Model Systems for Investigating Root Responses to Localized Water Availability.

Water is distributed unevenly in soil, creating fine-scale patterns that shift with soil structure and drying. To navigate this complexity, roots deploy localized adaptive responses that allow them to sense and respond to water availability with high spatial precision. These include alterations in root branching (hydropatterning and xerobranching) and directional growth responses (hydrotropism and xerotropism), all of which collectively optimize water capture. Despite their importance, reproducing these heterogeneous water cues in the laboratory is challenging because soil systems are structurally and hydraulically complex. To address this, a suite of agar-based model systems has been developed to provide reproducible, tractable, and quantifiable platforms for studying root water sensing. These approaches rely on manipulating water potential using solutes or air interfaces to mimic microscale environmental heterogeneity and trigger adaptive responses. In this chapter, we outline the conceptual principles underlying water-driven root adaptive responses and provide detailed protocols for five agar-based experimental systems designed to study hydrotropism, xerotropism, xerobranching, and hydropatterning. These methods support robust mechanistic investigation and can be adapted for both Arabidopsis and crop species.