Endophytes are nonpathogenic plant-associated bacteria that can play an important role in plant vitality and may confer resistance to abiotic or biotic stress. The effects of 5 endophytic bacterial strains isolated from pepper plants showing 1-aminocyclopropane-1-carboxylate deaminase activity were studied in sweet pepper under in vitro conditions. Four of the strains tested showed production of indole acetic acid. Plant growth, osmotic potential, free proline content, and gene expression were monitored in leaves and roots under control and mild osmotic stress conditions. All indole acetate producers promoted growth in Capsicum annuum L. ‘Ziegenhorn Bello’, from which they were isolated. Osmotic stress caused an increase in the content of free proline in the leaves of both inoculated and noninoculated plants. Inoculated control plants also revealed higher proline levels in comparison with noninoculated control plants. Differential gene expression patterns of CaACCO, CaLTPI, CaSAR82A, and putative P5CR and P5CS genes during moderate stress were observed, depending on the bacterium applied. Inoculation with 2 bacterial strains, EZB4 and EZB8 ( Arthrobacter sp. and Bacillus sp., respectively), resulted in a significantly reduced upregulation or even downregulation of the stress-inducible genes CaACCO and CaLTPI, as compared with the gene expression in noninoculated plants. This indicates that both strains reduced abiotic stress in pepper under the conditions tested.
Abstract Global banana production is affected by Fusarium wilt, a devastating disease caused by the soilborne root‐infecting fungus, Fusarium oxysporum f. sp. cubense (Foc). Fusarium wilt is notoriously difficult to manage because infection arises through complex below‐ground interactions between Foc, the plant, and the soil microbiome in the root–soil interface, defined as the rhizosphere. Interactions in the rhizosphere play a pivotal role in processes associated with pathogen development and plant health. Modulation of these processes through manipulation and management of the banana rhizosphere provides an auspicious prospect for management of Fusarium wilt. Yet, a fundamental understanding of interactions in the banana rhizosphere is still lacking. The objective of this review is to discuss the state‐of‐the‐art of the relatively scant data available on banana below‐ground interactions in relation to Fusarium wilt and, as a result, to highlight key research gaps. Specifically, we seek to understand (a) the biology of Foc and its interaction with banana; (b) the ecology of Foc, including the role of root‐exuded metabolites in rhizosphere interactions; and (c) soil management practices and how they modulate Fusarium wilt. A better understanding of molecular and ecological factors influencing banana below‐ground interactions has implications for the development of targeted interventions in the management of Fusarium wilt through manipulation of the banana rhizosphere.
Measurements of carbon dioxide (CO2)-evolution from soils are important in evaluating biomass and activity of soil microorganisms, as well as decomposition of soil organic matter. The Respicond VI is a fully computerized system allowing continuous measurement of CO2 evolution in short- and long-term soil incubation experiments in up to 96 incubation vessels. The measurement of CO2 evolution is based on the absorption of CO2 by an electrolyte (KOH solution) producing a change in the cell conductance measured using two electrodes. In this study, the Respicond VI was recalibrated yielding 174.5 mg CO2 as constant A expressing the theoretical maximum amount of CO2 absorbed in 10 ml 0.5 M KOH. This value of A corresponds to 34.9 mg CO2 ml-1 1 M KOH. The constant A does neither depend on the investigated incubation temperatures (5°C - 25°C) nor on the concentrations of the KOH solutions (0.5, 0.1, 0.05 M KOH). To eliminate any influence of changing incubation temperatures, either induced by uncertainties in temperature control or as a part of the experimental setup, on the conductance of KOH solution, a correction procedure was developed using a factor calculated from changing conductance of KOH solutions in incubation vessels without soil.
Partial least squares regression (PLSR) analysis was used for the generic prediction of soil organic C (Corg) content in Alfisols using diffuse reflectance Fourier-transform mid-infrared spectroscopy (DRIFT-MIRS). For prediction improvement, we tested the potential of Corg removal by heating and spectral subtraction of mineralogic signals in DRIFT-MIRS spectra. Eighty-seven Alfisol samples (Corg range 1–32 g kg−1) of contrasting geographic origin were heated (550°C) or left untreated (NT). Spectra from DRIFT-MIRS were recorded and spectral subtraction was performed by taking the difference between DRIFT-MIRS spectra of NT and heated soils. Prediction of Corg by PLSR was acceptable for NT (R2 = 0.93; root mean square error of cross validation = 0.20; residual prediction deviation = 3.7) but was not improved by heating and spectral subtraction. Additional studies are recommended to substantiate the potential of PLSR-based, generic quantification of Corg in unheated soils for soil groups other than Alfisols and to assess if heating is suited to advance generic prediction of Corg across soil orders with widely different mineralogic signatures.
Soil microbial community structure is determined by environmental conditions and influenced by other factors, such as the intensity of the land use management. Studies addressing the effect of environmental factors and management on grassland soil microbial communities at the continental scale are missing, and the wide range of ecosystem services provided by these ecosystems are thus also wanting. To address this knowledge gap, this study presents data on grassland soil microbial communities along a pan-European agro-ecological gradient. The transect included five geographical locations (Sweden, Germany, Switzerland, Portugal mainland, Portugal Azores). At each location, soils were collected in two regions characterized by favourable and less favourable conditions for plant growth. In each of these ten regions, grasslands along a gradient of management intensity were selected, i.e. grassland under intensive, less intensive and extensive management. Phospholipid fatty acid analysis (PLFA) was used to characterize the microbial community structure (PLFA pattern) in relation to climatic and soil properties. Over the whole geographical range, the environmental properties determined the soil microbial community structure. In Sweden and Switzerland, the regional growth conditions had the strongest influence on the soil microbial communities, while in Germany, Portugal mainland and Azores the management intensity was more important. Splitting up this whole community response into individual groups reveals that, in general, saprotrophic fungal biomarkers were highest in extensively managed grasslands while bacterial biomarkers differed mainly between the regions. We conclude that at the transect level, climate and soil properties were the most important factors influencing soil bacterial community structure, while soil fungal groups were more responsive to grassland management intensity. Overall agricultural sustainability could benefit from informed soil health promoting management practices, and this study contributes to such knowledge, showing the importance of management for the soil microbial biomass and community structure.
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