Browsing by Subject "PGPR"

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Fertilizer placement and the potential for its combination with bio-effectors to improve crop nutrient acquisition and yield
(2016) Nkebiwe, Peteh Mehdi; Müller, Torsten
Even when total nitrogen (N) and phosphorus (P) concentrations in most agricultural soils are high, the concentrations of plant-available N and P fractions are often inadequate for acceptable yield. In comparison to conventional fertilizer application by homogenous broadcast over the soil surface (with or without subsequent incorporation), fertilizer placement in defined soil areas/volumes close to seeds or crop roots is a more effective application method to enhance the plant-availability of applied fertilizers. Nevertheless, considerable root growth in subsurface nutrient patches or around concentrated fertilizer-depots (and/or improved nutrient influx rates in roots) is a prerequisite for improved uptake of placed nutrients. Furthermore, zones with intense rooting around placed fertilizer depots (“rhizosphere hotspots”) with high concentrations of organic nutrients released as root exudates may be favorable for the survival and establishment of inoculated plant-growth-promoting microorganisms (PGPMs), which mobilize nutrients in soil to favor plant growth. In the last three decades, several published field studies comparing fertilizer placement to fertilizer broadcast arrived at different and often conflicting results regarding their effects on yield and nutrient status of various crops. For this reason, the first task was to conduct a Meta-analysis on data in published peer-reviewed field studies on fertilizer placement that met a set of pre-defined criteria for inclusion. We investigated the relative effect of fertilizer placement for specific fertilizer formulations (e.g. NH4+ and CO(NH2)2 without or in combination with soluble P (HPO42-; H2PO4-); soluble K; solid or liquid manure) in a precise restricted area on surface or subsurface soil in comparison to fertilizer broadcast on yield, nutrient concentration and content in above-ground plant parts. We utilized data from a total of 40 field studies published between 1982 and 2015 (85% of studies published from 2000) that met our criteria. We used the method of “baseline contrasts” to compare different fertilizer placement treatments to fertilizer broadcast as a common control or baseline treatment. Results showed that overall, fertilizer placement led to +3.7% higher yields, +3.7% higher concentrations of nutrients in above-ground plant parts and +11.9% higher contents of nutrients also in above-ground plant parts than fertilizer broadcast application. Placement depth had a strong effect of the outcome of fertilizer placement because relative placement effects increased with increasing fertilizer placement depth. Composition of fertilizer formulations was also an important factor. High yields of fertilizer placement relative to fertilizer broadcast application were obtained for CO(NH2)2 in combination with soluble P (HPO42-; H2PO4-) (+27%) or NH4+ in combination with HPO42-; H2PO4- (+15%) (Nkebiwe et al., 2016 a: Field Crops Research 196: 389–401). The next aim was to investigate the effect of fertilizer placement in subsurface soil in combination with application of bio-effectors (BEs) (PGPMs and natural active substances such as humic acids and seaweed extracts) on root growth of crop plants, establishment of inoculated PGPM in the rhizosphere, grain and biomass production as well as plant nutrient status for maize (Zea mays L) and wheat (Triticum aestivum L) cultures. Through various pot and rhizobox experiments, we observed that placement of a subsurface concentrated NH4+-fertilizer depot stabilized with the nitrification inhibitor DMPP (3,4-di-methylpyrazolphosphate) induced dense rooting around the depot contributing to more efficient exploitation of the depot. For this, it was crucial the N persisted in the depot mainly as poorly mobile NH4+, in order to induce localized depot-zone root-growth as well as favorable chemical and biological changes in the rhizosphere to improve N and P uptake by crop plants. Through in vitro culture experiments on solid and liquid media, we could show that via acidification of the growth media, several selected microbial BEs were capable to solubilize sparingly soluble inorganic phosphates and also that these BEs showed considerable tolerance to high concentrations of NH4+ und DMPP. The latter indicated a potential for the BEs to colonize plant roots in NH4+-rich well rooted soil zones around a subsurface NH4+-fertilizer depot (Nkebiwe et al., 2016 c: Manuscript submitted). Through further pot experiments and four others experiments as Bachelor and Master theses conduction under my supervision, we observed that certain BEs that readily solubilized tri-calcium phosphates in vitro were able to mobilize rock phosphate (RP) applied in soil-based substrates when N was supplied as stabilized NH4++DMPP, thereby contributing to enhanced P uptake and growth of maize and wheat plants. The bacterial BE Pseudomonas sp. DSMZ 13134 and BE consortia products containing bacteria and fungi such as CombiFectorA were good candidates. BE-induced RP-solubilzation occurred mainly in substrates with low CaCO3 contents indicating low P sorption capacity for neutral and moderately alkaline soils. With CombiFectorA, maize P-acquisition from sewage sludge ash could be enhanced, thus increasing the efficiency of a sparingly soluble fertilizer based of recycled wastes. Possible explanations for the beneficial effects of best performing BEs to improve plant growth were enhanced solubility of sparingly soluble P fertilizers via acidification of the rhizosphere and release of nutrient-chelating substances as well as improvement of root growth for better spatial interception of nutrients (Nkebiwe et al., 2016 d: Manuscript in preparation). Alongside, more greenhouse and two field experiments (grain maize 2014 and maize silage 2015) were designed, planned, conducted and evaluated. A peer-reviewed paper from this work has already been published (Nkebiwe et al., 2016 b: Chemical and Biological Technologies in Agriculture 3:15). In the greenhouse and experiments, placement of a concentrated stabilized NH4+-fertilizer depot led to improved root and shoot growth, and increased shoot N and P contents. Through intense root growth of maize around the NH4+-depot, increased root-colonization by Pseudomonas sp. DSMZ 13134 close to seeds could be observed. In the field, many weeks after subsurface placement of the concentrated stabilized NH4+-depot, it could be shown that N considerably persisted in the depot-zone as NH4+, which strongly induced depot-zone root growth. Placement of the NH4+-depot led to +7.4 % increase in grain yield of maize (2014) and +5.8% increase in maize silage yield (2015) in comparison to fertilizer broadcast. Placement of Pseudomonas sp. DSMZ 13134 inoculum in the sowing row let to +7.1% increase in yield of maize silage (2015) in comparison to the non-inoculated control. In total, these results showed that precise placement of specific fertilizer formulations in combination with the application of selected PGPMs can lead to improved plant growth, improved N and P uptake with a potential to save resources.
A global network meta-analysis of the promotion of crop growth, yield, and quality by bioeffectors
(2022) Herrmann, Michelle Natalie; Wang, Yuan; Hartung, Jens; Hartmann, Tobias; Zhang, Wei; Nkebiwe, Peteh Mehdi; Chen, Xinping; Müller, Torsten; Yang, Huaiyu
Bioeffector (BE) application is emerging as a strategy for achieving sustainable agricultural practices worldwide. However, the effect of BE on crop growth and quality is still controversial and there is still no adequate impact assessment that determines factors on the efficiency of BE application. Therefore, we carried out a network metaanalysis on the effect of BEs using 1,791 global observations from 186 studies to summarize influencing factors and the impact of BEs on crop growth, quality, and nutrient contents. The results show that BEs did not only improve plant growth by around 25% and yield by 30%, but also enhanced crop quality, e.g., protein (55% increase) and soluble solids content (75% increase) as well as aboveground nitrogen (N) and phosphate (P) content by 28 and 40%, respectively. The comparisons among BE types demonstrated that especially non-microbial products, such as extracts and humic/amino acids, have the potential to increase biomass growth by 40–60% and aboveground P content by 54–110%. The soil pH strongly influenced the efficiency of the applied BE with the highest effects in acidic soils. Our results showed that BEs are most suitable for promoting the quality of legumes and increasing the yield of fruits, herbs, and legumes. We illustrate that it is crucial to optimize the application of BEs with respect to the right application time and technique (e.g., placement, foliar). Our results provide an important basis for future research on the mechanisms underlying crop improvement by the application of BEs and on the development of new BE products.
Interactions between non-symbiotic N2-fixing bacteria and plant roots in plant-microbial associations
(2009) Calvo Alegre, Olga-Cristina; von Wirén, Nicolaus
The development of biofertilizers on the basis of plant growth promoting rhizobacteria (PGPR) may be a promising approach to partially substitute costly and energy-consuming mineral fertilizers in agricultural plant production and to support agriculture in developing countries. A successful and competitive rhizosphere colonization of PGPR strains has been identified as a prerequisite for the expression of plant growth promoting effects. Apart from a wide range of external factors with an impact on the colonization process, such as soil properties, temperature, soil moisture and fertilization levels, in particular plant-microbial interactions may play an important role for the successful establishment of compatible associations. In this context, certain plant root exudates may act as signals to mediate bacterial responses with importance for root colonisation (e.g. motility and chemotaxis, production of extracellular polysaccharides). On the other hand, the induction of bacterial plant growth promotion may also depend on ability of the host plant to respond to the presence and the activity of the associated bacteria. It was therefore the aim of this thesis to investigate the contribution of putative PGPR to growth and N uptake in wheat plants and characterize the underlying mechanisms in root-bacterial associations. In the first part of this thesis, the contribution of various non-symbiotic diazotrophic rhizobacteria to plant growth promotion and N nutrition has been studied in a series of greenhouse pot inoculation experiments with wheat (Triticum aestivum L.). Different bacterial inoculants, plant genotypes, soil properties, water regimes and N fertilization levels have been varied as factors with potential impact on plant growth promotion by diazotrophs. The contribution of biological nitrogen fixation was assessed by the 15N dilution method. Plant growth and grain yield were influenced by the different N fertilization levels but no stimulation of growth or N uptake was note upon bacterial inoculation. These observations suggested a high degree of specificity or limiting factors, determining a successful plant-microbial association. The second part describes possible mechanisms that may be involved in the establishment of diazotrophs in the rhizosphere of suitable host plants. As an initial step of the colonization process, a targeted movement of the bacteria to the root surface is required and root exudates may act as attractants. Since dicarboxylic acids are known to exert chemotactic activity on diazotrophic bacteria, seed and root exudates of two graminaceous crops (Triticum aestivum L. and Zea mays L.) and for comparison also of a non-graminaceous plant species (Phaseolus vulgaris L.) were collected in hydroponic culture with and without N supply, and organic acid profiles in these root exudates were analysed. Bacterial motility assays were conducted with the major carboxylates detected in the root exudates of the selected plant species and compared to glucose and water, using Brevibacillus reuszeri as a model bacterium. Pure malate, which was found at high levels in root exudates of bean and wheat, and particularly malonate (bean) and t-aconitate (maize) stimulated the motility of Brevibacillus reuszeri as compared with glucose or water. A particularly intense promotion of bacterial motility was recorded in the presence of crude root exudates of wheat and maize plants grown under N limitation, which was not observed for root exudates of bean. However, this was not related with comparable changes of malate or t-aconitate concentrations in the root exudates. In wheat exudates, malate concentrations even decreased in response to N limitation. These findings suggest the presence of specific factors released in root exudates of N-deficient cereals, promoting the rhizosphere colonisation with B. reuszeri. For an identification of the respective factors, a more comprehensive profiling of the root exudates is necessary. In associations with diazotrophic bacteria, host plants are supplied with ammonium by the bacterial partner. This raised the question whether plant ammonium uptake systems have an impact on the efficiency of the association. To address this problem, an antisense approach was conducted with tomato, with characterised ammonium transporters (LeAMT1;1 and LeAMT1;2). The final goal was the inhibition of the ammonium transporters by production of LeAMT antisense lines to study their putative role in plant associations with diazotrophic bacteria. Northern blot analysis revealed a strong repression of LeAMT1;2 expression in three independent antisense lines associated with a lower ammonium uptake capacity under N-sufficient and N-deficient growth conditions. In contrast, LeAMT1,1 expression was only weakly repressed in antisense lines and there was no impact on N uptake. A faster decline of chlorophyll in older leaves indicates a physiological function of LeAMT1;1 and LeAMT1;2 in ammonium uptake and retrieval in shoot and root cells. The absence of consistent effects on N acquisition of the investigated antisense lines limited the suitability of this approach for studies on associations with diazotrophic bacteria.
Role of Bacillus spp. plant growth promoting properties in mitigating biotic and abiotic stresses in lowland rice (Oryza sativa L.)
(2023) Weinand, Tanja; El-Hasan, Abbas; Asch, Folkard
The ability of microorganisms to promote plant growth and mitigate abiotic and biotic stresses makes them an interesting tool for sustainable agriculture. Numerous studies aim to identify new, promising bacteria isolates. Traditional culture-based methods, which focus on selecting microorganisms with plant-growth-promoting traits, such as hormone production, nutrient solubilization, and antifungal properties, are widely used. This study aims to investigate the role of plant-growth-promoting properties in bacteria-mediated stress mitigation and the suitability of traditional culture-based methods as a screening tool for the identification of beneficial bacteria. To this end, we tested three endophytic Bacillus isolates, which have previously been shown to affect tolerance against iron toxicity in lowland rice, (a) for their effect on the resistance against brown spot disease, and (b) for plant-growth-promoting traits using common culture-based methods. Both B. pumilus isolates inhibited fungal growth in vitro and reduced brown spot disease in two of three rice cultivars in planta, although they tested negative for all plant-growth-promoting traits. While B. megaterium was negative for ACC deaminase activity and nutrient solubilization, it exhibited auxin production. Nevertheless, B. megaterium did not suppress brown spot disease in any of the three rice cultivars. This study shows that bacteria do not necessarily have to possess classical plant-growth-promoting properties in order to be beneficial to plants, and it emphasizes the limitation of common culture-based methods in effectively identifying beneficial bacteria. Moreover, our results highlight the significance of the interaction between bacteria and plant cultivars in determining the beneficial effects of Bacillus spp. on plants under biotic or abiotic stresses.