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Aims: This study investigated the influence of climate and soil on the exudation rate and polysaccharide composition of aerial nodal root mucilage from drought-resistant and drought-susceptible maize varieties. Methods: Two maize varieties were grown in two different soils (sandy-clay loam Acrisol and loam Luvisol) under simulated climatic conditions of their agroecological zones of origin in Kenya and Germany. The exudation rate of mucilage from the aerial nodal roots was quantified as dry weight per root tip per day and the mucilage was characterized for its polysaccharide composition. Results: On average, the mucilage exudation rate was 35.8% higher under the Kenyan semi-arid tropical than under the German humid temperate climatic conditions. However, cultivation in the loam Luvisol soil from Germany led to 73.7% higher mucilage exudation rate than cultivation in the sandy-clay loam Acrisol soil from Kenya, plausibly due to its higher microbial biomass and nutrient availability. The drought-resistant Kenyan maize variety exuded 58.2% more mucilage than the drought-susceptible German variety. On average, mucilage polysaccharides were composed of 40.6% galactose, 26.2% fucose, 13.1% mannose, 11% arabinose, 3.5% glucose, 3.2% xylose, 1.3% glucuronic acid, and 1% an unknown uronic acid. Overall, significantly higher proportions of the uronic acids were found in the mucilage of the plants grown in the Kenyan sandy-clay loam soil and under the Kenyan semi-arid tropical climatic conditions. Conclusions: Maize is able to enhance its mucilage exudation rate under warm climatic conditions and in soils of high microbial activity to mitigate water stress and support the rhizosphere microbiome, respectively. [ABSTRACT FROM AUTHOR]

Aims: Visualization of enzymatic activity links microbial functioning to localization in heterogeneous soil habitats. To assess enzymatic reactions in soil thin layer at the microscopic level, we developed a micro-zymography approach and tested it by visualization of the potential activity of phosphomonoesterase for aggregates collected from the rhizosphere of Zea mays L. Methods: We evaluated micro-zymography by applying fluorogenically-labeled substrate i) on individual soil aggregates freshly sampled from the rhizosphere, ii) on thin layers of aggregates (≈ 500 µm) saturated with substrate to assess the dynamics of phosphomonoesterase activity, and iii) on maize roots under laser scanning microscope upon the identified hotspots by membrane-based zymography. Results: We found super transparent silicon as the most appropriate fixative to prevent sample drying. We optimized microscope settings to eliminate the soil auto-fluorescence. The fluorescent signal shifted from the free liquid phase towards the aggregate boundaries within 30 min after substrate addition and was finally detectable at the surface of a few aggregates. This was probably due to higher microbial abundance and enzymatic activity on the soil aggregates compared to the liquid phase. The enzymatic activity appeared patchy at the aggregate and root surfaces indicating heterogeneous distribution of hotspots. Conclusions: The methodology including calibration, sample preparation, fixation, and monitoring was developed. The novel membrane-free micro-zymography approach is a promising tool to identify functional specificity and niche differentiation on roots and soil aggregates. This approach revealed unexplained complexity of competing processes (biochemical, hydrolytic, and physical) due to differently charged reaction products and enzyme-clay complexes. [ABSTRACT FROM AUTHOR]

Background and aims: Biopores as microbial hotspots provide additional nutrients to crops - but only if their roots grow within the biopores. Such reuse has never been quantified as pre-crop-specific biopores are hardly differentiated from the multitude of pre-existing biopores. Quantification requires e.g. radionuclide labelling of pre-crops (Cs, to label their biopores) and main crops (C, to detect new roots). Preliminary testing was performed on simulated biopore reuse: both nuclides given to the same plant were excreted into the same rhizosphere. Methods: Cichorium intybus (cv. Puna) and Medicago sativa (cv. Planet) were each sequentially labelled via the leaves with Cs and CO. β-signals were visualised by imaging of horizontal soil cuts - with and without shielding off the weaker C. Results: Both species allocated 7.1-9.4% of the Cs and 21-63% of the C below ground. The first image gave both activities; while the second gave only Cs. Subtracting the second from the first image gave the C distribution, resulting in successful separation of the signals. Thus, separate spatial representations of the roots were obtained. Main root locations by Cs and C showed a very high spatial overlap coefficient (> 0.95). Conclusions: Biopore reuse quantification likely becomes feasible with this sequential labelling and shielding approach. [ABSTRACT FROM AUTHOR]

Plants stimulate microbial enzyme production in the rhizosphere, regulating soil organic matter decomposition and nutrient cycling. The availability of labile organic compounds (i.e. exudates) and water is the main prerequisite for such microbial activity and enzyme production, thus shaping the rhizosphere. Root morphology (i.e., root hairs) and exudate composition define the spatial distribution of properties and functions in the rhizosphere. However, the role of root architecture and exudate composition in this spatial self-organization of the rhizosphere remains unknown. We coupled three in situ imaging approaches: 14C imaging to localize exudates, soil zymography to analyze enzyme activity distribution, and neutron radiography for water fluxes to trace the spatial structure of the rhizosphere of three maize genotypes (wild-type, mutant with defective root-hair prolongation rth3 , and mutant with reduced benzoxazinoid content in root exudates bx1). The co-localization of these three soil images revealed the pivotal role of both optimal water content (neutron radiography) and root exudation (14C imaging) for β-glucosidase production by the rhizosphere microbiome and its hydrolytic activity (zymography). Root hairs increased the exudate release and enlarged the spatial extent of increased β - glucosidase activity around the root axis by 35%, leading to a two-fold faster decomposition of 14C exudates compared to the mutant with defective root hairs. In contrast, benzoxazinoids suppressed β - glucosidase activity by 30%, reflecting decreased microbial activity, whereas their absence broadened the rhizosphere. Overall, root hairs in wild-type maize increased microbial activity (i.e. β - glucosidase production), whereas the benzoxazinoids in root exudates suppressed microorganisms. [Display omitted] • Zymography, 14C imaging, and neutron radiography were coupled to localize rhizosphere processes. • Exudates are released at root tip in wild-type maize, but along the whole root in rth3 and bx1 mutants. • Root hairs enlarged the spatial extent of increased β - glucosidase activity around the root by 35%. • Benzoxazinoids (secondary metabolites) suppressed β - glucosidase activity in the rhizosphere by 30%. • Hotspot co-localization revealed that exudates and water increased β - glucosidase activity. [ABSTRACT FROM AUTHOR]

Despite detailed investigations on the distinct biochemical properties of the rhizosphere and their effects on the availability of nutrients for plants, their biophysical aspects, particularly the effect of mucilage on the transport of water and nutrients are poorly understood. The aim of this study was to investigate the effect of mucilage on the diffusion coefficient of nutrients and consequently their transport in the soil and into the plant roots. Repeated phosphor imaging was used to monitor the temporospatial distribution of 137Cs (as an analog of K) within a modeled rhizosphere soil with and without mucilage (a sandy soil amended with mucilage extract from chia seed) under different soil water contents. The monitored profiles of activities were used to estimate the diffusion coefficient of soils with and without mucilage by solving a diffusion equation. Assuming an identical the effect of mucilage on diffusion coefficient of Cs and K, a diffusion-convection equation was numerically solved to predict the transport of K within the soil and its uptake by a single plant root during a soil drying cycle. To this end, the hydraulic and diffusive properties of the soil were parameterized based on the measured data and the K uptake and its concentration in soil were taken from literature data. The results of this study suggest that mucilage in the rhizosphere keeps the rhizosphere wetter and maintains the connectivity of the liquid phase during a soil drying cycle, and thereby could prevent a marked drop in the diffusion coefficient. The results of modeling of nutrient uptake by a single root showed that the presence of mucilage in the rhizosphere could favor nutrient uptake by the plant root. In the case of nutrients with low concentration in the soil solution, it prevents a marked concentration drop in the vicinity of the root as the soil dries and diffusion becomes restricted. This will delay the risk of nutrient deficiency to the plant root. In the case of nutrients with a high concentration in the soil solution, the presence of mucilage in the rhizosphere may delay the risk of salinity stress as the soil dries and the concentration of nutrients increases at the vicinity of the root surface. In conclusion, the results of this contribution show that mucilage may favor the transport of nutrient within the soil and their uptake by plant roots under drying soil condition. [ABSTRACT FROM AUTHOR]