Symbiotic interactions between chickpea (Cicer arietinum L.) genotypes and Mesorhizobium strains

Harnessing chickpea bacterial endophytes for improved plant health and fitness

Endophytic bacteria live asymptomatically inside the tissues of host plants without inflicting any damage. Endophytes can confer several beneficial traits to plants, which can contribute to their growth, development, and overall health. They have been found to stimulate plant growth by enhancing nutrient uptake and availability. They can produce plant growth-promoting substances such as auxins, cytokinins, and gibberellins, which regulate various aspects of plant growth and development. Endophytes can also improve root system architecture, leading to increased nutrient and water absorption. Some endophytes possess the ability to solubilize nutrients, such as phosphorus and potassium, making them more available for plant uptake, and fixing atmospheric nitrogen. Chickpea (Cicer arietinum) is a major legume crop that has mutualistic interactions with endophytes. These endophytes can benefit the chickpea plant in various ways, including higher growth, improved nutrient uptake, increased tolerance to abiotic and biotic stressors, and disease suppression. They can produce enzymes and metabolites that scavenge harmful reactive oxygen species, thus reducing oxidative stress. Moreover, several studies reported that endophytes produce antimicrobial compounds, lytic enzymes, and volatile organic compounds that inhibit the growth of fungal pathogens and trigger systemic defense responses in plants, leading to increased resistance against a broad range of pathogens. They can activate plant defense pathways, including the production of defense-related enzymes, phytoalexins, and pathogenesis-related proteins, thereby providing long-lasting protection. It is important to note that the diversity and function of chickpea-associated endophytes can vary depending on factors such as variety, geographical location, and environmental conditions. The mechanisms behind the plant-beneficial interactions are still being intensively explored. In this review, new biotechnologies in agricultural production and ecosystem stability were presented. Thus, harnessing chickpea endophytes could be exploited in developing drought-resistant cultivars that can maintain productivity in arid and semi-arid environments, crucial for meeting the global demand for chickpeas.

Comparative Genome Analysis of Polar Mesorhizobium sp. PAMC28654 to Gain Insight into Tolerance to Salinity and Trace Element Stress

In this study, Mesorhizobium sp. PAMC28654 was isolated from a soil sample collected from the polar region of Uganda. Whole-genome sequencing and comparative genomics were performed to better understand the genomic features necessary for Mesorhizobium sp. PAMC28654 to survive and thrive in extreme conditions and stresses. Additionally, diverse sequence analysis tools were employed for genomic investigation. The results of the analysis were then validated using wet-lab experiments. Genome analysis showed trace elements' resistant proteins (CopC, CopD, CzcD, and Acr3), exopolysaccharide (EPS)-producing proteins (ExoF and ExoQ), and nitrogen metabolic proteins (NarG, NarH, and NarI). The strain was positive for nitrate reduction. It was tolerant to 100 mM NaCl at 15 °C and 25 °C temperatures and resistant to multiple trace elements (up to 1 mM CuSO4·5H2O, 2 mM CoCl2·6H2O, 1 mM ZnSO4·7H2O, 0.05 mM Cd(NO3)2·4H2O, and 100 mM Na2HAsO4·7H2O at 15 °C and 0.25 mM CuSO4·5H2O, 2 mM CoCl2·6H2O, 0.5 mM ZnSO4·7H2O, 0.01 mM Cd(NO3)2·4H2O, and 100 mM Na2HAsO4·7H2O at 25 °C). This research contributes to our understanding of bacteria's ability to survive abiotic stresses. The isolated strain can be a potential candidate for implementation for environmental and agricultural purposes.

Nodulation and Growth Promotion of Chickpea by Mesorhizobium Isolates from Diverse Sources

The cultivation of chickpea (Cicer arietinum L.) in South Africa is dependent on the application of suitable Mesorhizobium inoculants. Therefore, we evaluated the symbiotic effectiveness of several Mesorhizobium strains with different chickpea genotypes under controlled conditions. The tested parameters included shoot dry weight (SDW), nodule fresh weight (NFW), plant height, relative symbiotic effectiveness (RSE) on the plant as well as indole acetic acid (IAA) production and phosphate solubilization on the rhizobia. Twenty-one Mesorhizobium strains and six desi chickpea genotypes were laid out in a completely randomized design (CRD) with three replicates in a glasshouse pot experiment. The factors, chickpea genotype and Mesorhizobium strain, had significant effects on the measured parameters (p < 0.001) but lacked significant interactions based on the analysis of variance (ANOVA). The light variety desi genotype outperformed the other chickpea genotypes on all tested parameters. In general, inoculation with strains LMG15046, CC1192, XAP4, XAP10, and LMG14989 performed best for all the tested parameters. All the strains were able to produce IAA and solubilize phosphate except the South African field isolates, which could not solubilize phosphate. Taken together, inoculation with compatible Mesorhizobium promoted chickpea growth. This is the first study to report on chickpea-compatible Mesorhizobium strains isolated from uninoculated South African soils with no history of chickpea production; although, their plant growth promotion ability was poorer compared to some of the globally sourced strains. Since this study was conducted under controlled conditions, we recommend field studies to assess the performance of the five highlighted strains under environmental conditions in South Africa.

Plant growth-promoting rhizobacterial biofertilizers for crop production: The past, present, and future

Recent decades have witnessed increased agricultural production to match the global demand for food fueled by population increase. Conventional agricultural practices are heavily reliant on artificial fertilizers that have numerous human and environmental health effects. Cognizant of this, sustainability researchers and environmentalists have increased their focus on other crop fertilization mechanisms. Biofertilizers are microbial formulations constituted of indigenous plant growth-promoting rhizobacteria (PGPR) that directly or indirectly promote plant growth through the solubilization of soil nutrients, and the production of plant growth-stimulating hormones and iron-sequestering metabolites called siderophores. Biofertilizers have continually been studied, recommended, and even successfully adopted for the production of many crops in the world. These microbial products hold massive potential as sustainable crop production tools, especially in the wake of climate change that is partly fueled by artificial fertilizers. Despite the growing interest in the technology, its full potential has not yet been achieved and utilization still seems to be in infancy. There is a need to shed light on the past, current, and future prospects of biofertilizers to increase their understanding and utility. This review evaluates the history of PGPR biofertilizers, assesses their present utilization, and critically advocates their future in sustainable crop production. It, therefore, updates our understanding of the evolution of PGPR biofertilizers in crop production. Such information can facilitate the evaluation of their potential and ultimately pave the way for increased exploitation.

Chickpea (Cicer arietinum L.) Seeds as a Reservoir of Endophytic Plant Growth-Promoting Bacteria

The seed microbiome, the primary source of inoculum for plants, may play an important role in plant growth, health and productivity. However, the structure and function of chickpea seed endophytes are poorly characterized. Bacteria with beneficial characteristics can be selected by the plant and transmitted vertically via the seed to benefit the next generation. Studying the diversity and multifunctionality of seed microbial communities can provide innovative opportunities in the field of plant-microbe interaction. This study aimed to isolate, identify and characterize culturable endophytic bacteria from chickpea (Cicer arietinum L.) seeds. Phylogenetic analysis based on 16S rDNA showed that the endophytic bacteria belong to the genera Mesorhizobium, Burkholderia, Bacillus, Priestia, Paenibacillus, Alcaligenes, Acinetobacter, Rahnella, Enterobacter, Tsukamurella, and Microbacterium. The most frequently observed genus was Bacillus; however, rhizobia typically associated with chickpea roots were also found, which is a novel finding of this study. Siderophore production and phosphorus solubilization were the most widespread plant growth-promoting features, while hydrogen cyanide production was relatively rare among the isolates. Most of the isolates possess two or more plant growth-promoting features; however, only Bacillus thuringiensis Y2B, a well-known entomopathogenic bacteria, exhibited the presence of all plant growth-promoting traits evaluated. Results suggest that endophytic bacteria such as Bacillus, Mesorhizobium, and Burkholderia may be vertically transferred from inoculated plants to seeds to benefit the next generation.

Distribution, Characterization and the Commercialization of Elite Rhizobia Strains in Africa

Grain legumes play a significant role in smallholder farming systems in Africa because of their contribution to nutrition and income security and their role in fixing nitrogen. Biological Nitrogen Fixation (BNF) serves a critical role in improving soil fertility for legumes. Although much research has been conducted on rhizobia in nitrogen fixation and their contribution to soil fertility, much less is known about the distribution and diversity of the bacteria strains in different areas of the world and which of the strains achieve optimal benefits for the host plants under specific soil and environmental conditions. This paper reviews the distribution, characterization, and commercialization of elite rhizobia strains in Africa.

Application of Recombinant Human scFv Antibody as a Powerful Tool to Monitor Nitrogen Fixing Biofertilizer in Rice and Legume

Bradyrhizobium is an endophytic bacterium under investigation as an efficient biofertilizer for sustainable legume-rice rotational cropping system. Monitoring and bio-imaging of this nitrogen fixing bacterium is essential for the study of plant-microbe evolution, soil microbiome, as well as quality control in organic farming. While phage display antibody technology has been widely used to generate recombinant antibody for myriad medical purposes, so far, this technology has been minimally applied in the agricultural sector. In this study, single-chain variable fragments (scFv) against two Bradyrhizobium strains SUTN9-2 (yiN92-1e10) and DOA9 (yiDOA9-162) were isolated from a human phage display antibody library. Specific binding of scFv was demonstrated by ELISA and confocal-immunofluorescence imaging techniques. Bradyrhizobium localization in both endophytic and bacteroid forms could be observed inside rice tissue and plant nodule, respectively. Moreover, successful application of the recombinant antibody for the evaluation of nodule occupancy was also demonstrated in comparison with standard GUS-staining method. The results of this study showed for the first time the potential use of human phage display scFv antibody for imaging and monitoring of Bradyrhizobium biofertilizer and thus could be further applied for point-of-detection of bacterial inoculum in the legume-rice rotational crop system.

IMPORTANCE Human scFv antibody generated from phage display technology was successfully used for the generation of specific recombinant antibodies: yiN92-1e10 and yiDOA9-162 for the detection of Bradyrhizobium strains SUTN9-2 and DOA9, respectively. These two recombinant scFv antibodies could be used for precise detection of the rhizobia both in symbiosis with legume and endophyte in rice tissue by ELISA and immunofluorescent staining, during legume-rice rotational cropping system in the field. This methodology can be further employed for the study of other plant-microbe interactions and monitoring of biofertilizer in diverse sustainable cropping systems as well as in precision agriculture.