Browsing by Subject "Wurzel"

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    Dissecting the genetic basis of root- and rhizosphere-related phosphorususe efficiency in European elite maize (Zea mays L.) lines and landraces
    (2021) Li, Xuelian; Ludewig, Uwe
    In agriculture, farmers massively apply P fertilizer to maintain high yield. Due to the long-term high fertilization rates and long-term organic residue accumulation, the total P pool per hectare has increased between 1900 and 2020. Since modern varieties have often been selected in high-nutrient input conditions for high yields, concerns are being raised that the beneficial traits for P uptake under a limited P supply will gradually decline in elite varieties. Regarding to maize (Zea mays L.), thousands of varieties have been bred since it was domesticated as a food product. It is an open question whether traits and genes related to P deficiency in European maize have changed since the Green Revolution, the start of hybrid breeding and high-intensity fertilization. This is the core research question of this dissertation. Here I present the analysis of roots in response to P deficiency using a diverse panel of European maize genotypes via several experiments. In Chapter I, we focus on whether maize seedlings of the flint and dent heterotic pools vary in the P acquisition and utilization since the onset of hybrid breeding using 34 genotypes in mini-rhizotrons. These genotypes included 16 flint lines that were released over more than five decades ago, 7 doubled haploid lines from the flint landraces (DH_LR), 8 dent lines, and 3 hybrids. Seedling P use efficiency (PUE) and related traits were measured and compared at two P levels in a calcareous soil. In Chapter II, we compared the root exudated organic acids and mycorrhizal fungi colonization degree among 24 genotypes which have been evaluated in Chapter I. These genotypes included 16 flint lines, 6 DH_LR and 2 old dent lines. Seedling colonization with arbuscular mycorrhizal fungi (AMF) and organic acid anion release were measured. P-uptake-related root traits were compared under P-sufficient and P-deficient conditions. In Chapter III, using nearly isogenic maize lines, the B73 wild type and the rth3 root hairless mutant, we quantified the effect of root hairs and AMF infection in a calcareous soil under P deficiency. Wild-type root hairs extended the rhizosphere for acid phosphatase activity by 0.5 mm compared with the rth3 hairless mutant. Total root length of the wild type was longer than that of rth3 under P deficiency. Higher AMF colonization and mycorrhiza-induced phosphate transporter gene expression were identified in the mutant under P deficiency, but plant growth and P acquisition were similar between mutant and the wild type. The mycorrhizal dependency of maize was 33 % higher than the root hair dependency. Root hairs and AMF inoculation are two alternative ways to increase Pi acquisition under P deficiency, but these two strategies compete with each other. In Chapter IV again two nearly isogenic maize lines, the B73 wild type and the rth2 root hairless mutant, were used to address the importance of root hairs during drought and under P deficiency. The results indicate that drought and P deficiency synergistically impair maize growth; while P concentrations were little affected by the loss of root hairs, the P content was massively reduced at combined stress, showing that P deficiency is much more severe under drought. In Chapter V, we first compared the root traits response to low P and high P of six preselected genotypes in European flint in Chapter I. We then generated RNA libraries from the roots of these lines under both low P and high P. Using an expressed genes matrix, we conducted a Weighted Genomic Coexpression Network Analysis (WGCNA), and detected general low P-induced modules and modules that were higher in founder flints. The P deficiency-responsive metabolic processes common to all six genotypes included: (1) acceleration of carbon supply for organic acid synthesis through glycolysis and TCA cycle; (2) alteration of lipid metabolism; (3) changes of activity of transmembrane transporters; (4) carotenoid metabolism. Additionally, the founder flint line EP1, F2 and doubled haploid landrace SM1 have their specific strategies and mechanism to cope with low P. Our findings well support other studies with transcriptome, proteome and metabolome experiments in maize and other species, and point to molecular events involved in the efficient alleviation of P stress in efficient maize accessions. Altogether, this study presents informative analyses in how maize genotypes with distinct breeding history adapt to P deficiency in regard of root, rhizosphere traits and root transcription. It showed correlation between phenotypic traits and gene transcription, which is much more complex than previously reported. It also opened a novel insight into molecular regulation on Pi utilization, resulting in promotion of vegetative biomass in P deficiency. These findings will also provide precious knowledge for plant breeders and agronomists who work on P research in maize and other cereal crops.
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    Effects of resource availability and quality on soil microorganisms and their carbon assimilation
    (2014) Kramer, Susanne; Kandeler, Ellen
    Soil microorganisms play a pivotal role in decomposition processes and therefore influence nutrient cycling and ecosystem function. Availability and quality of resources determines activity, growth and identity of substrate users. In agricultural systems, availability of resources is dependent on, for example, crop type, management, season, and depth. At depth substrate availability and microbial biomass decrease. However, there remain gaps in our understanding of C turnover in subsoil and how processes in the topsoil may influence abundance, activity, and function of microorganisms in deeper soil layers. With respect to substrate quality it is thought that bacteria are the dominant users of high quality substrates and more labile components whereas fungi are more important for the degradation of low quality and more recalcitrant substrates (i.e. cellulose, lignin). Therefore, this thesis was designed to increase our understanding of C turnover and the influence of both availability and quality of substrates on microorganisms in an agricultural soil. In the first and second studies, a recently established C3-C4 plant exchange field experiment was used to investigate the C flow from belowground (root) and aboveground (shoot litter) resources into the belowground food web. Maize plants were cultivated to introduce a C4 signal into the soil both by plant growth (belowground / root channel) and also by applying shoot litter (aboveground litter channel). To separate C flow from the shoot litter versus the root channel, maize litter was applied on wheat cultivated plots, while on half of the maize planted plots no maize litter was returned. Wheat cultivated plots without additional maize litter application served as a reference for the calculation of incorporated maize-C into different soil pools. Soil samplings took place in two consecutive years in summer, autumn and winter. Three depths were considered (0-10 cm: topsoil, 40-50 cm: rooted zone beneath the plough layer, 60-70 cm: unrooted zone). In the third study a microcosm experiment with substrates of different recalcitrance and complexity was carried out to identify primary decomposers of different plant litter materials (leaves and roots) during early stages of decomposition (duration of 32 days) and to follow the C flow into the next higher trophic level (protozoa).
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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.
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    Regulation of phosphate deficiency-induced carboxylate exudation in cluster roots of white lupin (Lupinus albus L.)
    (2005) Kania, Angelika; Römheld, Volker
    In many tropical and subtropical areas crop production is severely limited by a deficiency of plant-available phosphorus (P) in the soils. Therefore plant mechanisms to mobilize the sparingly soluble P fraction are of high interest. One such mechanism of P-deficient plants is the exudation of carboxylates and protons from roots. White lupin (Lupinus albus L.) was chosen as a model system to investigate plant metabolism under P deficiency which enable the plant to release ex-traordinarily high amounts of citrate and protons from its cluster roots (bottlebrush-like clusters of short rootlets of determinate growth which form along secondary lateral roots). The aim of this work was to determine the reasons for the high citrate acccumulation observed in mature cluster roots of P-deficient white lupin and to characterize the regulation of citrate release. A threshold citrate concentration is seen as a prerequisite for the transient pulse of intense citrate exudation associated with rhizosphere acidification which occurs over a time period of 2-3 days. Biochemical changes on the anabolic side of citrate metabolism such as increased activities of phosphoenolpyruvate carboxylase (PEP-C) or malate dehydrogenase (MDH) cannot solely ex-plain the very high citrate accumulation observed during cluster root development, although these reactions supply the cluster roots with citrate precursors. In addition, pyruvate concentra-tions decrease in developing cluster roots, probably in relation to the decreasing malic enzyme activities in the respective clusters. Citrate accumulation might also be caused by an impaired citrate turnover. Aconitase, the en-zyme catalyzing the turnover of citrate via cis-aconitate to isocitrate, showed decreasing activi-ties during cluster root development. NADP-isocitrate dehydroganase (NADP-ICDH) activities, as the next metabolic reaction which oxidizes isocitrate to 2-oxoglutarate, paralleled aconitase activities in all the different root segments investigated, although on a two- to threefold higher level. For this, aconitase rather than NADP-ICDH activities seem to limit citrate turnover. Spe-cific activities of aconitase and NADP-ICDH were the same in all the root segments investi-gated. Aconitase is rapidly inactivated by H₂O₂, which can be produced at increased rates under P limitation. However, neither H₂O₂ concentrations nor malondialdehyde concentrations as a marker for lipid peroxidation under oxidative stress were increased in clusters with low aconitase activities. Artifical inhibition of aconitase by incubating young cluster roots with high amounts of externally applied H₂O₂ did not change citrate and malate concentrations in these root seg-ments. However, a strong increase in citrate concentrations and a strong decrease in malate con-centrations in young cluster roots, together with high citrate exudation rates, could be observed when monofluoroacetate (MFA) as another aconitase inhibitor was applied. Inhibition of the aconitase enzyme therefore forced still young clusters to react like mature ones. This hints to aconitase as a key metabolic step in citrate turnover. High rates of carboxylate exudation were measured even from seedling root tips when incubated with MFA. Decreasing dehydrogenase activities as found during cluster root development by in situ staining with formazan were independent of the substrate supplied (citrate, aconitate, isocitrate, succinate, malate). This is in accordance with the decreasing enzyme activities measured in the different root segments such as aconitase, NADP-ICDH or malic enzyme in vitro. A reduced nitrate re-ductase (NR) activity under P deficiency, resulting in a lower drainoff of 2-oxoglutarate for N assimilation, seems not to play an important role for citrate accumulation, since an artificial NR inhibition with tungstate did not significantly increase citrate concentrations in young cluster roots. The change from malate to citrate accumulation during cluster root development is paralleled by a reduction in ATP-citrate lyase (ACL) activity, an enzyme cleaving citrate to oxaloacetate and acetyl-CoA. The good correlation between the citrate/malate ratio in root exudates and ACL ac-tivities indicates that ACL plays a key role as a metabolic switch between malate and citrate ac-cumulation during cluster root development under P deficiency. The enzyme might prevent high citrate concentrations under less severe P deficiency, when ACL activity is not limited by ATP availability. The attempt to inhibit the ACL enzyme by application of hydroxycitrate (HC) did not show any effect on citrate or malate concentrations in the young cluster roots. However, HC was probably not taken up into the root cells and could therefore not exert any inhibitory effects. Decreasing total respiration rates as found for developing cluster roots might affect citrate accu-mulation directly by reduced consumption of citrate in the TCA cycle or indirectly by H₂O₂-induced inhibition of aconitase activity. However, a reduced respiration rate did not result in higher H₂O₂ concentrations in white lupin. Cytochrome pathway capacity decreased parallel to total respiration, suggesting that the cytochrome pathway determines total respiration. An in-crease in alternative oxidase (AOX) capacity did take place in cluster roots, but was not high enough to compensate for the decreased cytochrome capacity. The AOX enzyme often occurs under P deficiency or under oxidative stress, probably to bypass a limiting Pi-and ADP-dependent cytochrome pathway. The amount of the AOX protein, determined by immunodetec-tion, paralleled AOX capacity. However, the availability of Pi and adenylates was not limiting for total respiration, since uncoupling oxidative phosphorylation with CCCP did not increase the respiration rate. The citrate/malate ratio in young clusters with high rates of respiration and low inherent levels of citrate accumulation was only slightly increased by short-term application (4 -8 h) of azide and SHAM as respiration inhibitors. The concomitant release of citrate and protons from mature cluster roots of P-deficient white lupin plants hints to a common regulation of citrate exudation and H+-ATPase activity in this specific root zone. Highly purified inside-out plasma membrane (PM) vesicles were isolated in a membrane-physiological approach to determine H+-ATPase characteristics involved in citrate exudation under P deficiency. Increased hydrolytic activity of the PM H+-ATPase derived from P-deficient plants parallels an increase in rhizosphere acidification and citrate exudation and hints to a causal relationship. Western blot analysis revealed a higher H+-ATPase protein amount under P deficiency. The op-timum pH of the H+-ATPase was shifted towards more acidic conditions under P-deficiency, which might be an adaptation to the supposedly decreased cytosolic pH brought about by the pH stat mechanism when carboxylates accumulate. Lower citrate concentrations (2 mM) stimulated PM vesicle acidification even in the absence of ATP, which was further enhanced by the addi-tion of Mg-ATP, and particularly expressed in PM vesicles isolated from roots of P-deficient plants. Accordingly, 14C-citrate was taken up at higher rates into vesicles derived from P-deficient white lupin compared with vesicles of P-sufficient control plants. Therefore citrate transport predominantly occurs in roots of P-deficient plants, and is linked with the activity of the PM H+-ATPase to maintain the electrochemical potential gradient which is reduced by citrate export out of the cell. Citrate exudation combined with an increase in H+-ATPase activity seems to prevent citrate accumulation up to concentrations which might exert inhibitory effects on the PM H+-ATPase. Such an inhibition was seen by diminished intravesicular proton accumulation, detected with the pH probe acridine orange, when 5 mM citrate were applied to the vesicle preparation. No such inhibitory effects were observed by malate application, which hints to a citrate-specific reaction. Lowering the cytosolic pH by external application of propionate stimulated citrate and malate exudation in non-cluster laterals and in young clusters. Therefore a causal relationship might exist between citrate accumulation and exudation by acidification of the cytosol. The threshold citrate concentration at which citrate exudation is triggered perhaps is reached when citrate ac-cumulation leads to acidification of the cytosol. Carboxylate exudation in young cluster roots and seedling root tips hints to a putative anion channel which already exists in young tissue and might be regulated in relation with H+-ATPase activity and cytosolic pH. Protoplasts, isolated from mature cluster roots, did only give very low yield and were not viable for seals high enough for patch-clamp studies. This might be due to the fast senescence in the developing clusters which also seems to change membrane integrity. High yields could only be gained from seedling root tips, or cotyledons. Similarly, protoplast isolation from root hairs also was only possible from seedling root tips or non-cluster lateral root tips, but even not from just emerging root hairs of young cluster roots. To determine the influence of a second growth factor in addition to P deficiency on citrate me-tabolism, white lupin was cultivated in nutrient solution and in rhizoboxes at ambient (400 μmol mol-1) and at elevated (800 μmol mol-1) atmospheric CO₂ concentrations. Plant development was accelerated at elevated CO₂ concentrations, and P deficiency and senes-cence symptoms such as yellowing, wilting, and abscission of leaves could be seen much earlier. When cultivated in nutrient solution, shoot growth was rather unaffected by the CO₂ concentra-tion, whereas root growth was much faster at elevated CO₂. Quite contrary, shoot growth was slightly higher at elevated CO₂ concentrations in plants in rhizobox culture, but root growth was unchanged. However, the harvest of a higher amount of cluster roots from plants at elevated CO₂ or the calcareous soil might have reduced root growth of the plants grown in rhizoboxes. Higher root/shoot ratios under P deficiency were further increased at elevated CO₂ concentra-tions. The amount of clusters was higher in plants grown in nutrient solution at 800 μmol mol-1 CO₂ from day 21 to day 33 after sowing, but thereafter the differences disappeared. No signifi-cant differences between CO₂ treatments were observed for the proportion of cluster roots rela-tive to the whole root system. Independent of the cultivation method, root exudation per cluster or per cluster root weight was unchanged by the elevated CO₂ concentration. The distribution of citrate and malate exudation in different cluster root segments with decreasing malate exudation and a peak of citrate exudation in mature clusters was also confirmed at 800 μmol mol-1 CO₂. The increased carbon distribution into the root at 800 μmol mol-1 CO₂, seen in a higher root/shoot ratio, was not transformed into higher exudation rates from the single cluster. Acid and alkaline phosphatase activities in the rhizosphere of L. albus continually increased during cluster root development independent of the CO₂ supply. Phosphatase activities and carboxylate accumulation and exudation rates were essentially un-changed by different atmospheric CO₂ concentrations. This might be due to also unaltered Pi concentrations in the respective root segments, because internal P concentrations seem to deter-mine these parameters. Since citrate accumulation and exudation probably depends on citrate degradation, which is not influenced by the amount of carbon supplied for anabolic processes, elevated CO₂ concentrations do rather not change citrate concentration and exudation. Accord-ingly, no significant effects of different CO₂ concentrations were seen on microbial diversity in the rhizosphere of white lupin. So far, no single cause or mechanism was found to be responsible for the high citrate concentra-tions measured in mature cluster roots, although citrate degradation seems to be important and aconitase probably plays a key role. A general impairment of metabolism due to decreasing con-centrations of Pi, adenylates, RNA, and proteins rather seems to bring about decreasing enzyme activities and reduced respiration. Various regulatory mechanisms via phosphorylation/ dephos-phorylation, phytohormones, nitric oxide, or others also have to be considered.
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    Soil microbial assimilation and turnover of carbon depend on resource quality and availability
    (2017) Müller, Karolin; Kandeler, Ellen
    The decomposition of soil organic carbon (SOC), which is predominantly performed by soil microorganisms, is an important process in global carbon (C) cycling. Despite the importance of microbial activity to the global C budget, the effects of resource quality and availability on soil microorganisms are little understood. Most of this plant-derived C enters the soil organic C pool via incorporation into soil microorganisms, but the subsequent fate of C is rarely reported. Therefore, soil microbial biogeochemistry is still highly uncertain in earth system models. The study presented in Chapter 5 used a field experiment established in 2009 to investigate C flow at three soil depths over five consecutive years after a C3 to C4 crop exchange. Root-derived C (belowground pathway) was introduced by the cropping of maize plants, whereas shoot-derived C (aboveground pathway) was introduced by application of shoot litter to the soil surface. The proportion of maize-derived C varied between the different soil pools with lower incorporation into SOC and EOC (extractable organic C) and higher incorporation ratios of maize C into microbial groups. Although root-C input was three times higher than shoot-C input, similar relative amounts of maize-C were found in microorganisms. Both root and shoot C were transferred to a depth of 70 cm. At all three depths, fungi utilized the provided maize C to a greater extent than did either Gram-positive or Gram-negative bacteria. Fungal biomass was labeled with maize-C to 78% after the fifth vegetation period, indicating preferential utilization of litter-derived C by saprotrophic fungi. The second study investigated, in a microcosm experiment, the effects of decreasing resource quality on microorganisms during plant residue decomposition at the soil-litter interface. Reciprocal transplantation of labeled 13C and unlabeled 12C maize litter to the surface of soil cores allowed us to follow C transfer and subsequent C turnover from residues into microbial biomass of fundamental members (bacteria and fungi) of the detritivore food web during three stages of the litter decomposition process. Quality (i.e. age) of the maize litter influenced C incorporation into bacteria and fungi. Labile C from freshly introduced litter was incorporated by both groups of microorganisms, whereas saprotrophic fungi additionally used complex C in the intermediate stage of decomposition. Bacteria responded differentially to the introduced litter; either by turnover of litter C in their phospholipid fatty acids (PLFAs) over time, or by storage and/or reuse of previous microbially released C. Saprotrophic fungi, however, showed a distinct litter C turnover in the fungal PLFA. The mean residence time of C in the fungal biomass was 32 to 46 days; the same or shorter time than in bacterial PLFAs. In the third study, presented in Chapter 7, another field experiment was conducted to distinguish herbivore- from detritus-based food chain members over two consecutive years. Three treatments were established: maize as crop plant, maize shoot litter application, and fallow without C input. This provided root-derived C, shoot-derived C, and autochthonous organic matter, respectively, as the main C resource. The altered C supply due to plant removal had less severe effects on the micro-food web structure than expected. In the first growing season, nematode abundance under plant cultivation was similar to that under litter and fallow conditions. After the second harvest, the abundance of detritivore food chain members increased, reflecting the decomposition of root residues. Bacteria and fungi showed a marked resilience to changed C availability. Results of this experiment suggest considerable micro-food web resilience to altered C and nutrient availability, and indicate that organic matter from previous vegetation periods was successfully utilized to overcome C deprivation. In conclusion, this thesis provides new insights into microbially mediated decomposition processes at different time scales and at different soil depths. Stable isotope probing combined with biomarker analysis enabled us to study C fluxes between biotic and soil C pools to separate the contributions of bacteria and fungi to soil C cycling. These results can be used as a basis for an empirical model of C flow through the entire soil food web.
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    Transporters mediating ammonium uptake in plants and their regulation by the abiotic stress signaling pathway
    (2023) Porras Murillo, Romano; Ludewig, Uwe
    Nitrogen nutrition refers to the uptake, assimilation, and utilization of ammonium, nitrate, and organic nitrogen sources. Ammonium is energetically a more cost-effective nitrogen source than nitrate but can be toxic for plants, and its use by plants is regulated at different levels. Ammonium transporters (AMTs) take up ammonium and are localized primarily in plant roots, working as trimers in the plasma membrane. Under high external ammonium concentrations, phosphorylation in AMTs C-termini shuts down transport to avoid toxicity. This phosphorylation is performed by CIPK23, a kinase shown to be inhibited by Clade A PP2Cs. This study aimed to characterize AMTs from wheat and analyze their transcriptional response to ammonium. Another aim was to determine the role of clade A PP2Cs and PYR/PYL receptor proteins for abscisic acid in ammonium nutrition. Chapter I describes the physiological responses of winter wheat to different nitrogen sources and ammonium concentrations. The plants mainly used root morphological responses to adapt to differences in the nitrogen source. High external concentrations of ammonium reduced plant growth, while these conditions induced the expression of TaAMT1;1 and TaAMT1;2. In Chapter II, we studied the capacity of TaAMT2s to transport ammonium and their transcriptional responsiveness to ammonium nutrition. From the six TaAMT2s, only TaAMT2;1 could transport ammonium in a yeast complementation line. Besides, its expression in roots is lower under ammonium than under nitrate. The expression pattern among the remaining TaAMT2s (TaAMT2;2-TaAMT2;6) is similar, with higher expression under ammonium, in both roots and leaves, compared to nitrate. Chapter III focused on the role of the PP2C phosphatase ABI1 (ABA-insensitive 1) in ammonium nutrition and the effect of external ammonium concentrations on ABA concentrations. Ammonium increased ABA concentrations in roots by activating ABA-GE, meaning ammonium toxicity could be sensed as abiotic stress through ABA. Without ammonium, ABI1 dephosphorylates AMTs and inhibits CIPK23; with ammonium, ABA-PYR/PYL complex-mediated inhibition of ABI1 releases CIPK23 to phosphorylate AMTs and avoids ammonium toxicity. Finally, in Chapter IV, we studied the role of AIP1 and its ammonium-dependent regulator, PYL8, in nitrogen nutrition. We described the function of AIP1, which was redundant to ABI1 in AMT regulation. Based on ammonium-dependent root architecture changes, and higher auxin accumulation in pyl8-1 root tips compared to the wild type, we suggest that PYL8 is involved in root-phenotype modulation in an ammonium-dependent manner.