Symbiotic ability of diverse Frankia strains on Casuarina glauca plants in hydroponic conditions

Silver Nanoparticles Alter Soil Microbial Community Compositions and Metabolite Profiles in Unplanted and Cucumber-Planted Soils

The rapid development of nanotechnology makes the environmental impact assessment a necessity to ensure the sustainable use of engineered nanomaterials. Here, silver nanoparticles (AgNPs) at 100 mg/kg were added to soils in the absence or presence of cucumber (Cucumis sativa) plants for 60 days. The response of the soil microbial community and associated soil metabolites was investigated by 16S rRNA gene sequencing and gas chromatography-mass spectrometry (GC-MS)-based metabolomics, respectively. The results show that AgNP exposure significantly increased the soil pH in both unplanted and cucumber-planted soils. The soil bacterial community structure was altered upon Ag exposure in both soils. Several functionally significant bacterial groups, which are associated with carbon, nitrogen, and phosphorus cycling, were compromised by AgNPs in both unplanted and cucumber-planted soils. Generally, plants played a limited role in mediating the impact of AgNPs on the bacterial community. Soil metabolomic analysis showed that AgNPs altered the metabolite profile in both unplanted and cucumber-planted soils. The significantly changed metabolites are involved in sugar and amino acid-related metabolic pathways, indicating the perturbation of C and N metabolism, which is consistent with the bacterial community structure results. In addition, several fatty acids were significantly decreased upon exposure to AgNPs in both unplanted and cucumber-planted soils, suggesting the possible oxidative stress imposed on microbial cell membranes. These results provide valuable information for understanding the biological and biochemical impact of AgNP exposure on both plant species and on soil microbial communities; such understanding is needed to understand the risk posed by these materials in the environment.

Research Advances of Beneficial Microbiota Associated with Crop Plants

Plants are associated with hundreds of thousands of microbes that are present outside on the surfaces or colonizing inside plant organs, such as leaves and roots. Plant-associated microbiota plays a vital role in regulating various biological processes and affects a wide range of traits involved in plant growth and development, as well as plant responses to adverse environmental conditions. An increasing number of studies have illustrated the important role of microbiota in crop plant growth and environmental stress resistance, which overall assists agricultural sustainability. Beneficial bacteria and fungi have been isolated and applied, which show potential applications in the improvement of agricultural technologies, as well as plant growth promotion and stress resistance, which all lead to enhanced crop yields. The symbioses of arbuscular mycorrhizal fungi, rhizobia and Frankia species with their host plants have been intensively studied to provide mechanistic insights into the mutual beneficial relationship of plant–microbe interactions. With the advances in second generation sequencing and omic technologies, a number of important mechanisms underlying plant–microbe interactions have been unraveled. However, the associations of microbes with their host plants are more complicated than expected, and many questions remain without proper answers. These include the influence of microbiota on the allelochemical effect caused by one plant upon another via the production of chemical compounds, or how the monoculture of crops influences their rhizosphere microbial community and diversity, which in turn affects the crop growth and responses to environmental stresses. In this review, first, we systematically illustrate the impacts of beneficial microbiota, particularly beneficial bacteria and fungi on crop plant growth and development and, then, discuss the correlations between the beneficial microbiota and their host plants. Finally, we provide some perspectives for future studies on plant–microbe interactions.

Establishment of Actinorhizal Symbiosis in Response to Ethylene, Salicylic Acid, and Jasmonate

Phytohormones play a crucial role in regulating plant developmental processes. Among them, ethylene and jasmonate are known to be involved in plant defense responses to a wide range of biotic stresses as their levels increase with pathogen infection. In addition, these two phytohormones have been shown to inhibit plant nodulation in legumes. Here, exogenous salicylic acid (SA), jasmonate acid (JA), and ethephon (ET) were applied to the root system of Casuarina glauca plants before Frankia inoculation, in order to analyze their effects on the establishment of actinorhizal symbiosis. This protocol further describes how to identify putative ortholog genes involved in ethylene and jasmonate biosynthesis and/or signaling pathways in plant, using the Arabidopsis Information Resource (TAIR), Legume Information System (LIS), and Genevestigator databases. The expression of these genes in response to the bacterium Frankia was analyzed using the gene atlas for Casuarina-Frankia symbiosis (SESAM web site).

Symbiotic Performance of Diverse Frankia Strains on Salt-Stressed Casuarina glauca and Casuarina equisetifolia Plants

Symbiotic nitrogen-fixing associations between Casuarina trees and the actinobacteria Frankia are widely used in agroforestry in particular for salinized land reclamation. The aim of this study was to analyze the effects of salinity on the establishment of the actinorhizal symbiosis between C. glauca and two contrasting Frankia strains (salt sensitive; CcI3 vs. salt tolerant; CeD) and the role of these isolates in the salt tolerance of C. glauca and C. equisetifolia plants. We show that the number of root nodules decreased with increasing salinity levels in both plants inoculated with CcI3 and CeD. Nodule formation did not occur in seedlings inoculated with CcI3 and CeD, at NaCl concentrations above 100 and 200 mM, respectively. Salinity also affected the early deformation of plant root hairs and reduced their number and size. In addition, expression of symbiotic marker Cg12 gene, which codes for a subtilase, was reduced at 50 mM NaCl. These data suggest that the reduction of nodulation in C. glauca under salt stress is in part due to inhibition of early mechanisms of infection. We also show that prior inoculation of C. glauca and C. equisetifolia with Frankia strains CcI3 and CeD significantly improved plant height, dry biomass, chlorophyll and proline contents at all levels of salinity tested, depending on the Casuarina-Frankia association. There was no correlation between in vitro salt tolerance of Frankia strains and efficiency in planta under salt-stressed conditions. Our results strongly indicate that increased N nutrition, photosynthesis potential and proline accumulation are important factors responsible for salt tolerance of nodulated C. glauca and C. equisetifolia.

Permanent Draft Genome Sequence for Frankia sp. Strain CeD, a Nitrogen-Fixing Actinobacterium Isolated from the Root Nodules of Casuarina equistifolia Grown in Senegal

Frankiastrain CeD is a member ofFrankialineage Ib that is able to reinfect plants of theCasuarinafamilies. Here, we report a 5.0-Mbp draft genome sequence with a G+C content of 70.1% and 3,847 candidate protein-encoding genes.