MUCILAGE: the hydraulic bridge between roots and soil

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Water is the source for plant growth. In times of climate change, water scarcity will endanger food production worldwide. One mitigation strategy for sustaining food production is to improve the capability of crops to capture water from soils. The central hypothesis of our project is that mucilage exuded by roots favors the water flow into roots and it increases plant drought tolerance. We hypothesize that mucilage alters the physicochemical properties of the rhizosphere, in particular its water holding capacity and hydraulic conductivity, and that these alterations affect the water flow across the rhizosphere. Thanks to its high water holding capacity, mucilage maintains the rhizosphere wet and helps roots to stay in hydraulic contact with soil during drought, improving the hydraulic conductivity of the root-soil interface. However, as mucilage dries, it becomes hydrophobic and it may reduce the local root uptake. It is not clear whether such temporary hydrophobicity is a problem for the acquisition of water or, on the contrary, it is a strategy of plants to not lose water when the soil becomes too dry. Furthermore, it is not known how such characteristics depend on plant species and soil types. Objective of our project is to understand the mechanistic role of mucilage for the regulation of water supply to plants. We structured the project in three sub-projects with the following tasks: 1) to measure the physiochemical properties of mucilage, and in particular the binding characteristics and mobility of water in mucilage; to this end we will employ1H-NMR-relaxometry, diffusometry and differential scanning calorimetry. 2) To measure the water retention curve and the hydraulic conductivity of soil-gel mixtures; to this end we will mix PGA-Ca (model for mucilage) and real mucilage (collected from different Fabaceae and Poaceae) with soils of different particle size distribution. In a first step, the gel will be homogeneously mixed with sand; then it will be added around artificial roots (suction cups) to mimic mucilage distribution in the rhizosphere. 3) To measure the effects of mucilage and rhizosphere on root water uptake. To this end a root pressure probe will be applied to single roots grown in soils. The measurements will be coupled with neutron radiography of water distribution in the rhizosphere. The experiments will be simulated with a numerical model of water flow into a single root including mucilage-rhizosphere dynamics. The three sub-projects are planned to stepwise link the processes at the supramolecular scale with the macroscopic soil hydraulic properties and root water uptake. The expected outcome of the project is the understanding of the role of mucilage on soil-plant water relations and its importance for improving plant drought tolerance.