Inhibitory activity of bacterial lipopeptides against Fusarium oxysporum f.sp. Strigae.

This study investigated the influence of bacterial cyclic lipopeptides (LP; surfactins, iturins, fengycins) on microbial interactions. The objective was to investigate whether the presence of bacteria inhibits fungal growth and whether this inhibition is due to the release of bacterial metabolites, particularly LP. Selected endophytic bacterial strains with known plant-growth promoting potential were cultured in the presence of Fusarium oxysporum f.sp. strigae (Fos), which was applied as model fungal organism. The extracellular metabolome of tested bacteria, with a focus on LP, was characterized, and the inhibitory effect of bacterial LP on fungal growth was investigated. The results showed that Bacillus velezensis GB03 and FZB42, as well as B. subtilis BSn5 exhibited the strongest antagonism against Fos. Paraburkholderia phytofirmans PsJN, on the other hand, tended to have a slight, though non-significant growth promotion effect. Crude LP from strains GB03 and FZB42 had the strongest inhibitory effect on Fos, with a significant inhibition of spore germination and damage of the hyphal structure. Liquid chromatography tandem mass spectrometry revealed the production of several variants of iturin, fengycin, and surfactin LP families from strains GB03, FZB42, and BSn5, with varying intensity. Using plate cultures, bacillomycin D fractions were detected in higher abundance in strains GB03, FZB42, and BSn5 in the presence of Fos. Additionally, the presence of Fos in dual plate culture triggered an increase in bacillomycin D production from the Bacillus strains. The study demonstrated the potent antagonistic effect of certain Bacillus strains (i.e., GB03, FZB42, BSn5) on Fos development. Our findings emphasize the crucial role of microbial interactions in shaping the co-existence of microbial assemblages.

Determining the footprint of breeding in the seed microbiome of a perennial cereal.

BACKGROUND: Seed endophytes have a significant impact on plant health and fitness. They can be inherited and passed on to the next plant generation. However, the impact of breeding on their composition in seeds is less understood. Here, we studied the indigenous seed microbiome of a recently domesticated perennial grain crop (Intermediate wheatgrass, Thinopyrum intermedium L.) that promises great potential for harnessing microorganisms to enhance crop performance by a multiphasic approach, including amplicon and strain libraries, as well as molecular and physiological assays. RESULTS: Intermediate wheatgrass seeds harvested from four field sites in Europe over three consecutive years were dominated by Proteobacteria (88%), followed by Firmicutes (10%). Pantoea was the most abundant genus and Pantoea agglomerans was identified as the only core taxon present in all samples. While bacterial diversity and species richness were similar across all accessions, the relative abundance varied especially in terms of low abundant and rare taxa. Seeds from four different breeding cycles (TLI C3, C5, C704, C801) showed significant differences in bacterial community composition and abundance. We found a decrease in the relative abundance of the functional genes nirK and nifH as well as a drop in bacterial diversity and richness. This was associated with a loss of amplicon sequence variants (ASVs) in Actinobacteria, Alphaproteobacteria, and Bacilli, which could be partially compensated in offspring seeds, which have been cultivated at a new site. Interestingly, only a subset assigned to potentially beneficial bacteria, e.g. Pantoea, Kosakonia, and Pseudomonas, was transmitted to the next plant generation or shared with offspring seeds. CONCLUSION: Overall, this study advances our understanding of the assembly and transmission of endophytic seed microorganisms in perennial intermediate wheatgrass and highlights the importance of considering the plant microbiome in future breeding programs.

Phosphate-Solubilizing Bacillus sp. Modulate Soil Exoenzyme Activities and Improve Wheat Growth.

Phosphorus (P) is a vital mineral nutrient in agriculture and its deficiency results in reduced growth, yield, and grain quality in cereals. Much of the applied P in agriculture becomes fixed in soils, limiting its accessibility to plants. Thus, investigating sustainable strategies to release fixed P resources and enhance plant uptake is crucial. This study explored how plant-associated bacteria employ phosphate solubilizing mechanisms to improve P availability. The growth patterns of four bacterial strains, namely Bacillus subtilis ZE15 and ZR3, along with Bacillus megaterium ZE32 and ZR19, were examined in Pikovskaya's broth culture with and without the addition of insoluble phosphorus (P). In the absence of P amendment, most strains reached a stationary growth phase by the fourth day. However, their responses diverged when exposed to P-amended media. Particularly, ZE15 demonstrated the highest P solubilization capability, achieving up to 130 µg mL-1 solubilization in vitro. All strains produced organic acids in Pikovskaya's broth culture. A comparison of the influence of Ca3(PO4)2 revealed significantly greater organic acid quantities in the presence of insoluble P. Notably, strain ZE15 exhibited the highest phosphate esterase activity (3.65 nmol g-1 dry matter), while strain ZE32 showed the highest ß-D glucosidase activity (2.81 nmol g-1 dry matter) in the presence of insoluble P. The ability of Bacillus species to solubilize P in combination with increased exoenzyme activity in the rhizosphere could be used in future studies to support P uptake through enhanced solubilization and mineralization.

Root exudate fingerprint of Brachiaria humidicola reveals vanillin as a novel and effective nitrification inhibitor.

Introduction: Biological Nitrification Inhibition (BNI) is defined as the plant-mediated control of soil nitrification via the release of nitrification inhibitors. BNI of Brachiaria humidicola (syn. Urochloa humidicola) has been mainly attributed to root-exuded fusicoccane-type diterpenes, e.g., 3-epi-brachialactone. We hypothesized, however, that BNI of B. humidicola is caused by an assemblage of bioactive secondary metabolites. Methods: B. humidicola root exudates were collected hydroponically, and metabolites were isolated by semi-preparative HPLC. Chemical structures were elucidated by HRMS as well as 1D and 2D NMR spectroscopy. Nitrification inhibiting potential of isolated metabolites was evaluated by a Nitrosomonas europaea based bioassay. Results and discussion: Besides previously described brachialactone isomers and derivatives, five phenol and cinnamic acid derivatives were identified in the root exudates of B. humidicola: 2-hydroxy-3-(hydroxymethyl)benzaldehyde, vanillin, umbelliferone and both trans- and cis-2,6-dimethoxycinnamic acid. Notably, vanillin revealed a substantially higher nitrification inhibiting activity than 3-epi-brachialactone (ED50 ∼ 12.5 µg·ml-1, ED80 ∼ 20 µg·ml-1), identifying this phenolic aldehyde as novel nitrification inhibitor (NI). Furthermore, vanillin exudation rates were in the same range as 3-epi-brachialactone (1-4 µg·h-1·g-1 root DM), suggesting a substantial contribution to the overall inhibitory activity of B. humidicola root exudates. In relation to the verification of the encountered effects within soils and considering the exclusion of any detrimental impact on the soil microbiome, the biosynthetic pathway of vanillin via the precursor phenylalanine and the intermediates p-coumaric acid/ferulic acid (precursors of further phenolic NI) might constitute a promising BNI breeding target. This applies not only to Brachiaria spp., but also to crops in general, owing to the highly conserved nature of these metabolites.

Iron necessity for chlamydospore germination in Fusarium oxysporum f. sp. cubense TR4.

Fusarium wilt disease of banana, caused by the notorious soil-borne pathogen Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), is extremely difficult to manage. Manipulation of soil pH or application of synthetic iron chelators can suppress the disease through iron starvation, which inhibits the germination of pathogen propagules called chlamydospores. However, the effect of iron starvation on chlamydospore germination is largely unknown. In this study, scanning electron microscopy was used to assemble the developmental sequence of chlamydospore germination and to assess the effect of iron starvation and pH in vitro. Germination occurs in three distinct phenotypic transitions (swelling, polarized growth, outgrowth). Outgrowth, characterized by formation of a single protrusion (germ tube), occurred at 2 to 3 h, and a maximum value of 69.3% to 76.7% outgrowth was observed at 8 to 10 h after germination induction. Germination exhibited plasticity with pH as over 60% of the chlamydospores formed a germ tube between pH 3 and pH 11. Iron-starved chlamydospores exhibited polarized-growth arrest, characterized by the inability to form a germ tube. Gene expression analysis of rnr1 and rnr2, which encode the iron-dependent enzyme ribonucleotide reductase, showed that rnr2 was upregulated (p < 0.0001) in iron-starved chlamydospores compared to the control. Collectively, these findings suggest that iron and extracellular pH are crucial for chlamydospore germination in Foc TR4. Moreover, inhibition of germination by iron starvation may be linked to a different mechanism, rather than repression of the function of ribonucleotide reductase, the enzyme that controls growth by regulation of DNA synthesis.

How feedback loops between meso- and macrofauna and organic residues contrasting in chemical quality determine decomposition dynamics in soils.

The concept of feedback loops between changes in chemical quality of decomposing organic residues and changes in faunal communities was employed in studying how such feedback loops, representing distinct ecological successional stages, determine decomposition dynamics in soils. A 52-week litterbag decomposition study was superimposed onto an 18-year long term field experiment. Four types of organic residues contrasting in chemical quality (i.e., nitrogen (N), lignin, polyphenols, cellulose) were incorporated into soil annually to assess decomposition and associated meso- and macrofauna communities. In the first 4 weeks after residue incorporation (loop #1), the abundances (densities) of both mesofauna and macrofauna were positively influenced by labile cellulose and N. The mesofauna Collembola and Acari contributed 70-100% and 0-30% to the decomposition, respectively, while the macrofauna beetles and flies contributed 20-90% and 10-66%, respectively. The abundances were highest under groundnut (high N, low lignin) ([1.35 and 0.85 individual number (g dry litter)-1] for mesofauna and macrofauna, respectively). The presence of macrofauna at week 2 led to a mass loss (R2 = 0.67**), indicating that macrofauna preceded mesofauna in degrading residue. In week 8 (transition of loop #2 to #3), only macrofauna (beetles dominated contributing 65%) played an important role in lignin decomposition (R2 = 0.56**), resulting in a mass loss (R2 = 0.52**). In week 52 (loop #4) macrofauna, ants (Formicidae) replaced beetles as the dominant decomposers showing a feedback reaction to availability of protected cellulose. The Formicidans contributed 94% to the decomposition and influenced losses of mass (R2 = 0.36*) and N (R2 = 0.78***). The feedback loop concept provides a more comprehensive "two-sided" view into decomposition, as regulated simultaneously by two factors, than earlier "one-sided" approaches to soil fauna-mediated decomposition.

Rhizophagus irregularis improves Hg tolerance of Medicago truncatula by upregulating the Zn transporter genes ZIP2 and ZIP6.

Mercury (Hg) pollution of soils is a critical environmental problem. To rehabilitate Hg contaminated soils, arbuscular mycorrhizal (AM) fungi-based phytoremediation may be supportive, yet the functional potential of AM fungi in response to Hg exposure is unclear. In a greenhouse experiment, we assessed the response of Medicago truncatula (Hg tolerance index (TI), Hg partitioning) to different Hg concentrations [0 (Hg0), 25 (Hg25), 50 (Hg50) µg g-1] in treatments with (AM) and without (NM) inoculation of Rhizophagus irregularis. Additionally, zinc (Zn) uptake and the expression of two Zn transporter genes (ZIP2, ZIP6) were examined because Zn is an essential element for plants and shares the same outer electronic configuration as Hg, implying potential competition for the same transporters. The results showed that AM plants had a higher TI than NM plants. Plant roots were identified as dominant Hg reservoirs. AM inoculation reduced the root Hg concentration under Hg50 compared to the NM treatment. There was an interaction between Hg treatment and AM inoculation on Hg stem concentration, i.e., at Hg25, AM inoculation decreased the Hg translocation from roots to stems, while Hg translocation was increased at Hg50 compared to the NM treatment. Zn acquisition was improved by R. irregularis. The negative relationship between Hg and Zn concentrations in the roots of AM and NM plants implied potential competition for the same transporters, although the expression of Zn transporters was upregulated by AM inoculation at all Hg levels. In conclusion, this baseline study demonstrated that R. irregularis may play an important role in Hg tolerance of M. truncatula, suggesting its potential for Hg-contaminated phytoremediation.

Bacterial secondary metabolites: possible mechanism for weed suppression in wheat.

Chemical weed control is an effective method, but has proved hazardous for humans, environment, and soil biodiversity. Use of allelopathic bacteria may be more efficient and sustainable weed control measure. The bacterial inoculants have never been studied in context of their interaction with weed root exudates and precursor-dependent production of the natural phytotoxins (cyanide, cytolytic enzymes and auxin) by these strains to understand their weed suppression and wheat growth promotion abilities. Therefore, root exudates of Avena fatua, Phalaris minor, Rumex dentatus, and wheat were quantified and their role in microbial root colonization and secondary metabolite production, i.e., cyanide, cytolytic enzymes, phenolics, and elevated auxin concentration, was studied. The results depicted l-tryptophan and glycine as major contributors of elevated cyanide and elevated levels in weed rhizosphere by the studied Pseudomonas strains, through their higher root colonization ability in weeds as compared with wheat. Furthermore, the higher root colonization also enhanced p-coumaric acid (photosynthesis inhibitor by impairing cytochrome c oxidase activity in plants) and cytolytic enzyme (root cell wall degradation) concentration in weed rhizosphere. In conclusion, the differential root colonization of wheat and weeds by these strains is responsible for enhancing weed suppression (enhancing phytotoxic effect) and wheat growth promotion (lowering phytotoxic effect).

De novo synthesis of ferrichrome by Fusarium oxysporum f. sp. cubense TR4 in response to iron starvation.

Manipulation of iron bioavailability in the banana rhizosphere may suppress Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense (Foc). However, iron starvation induced by application of synthetic iron chelators does not effectively suppress Fusarium wilt. It is unclear whether Foc can subvert iron chelators and thereby evade iron starvation through the synthesis of iron-scavenging secondary metabolites, called siderophores. In vitro studies were conducted using iron-deficient growth medium and medium supplemented with a synthetic iron chelator, 2,2'-dipyridyl, to mimic iron starvation in Foc Tropical Race 4 (Foc TR4). Concentration of extracellular siderophores increased three-fold (p < 0.05) in the absence of iron. Liquid chromatography-mass spectrometry analysis detected the hydroxamate siderophore, ferrichrome, only in the mycelia of iron-starved cultures. Moreover, iron-starved cultures exhibited a reduction in total cellular protein concentration. In contrast, out of the 20 proteinogenic amino acids, only arginine increased (p < 0.05) under iron starvation. Our findings suggest that iron starvation does not cause a remodelling of amino acid metabolism in Foc TR4, except for arginine, which is required for biosynthesis of ornithine, the precursor for siderophore biosynthesis. Collectively, our findings suggest that biosynthesis of siderophores, particularly ferrichrome, could be a counteractive mechanism for Foc TR4 to evade iron starvation.

Diacetoxyscirpenol, a Fusarium exometabolite, prevents efficiently the incidence of the parasitic weed Striga hermonthica.

BACKGROUND: Certain Fusarium exometabolites have been reported to inhibit seed germination of the cereal-parasitizing witchweed, Striga hermonthica, in vitro. However, it is unknown if these exometabolites will consistently prevent S. hermonthica incidence in planta. The study screened a selection of known, highly phytotoxic Fusarium exometabolites, in identifying the most potent/efficient candidate (i.e., having the greatest effect at minimal concentration) to completely hinder S. hermonthica seed germination in vitro and incidence in planta, without affecting the host crop development and yield. RESULTS: In vitro germination assays of the tested Fusarium exometabolites (i.e., 1,4-naphthoquinone, equisetin, fusaric acid, hymeglusin, neosolaniol (Neo), T-2 toxin (T-2) and diacetoxyscirpenol (DAS)) as pre-Striga seed conditioning treatments at 1, 5, 10, 20, 50 and 100 µM, revealed that only DAS, out of all tested exometabolites, completely inhibited S. hermonthica seed germination at each concentration. It was followed by T-2 and Neo, as from 10 to 20 µM respectively. The remaining exometabolites reduced S. hermonthica seed germination as from 20 µM (P < 0. 0001). In planta assessment (in a S. hermonthica-sorghum parasitic system) of the exometabolites at 20 µM showed that, although, none of the tested exometabolites affected sorghum aboveground dry biomass (P > 0.05), only DAS completely prevented S. hermonthica incidence. Following a 14-d incubation of DAS in the planting soil substrate, bacterial 16S ribosomal RNA (rRNA) and fungal 18S rRNA gene copy numbers of the soil microbial community were enhanced; which coincided with complete degradation of DAS in the substrate. Metabolic footprinting revealed that the S. hermonthica mycoherbicidal agent, Fusarium oxysporum f. sp. strigae (isolates Foxy-2, FK3), did not produce DAS; a discovery that corresponded with underexpression of key genes (Tri5, Tri4) necessary for Fusarium trichothecene biosynthesis (P < 0.0001). CONCLUSIONS: Among the tested Fusarium exometabolites, DAS exhibited the most promising herbicidal potential against S. hermonthica. Thus, it could serve as a new biocontrol agent for efficient S. hermonthica management. Further examination of DAS specific mode of action against the target weed S. hermonthica at low concentrations (≤ 20 µM), as opposed to non-target soil organisms, is required.

Genetic diversity and community composition of arbuscular mycorrhizal fungi associated with root and rhizosphere soil of the pioneer plant Pueraria phaseoloides.

The pioneering plant Pueraria phaseoloides had a strong modulation effect on arbuscular mycorrhizal fungi (AMF) communities. Irrespective of geographical location, community composition of AMF in rhizosphere soil differed from that of the root. Co-occurrence network analysis revealed two AMF keystone species in rhizosphere soil (Acaulospora) and roots (Rhizophagus) of P. phaseoloides.

Rhizosphere pH and cation-anion balance determine the exudation of nitrification inhibitor 3-epi-brachialactone suggesting release via secondary transport.

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH4 + nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH4 + by K+ revealed that the NH4 + effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH4 + uptake (R2 = 0.92 for high ~6.8 and R2 = 0.84 for low ~4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH4 + availability, but merely as the result of associated changes in ΔP.

Bioaccumulation and potential ecotoxicological effects of trace metals along a management intensity gradient in volcanic pasturelands.

The particularities of volcanic soils raise the need to better understand the link between soil agricultural management intensity and trace metal bioaccumulation. The Azores are a region characterized by volcanic soils, which were changed in different degrees according to the intensity of the agricultural practices. The main objective of this study was to assess the potential ecotoxicological effects of the trace metals present in volcanic pastureland soils along a gradient of management intensity (i.e., semi-natural, permanent and reseeded), using earthworms (Eisenia fetida) as biological indicators. For this purpose earthworms were exposed during 7, 14, 28 and 56 days to soils from the three types of pastures. At each exposure time, we quantified trace element bioaccumulation (As, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, Pb, Rb, U, V and Zn) and the activities of superoxide dismutase and acetylcholinesterase in earthworm tissues. Overall, the results showed that the type of pastureland management significantly increased the soil contents in trace metals: V, Co, Ni and Zn in semi-natural pasturelands; As, Cd and Hg in reseeded pasturelands; and, Rb and U in both permanent and reseeded pasturelands. The soil physicochemical properties observed in the reseeded pastureland systems (higher electric conductivity values associated with a moderately acid pH value) modulated the metal bioavailability, from soil to biota, leading to a greater Hg bioaccumulation in earthworm tissues. The long-term exposure (56 days) of earthworms to reseeded pastureland soil was associated with adverse biological effects (intensification of AChE activity and decrease of SOD activity), encompassing key processes such as neurotransmission and antioxidant defence mechanisms in resident soil biota (earthworms). This study point towards the increased importance of semi-natural and permanent pastureland management, over the intensive management (reseeded pasturelands), in favour of more sustainable ecosystems.

Brachialactone isomers and derivatives of Brachiaria humidicola reveal contrasting nitrification inhibiting activity.

Biological Nitrification Inhibition (BNI) of Brachiaria humidicola has been mainly attributed to the root-exuded fusicoccane-type diterpene brachialactone. We hypothesized, however, that according to the high diversity of fusicoccanes described for plants and microorganisms, BNI of B. humidicola is caused by an assemblage of bioactive fusicoccanes. B. humidicola root exudates were collected hydroponically and compounds isolated by semi-preparative HPLC. Chemical structures were revealed by spectroscopic techniques, including HRMS as well as 1D and 2D NMR. Nitrification inhibiting (NI) potential of isolated compounds was evaluated by a Nitrosomonas europaea based bioassay. Besides the previously described brachialactone (1), root exudates contained 3-epi-brachialactone (2), the C3-epimer of 1 (m/z 334), as well as 16-hydroxy-3-epi-brachialactone (3) with an additional hydroxyl group at C16 (m/z 350) and 3,18-epoxy-9-hydroxy-4,7-seco-brachialactone (4), which is a ring opened brachialactone derivative with a 3,18 epoxide ring and a hydroxyl group at C9 (m/z 332). The 3-epi-brachialactone (2) showed highest NI activity (ED50 ~ 20 µg mL-1, ED80 ~ 40 µg mL-1), followed by compound 4 with intermediate (ED50 ~ 40 µg mL-1), brachialactone (1) with low and compound 3 without activity. In coherence with previous reports on fusicoccanes, stereochemistry at C3 was of high relevance for the biological activity (NI potential) of brachialactones.

Population genetic structure and marker-trait associations in East and West African Striga hermonthica with varying phenotypic response to Fusarium oxysporum f. sp. strigae isolates Foxy-2 and FK3.

To examine the genetic basis for the variable susceptibility of Striga hermonthica from differing zones of sub-Saharan Africa to Fusarium oxysporum f. sp. strigae (Fos) isolates Foxy-2 and FK3, 10 S. hermonthica populations from Eastern and Western Africa were phenotyped for their susceptibility response to Foxy-2 and FK3, and then genotyped with 22 simple sequence repeat (SSR) markers. There is low genetic differentiation between East African and West African S. hermonthica populations (i.e. the proportion of the total genetic variance contained in the subpopulation relative to the total genetic variance, FST  = 0.012, P < 0.05), but intermediate genetic differentiation (FST  = 0.143, P < 0.01) underlies the S. hermonthica groups that are differentiated by their phenotypic responses to Fos isolates. An expressed sequence tag SSR (EST-SSR) marker Y53 (P < 0.01) and a genomic SSR marker E1009 (P < 0.05) were associated with the S. hermonthica class susceptible to Foxy-2 and FK3 (group A). A divergent S. hermonthica class, consisting of groups with intermediate susceptibility to Foxy-2 (group B) and susceptibility to either FK3 (group C) or Foxy-2 (group D), showed no marker-trait association, instead demonstrated linkage disequilibrium decay. Owing to point substitutions and insertion-deletion mutations, the unique, protein-coding nucleotide sequence at the E1009 locus in group A was partly dissimilar to group B, but was totally distinct from groups C and D. These findings implied that the inconsistent effectiveness of a Fos isolate is better explained by genomic variation in S. hermonthica, rather than by S. hermonthica sampling zones.

Nitrate reductase activity in leaves as a plant physiological indicator of in vivo biological nitrification inhibition by Brachiaria humidicola.

The tropical forage grass Brachiaria humidicola (Bh) controls soil microbial nitrification via biological nitrification inhibition (BNI). The aim of our study was to verify if nitrate reductase activity (NRA) in Bh roots or leaves reflects in vivo performance of BNI in soils. NRA was measured in roots and leaves of contrasting accessions and apomictic hybrids of Bh grown under controlled greenhouse and natural field conditions. Nitrate (NO3-) contents were measured in soil solution and in Bh stem sap to validate NRA data. Potential soil nitrification rates (NRs) and leaf δ15N values were used to verify in vivo BNI by the NRA assay in the field study. NRA was detected in Bh leaves rather than roots, regardless of NO3- availability. NRA correlated with NO3- contents in soils and stem sap of contrasting Bh genotypes substantiating its reflectance of in vivo BNI performance. Additionally, leaf NRA data from the field study significantly correlated with simultaneously collected NRs and leaf δ15N data. The leaf NRA assay facilitated a rapid screening of contrasting Bh genotypes for their differences in in vivo performance of BNI under field and greenhouse conditions, but inconsistency of the BNI potential by Bh germplasm was observed. Among Bh genotypes tested, leaf NRA was closely linked with nitrification activity, and consequently with actual BNI performance. It was concluded that NRA in leaves of Bh can serve as an indicator of in vivo BNI activity when complemented with established BNI methodologies (δ15N, NRs) under greenhouse and field conditions.

Litter decomposition driven by soil fauna, plant diversity and soil management in urban gardens.

In the face of growing urban densification, green spaces in cities, such as gardens, are increasingly important for biodiversity and ecosystem services. However, the influences of urban green space management on biodiversity and ecosystem functioning (BEF) relationships is poorly understood. We investigated the relationship between soil fauna and litter decomposition in 170 urban garden sites along a gradient of urbanisation intensity in the city of Zurich, CH. We used litter bags of 1 and 4 mm mesh size to evaluate the contribution of soil meso- and macrofauna on litter decomposition. By using multilevel structural equation models (SEM), we investigated direct and indirect environmental effects and management practices on litter decomposition and litter residue quality. We evaluated the role of taxonomic, functional and phylogenetic diversity of soil fauna species on litter decomposition, based on a sample of 120 species (81,007 individuals; 39 collembola, 18 earthworm, 16 isopod, 47 gastropod species). We found highest litter decomposition rates using 4 mm mesh size litter bags, highlighting the importance of soil macrofauna. Urban warming, a proxy for urbanisation intensity, covaried positively, whereas soil disturbances, such as intensive soil and crop management, were negatively correlated with decomposition rates. Interestingly, soil fauna species richness decreased, with the exception of gastropods, and soil fauna abundance increased with urban warming. Our data also show that plant species richness positively affected litter decomposition by increasing soil fauna species richness and microbial activity. A multivariate analysis of organic compounds in litter residues confirmed the importance of soil fauna species richness and garden management on litter decomposition processes. Overall, we showed, that also in intensively managed urban green spaces, such as gardens, biodiversity of plants and soil fauna drives key ecosystem processes. Urban planning strategies that integrate soil protecting management practices may help to maintain important ecosystem services in this heavily used urban environment.

Perspectives of Microbial Inoculation for Sustainable Development and Environmental Management.

How to sustainably feed a growing global population is a question still without an answer. Particularly farmers, to increase production, tend to apply more fertilizers and pesticides, a trend especially predominant in developing countries. Another challenge is that industrialization and other human activities produce pollutants, which accumulate in soils or aquatic environments, contaminating them. Not only is human well-being at risk, but also environmental health. Currently, recycling, land-filling, incineration and pyrolysis are being used to reduce the concentration of toxic pollutants from contaminated sites, but too have adverse effects on the environment, producing even more resistant and highly toxic intermediate compounds. Moreover, these methods are expensive, and are difficult to execute for soil, water, and air decontamination. Alternatively, green technologies are currently being developed to degrade toxic pollutants. This review provides an overview of current research on microbial inoculation as a way to either replace or reduce the use of agrochemicals and clean environments heavily affected by pollution. Microorganism-based inoculants that enhance nutrient uptake, promote crop growth, or protect plants from pests and diseases can replace agrochemicals in food production. Several examples of how biofertilizers and biopesticides enhance crop production are discussed. Plant roots can be colonized by a variety of favorable species and genera that promote plant growth. Microbial interventions can also be used to clean contaminated sites from accumulated pesticides, heavy metals, polyaromatic hydrocarbons, and other industrial effluents. The potential of and key processes used by microorganisms for sustainable development and environmental management are discussed in this review, followed by their future prospects.

Low 15N Natural Abundance in Shoot Tissue of Brachiaria humidicola Is an Indicator of Reduced N Losses Due to Biological Nitrification Inhibition (BNI).

The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate ( NO 3 - ) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced NO 3 - losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the 15N isotope and leads to 15N depleted NO 3 - , but 15N enriched NH 4 + in soils. Leaching of 15N depleted NO 3 - enriches the residual N pool in the soil with 15N. We hypothesized that altered nitrification and NO 3 - leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil 15N natural abundance (δ15N), which in turn is reflected in distinct δ15N in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δ15N of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δ15N in several Bh genotypes and leached NO 3 - amounts and shoot N uptake. Additionally, plant 15N and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf δ15N of young leaves (regrown after cutback) of Bh and combined it with nitrification rates (NRs) of incubated soil to test whether there is a direct relationship between plant δ15N and BNI. Increased leached NO 3 - was positively correlated with higher δ15N in Bh, whereas the correlation between shoot N uptake and shoot δ15N was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δ15N declined and shoot N% increased over time. Leaf δ15N of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δ15N of Bh genotypes over time reflects the long-term effect of BNI as linked to lower NO 3 - formation and reduced NO 3 - leaching. Accordingly, a low δ15N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.

Genetic diversity and symbiotic effectiveness of Bradyrhizobium strains nodulating selected annual grain legumes growing in Ethiopia.

Vigna unguiculata, Vigna radiata and Arachis hypogaea growing in Ethiopia are nodulated by a genetically diverse group of Bradyrhizobium strains. To determine the genetic identity and symbiotic effectiveness of these bacteria, a collection of 36 test strains originating from the root nodules of the three hosts was investigated using multilocus sequence analyses (MLSA) of core genes including 16S rRNA, recA, glnII, gyrB, atpD and dnaK. Sequence analysis of nodA and nifH genes along with tests for symbiotic effectiveness using δ15N analysis were also carried out. The phylogenetic trees derived from the MLSA grouped most test strains into four well-supported distinct positions designated as genospecies I-IV. The maximum likelihood (ML) tree that was constructed based on the nodA gene sequences separated the entire test strains into two lineages, where the majority of the test strains were clustered on one of a well-supported large branch that comprise Bradyrhizobium species from the tropics. This clearly suggested the monophyletic origin of the nodA genes within the bradyrhizobia of tropical origin. The δ15N-based symbiotic effectiveness test of seven selected strains revealed that strains GN100 (δ15N=0.73) and GN102 (δ15N=0.79) were highly effective nitrogen fixers when inoculated to cowpea, thus can be considered as inoculants in cowpea production. It was concluded that Ethiopian soils are a hotspot for rhizobial diversity. This calls for further research to unravel as yet unknown bradyrhizobia nodulating legume host species growing in the country. In this respect, prospective research should also address the mechanisms of symbiotic specificity that could lead to high nitrogen fixation in target legumes.