Waterlogging tolerance: nonsymbiotic haemoglobin-nitric oxide homeostasis and antioxidants.

Waterlogging is a serious problem which affects crop growth and yield in low-lying, rainfed areas. Most of the rainy-season crops, especially legumes, and to a lesser extent maize and rice are affected by flooding leading to hypoxic or even anoxic conditions. Lack of oxygen shifts the energy metabolism from aerobic mode to anaerobic mode, which in turn adversely affects nutrient and water uptake; so the plants show wilting even when surrounded by excess water. Gaseous plant hormone ethylene plays an important role in modifying the plant's response to oxygen deficiency. Plants normally adapted to growing under waterlogged conditions have mechanisms to cope with this stress in the form of aerenchyma formation, increased availability of soluble sugars, greater activity of glycolytic pathway and fermentation enzymes. Both alcoholic and lactic acid fermentation help in maintaining lower redox potential (low NADH/NAD ratio), and thereby play an important role in anaerobic stress tolerance. Other important biomolecules which are induced under waterlogging, are nonsymbiotic haemoglobins and nitric oxide. Interaction of nonsymbiotic haemoglobins and nitric oxide has been suggested as an alternative to the fermentation pathway, which also rules out the production of toxic alcohol and lactic acid, the latter being the major cause of cytoplasmic acidosis. Waterlogging stress also results in the production of reactive oxygen species, and induction of antioxidant defence enzymes. Waterlogging-induced production of ethylene, H2O2 and NO is also involved in signalling and induction of various defence-related genes leading to synthesis of proteins/enzymes imparting hypoxia tolerance. [ABSTRACT FROM AUTHOR]