Evaluation of Symbiotic Association between Various Rhizobia, Capable of Producing Plant-Growth-Promoting Biomolecules and Mung Bean for Sustainable Production

To feed the increased world population, sustainability in the production of crops is the need of the hour, and exploration of an effective symbiotic association of rhizobia with legumes may serve the purpose. A laboratory-scale experiment was conducted to evaluate the symbiotic effectiveness of twenty wild rhizobial isolates (MR1–MR20) on the growth, physiology, biochemical traits, and nodulation of mung bean to predict better crop production with higher yields. Rhizobial strain MR4 resulted in a 52% increase in shoot length and 49% increase in shoot fresh mass, while MR5 showed a 30% increase in root length, with 67% and 65% improvement in root fresh mass by MR4 and MR5, respectively, compared to uninoculated control. Total dry matter of mung bean was enhanced by 73% and 68% with strains MR4 and MR5 followed by MR1 and MR3 with 60% increase in comparison to control. Rhizobial strain MR5 produced a maximum (25 nodules) number of nodules followed by MR4, MR3, and MR1 which produced 24, 23, and 21 nodules per plant. Results related to physiological parameters showed the best performance of MR4 and MR5 compared to control among all treatments. MR4 strain helped the plants to produce the lowest values of total soluble protein (TSP) (38% less), flavonoids contents (44% less), and malondialdehyde (MDA) contents (52% less) among all treatments compared to uninoculated control plants. Total phenolics contents of mung bean plants also showed significantly variable results, with the highest value of 54.79 mg kg−1 in MR—inoculated plants, followed by MR5- and MR1-inoculated plants, while the minimum concentration of total phenolics was recorded in uninoculated control plants of mung bean. Based on the results of growth promotion, nodulation ability, and physiological and biochemical characteristics recorded in an experimental trial conducted under gnotobiotic conditions, four rhizobial isolates (MR1, MR3, MR4, and MR5) were selected using cluster and principal component analysis. Selected strains were also tested for a variety of plant-growth-promoting molecules to develop a correlation with the results of plant-based parameters, and it was concluded that these wild rhizobial strains were effective in improving sustainable production of mung bean.

Differential Expression of Paraburkholderia phymatum Type VI Secretion Systems (T6SS) Suggests a Role of T6SS-b in Early Symbiotic Interaction

Paraburkholderia phymatum STM815, a rhizobial strain of the Burkholderiaceae family, is able to nodulate a broad range of legumes including the agriculturally important Phaseolus vulgaris (common bean). P. phymatum harbors two type VI Secretion Systems (T6SS-b and T6SS-3) in its genome that contribute to its high interbacterial competitiveness in vitro and in infecting the roots of several legumes. In this study, we show that P. phymatum T6SS-b is found in the genomes of several soil-dwelling plant symbionts and that its expression is induced by the presence of citrate and is higher at 20/28°C compared to 37°C. Conversely, T6SS-3 shows homologies to T6SS clusters found in several pathogenic Burkholderia strains, is more prominently expressed with succinate during stationary phase and at 37°C. In addition, T6SS-b expression was activated in the presence of germinated seeds as well as in P. vulgaris and Mimosa pudica root nodules. Phenotypic analysis of selected deletion mutant strains suggested a role of T6SS-b in motility but not at later stages of the interaction with legumes. In contrast, the T6SS-3 mutant was not affected in any of the free-living and symbiotic phenotypes examined. Thus, P. phymatum T6SS-b is potentially important for the early infection step in the symbiosis with legumes.

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis with Legumes

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes that involves the rhizobial infection of roots and the bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role for EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia, and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Interplay of Rhizobial Inoculants and Fungicide on Chickpea (Cicer Arietinum) and Faba Bean (Vicia Faba L.) Nodulation and Seed Yield in Central Ethiopian Highland

Field and green house experiments were conducted on faba bean and chickpea during 2016-2017 to investigate the effect of fungicides and rhizobial inoculant interaction on nodulation and biomass accumulation of chickpea under Vertisol condition and (ii) faba bean under Nitisol condition. Chickpea seed was treated with Apron Star, Imidalm and both, and co-dressed with EAL-029 rhizobia simultaneously or a week later. Likewise, in one of the two sets, faba bean seed was treated with Apron Star and simultaneously dressed with FB-1017 or FB-1035 rhizobial strain. The other set had the same strains as pre inoculant and sprayed with Mancozeb at 30th day after sowing. Sole inoculants and N (faba bean) were used as check. The application rates of Apron Star, Imidalm, and inoculant were 2.5, 0.75, and 3.12 g kg-1 of seed while for Mancozeb is 2.5kg/ha. All treatments were replicated 4x and laid in RCB design. The result generally depicted that Apron Star application was compatible to EAL-029 rhizobia on chickpea. Staggered dressing of Apron Star and EAL-029 had better chickpea shoot dry matter accumulation. With regards to faba bean, co-dressing of Apron Star with FB-1017 or FB-1035 produced the highest nodulation. This confirmed the synergy of Apron Star with FB-1017 on Nitisol of central high land of Ethiopia. Moreover, spraying mancozeb on the 30th day after sowing to FB-1017 or FB-1035 preinoculated faba bean plant showed enhanced seed yield on Nitisol.

Rhizobial Exopolysaccharides: Genetic Regulation of Their Synthesis and Relevance in Symbiosis With Legumes

Rhizobia are soil proteobacteria able to engage in a nitrogen-fixing symbiotic interaction with legumes which involves root rhizobial infection and bacterial invasion of new organs formed by the plant in response to the presence of appropriate bacterial partners. This interaction relies on a complex molecular dialogue between both symbionts. Bacterial N-acetyl-glucosamine oligomers called Nod factors are indispensable in most cases for early steps of the symbiotic interaction. In addition, different rhizobial surface polysaccharides, such as exopolysaccharides (EPS), may also be symbiotically relevant. EPS are acidic polysaccharides located out of the cell with little or no cell association that carry out important roles both in free-life and in symbiosis. EPS production is very complexly modulated and, frequently, co-regulated with that of Nod factors, but the type of co-regulation varies depending on the rhizobial strain. Many studies point out a signalling role of EPS-derived oligosaccharides in root infection and nodule invasion but, in certain symbiotic couples, EPS can be dispensable for a successful interaction. In summary, the complex regulation of the production of rhizobial EPS varies in different rhizobia and the relevance of this polysaccharide in symbiosis with legumes depends on the specific interacting couple.

Bio-protective effect of a root-nodulating Rhizobium etli strain in common bean (Phaseolus vulgaris) against Meloidogyne incognita and Radopholus similis in an in vitro autotrophic tripartite culture system

Summary The bio-protective effect of a root-nodulating strain (CNPAF 512) of the nitrogen-fixing rhizobium, Rhizobium etli, against both a sedentary (Meloidogyne incognita) and a migratory (Radopholus similis) endoparasitic nematode in common bean (Phaseolus vulgaris) was examined using an in vitro autotrophic tripartite culture system. Two in vitro assays were carried out with each of the nematode species. Each assay consisted of two treatments: the plants were either inoculated with the rhizobial strain or remained non-inoculated (control plants). To examine the effect of either pre- or simultaneous inoculation of the rhizobial strain on the reproduction of M. incognita and R. similis, one assay was carried out in which the nematodes were inoculated 3 weeks after rhizobial inoculation while another assay was carried out in which the nematodes were inoculated simultaneously with the rihizobial strain. Both pre-inoculation and simultaneous inoculation with R. etli CNPAF 512 significantly suppressed the reproduction of both M. incognita and R. similis.

OnfD, an AraC-Type Transcriptional Regulator Encoded by Rhizobium tropici CIAT 899 and Involved in Nod Factor Synthesis and Symbiosis

ABSTRACT Rhizobium tropici CIAT 899 is a broad-host-range rhizobial strain that establishes symbiotic interactions with legumes and tolerates different environmental stresses such as heat, acidity, or salinity. This rhizobial strain produces a wide variety of symbiotically active nodulation factors (NF) induced not only by the presence of plant-released flavonoids but also under osmotic stress conditions through the LysR-type transcriptional regulators NodD1 (flavonoids) and NodD2 (osmotic stress). However, the activation of NodD2 under high-osmotic-stress conditions remains elusive. Here, we have studied the role of a new AraC-type regulator (named as OnfD) in the symbiotic interaction of R. tropici CIAT 899 with Phaseolus vulgaris and Lotus plants. We determined that OnfD is required under salt stress conditions for the transcriptional activation of the nodulation genes and therefore the synthesis and export of NF, which are required for a successful symbiosis with P. vulgaris. Moreover, using bacterial two-hybrid analysis, we demonstrated that the OnfD and NodD2 proteins form homodimers and OnfD/NodD2 form heterodimers, which could be involved in the production of NF in the presence of osmotic stress conditions since both regulators are required for NF synthesis in the presence of salt. A structural model of OnfD is presented and discussed. IMPORTANCE The synthesis and export of rhizobial NF are mediated by a conserved group of LysR-type regulators, the NodD proteins. Here, we have demonstrated that a non-LysR-type regulator, an AraC-type protein, is required for the transcriptional activation of symbiotic genes and for the synthesis of symbiotically active NF under salt stress conditions.

Amelioration of phlorizin-induced autotoxicity in Malus hupehensis by rhizobium

Background: Phlorizin can cause autotoxic reaction in apple plant, resulting in severe yield losses. It is noted that rhizobium can both degrade phenolic compounds and form mutually beneficial relationships with non-legumes. However, phenolic-degrading rhizobia have rarely been studied for its potential to alleviate the autotoxicity in apple plant.Results: The aim of this study is to determine the capability of phenolic-degrading rhizobia for eliminate the toxic effects caused by phlorizin in Malus hupehensis. Results showed bioaugmentation with rhizobial strain y5077 not only efficiently degrades phlorizin, but also eliminates growth inhibition caused by phlorizin. Rhizobia inoculation enhanced melatonin accumulations and reduced H2O2 and malondialdehyde production. Meantime, activities of antioxidant enzymes and the expression of genes in the ascorbate-glutathione cycle were maintained at a lower level under phlorizin stress. In addition, exogenous melatonin treatment abolished the induction of antioxidant enzymes by phlorizin. The results showed that rhizobia inoculation could alleviate phlorizin-induced oxidative stress by inducing melatonin production.Conclusions: The facts suggested phenolic-degrading rhizobia might be exploited for soil bioremediation in autotoxic apple orchard.