Browsing by Subject "Rhizosphere"

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Drought-induced processes in the rhizosphere of maize (Zea mays L.)
(2023) Käsbauer, Lena; Zörb, Christian
Drought events are increasing due to climate change, resulting in significant yield losses. Many breeding strategies focus on drought resistance to avoid these yield losses or complete crop failure. Additionally, to improve drought resistance under soil desiccation, the soil and particularly rhizosphere processes are more and more in the focus of research. Specifically, linkages between the diverse and highly dynamic interactions of soil, plant, and microorganism community must be understood. This thesis thus aims to answer the following research questions: i) Are root hairs relevant for water uptake, and what role do they play under drought? ii) Does local drought in Zea mays result in distinguishable systemic and local metabolic and physiological responses, as well as compensatory water uptake? iii) Do the physico-chemical properties of Zea mays mucilage differ between two common collection systems? In the first part, published studies considering root hairs in nutrient and water uptake were summarized, and show a high plasticity of root hairs under different nutrient and water availability states. This plasticity was apparent through changes in root hair morphology and development. Furthermore, the role of root hairs in water uptake is under discussion due to variable results from different studies and crop species. Nevertheless, it seems that overall root hairs improve drought resilience. Furthermore, a better nutrient uptake and mucilage exudation by root hairs and thus an increased drought stability is discussed. This suggests a beneficial role of root hairs for drought stress robustness. In the second part, local and systemic drought responses of maize and their effect on rhizosphere processes were assessed in a split-root experiment. The root system of maize was separated into two differently watered (watered, drought stressed) rhizobox chambers. The local drought treatment was performed for 10 days. Under these conditions, the local drought led to a local and systemic response through osmotic adjustment. Osmolarity increased in the shoot, while increased proline concentrations and slight changes in root exudates indicated a local response in the drought stressed root compartment. This metabolic adjustment contributed to a hydraulic redistribution of water between the root halves and enhanced water availability. Comparing the physico-chemical properties of maize mucilage collected by two common collection systems emphasized the impact of mucilage collection when interpreting the role of mucilage in rhizosphere processes. The mucilage differed in terms of physico-chemical properties, which included contact angle, viscosity, surface tension (physical) and nutrient content, pH, polysaccharide polymer length, and neutral sugar composition (chemical). The mucilage was collected in two ways: 1) from primary and seminal roots of seedlings growing in a semi-sterile aeroponic system and 2) from airborne brace roots of maize growing on sandy soil. The two collection systems differed in terms of plant age, environment (sterility, light availability, air humidity), and root type. The higher viscosity of the brace root mucilage may have reflected the drier air humidity surrounding the root and therefore the need to enhance water holding capacity. Non-sterile conditions during brace root mucilage collection probably resulted in higher shares of hexoses, while semi-sterile conditions may explain the lack of mannose in the aeroponic mucilage. Brace root mucilage may therefore have a greater relevance during soil desiccation than aeroponic mucilage. In summary, this work helps to fill knowledge gaps in understanding and linking rhizosphere processes by i) providing a state-of-the-art summary of root hair plasticity related to nutrient and water availability and concluding a beneficial role of root hairs in drought robustness, ii) showing local and systemic osmotic adjustment and hydraulic redistribution under local drought, and iii) emphasizing the role of the mucilage collection systems when interpreting the role of mucilage in rhizosphere processes
Impacts of the fungal bio-control agent Fusarium oxysporum f.sp. strigae on plant beneficial microbial communities in the maize rhizosphere
(2016) Musyoki, Mary Kamaa; Cadisch, Georg
Striga hermonthica causes severe yield reduction in cereal crop production in Sub-Saharan Africa. Intergrated Striga management has been proposed as one of the best options to control striga. Along this line, the use of biocontrol agent (BCA) Fusarium oxysporum f.sp. strigae (Foxy-2) has been proven as an effective and environmental friendly management strategy. It is well established that a prerequisite for a successful BCA is sufficient risk assessment analysis. Towards this direction, Foxy-2 was assessed for its potential non-target impacts on the abundance, community structure of bacterial and archaeal nitrifying prokaryotes as well as enzymatic activities of proteolytic bacteria. Maize rhizosphere soils treated with or without Foxy-2, Striga and high quality organic residues (i.e., Tithonia diversifolia) as N source were evaluated by quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (TRFLP). It was observed that Foxy-2 had a promoting effect on archaeal abundance under controlled conditions in sandy soils. Furthermore, crop growth stage, seasonality and soil type had a strong effect on abundance and community structure of nitrifying prokaryotes over Foxy-2 inoculation. In addition proteolytic enzymatic activities analysis showed that Foxy-2 did not affect their activities. Correlation analysis also showed that abundance and community structure on nitrifying communities positively correlated with extractable organic carbon, extractable organic nitrogen and soil pH, while proteolytic enzymatic activities correlated with extractable organic nitrogen and soil ammonium. It was concluded that Foxy-2 is compatible with nitrifying prokaryotes and proteolytic enzymatic activities.
Mechanistic aspects of the eco-physiology of Fusarium oxysporum f. sp. cubense TR4
(2023) Were, Evans; Rasche, Frank
Banana and plantain (Musa spp.), here termed as bananas, are a source of food security and income for more than 400 million people globally. Banana production is threatened by Fusarium wilt disease, caused by the soilborne root-infecting fungal pathogen Fusarium oxysporum f. sp. cubense (Foc). Foc Tropical Race 4 (Foc TR4) is considered the most virulent race of Foc and has gained notoriety due to its inexorable spread and devastating impact on banana cultivation. Host infection occurs when pathogen propagules, called chlamydospores, germinate and produce hyphae that penetrate host roots and subsequently invade host tissues. Infection occurs in a narrow zone of soil immediately adjacent to the roots, called rhizosphere. The rhizosphere is notable for the extensive interactions between roots, the microbiome, and soil physico-chemical factors. Banana rhizosphere interactions are poorly understood, yet profoundly influence infection and development of Fusarium wilt. It is speculated that a better understanding of banana rhizosphere interactions will improve management of Fusarium wilt through the reduction of the abundance and/or efficacy of inoculum or enhance the disease suppressiveness of soils. Hence, the overarching objective of this doctoral study was to contribute to the fundamental ecological understanding of banana rhizosphere interactions related to Foc. The first study of this thesis analysed literature from four electronic databases (AGRIS, CAB Direct, SciVerse Scopus, ProQuest) to bring together the relatively scant data available on banana rhizosphere interactions and to highlight the key knowledge gaps. Analysis of 2,281 publications revealed the complexity of banana rhizosphere interactions and the driving factors of Fusarium wilt, for which the mechanisms remain poorly understood. Data from the literature shows that management of Fusarium wilt through rhizosphere manipulation is a dominant element albeit with limited success in the field. Notably, the data from literature shows that biological control agents (bacterial and fungal strains) are highly effective in vitro and in the greenhouse with a mean efficacy of 77.1% and 73.5%, respectively, but efficacy remains below 25.0% under field conditions. The second study of this thesis provides empirical evidence for suppression of Foc TR4 by root-secreted phenolic acids of non-host plants. Hydroponic culture and targeted metabolite analysis of root exudates of two legumes, Desmodium uncinatum and Mucuna pruriens, identified phenolic compounds such as benzoic-, t-cinnamic-, and p-hydroxybenzoic acid with inhibitory potential. These phenolic compounds suppressed Foc TR4 by inhibition of chlamydospore germination, production of new spores, and hyphal growth, and specifically also the biosynthesis of fusaric acid and beauvericin toxins, which are essential in the biology of the fungus. The third study of this thesis provides empirical evidence that the process of chlamydospore germination in Foc TR4 is developmentally orchestrated and iron-dependent. Scanning electron microscopy showed that iron-starved chlamydospores are unable to form a germ tube and exhibit reduced metabolic activity. Moreover, germination exhibits plasticity regarding extracellular pH, where over 50% germination occurs between pH 3 and pH 11. This suggests that disease suppression by manipulation of soil pH may not necessarily act via alteration of iron bioavailability. The requirement for iron was further investigated by assessing the expression of two genes (rnr1 and rnr2) that encode ribonucleotide reductase (RNR), the enzyme that controls cell growth through DNA synthesis. Expression of rnr2 was significantly induced in iron-starved chlamydospores compared to the control. The fourth study assessed the production of microbial iron-sequestering metabolites (siderophores) as a potential mechanism to counteract iron starvation. Specifically, ferrichrome, a hydroxamate siderophore, was synthesized exclusively in the mycelia of iron-starved cultures, which suggests de novo biosynthesis. Moreover, amino acid precursors for siderophore biosynthesis (ornithine, arginine) were altered by iron starvation. Collectively, this doctoral thesis extends the fundamental understanding of the biology and ecology of Foc TR4 and provides a base for realizing the potential of rhizosphere manipulation for management of Fusarium wilt.
Microbial consortia as inoculants for improvedcrop performance
(2020) Bradácová, Klára; Neumann, Günter
The use of microbial consortia products (MCP) based on combinations of different strains of plant growth-promoting microorganisms (PGPM) and frequently also on non-microbial bio-stimulants (BS) with complementary beneficial properties, is discussed as a strategy to increase the efficiency and the flexibility of BS-based crop production strategies under variable environmental conditions. Moreover, MCP application aims at the restoration of plant-beneficial, soil biological processes disturbed by soil degradation and intensive use of agro-chemicals. This PhD thesis was initiated to characterize the modes of action and the potential advantages of a representative commercial MCP formulation over selected single strain PGPM inoculants, with documented effects on plant growth promotion and pathogen suppression. In total, nine pot and field experiments were conducted with three crops (maize, spring wheat, tomato) on seven different soils with three organic and inorganic fertilization regimes. Only in one out of nine experiments conducted in this thesis, clear evidence for superior MCP performance was detectable in a drip-irrigated tomato field experiment conducted under the challenging environmental conditions of the Negev desert in Israel (Bradáčová et al., 2019c). This finding demonstrates that MCP inoculants can exhibit an advantage over single strain inoculants but not as a general feature. Selective interactions with the type and dosage of the selected fertilizers, as well as avoidance of inhibitory effects on root growth during MCP rhizosphere establishment, have been identified as critical factors. A further characterization of the conditions, promoting beneficial plant-MCP interactions is mandatory for a more targeted and reproducible MCP application.
Rhizosphere processes as determinants for glyphosate damage of non-target plants
(2010) Bott, Sebastian; Neumann, Günter
Due to low production costs and high herbicidal efficiency, glyphosate is the most widely used wide-spectrum herbicide. Glyphosate acts as a non-selective, total herbicide by inhibiting the biosynthesis of aromatic amino acids. Apart from glyphosate drift contamination, risks of glyphosate toxicity to crop plants and other non-target organisms are generally considered as marginal, because glyphosate is almost instantaneously inactivated by adsorption to the soil matrix and rapid microbial/chemical degradation in the soil solution. However, in the recent past, an increasing number of yet unexplained observations on significant damage of crop plants have been reported in the literature and by farmers, suggesting gaps in the risk assessment, with respect to the fate glyphosate in the rhizosphere and the interaction with rhizosphere processes. According to these observations, the aim of present study was a systematic evaluation of potential rhizosphere effects of glyphosate, including direct toxicity, risks of re-mobilisation by fertiliser application, potential role of pathogens and allelopathic compounds, and interactions with micronutrients, both in glyphosate-sensitive and transgenic glyphosate-resistant crops. A series of field trials in reduced soil tillage cropping systems as well as green-house experiments on soils with contrasting properties with sunflower, winter wheat and soybean, consistently revealed a close clausal relationship between crop damage and (a) short waiting times between glyphosate application on target weeds and subsequent sowing of crops and (b) the density and speed of decay of glyphosate-treated weeds. The results suggested that damage of crop plants is induced by a rhizosphere transfer of glyphosate from weeds to subsequently sown crops. This transfer might take place by contact contamination due to exudation of glyphosate from living roots of treated weeds and/or release during decomposition of the root residues. A comparison between phytotoxic effects of glyphosate and aminomethylphosphonic acid (AMPA) as major metabolite of glyphosate in soils, revealed high toxicity in case of root exposure to glyphosate, but not to AMPA. By contrast, a significant decline of germination was induced by seed exposure to AMPA, while germination was not affected by glyphosate treatments. The observed differences in sensitivity to glyphosate and AMPA in different stages of plant development may explain variable symptoms of crop damage under field conditions, ranging from growth depressions and chlorosis to reduced field emergence. The results of the present study further suggest that risks for crop damage associated with rhizosphere transfer of glyphosate are additionally influenced by a range of environmental factors, such as growth season (spring or fall application), temperature, soil moisture, redox potential of soils and soil microbial activity. These factors might shorten or prolongate the time window for crop damage of glyphosate contact contamination in the rhizosphere under field conditions. Model experiments investigating the sensitivity of different plant species to glyphosate root exposure, revealed significant differences between winter wheat, maize and soybean in terms of glyphosate-induced plant damage but also in their ability for recovery from glyphosate damage suggesting marked genotypic differences in the expression of damage symptoms also under field conditions. In agreement with previous investigations, results of the present study indicated a rapid inactivation of glyphosate by adsorption to the soil matrix. Glyphosate adsorption in soils seem to be mainly mediated by the phosphonate group of the molecule in a way similar to the adsorption of inorganic phosphate. Accordingly glyphosate re-mobilisation is possible via ligand exchange by phosphate application. The results of the present study have demonstrated for the first time that depending on soil properties also the application of fertiliser phosphate is able to re-mobilise glyphosate in sufficient quantities to mediate crop damage in pot experiments. This finding suggest, that re-mobilisation of glyphosate potentially by fertiliser P or root-induced chemical modifications for P and Fe mobilisation needs to be considered as additional potential rhizosphere pathway for glyphosate damage to non-target plants. Field trials and model experiments under soil and hydroponic conditions consistently revealed a significantly impaired nutritional status of glyphosate-sensitive but also glyphosate-resistant crops. However, depending on the culture conditions different mineral nutrients were affected by the glyphosate treatments and plant damage was not related with a certain nutrient deficiency. These findings suggest that damaged root growth, induced by glyphosate toxicity, rather than specific interactions with certain mineral nutrients are responsible for the observed impairment of nutrient acquisition. In conclusion, results of the present study highlight that risks for crop damage associated with glyphosate toxicity in the rhizosphere can be substantial and is influenced by factors such as waiting time after herbicide application, weed density, cropping systems, fertilizer management, genotypic differences, and probably also environmental factors including temperature, soil moisture, and soil microbial activity. The independency between these factors is so far not entirely clear but should be investigated in future studies. Nevertheless, results of present study suggest that risks could be minimized by simple management tools such as the consideration of waiting times between application of glyphosate and sowing of crops particularly in case of high weed densities and alternation of herbicides to reduce not only risk for remobilization of glyphosate but also problems associated to the selection of glyphosate-resistant weeds.
The role of soil properties and fertilization management in pathogen defense and plant microbial interactions in the rhizosphere of lettuce (Lactuca sativa L.)
(2022) Windisch, Saskia Helen; Neumann, Günter
Soil microorganisms are involved in nearly all relevant soil processes and considered as key players in agro-ecosystems. This is particularly relevant for the rhizosphere which is created by the activity of plant roots with dynamic impact on microbial communities, their diversity and activity. Both, beneficial but also pathogenic plant-microbial interactions in the rhizosphere are driven by root exudates and other root-induced modifications in rhizosphere chemistry, which are highly variable in space, time, composition and intensity. The physicochemical properties of the rhizosphere are influenced by numerous external factors including nutrient availability, biotic and abiotic stress, soil properties or plant genotypic variation but the related consequences for plant-microbial interactions and the consequences for plant performance and health status are still poorly understood. In this context the present study was initiated to investigate (i) the influence of the soil type on root exudation and the composition of the rhizosphere solution (ii) their impact on interactions with soil pathogens and beneficial rhizosphere microorganisms and (iii) the effect of long-term fertilization strategies (organic vs. mineral fertilization), using lettuce (Lactuca sativa) as a well-characterized model plant for studies on plant-microbial interactions in the rhizosphere.