Molecular and hormonal regulation of plant responses to waterlogging stress: From fundamental mechanisms to potential strategies for engineering crop tolerance.

Waterlogging is a major environmental constraint that severely restricts plant growth by causing oxygen deprivation and metabolic disruption in the root zone. In response, plants under hypoxia activate sophisticated physiological and molecular mechanisms mediated by phytohormones. Although key hormones, including ethylene, auxin, gibberellin, abscisic acid, cytokinin, jasmonic acid, salicylic acid, and brassinosteroids, are known to play central roles in regulating adaptation to waterlogging, our understanding of how their intricate crosstalk orchestrates adaptive decisions remains fragmented. In this review, we synthesize recent advances to construct an integrated framework that spans initial oxygen sensing and hormone-driven morphological adaptations to critical metabolic and physiological adjustments under waterlogging. In particular, we introduce the novel concept of "stress-metabolic integration and hormonal allocation" under waterlogging-induced hypoxia, which explains how redox state, carbon status, and hormones are quantitatively integrated to determine cellular fate and adaptive outcomes. We also highlight how synergistic and antagonistic interactions among major hormones fine-tune the balance between survival strategies and growth repression. Furthermore, we describe how hormone treatments can be integrated with practical and effective agronomic strategies to enhance crop waterlogging tolerance under field conditions. Finally, we identify critical knowledge gaps and propose transformative research directions, offering a blueprint for the rational design of "climate-resilient crops" through targeted manipulation of molecular and hormonal networks in practical agriculture.