Waterlogging and hypoxia in plant abiotic stress.

Waterlogging is a major abiotic stress affecting 12-27% of global cultivated land and causing average yield losses of 20-33% in cereals and legumes. This poses a serious threat to food security in flood-prone regions. Soil saturation reduces gas diffusion nearly 10,000-fold compared to air. This rapidly induces hypoxia or anoxia in the root zone within hours to days. This oxygen limitation forces a metabolic shift from efficient aerobic respiration (approximately 36 ATP per glucose) to anaerobic fermentation (approximately 2 ATP per glucose). This shift results in severe energy deficits, cytoplasmic acidosis, and accumulation of toxic metabolites such as ethanol, acetaldehyde, and organic acids. Plants exhibit diverse adaptive responses to flooding stress, including morphological modifications such as aerenchyma formation, adventitious root development, hypertrophied lenticels, and shoot elongation in deepwater rice. These modifications facilitate internal oxygen transport and gas exchange. At the biochemical level, fermentative enzymes including alcohol dehydrogenase and pyruvate decarboxylase are upregulated, while antioxidant defense systems mitigate oxidative damage during reoxygenation. Hormonal crosstalk among ethylene, abscisic acid, auxin, and gibberellins regulates the balance between escape and quiescence strategies. At the molecular level, oxygen sensing is primarily mediated by the N-degron pathway through Group VII Ethylene Response Factors (ERF-VIIs). Plant cysteine oxidases regulate oxygen-dependent degradation under normoxia, while hypoxia stabilizes ERF-VIIs to activate anaerobic gene expression. Nitric oxide further modulates ERF-VII proteolysis and stability. The