Plant anaerobic stress as a novel trend in ecological physiology, biochemistry, and molecular biology: 2. Further development of the problem

This review is a logical development of a previous publication, which summarized the main results of the early period of the systematic and active studying of hypoxic and anoxic stresses in plants. These studies laid a foundation for a new scientific discipline in biology, the investigation relevant to plant anaerobic stress. This review considers a further development of this trend when the investigations embraced a wider set of topics and the discipline acquired an international recognition. The results obtained during last decades by physiologists, biochemists, and molecular biologists engaged in the problem of plant anaerobic stress confirmed the correctness of a concept of the two principal strategies of plant adaptation to hypoxia and anoxia conditions. They are true tolerance manifesting at the molecular level under conditions of oxygen deficiency or its absence and apparent tolerance, which is realized by avoidance of anaerobiosis due to the long-distance oxygen transport. Therefore, experimental material available now is considered and discussed in this review mainly in the light of these principal notions. Especial attention is paid to the role of stress proteins, which synthesis is induced under hypoxia and anoxia. The results of these experiments confirmed earlier conclusions about the key role of energy (glycolysis and alcoholic fermentation) and carbohydrate (mobilization and utilization of reserved carbohydrates) metabolism in plant adaptation to oxygen deficiency or its absence from the environment. The phenomenon of hypoxic acclimation and its role in plant adaptation to anoxia are also considered. Along with these topics, a further development of pH-stat theory is discussed. A special attention is paid to plant strategy realized by the formation of the net of air-filled spaces (aerenchyma) and long-distance oxygen transport from aerated plant parts to those located in anaerobic environment (apparent tolerance). Among other important aspects, we consider (1) post-anaerobic plant injury by free oxygen radicals; (2) the physiological role of alternative pathways of plant adaptation (nitrate reduction and lipid synthesis); (3) the phenomenon of the adaptation syndrome in plants and possible molecular mechanisms of its realization; and (4) some biotechnological advances in the field of genetic and cell engineering used for the creation of plants more tolerant to anaerobic stress.