Unearthing the hidden organ: vital role of the root in drought tolerance of plants.

Terrestrial plants evolved different adaptation mechanisms to drought, a primary survival challenge. While shoot traits, including stomatal closure, have been the major focus of drought research, the role of the root has lately been in the limelight. Being the first organ exposed to water scarcity in the soil, the root acts as the drought sensor, relaying the information to the shoot through synchronized hormonal, hydraulic, and electrical signals. This review encompasses recent genetic screens focused on root traits, modern approaches to phenotyping root system architecture, and molecular signaling events in roots for drought tolerance. Molecular mechanisms of root responses, including tropism and hydropatterning, and the intricacies of root-to-shoot communication under drought are enumerated. The review discusses root adaptations in extremophytes, including extraordinary root depths and spreads, aerial roots to capture moisture from the atmosphere, histomorphological root adaptations including lignification, suberization, and aerenchyma formation, determinate root development in Cactaceae, and root signaling in resurrection plants. The vital roles of the rhizomicrobiome and root exudates in augmenting the root tolerance responses to drought from individual plants to the ecosystem level are elaborated. The research gap in drought memory studies in roots, examining spatio-temporal changes in root apex and the coordination of root phenotypic plasticity with contemporary phenotyping techniques, is highlighted, anticipating the contribution of future single-cell omics technologies. Finally, how state-of-the-art root stress biology can be translated into manipulating root traits for crop drought tolerance through genome editing and rhizomicrobiome engineering is discussed.