Browsing by Subject "Arbuscular mycorrhizal fungi"

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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.
Ecological and molecular characteristics of arbuscular mycorrhizal fungi (AMF) on mercury phytoremediation
(2023) Guo, Yaqin; Frank, Rasche
Environmental pollution caused by harmful chemicals represents a major challenge worldwide. Among these, heavy metals (HM) in soils are of particular concern due to their persistence, toxicity, and bioaccumulation which can significantly threaten human health, plant growth, and ecosystem integrity. Phytoremediation, which uses plants to extract pollutants from soils, has been recognized as a promising approach to remediate HM-contaminated soils. Arbuscular mycorrhizal fungi (AMF)-assisted phytoremediation has shown great potential to enhance plant growth and metal uptake by forming a mutual association between plant roots and AMF, which can improve nutrient uptake and tolerance to environmental stress. Despite its potential, however, the effectiveness of this approach can be limited by various factors, such as environmental and geographic factors, soil properties, and plant-microbe interactions. An advanced fundamental understanding of both ecological and molecular characteristics of this technology is thus crucial to improve its effectiveness and application potential. Therefore, the impetus of this doctoral thesis was to investigate the potential of AMF in phytoremediation of soils contaminated with HM, with a particular focus on mercury (Hg) remediation. The first study (Chapter 2) contributes to the ecological understanding of AMF in a degraded ecosystem. In this study, two geographically distinct, abandoned gold mining locations in Ghana were selected and the genetic diversity and composition of AMF communities both in the rhizosphere and roots of the pioneer plant Pueraria phaseoloides (Roxb.) Benth. (tropical kudzu) were analyzed using a metagenomic sequencing approach. To determine the primary factor shaping AMF communities, both biotic (plant identity) and abiotic factors (geographic locations and soil conditions) were examined. In total, 195 amplicon sequence variants (ASV) affiliated to eight genera of the phylum Glomeromycota were identified. The root compartment showed a lower diversity than the rhizosphere soils and a difference of AMF compositions between the two compartments was detected irrespective of geographical location. Moreover, co-occurrence network analysis revealed two different keystone species in the two compartments, i.e., Acaulospora in rhizosphere soil and Rhizophagus in roots. The high abundance of Rhizophagus in the roots of P. phaseoloides was the result of a good match of functions between plant and AMF. Collectively, the results indicated that plant compartment (root versus rhizosphere) is the main factor shaping AMF communities associated with P. phaseoloides. The second study (Chapter 3) comprises a research synthesis of the role of AMF in zinc (Zn), cadmium (Cd), and Hg bioremediation. The study assumed that mycorrhization plays a role in modulating the uptake of Hg, facilitated by Zn and/or Cd transporters. The synthesis demonstrated that AMF have the ability to regulate the transporters responsible for Zn and Cd uptake and transport, such as ZIP (zinc-iron permease or ZRT-IRT-like protein), CDF (cation diffusion facilitator), NRAMP (natural resistance-associated macrophage proteins), and HMA (heavy metal ATPase). This regulation can either enhance or inhibit the uptake and transport of Zn or Cd. The extent of this regulation is influenced by multiple factors, such as the plant species, the species of AMF involved, and soil conditions, including pH and elements such as phosphorus (P). It was concluded that future research is needed to investigate the optimal environmental conditions under which AMF are effective in Hg remediation for appropriate application. The third study (Chapter 4) offers essential insights into the distinct functions of AMF symbiosis in Hg partitioning in plants. This relationship was assessed in the context of Zn uptake mechanisms and the expression of two Zn transporter genes (ZIP2 and ZIP6). Zn is crucial for plants and has a similar outer electronic configuration as Hg, which implies a potential competition for the same transporters. In a greenhouse experiment, plants of Medicago truncatula were exposed to different Hg concentrations with and without inoculation of the AMF species Rhizophagus irregularis. This study demonstrated that mycorrhizal symbiosis improved plant Hg tolerance under Hg exposure, but the specific roles of mycorrhizal symbiosis in Hg partitioning depended on Hg concentrations in the substrate. A negative relationship between Hg and Zn concentrations in roots was observed, although the expression of Zn transporters (ZIP2 and ZIP6) by mycorrhizal inoculation was upregulated irrespective of Hg concentrations in the substrate. More importantly, mycorrhizal colonization reduced Hg concentrations in leaves compared to controls, regardless of Hg concentrations in the substrate. This study demonstrated that mycorrhizal symbiosis influences Hg uptake in M. truncatula and highlights the importance of AMF in phytoremediation. Overall, this doctoral thesis extends the understanding of AMF in phytoremediation with insights from both ecological and molecular perspectives and provides a knowledge basis to realize the potential and implementation of this technology.
Growth responses of three European weeds on different AMF species during early development
(2022) Säle, Verena; Sieverding, Ewald; Oehl, Fritz
Arbuscular mycorrhizal fungi (AMF) have multiple functions in agroecosystems and affect many processes below- and aboveground, including plant productivity. Mycorrhizal symbiosis is not necessarily beneficial for the host plant and the growth response can be not only positive but also neutral or negative. Among other factors, the responsiveness of plants to AMF depends on the plant-fungus combination. To find out whether the AMF species or isolate is a decisive factor for growth responses of weeds, 44 AMF isolates were tested in a pot experiment for their effects on three agricultural weeds: Echinochloa crus-galli, Solanum nigrum and Papaver rhoeas. The 44 isolates cover 18 AMF species from 13 genera and all 5 orders of the Glomeromycota. The aboveground biomass of the weeds was determined after different times of growth of each weed. In most cases, the effects of AMF isolates on weed growth were negative or neutral. We conclude that some weed species do not benefit from AMF in terms of growth. AMF species can even cause negative growth responses, an effect that may be of practical interest for organic farming where the aim is to obtain a high diversity and concentration of native AMF for the benefit of the cultivated crops without increasing the labor for mechanical weeding.