Metabolic Acclimation of Root Tips to Oxygen Deficiency

Survival of roots in O2 deficient media appears to depend on two completely opposing stratagems; the one involving cell survival, the other, cell death. In the apical zone of maize primary roots, the ability of cells to tolerate hypoxia or anoxia is paramount so that the meristem is not destroyed. Transcriptional and post-transcriptional regulation of protein synthesis ensures that enzymes of the glycolytic and ethanolic fermentation pathway are abundant in seedling root tips that have previously acclimated to low O2. By contrast, anaerobically shocked cells are much less able to tolerate anoxia and quickly succumb. Acclimation is associated with higher anaerobic respiration rates, greater energy metabolism and improved survival after transfer to anoxia. Hypoxia also sets in motion an entirely different set of responses in the zone close to the tip where cell elongation is ceasing, resulting in a change of root structure. Ethylene-dependent lysis of cortical cells is induced, these cells being selectively sacrificed to form aerenchyma and thereby improve the internal oxygenation of the root. The significance of the hypoxic induction of changes in the activities of various key enzymes associated with cell death and lysis is discussed.