MUCILAGE: the hydraulic bridge between roots and soil

水是植物生长的源泉。在气候变化时期，缺水将危及全球粮食生产。维持粮食生产的一个缓解战略是提高作物从土壤中获取水分的能力。我们项目的中心假设是，根部分泌的粘液有利于水分流入根部，从而提高植物的抗旱性。我们假设，粘液改变了根际的物理化学性质，特别是其保水能力和水力传导性，并且这些改变影响了穿过根际的水流。由于其高持水能力，粘液保持根际湿润，并帮助根在干旱期间保持与土壤的水力接触，提高根-土壤界面的水力传导性。然而，随着粘液的干燥，它变得疏水性，可能会降低局部根的摄取。目前尚不清楚这种暂时的疏水性是获取水分的问题，还是相反，当土壤变得过于干燥时，这是植物的一种不失去水分的策略。此外，还不知道这些特征如何取决于植物种类和土壤类型。我们项目的目的是了解粘液在调节植物供水方面的机械作用。我们将该项目分为三个子项目，任务如下：1)测量粘液的物理化学性质，特别是粘液中水的结合特性和流动性；为此，我们将使用1H-核磁共振弛豫法、扩散法和差示扫描量热法。2)为了测量土壤-凝胶混合物的持水曲线和导水率，我们将PGA-Ca(粘胶模型)和来自不同豆科和禾本科的真实粘液混合在不同粒度分布的土壤中。在第一步中，凝胶将与沙子均匀混合；然后将其添加到人造根(吸盘)周围，以模拟粘液在根际的分布。3)测定粘液和根际对根系吸水的影响。为此，根压力探头将应用于在土壤中生长的单个根。这些测量将与根际水分分布的中子射线照相术相结合。实验将用一个包含粘液-根际动力学的水流入单个根的数值模型来模拟。这三个子项目计划逐步将超分子尺度上的过程与宏观土壤水力性质和根系吸水量联系起来。该项目的预期成果是了解粘液在土壤-植物水分关系中的作用及其对提高植物抗旱性的重要性。

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.