Hyperhydricity Syndrome in In Vitro Plants: Mechanisms, Physiology, and Control

Understanding the physiological characteristics of hyperhydric plantlets is ultimately necessary since hyperhydricity results in financial loss for in vitro plants from a commercial perspective. Although many studies report the possible causes and symptoms of hyperhydricity, knowledge of it remains limited. This review aims to provide an integrated overview of this phenomenon and outline the perspectives for its prevention. First, we summarize the factors of in vitro hyperhydricity, including gelling agents, growth regulators, vessel ventilation and gas exchange, light, and osmotic conditions. Second, we describe physiological and internal changes commonly observed in hyperhydric plants, such as ROS/ethylene imbalance, altered antioxidant capacity, defects in the cell wall, and lignification. Third, we outline ultrastructural characteristics and accumulate HPLC findings to recognize the metabolite profiles of hyperhydric plantlets. Fourth, we introduce emerging AI-assisted MLM (machine learning model) approaches to detect and optimize the culture parameters to prevent hyperhydricity. Finally, we evaluate the strategies for the protection of the culture from hyperhydric conditions. This structured overview intends to reduce hyperhydricity in commercial and research settings.