Metabolic Acclimation to Anoxic Conditions and Biochemical Basis of Death
Exposure to hypoxic conditions induces a metabolic acclimation which improves the survival of maize root systems under subsequent extreme anoxic conditions. The regulation of cytoplasmic pH is considered to be the major factor in plant tissue survival under anoxia. The rapid acidification of the cytosol following transfer to anoxic conditions has been attributed to lactic acid accumulation. However, the pH drop and accumulation rate of lactic acid have different kinetics. This implies that lactic acid alone cannot account for the initial cytoplasmic acidification in anoxic maize root tips. Nevertheless, lactic acid might play a decisive role in the susceptibility of plant tissues to anoxia by producing an overacidification of the cytoplasm during the first hour of the anaerobic incubation after the main drop of pH. Hypoxically pretreated (acclimated) maize root tips accumulate less lactic acid and have higher cytoplasmic pH values under anoxia than non- acclimated ones. This lower accumulation is the result of an increased efflux of lactic acid and reduced production. The higher efflux of lactate from acclimated root tips is abolished by the presence of cycloheximide during the hypoxic pretreatment, suggesting that protein synthesis might be involved in this excretion. Similar strategies are encountered in some cereal organs reported to be very resistant to anoxic conditions; for example, rice embryos produce only trace amounts of lactic acid while barley aleurone tissues produce large amounts of lactic acid which is efficiently excreted into the medium. The mechanism of lactic acid excretion induced in maize root tips by the hypoxic pretreatment, either carrier-mediated or triggered by the pH gradient, may be an important determinant of the tolerance of plant tissues to anoxic conditions.
KeywordsPorosity Toxicity Sucrose Fermentation Maize
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