Nucleotide Levels Do Not Critically Determine Survival of Maize Root Tips Acclimated to a Low-Oxygen Environment

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Abstract
We tested the hypothesis that ATP levels and energy charge determine the resistance of maize (Zea mays) root tips to anoxia. We focused on root tips of whole maize seedlings that had been acclimated to low O2 by exposure to an atmosphere of 3% (v/v) O2 in N2. Acclimated anoxic root tips characteristically have higher ATP levels and energy charge and survive longer under anoxia than nonacclimated tips. We poisoned intact, acclimated root tips with either fluoride or mannose, causing decreases in ATP and energy charge to values similar to or, in most cases, below those found in nonacclimated anoxic tips. With the exception of the highest fluoride concentration used, the poisoned, acclimated tips remained much more tolerant of anoxia than nonacclimated root tips. We conclude that high ATP and energy charge are not components critical for the survival of acclimated root tips during anoxia. The reduced nucleotide status in poisoned, acclimated root tips had little effect on cytoplasmic pH regulation during anoxia. This result indicates that in anoxic, acclimated root tips either cytoplasmic pH regulation is not dominated by ATP-dependent processes or these processes can continue in vivo largely independently of any changes in ATP levels in the physiological range. The role of glycolytic flux in survival under anoxia is discussed.