Packed Granular Microgels as a Transparent Soil Medium for Biophysical Plant Studies

The rhizosphere is a dynamic ecosystem shaped by the interactions between plant roots, their exudates, microorganisms, and the soil that is critical for both the health of the plant and the sequestration of carbon from the atmosphere. However, spatial-temporal studies of plant roots and the plant-environment interactions that occur within the rhizosphere are hindered by the opaque nature of soil. While hydrogel networks, such as agar, have been used as transparent soil substrates, they often do not recapitulate the granular characteristics of soil and prevent direct interactions with the plant roots. Here, we present an optically transparent, granular medium consisting of concentrated packings of hydrogel particles, commonly called microgels, and demonstrate their use as a transparent medium to visualize the macroscopic root system network in vitro for rapid, temporal root phenotyping. Unlike macroscopically crosslinked hydrogel networks, packed microgels have structural and material properties that closely imitate the granular characteristics of soil. Furthermore, these packings undergo localized, reversible yielding when subjected to shear stresses greater than their yield stress, enabling direct interactions with the root system without disturbing the macroscopic material properties of the substrate. We envision these microgels as a transformative tool to enable in vitro spatial-temporal biophysical studies of plant-environment interactions within the rhizosphere.