Plant roots redesign the rhizosphere to alter the three‐dimensional physical architecture and water dynamics

Abstract

The mechanisms controlling the genesis of rhizosheaths are not well understood, despite their importance in controlling the flux of nutrients and water from soil to root. Here, we examine the development of rhizosheaths from drought-tolerant and drought-sensitive chickpea varieties; focusing on the three-dimensional characterization of the pore volume (> 16 μm voxel spatial resolution) obtained from X-ray microtomography, along with the characterization of mucilage and root hairs, and water sorption. We observe that drought-tolerant plants generate a larger diameter root, and a greater and more porous mass of rhizosheath, which also has a significantly increased water sorptivity, as compared with bulk soil. Using lattice Boltzmann simulations of soil permeability, we find that the root activity of both cultivars creates an anisotropic structure in the rhizosphere, in that its ability to conduct water in the radial direction is significantly higher than in the axial direction, especially in the drought-tolerant cultivar. We suggest that significant differences in rhizosheath architectures are sourced not only by changes in structure of the volumes, but also from root mucilage, and further suggest that breeding for rhizosheath architectures and function may be a potential future avenue for better designing crops in a changing environment.

Cereal root exudates contain highly structurally complex polysaccharides with soil‐binding properties

“…High-molecular weight glycan exudates, soil-binding factors, rhizosheaths and the rhizosphere Rhizosheaths have been a focus for much recent work and consideration of their potential to be important factors associated with enhanced nutrient and water uptake, and are a potential breeding target particularly for water-limited grain crops (George et al, 2014;York et al, 2016;Adu et al, 2017;Brown et al, 2017;Rabbi et al, 2018;Young and Bengough, 2018). Understanding the mechanistic basis of their formation and stabilisation is therefore of broad economic and social interest.…”

“…Understanding the mechanistic basis of their formation and stabilisation is therefore of broad economic and social interest. Adhesive mucilage is recognised as an important component of rhizosheaths (Ahmed et al, 2018;Rabbi et al, 2018). Root hair length of the same genotype can vary between soil conditions and can influence rhizosheath mass (Haling et al, 2014) there is clearly potential for adhesive secretions to contribute to this.…”

“…Root systems anchor plants in soils, and access water and nutrients through developmentally plastic root architectures that also dynamically interact with soil biota. Roots influence the biology and chemistry of surrounding zones of soil known as rhizospheres (Walker et al, 2003;Rabbi et al, 2018). It is well established that roots release exudates that include a range of low-and high-molecular-weight (HMW) compounds that are likely to influence a wide range of soil properties (Walker et al, 2003;Naveed et al, 2017).…”

“…Factors involved in the maintenance of rhizosheaths may increase soil quality in general by increasing water flow, and thus nutrient flow and soil aeration through more frequent pores (Tisdall and Oades, 1982;Benard et al, 2019). During periods of drought, some grasses can increase the thickness of their rhizosheath, potentially in an effort to secure water uptake (Watt et al, 1994;Rabbi et al, 2018;Liu et al, 2019).…”

Crop Improvement from Phenotyping Roots: Highlights Reveal Expanding Opportunities

“…Genetic regulators of growth patterns of roots along the edges of pores, mediated by moisture, was simulated and tested ( Figure 5B, right) [105]. X-ray CT detectors allow the rhizosphere fine physical structure to be measured as it is altered by the root in situ in soil, along with predictions on water and nutrient uptake [87,[106][107][108]. But how can the large number of variables in the rhizosphere be systematically combined for the understanding of processes?…”