Root exudation and the biophysics of the rhizosphere

在今后几十年中，作物产量必须增加，以满足不断增长的世界人口的粮食需求。以可持续的方式确保粮食生产的一个战略是种植具有更高养分和水利用效率的植物。根际（靠近根部的土壤区域）在水分和养分吸收中起着至关重要的作用：在根际，根分泌物增强微生物活性和养分吸收，有利于植物养分效率。该项目的基本假设是，分泌物与植物积极相互作用的程度取决于它们在根周围的分布。我们的目标是了解哪些因素影响根际根系分泌物的时空分布。我们计划研究根系分泌的粘液对低分子量分泌物在根际扩散的影响。与土壤相比，胶浆已被证明可以改变根际与水有关的性质。这是可能的，粘液强烈影响根分泌物在根际的分布。我们假设：1）在湿润土壤中，粘液减少了分泌物的扩散，并有助于将它们保持在根表面附近，并增加与植物的相互作用; 2）在干燥土壤中，粘液增加了根际含水量，因此，它增强了根分泌物的扩散，增强了根际延伸。随着时间的推移，粘液经历生理化学变化，并转化为一种称为粘液的物质。我们假设根分泌物的扩散率随着粘液胶年龄的增加而降低。此外，我们将测试根系吸水是否影响分泌物的分布和含水量是否影响分泌量。我们计划在根箱中结合玉米标记、14 C成像和中子照相进行实验。在第一个工作包（WP 1）中，我们将测试水对流（即根吸水）和土壤含水量对根分泌物分布的影响，我们将根据含水量量化根沉积。在WP 2中，我们将使用14 C成像和中子射线照相术可视化和量化根系分泌物、含水量和粘液沿着根系的分布（二维方法）。在WP 3中，我们将研究粘液如何影响根际分泌物在简化系统中的分布。在WP 4中，我们将测量根分泌物和粘液的降解。最后，将实施一个数值模型来模拟水流、土壤性质、粘液和降解对渗出物空间分布的影响（WP 5）。新的实验和计算方法的结合将使我们能够解开根系分泌和土壤中粘液释放的相互作用。这将导致更好地了解土壤-根系界面的生物化学和物理过程，而这些过程反过来又会影响植物水分和养分的利用效率。

During the next decades crop production has to increase to meet the food demand of the growing world population. One strategy to secure food production in a sustainable way is to grow plants with higher nutrient and water use efficiency. The rhizosphere, the region of soil near the roots, plays a crucial role in water and nutrient uptake: in the rhizosphere, root exudates enhance microbial activity and uptake of nutrients, favoring plant nutrient efficiency. The underlying hypothesis of this project is that the degree to which exudates can positively interact with plants depends on their distribution around the root. Our objective is to understand what factors affect the spatial and temporal distribution of root exudates in the rhizosphere. We plan to investigate the effect of root exuded mucilage on the diffusion of low molecular weight exudates across the rhizosphere. Mucilage has been shown to change the water related properties of the rhizosphere compared to the bulk soil. It is likely that mucilage strongly affects root exudates distribution in the rhizosphere. We hypothesize that: 1) in wet soils mucilage decreases the diffusion of exudates and it helps to maintain them near the root surface and to increase interaction with plants; 2) in dry soils mucilage increases the rhizosphere water content and, consequently, it enhances the diffusion of root exudates, enhancing rhizosphere extension. Over time, mucilage undergoes physiochemical alterations and it is transformed into a substance called mucigel. We assume that diffusivity of root exudates is reduced with increasing age of mucigel. Furthermore, we will test whether root water uptake affects exudate distribution and whether water content influences the amount of exudation. We plan to perform experiments in rhizoboxes combining labeling of maize, 14C imaging and neutron radiography. In the first work package (WP1) we will test the effect of water convection (i.e. root water uptake) and soil water content on root exudate distribution we will quantify rhizodepostion depending on water content. In WP2 we will visualize and quantify the distribution of root exudates, water content and mucilage along the root system (two-dimensional approach) using 14C imaging and neutron radiography. In WP3 we will study how mucilage affects the distribution of root exudates in the rhizosphere in a simplified system. In WP4 we will measure the degradation of root exudates and mucilage. Finally, a numerical model will be implemented to simulate the effects of water flow, soil properties, mucilage and degradation on the spatial distribution of exudates (WP5). The combination of new experimental and computational methods will enable us to disentangle the interactions of root exudation and mucilage release in soil. This will lead to better understanding of biochemical and physical processes at the soil-root interface which in turn affect plant water and nutrient use efficiency.