retention.

Development of RR soybean in function of glyphosate doses and Bradyrhizobium inoculation

ABSTRACT Soybean has traditionally been produced in systems that include the use of herbicides, often in higher than recommended doses. The process of symbiotic nitrogen fixation in legumes can be hampered by these herbicides, both by direct effects on rhizobia and indirect effects on the host plant. An outdoor experiment was performed to evaluate the effects of different doses of a glyphosate herbicide on Bradyrhizobium strains and biological nitrogen fixation in soybean BMX Potência RR plants. Soybean seeds were inoculated with Bradyrhizobium elkanii (SEMIA 5019) and Bradyrhizobium japonicum (SEMIA 5079) strains in a commercial liquid inoculant. The treatments consisted of the absence and presence of Bradyrhizobium genotypes inoculated via seed and four doses of the herbicide glyphosate applied on the leaves (0, 1.0, 2.0, and 4.0 L ha-1 of the commercial product) at the V3 stage. The leaf chlorophyll index of inoculated RR soybean plants did not change on the application of glyphosate and, regardless of inoculation, plants had the capacity to recover from the effects of glyphosate application, without impaired development.

Leveraging biological nitrogen fixation and soybean yield by planting duration under varying onset of rainy seasons in Uganda

Rainfall distribution and onset of rainy seasons, both of which are key determinants of the decision to plant most annual crops in sub-Saharan Africa (SSA), are increasingly difficult to predict due to climate change. Consequently, planting schedules affect both crops and vital natural systems such as biological nitrogen fixation in legumes. The objective of this study was to establish the optimum planting duration for soybean (Glycine max L.), from the onset of the rainy season, in the context of nodulation and grain yield, and diversity of rhizobia in rhizosphere soil and nodules. A field experiment was set up with treatments, Bradyrhizobia inoculated and non- inoculated soybean variety Maksoy3N planted at four dates; 0, 10, 20 and 30 days from the onset of rain season. Diversity of rhizobia in the rhizosphere soil and nodules were fingerprinted using Single Strand Conformation Polymorphism (SSCP) method. Inoculation significantly (P&lt;0.05) increased the number but not effectiveness of nodules and grain yield; which was attributed to presence of native strains in the soil. Planting after 10 days following the onset of a rainy season significantly (P&lt;0.05) reduced grain yields and total yield loss was realised when seasonal cumulative rainfall was about 500 mm. Planting after 20 days from onset of rainy season significantly (P&lt; 0.05) reduced the total number of nodules. A total of 8 and 12 clusters of bacteria from the soil and nodules at 5% dissimilarity, respectively, were recorded. The Bradyrhizobia species consisted of B. japonicum, B. japonicum (USDA 6), B. elkanii, B. liaoningense, B. canariense, and B. yuanmingense.Key words: Climate change, inoculation, nodulation, rhizobia

Short-term soil nutrient and plant community responses to the eradication of a nitrogen fixing tree, Lupinus arboreus

Invasive plant eradication aims to restore an original state by eliminating the plant and its alteration of ecosystem structure and function. The method by which an invasive plant is eradicated is known to influence restoration success; however, success may be impeded by a legacy effect. We sought to examine how the soil nutrient pool and plant community assembly responded to the eradication of an invasive nitrogen-fixing legume, Lupinus arboreus, using three different techniques. We applied herbicide to six mature plants and with a handsaw removed the above ground biomass of 12 mature plants; six of these were left in situ to decompose and six were removed from site. Soil nutrient concentrations were measured, and plant community composition was monitored, in the treatment and reference plots over the following 13 months. Eradication resulted in a significant increase in extractable ammonium-N and potassium, in contrast nitrate-N declined. The plant community changed as a result of felling L. arboreus, and the removal of all biomass initially resulted in an increase in native plant cover. By the end of the experimental period, however, L. arboreus seedlings were increasing in abundance and native cover was declining. The application of herbicide resulted in the slow decay of the mature plant and as such delayed an increase in exotic species cover and inhibited L. arboreus germination. Nutrient concentrations in the eradication plots were not significantly different from the reference plots at the end of the experimental period. The impact of L. arboreus on the soil nutrient pool remains after eradication, a legacy effect which will hinder restoration of the original structure and function in the long term.

In silico structural homology modeling of nif A protein of rhizobial strains in selective legume plants.

Symbiosis is a complex genetic regulatory biological evolution which is highly specific pertaining to plant species and microbial strains. Biological nitrogen fixation in legumes is a functional combination of nodulation by nod genes and regulation by nif, fix genes. Three rhizobial strains (Rhizobium leguminosarum, Bradyrhizobium japonicum, and Mesorhizobium ciceri) that we considered for in silico analysis of nif A are proved to be the best isolates with respect to N2 fixing for ground nut, chick pea and soya bean (in vitro) out of 47 forest soil samples. An attempt has been made to understand the structural characteristics and variations of nif genes that may reveal the factors influencing the nitrogen fixation. The primary, secondary and tertiary structure of nif A protein was analyzed by using multiple bioinformatics tools such as chou-Fasman, GOR, ExPasy ProtParam tools, Prosa -web. Literature shows that the homology modeling of nif A protein have not been explored yet which insisted the immediate development for better understanding of nif A structure and its influence on biological nitrogen fixation. In the present predicted 3D structure, the nif A protein was analyzed by three different software tools (Phyre2, Swiss model, Modeller) and validated accordingly which can be considered as an acceptable model. However further in silico studies are suggested to determine the specific factors responsible for nitrogen fixing in the present three rhizobial strains.

Smart subsidies for catchment conservation in Malawi

Conservation agriculture (CA) is a management paradigm in which soil is covered outside of cropping seasons, minimally disturbed, and recharged with nitrogen-fixing legumes. Finding effective ways to encourage CA is a centuries-old problem playing out acutely today in Sub-Saharan Africa. To better understand this issue, we have collected data on rural livelihoods and CA adoption during a two-year intervention in southern Malawi. The intervention evaluated rates of CA adoption under two different structures of payment and three levels of monitoring. The dataset includes a baseline and endline survey covering 1,900 households, along with surveys conducted with participants opting into the intervention. Baseline and endline questions included modules on farm-level inputs and production at the plot-crop level; plot characteristics; household composition and assets; savings, loans, and other sources of income; neighborhood characteristics; and perceptions regarding CA. Registration questionnaires in the intervention included detailed assessments of recent production in plots being registered to the intervention, and basic information for all other plots; and basic information on household structure and assets.

English

Legumes (Fabaceae) are plants with the distinct ability to fix atmospheric nitrogen, the atmospheric nitrogen fixation by legumes is known as biological nitrogen fixation. Biological nitrogen fixation is the process whereby atmospheric nitrogen is reduced to ammonia in the presence of the enzyme nitrogenase. Nitrogen fixation in legumes starts with the formation of nodules. Inside the nodules, nitrogen fixation done by the bacteria (Rhizobia), and the ammonia (NH3) produced is absorbed by plant. The symbiotic relationship between a bacterium and a plant makes legumes special plants, which offer benefits when included in farming systems. These benefits are ecosystem, economic and environmental benefits. Inclusion of forage legumes in the form of intercropping in low-input grassland mixtures improves forage quantity, quality and soil fertility trough addition of nitrogen (N) from N2-fixation. Intercropping is a multiple cropping practice, which involves growing two or more crops in proximity. Legumes also improve the nutritive value of the low quality native pastures grown with them and are important component of farming system since they have high nutritive value and able to rehabilitate nutrient depleted soil. There are various factors affecting legume growth and development and these factors need to be taken into account when planning to grow legumes. These factors include pedoclimatic factors especially those associated with the soil acid complex. These factors are known as physical, chemical, biological and environmental factors. The improvement of forage quantity and quality through forage legume inclusion is crucial for improved animal performance, which is a goal of all livestock farmers. The inclusion of forage legumes in low-input grassland mixtures is vital to improve biomass production, forage quality and ultimately soil fertility. The improvement of forage quantity and quality is crucial for improved animal performance, which is a goal of every livestock farmer. Forage legumes have the potential to improve the diets of ruminants because they increase the crude protein (CP) concentration of the herbage mixture relative to that of grass monocultures. &nbsp; Key words: Legumes, biological nitrogen fixation, forage quality, forage quantity.

Legume abundance along successional and rainfall gradients in Neotropical forests.

The nutrient demands of regrowing tropical forests are partly satisfied by nitrogen-fixing legume trees, but our understanding of the abundance of those species is biased towards wet tropical regions. Here we show how the abundance of Leguminosae is affected by both recovery from disturbance and large-scale rainfall gradients through a synthesis of forest inventory plots from a network of 42 Neotropical forest chronosequences. During the first three decades of natural forest regeneration, legume basal area is twice as high in dry compared with wet secondary forests. The tremendous ecological success of legumes in recently disturbed, water-limited forests is likely to be related to both their reduced leaflet size and ability to fix N2, which together enhance legume drought tolerance and water-use efficiency. Earth system models should incorporate these large-scale successional and climatic patterns of legume dominance to provide more accurate estimates of the maximum potential for natural nitrogen fixation across tropical forests.

Challenges in Using Precision Agriculture to Optimize Symbiotic Nitrogen Fixation in Legumes: Progress, Limitations, and Future Improvements Needed in Diagnostic Testing

Precision agriculture (PA) has been used for ≥25 years to optimize inputs, maximize profit, and minimize negative environmental impacts. Legumes play an important role in cropping systems, by associating with rhizobia microbes that convert plant-unavailable atmospheric nitrogen into usable nitrogen through symbiotic nitrogen fixation (SNF). However, there can be field-level spatial variability for SNF activity, as well as underlying soil factors that influence SNF (e.g., macro/micronutrients, pH, and rhizobia). There is a need for PA tools that can diagnose spatial variability in SNF activity, as well as the relevant environmental factors that influence SNF. Little information is available in the literature concerning the potential of PA to diagnose/optimize SNF. Here, we critically analyze SNF/soil diagnostic methods that hold promise as PA tools in the short–medium term. We also review the challenges facing additional diagnostics currently used for research, and describe the innovations needed to move them forward as PA tools. Our analysis suggests that the nitrogen difference method, isotope methods, and proximal and remote sensing techniques hold promise for diagnosing field-level variability in SNF. With respect to soil diagnostics, soil sensors and remote sensing techniques for nitrogen, phosphorus, pH, and salinity have short–medium term potential to optimize legume SNF under field conditions.

Epidermal auxin biosynthesis facilitates rhizobial infection in Lotus japonicus.

Symbiotic nitrogen fixation in legumes requires nodule organogenesis to be coordinated with infection by rhizobia. The plant hormone auxin influences symbiotic infection, but the precise timing of auxin accumulation and the genetic network governing it remain unclear. We used a Lotus japonicus optimised variant of the DII-based auxin accumulation sensor and identified a rapid accumulation of auxin in the epidermis, specifically in the root hair cells. This auxin accumulation occurs in the infected root hairs during rhizobia invasion, while Nod factor application induces this response across a broader range of root hairs. Using the DR5 auxin responsive promoter, we demonstrate that activation of auxin signalling also occurs specifically in infected root hairs. Analysis of root hair transcriptome data identified induction of an auxin biosynthesis gene of the Tryptophan Amino-transferase Related (LjTar1) family following both bacteria inoculation and Nod factor treatment. Genetic analysis showed that both expression of the LjTar1 biosynthesis gene and the auxin response requires Nod factor perception, while common symbiotic pathway transcription factors are only partially required or act redundantly to initiate auxin accumulation. Using a chemical genetics approach, we confirmed that auxin biosynthesis has a functional role in promoting symbiotic infection events in the epidermis.

Genetic Characterization and Diversity of Rhizobium Isolated From Root Nodules of Mid-Altitude Climbing Bean (Phaseolus vulgaris L.) Varieties.

The increasing interest in the use of rhizobia as biofertilizers in smallholder agricultural farming systems of the Sub-Saharan Africa has prompted the identification of a large number of tropical rhizobia strains and led to studies on their diversity. Inoculants containing diverse strains of rhizobia have been developed for use as biofertilizers to promote soil fertility and symbiotic nitrogen fixation in legumes. In spite of this success, there is paucity of data on rhizobia diversity and genetic variation associated with the newly released and improved mid-altitude climbing (MAC) bean lines (Phaseolus vulgaris L.). In this study, 41 rhizobia isolates were obtained from the root nodules of MAC 13 and MAC 64 climbing beans grown in upper and lower midland agro-ecological zones of Eastern Kenya. Eastern Kenya was chosen because of its high production potential of diverse common bean cultivars. The rhizobia isolates were characterized phenotypically on the basis of colony morphology, growth and biochemical features. Rhizobia diversity from the different regions of Eastern Kenya was determined based on the amplified ribosomal DNA restriction analysis (ARDRA) of PCR amplified 16S rRNA genes using Msp I, EcoR I, and Hae III restriction enzymes. Notably, native rhizobia isolates were morphologically diverse and grouped into nine different morphotypes. Correspondingly, the analysis of molecular variance based on restriction digestion of 16S rRNA genes showed that the largest proportion of significant (p < 0.05) genetic variation was distributed within the rhizobia population (97.5%) than among rhizobia populations (1.5%) in the four agro-ecological zones. The high degree of morphological and genotypic diversity of rhizobia within Eastern Kenya shows that the region harbors novel rhizobia strains worth exploiting to obtain strains efficient in biological nitrogen fixation with P. vulgaris L. Genetic sequence analysis of the isolates and testing for their symbiotic properties should be carried out to ascertain their identity and functionality in diverse environments.