Legume plant defenses and nutrients mediate indirect interactions between soil rhizobia and chewing herbivores

Soil bacteria that form mutualisms with plants, such as rhizobia, affects susceptibility of plants to herbivores and pathogens. Soil rhizobia also promote nitrogen fixation, which mediates host nutrient levels and defenses. However, whether aboveground herbivores affect the function of soil rhizobia remains poorly understood. We assessed reciprocal interactions between Sitona lineatus, a chewing herbivore, and pea (Pisum sativum) plants grown with or without rhizobia (Rhizobium leguminosarum biovar viciae). We also examined the underlying plant-defense and nutritional mechanisms of these interactions. In our experiments, soil rhizobia influenced feeding and herbivory by chewing herbivores. Leaf defoliation by S. lineatus was lower on plants treated with rhizobia, but these insects had similar amino acid levels compared to those on un-inoculated plants. Plants grown with soil rhizobia had increased expression of gene transcripts associated with phytohormone-mediated defense, which may explain decreased susceptibility to S. lineatus. Rhizobia also induced expression of gene transcripts associated with physical and antioxidant-related defense pathways in P. sativum. Conversely, S. lineatus feeding reduced the number of root nodules and nodule biomass, suggesting a disruption of the symbiosis between plants and rhizobia. Our study shows that aboveground herbivores can engage in mutually antagonistic interactions with soil microbes mediated through a multitude of plant-mediated pathways.

Drought Tolerant Enterobacter sp./Leclercia adecarboxylata Secretes Indole-3-acetic Acid and Other Biomolecules and Enhances the Biological Attributes of Vigna radiata (L.) R. Wilczek in Water Deficit Conditions

Drought or water stress is a limiting factor that hampers the growth and yield of edible crops. Drought-tolerant plant growth-promoting rhizobacteria (PGPR) can mitigate water stress in crops by synthesizing multiple bioactive molecules. Here, strain PAB19 recovered from rhizospheric soil was biochemically and molecularly characterized, and identified as Enterobacter sp./Leclercia adecarboxylata (MT672579.1). Strain PAB19 tolerated an exceptionally high level of drought (18% PEG-6000) and produced indole-3-acetic acid (176.2 ± 5.6 µg mL−1), ACC deaminase (56.6 ± 5.0 µg mL−1), salicylic acid (42.5 ± 3.0 µg mL−1), 2,3-dihydroxy benzoic acid (DHBA) (44.3 ± 2.3 µg mL−1), exopolysaccharide (204 ± 14.7 µg mL−1), alginate (82.3 ± 6.5 µg mL−1), and solubilized tricalcium phosphate (98.3 ± 3.5 µg mL−1), in the presence of 15% polyethylene glycol. Furthermore, strain PAB19 alleviated water stress and significantly (p ≤ 0.05) improved the overall growth and biochemical attributes of Vigna radiata (L.) R. Wilczek. For instance, at 2% PEG stress, PAB19 inoculation maximally increased germination, root dry biomass, leaf carotenoid content, nodule biomass, leghaemoglobin (LHb) content, leaf water potential (ΨL), membrane stability index (MSI), and pod yield by 10%, 7%, 14%, 38%, 9%, 17%, 11%, and 11%, respectively, over un-inoculated plants. Additionally, PAB19 inoculation reduced two stressor metabolites, proline and malondialdehyde, and antioxidant enzymes (POD, SOD, CAT, and GR) levels in V. radiata foliage in water stress conditions. Following inoculation of strain PAB19 with 15% PEG in soil, stomatal conductance, intercellular CO2 concentration, transpiration rate, water vapor deficit, intrinsic water use efficiency, and photosynthetic rate were significantly improved by 12%, 8%, 42%, 10%, 9% and 16%, respectively. Rhizospheric CFU counts of PAB19 were 2.33 and 2.11 log CFU g−1 after treatment with 15% PEG solution and 8.46 and 6.67 log CFU g−1 for untreated controls at 40 and 80 DAS, respectively. Conclusively, this study suggests the potential of Enterobacter sp./L. adecarboxylata PAB19 to alleviate water stress by improving the biological and biochemical features and of V. radiata under water-deficit conditions.

Penicillium simplicissimum NL-Z1 Induced an Imposed Effect to Promote the Leguminous Plant Growth

It is found effective for phytoremediation of the guest soil spraying method by adding microbes to promote the growth of arbor leguminous plant on a high and steep rock slope. However, its underlying mechanisms remain elusive. Here, some experiments were conducted to explore the multifunctions of Penicillium simplicissimum NL-Z1 on rock weathering, nodule growth, and beneficial microbial regulation. The results show that P. simplicissimum NL-Z1 significantly increased the release of phosphorus, potassium, calcium, and magnesium from the rock by 226, 29, 24, and 95%, respectively, compared with that of the control. A significant increase of 153% in Indigofera pseudotinctoria Matsum nodule biomass, accompanied by an increase of 37% in the leguminous plant biomass was observed in the P. simplicissimum NL-Z1 treatment than in the control treatment. Interestingly, even though P. simplicissimum NL-Z1 itself became a minor microbial community in the soil, it induced a significant increase in Mortierella, which, as a beneficial microbe, can promote phosphate-solubilizing and plant growth. The results suggest that P. simplicissimum NL-Z1 could induce an imposed effect to promote leguminous plant growth, which may be conducive to the development of the phytoremediation technique for high and steep rock slope. The study provides a novel thought of using the indirect effect of microbes, i.e., promoting other beneficial microbes, to improve soil environment.

Impacts of Rhizobium Strain Ar02 on the Nodulation, Growth, Nitrogen (N2) Fixation Rate and ion Accumulation in Phaseolus vulgaris L. under Salt Stress

Background: Phaseolus vulgaris L. -rhizobia symbiosis has effectively enhanced common bean productivity via multiple biological mechanisms. This study aims to assess the impacts of the strain of Rhizobium on the nodulation, growth, nitrogen (N2) fixation rate and ion accumulation within Phaseolus vulgaris L. under salt stress. Methods: The Coco Blanc cultivar of the common bean was inoculated with the Ar02 rhizobia strain at 15 days after germination. Bean plants were inoculated in perlite culture to which salt was added in concentrations of 0, 25, 50 and 75 mmol L-1 NaCl. Result: Inoculation with the Ar02 rhizobia strain led to infective and effective symbiosis with the common bean plants exposed to saline solutions and non-saline solutions, respectively. Nodule biomass and nitrogen content declined under salt stress but maintained a higher number of nodules and nodule biomass at 75 mM NaCl. Plant root and shoot length increased with higher biomass under saline conditions, significantly more than the non-inoculated plant without salt. However, the progressive addition of NaCl reduced the growth of the root and shoot and the biomass within the inoculated plant. Salinity led to increased Na+ within the plant’s shoot, along with a reduction in Ca+2 and K+ concentrations. The shoot’s Ca+2, Na+ and K+ content were higher in the inoculated plant than the non-inoculated. The salt tolerance in common bean plants inoculated with Ar02 rhizobia was linked with the plant’s capability to sustain nodulation and enhance Na+ concentration in the shoot. Furthermore, salt tolerance within the same variety inoculated with Rhizobium was linked to a decline in the Ca+ and K+ concentrations in the shoot region of salt-exposed plants.

Nodulation Alleviates Water Stress in Vachellia Sieberiana

Species in the genus Vachellia (Fabaceae) have a global tropical and sub-tropical distribution. Numerous Vachellia species are currently observed to be expanding their indigenous ranges and increasing in dominance globally, suggesting an overarching driver. Most Vachellia species enhance nitrogen uptake mutualistically via specialized root nodule structures. Nodules contain N2-fixing rhizobia that consume host supplied carbon to catalyse atmospheric N2 into a plant useable form, a key element in plant growth. The rhizobial mutualism of some Vachellia species may be vital to understanding changing patterns of ecological success observed across the savanna precipitation gradient. Here, we investigated how the seedling root development and physiology of two dominant savanna woody species, the arid-adapted Vachellia erioloba and the mesic-adapted Vachellia sieberiana, responded to simulated drought events. Seedlings of both species were grown at 4%, 8% and 16% soil moisture content (SMC) for four months. Seedling growth and allometry of arid-adapted V. erioloba was unresponsive to water stress treatments, and no nodulation was observed, reflecting a fixed higher relative investment in belowground biomass. In contrast, V. sieberiana roots were nodulated, but developed the highest nodule biomass and growth rate when grown at the lowest soil moisture (4% SMC). These patterns suggest that effective life history strategies for the arid-30 adapted species precludes the need for rhizobial mutualism, possibly due to more “open” N cycling and lower competitive interactions in arid systems, while the more “closed” N cycling in mesic savannas, and higher competitive stress, may favour nodulation, especially under low water supply that limits root access to soil nitrogen, and signals a more competitive environment and an advantage from N2-fixing.

Lower nodule biomass with increased nitrogenase efficiency in Robinia pseudoacacia seedlings when grown under low soil phosphorus conditions

Symbiotic nitrogen (N) fixation is the largest non-anthropogenic N input to many terrestrial ecosystems. The energetic expense of symbiotic N fixation suggests soil phosphorus (P) availability may regulate symbiotic nitrogen fixation directly through nodule development and function, and/or indirectly through plant growth. Since P availability is heterogenous in the landscape, we sought to understand if symbiotic nitrogen fixation responds to both P availability and heterogeneity. To test how P availability affects symbiotic nitrogen fixation, we grew Robinia pseudoacacia seedlings under high (8.1 g P m−2) and low (0.2 g P m−2) conditions. Soil P heterogeneity was simulated by splitting roots into soil patches receiving P or no-P fertilizer. At the whole plant level, P availability limited seedling and nodule biomass. However, the low P treatment had higher nitrogenase efficiency (acetylene reduced (AR) g−1 nodule; a nodule efficiency proxy). High P seedlings had significantly more root and nodule biomass in the patches directly receiving P fertilizer, but patch proliferation was absent in the low P treatment. AR g−1 seedling did not differ between P treatments, suggesting P indirectly limited symbiotic nitrogen fixation through plant growth, rather than directly limiting symbiotic nitrogen fixation. This relatively consistent AR g−1 seedling across treatments demonstrates the ability of seedlings to respond to low P conditions with increased nitrogenase efficiency.

Effect of Soil Available Phosphorus Levels on Chickpea (Cicer arietinum L.) - Rhizobia Symbiotic Association

Background: Growing chickpea (Cicer arietinum) plants is affected by several environmental constraints as osmotic stress and nutrients deficiency particularly phosphorus (P). For other legume species, it was confirmed that P deficiency affects negatively their rhizobial symbiosis. The purpose of this study was to assess the effect of soil available P level on chickpea-rhizobia symbiosis under field conditions at Oualidia region of Morocco. Methods: Ten farmers’ fields with different soil available P levels were considered to carry out this study based on samples of 10 plants per plot. Result: The results showed that the plants from soil 7, with the lowest pH and the highest available P level (23.52ppm), presented high shoot dry weight (38.3 g/plant). Meanwhile the soil 5 with the lowest available P content showed low plant growth. The shoot P content was positively linked to soil P level but nodule biomass showed an irregular variation with soil available P level. Furthermore, it was confirmed that adequate plant P nutrition results in high chickpea yield and it was the case for plants from soil 7 presenting a mean yield of 62 seeds per plant). Finally, strong correlation was noted between yield and phosphorus concentration in soil (r=0.94).

Interaction of Biochar Type and Rhizobia Inoculation Increases the Growth and Biological Nitrogen Fixation of Robinia pseudoacacia Seedlings

Adding biochar to soil can change soil properties and subsequently affect plant growth, but this effect can vary because of different feedstocks and methods (e.g., pyrolysis or gasification) used to create the biochar. Growth and biological nitrogen fixation (BNF) of leguminous plants can be improved with rhizobia inoculation that fosters nodule development. Thus, this factorial greenhouse study examined the effects of two types of biochar (i.e., pyrolysis and gasification) added at a rate of 5% (v:v) to a peat-based growth substrate and rhizobia inoculation (yes or no) on Robinia pseudoacacia (black locust) seedlings supplied with 15NH415NO3. Seedling and nodule growth, nitrogen (N) content, and δ15N × 1000 were evaluated after 3 months. While addition of biochar without inoculation had no effect on seedling growth, inoculation with rhizobia increased seedling growth, BNF, and N status. Inoculated seedlings had reduced δ15N, indicating that N provided via fertilization was being diluted by N additions through BNF. Biochar type and inoculation interacted to affect seedling growth. Combining inoculation with either biochar type increased seedling leaf, stem, and total biomass, whereas gasifier biochar and inoculation improved all seedling growth variables and nodule biomass.

The impact of a native hemiparasite on a major invasive shrub is affected by host size at time of infection

Many studies have investigated the effect of parasitic plants on their hosts; however, few have examined how parasite impact is affected by host size. In a glasshouse experiment, we investigated the impact of the Australian native hemiparasitic vine, Cassytha pubescens, on a major invasive shrub, Ulex europaeus, of different sizes. Infected plants had significantly lower total, shoot, and root biomass, but the parasite’s impact was more severe on small than on large hosts. When infected, small but not large hosts had significantly lower nodule biomass. Irrespective of size, infection significantly decreased the host shoot/root ratio, pre-dawn and midday quantum yields, maximum electron transport rates, and carbon isotope composition, and the host nodule biomass per gram of root biomass significantly increased in response to infection. Infection did not affect host foliar nitrogen concentration or midday shoot water potential. Parasite biomass was significantly lower on small relative to large hosts, but was similar when expressed on a per gram of host total biomass basis. Parasite stem nitrogen, phosphorus, and potassium concentrations were significantly greater when C. pubescens was growing on small than on large hosts. Our results clearly show that C. pubescens strongly decreases performance of this major invasive shrub, especially when hosts are small. This suggests that C. pubescens could be used most effectively as a native biocontrol when deployed on smaller hosts.

Polyamines produced by Sinorhizobium meliloti Rm8530 contribute to symbiotically relevant phenotypes ex planta and to nodulation efficiency on alfalfa

In nitrogen-fixing rhizobia, emerging evidence shows significant roles for polyamines in growth and abiotic stress resistance. In this work we show that a polyamine-deficient ornithine decarboxylase null mutant (odc2) derived from Sinorhizobium meliloti Rm8530 had significant phenotypic differences from the wild-type, including greatly reduced production of exopolysaccharides (EPS; ostensibly both succinoglycan and galactoglucan), increased sensitivity to oxidative stress and decreased swimming motility. The introduction of the odc2 gene borne on a plasmid into the odc2 mutant restored wild-type phenotypes for EPS production, growth under oxidative stress and swimming. The production of calcofluor-binding EPS (succinoglycan) by the odc2 mutant was also completely or mostly restored in the presence of exogenous spermidine (Spd), norspermidine (NSpd) or spermine (Spm). The odc2 mutant formed about 25 % more biofilm than the wild-type, and its ability to form biofilm was significantly inhibited by exogenous Spd, NSpd or Spm. The odc2 mutant formed a less efficient symbiosis with alfalfa, resulting in plants with significantly less biomass and height, more nodules but less nodule biomass, and 25 % less nitrogen-fixing activity. Exogenously supplied Put was not able to revert these phenotypes and caused a similar increase in plant height and dry weight in uninoculated plants and in those inoculated with the wild-type or odc2 mutant. We discuss ways in which polyamines might affect the phenotypes of the odc2 mutant.