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Rhizosphere engineering Improving plant tolerance to drought by modifying the physical and biological properties of the rhizosphere

dc.contributor.advisorKuzyakov, Yakov Prof. Dr.
dc.contributor.authorAhmadi, Katayoun
dc.titleRhizosphere engineering Improving plant tolerance to drought by modifying the physical and biological properties of the rhizospherede
dc.contributor.refereeCarminati, Andrea Prof. Dr.
dc.description.abstractengAmong various biotic and abiotic stresses, drought is one of the most limiting factors compromising plant growth and crop productivity. Mobility and availability of nutrients are controlling factors for plant growth and development and they are strongly limited by soil drying. An additional factor that is negatively influenced by low soil water contents is microbial activity, which is one of the main drivers of nutrient availability to the plants. To cope with soil drying and its adverse direct and indirect effects on water and nutrient availability, it has been proposed that plant roots modify their environment in which they grow, the so-called rhizosphere, to improve the mobility and the availability of nutrients. These modifications of physical and biogeochemical properties of the rhizosphere have been attributed to root exudation and the secretion of mucilage. Mucilage is a bio-polymeric and gel-like substance released by the roots. Mucilage is capable of adsorbing a large volume of water, thereby increasing the water retention of the rhizosphere. It also maintains the contact between the root surface and the soil matrix by enhancing soil aggregation around the roots, as manifested by the formation of the so-called rhizosheath, a cohesive layer of soil particles adhering to the root surface. Rhizosheath has been proposed to play a crucial role in increasing plant tolerance to water deficit by maintaining the contact between roots and soil. A moist rhizosphere and optimal contact between root and soil could facilitate transport of resources to the roots as soil dries. However, recent studies have shown that mucilage makes the rhizosphere of some plant species water repellent, which might limit the fluxes of water across the root-soil interface during repeated drying-wetting. In summary, it is still unclear to what extent mucilage properties and its water repellency influences water dynamics, nutrient uptake and microbial and enzyme activity in the rhizosphere. The objective of this thesis was to explore strategies for improving plant drought tolerance by modifying the biophysical properties of the rhizosphere. To this end, I focused on potential ways to alter mucilage properties and their impact on rhizosphere physical and biogeochemical processes. The first objective of this thesis was to ascertain if plant tolerance to soil drying could be improved by increasing the mechanical stability of the rhizosheath and if this could be achieved by affecting mucilage swelling and its viscosity. In the second and third sections of this study, I investigated the effects of reduced rhizosphere hydrophobicity on i) microbial and enzyme activities in the rhizosphere and on ii) mobility and availability of nutrients and consequently plant performance under dry condition. Initially, the concept of rhizoligand, a new way to engineer rhizospheric properties, was developed. A rhizoligand was defined as an additive that: i) interacts with the mucilage network in the rhizosphere increasing its viscosity and thus increasing rhizosheath mass and the contact between roots and soil; and that ii) decreases the water repellency of rhizosphere. A commercial surfactant was tested and selected as prototype of rhizoligands for the experiments carried out in this thesis. The rhizoligand concept was tested through a series of experiments. Firstly, the capability of the tested surfactant to induce new cross linkage in the network of mucilage was tested using an analogue of root mucilage, mucilage from chia seeds. The surfactant significantly reduced the final swelling of mucilage. Secondly, the ability of the surfactant to enhance the rewetting rate of rhizosphere was tested with lupines growing in sandy soils. To this end, the neutron radiography was used to in situ monitor rhizosphere soil water dynamics. The results showed that under dry condition, the rhizosphere became hydrophobic, while the application of the used surfactant reduced its hydrophobicity and homogeneously rewetted the rhizosphere. These two preliminary tests proved that the selected surfactant behaved as rhizoligand. Then, in the first test with the rhizoligand we addressed the question whether rhizoligand enhances rhizosheath formation and the carbon content in the rhizosheath. White lupins were grown in sand and were exposed to six drying-rewetting cycles. Half of the plants were irrigated with water and the other half with the rhizoligand. The radius of rhizosheath was quantified by scanning the roots and analyzing the rhizosheath using the software WinRhizo. Rhizoligand application increased rhizosheath formation by 1.64 times. Additionally, the total carbon contained in the rhizosheath of plants irrigated with rhizoligand was significantly greater than in the rhizosheath of plants that were not treated with the rhizoligand. The second part of this thesis addressed the effect of rhizoligand on microbial and enzyme activity in the rhizosphere. It was hypothesized that the reduced hydrophobicity of the rhizosphere and the enhanced formation of rhizosheath created a favorable environment around the roots, with greater moisture and greater amount of carbon and mucilage in the rhizoligand-amended soil, and therefore stimulated microbial and enzyme activities in the rhizosphere. In agreement with this hypothesis, activities of the chitinase, sulfatase, and β-glucosidase were 4, 7.9, and 1.5 times greater in the rhizosphere of plants irrigated with rhizoligand than water. Similarly, microbial biomass C and microbial biomass N increased by 1.57 and 3 times in the rhizosphere of plants under rhizoligand application in comparison to the rhizosphere of reference plants, respectively. The effects of rhizoligand on the distribution of enzyme activities was also visualized using zymography. Application of rhizoligand i) increased the β-glucosidase and phosphatase activities by 5.3 and 2.9 times in the regions close to the roots (0-0.5 mm distance from the root surface), and ii) enlarged the area with high enzyme activity 1.46-fold for β-glucosidase and 1.2-fold for phosphatase. The enlarged area with high enzyme activity around the roots in amended-rhizoligand plants in comparison to reference plants was attributed to greater rhizosheath thickness of plants irrigated with rhizoligand. The third section of this thesis addressed the impact of rhizoligand on nutrient uptake. Plants amended with rhizoligand had higher nutrient content on a plant biomass basis (g plant-1) in comparison to control plants (plants not amended with rhizoligand). Fe content increased by 51% and Mn content increased by 46%. Additionally, root biomass was greater in the rhizoligand amended plants relative to control plants. Greater plant nutrient acquisition was explained as a result of multiple factors: i) higher biological activity (as shown in the section above) which lead to increase nutrient availability; ii) greater soil water content in the rhizosphere and consequently greater nutrient mobility; and iii) greater rhizosheath thickness which maintained the roots in contact with the soil and reduced root mortality during severe drying cycles (in fact, rhizosheath acts as a cylindrical protective layer covering the root surface and maintaining roots hydrated). In conclusion, application of rhizoligand improves plant performance by: i) reducing rhizosphere water repellency, ii) increasing the mechanical stability of the rhizosheath, iii) increasing the microbial and enzyme activities in the rhizosphere, and iv) improving plant nutrient acquisition. Such improvements are triggered by the interaction between mucilage and the applied rhizoligand, which binds the mucilage network and increases its viscosity, creating a new matrix at the root-soil interface. We propose the rhizoligand concept as an effective approach to engineer the rhizosphere properties and to improve plant tolerance to water
dc.contributor.coRefereeDippold, Michaela Prof. Dr.
dc.contributor.thirdRefereeBlagodatskaya, Evgenia PD Dr.
dc.subject.engRhizosphere engineeringde
dc.affiliation.instituteFakultät für Agrarwissenschaftende
dc.subject.gokfullLand- und Forstwirtschaft (PPN621302791)de

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