Hypoxic Induction of Anoxia Tolerance in Roots of Adh1 Null Zea mays L

Seedlings of alcohol dehydrogenase 1 null mutants (Adh1-) of Zea mays L., which fail to synthesize alcohol dehydrogenase 1 (ADH1) isozymes, were hypoxically acclimated by 18 h of exposure to an atmosphere of 4% (v/v) O2 in N2 at 25[deg]C. Their ability to tolerate subsequent anoxia by exposure to anaerobic (O2-free) conditions was compared with that of unacclimated seedlings that were transferred immediately from an atmosphere of 40% (v/v) O2 to anaerobic conditions. Only 10% of the root tips of unacclimated seminal roots survived 6 h of anoxia, whereas 70% of the hypoxically acclimated root tips were viable at 24 h. During anoxia, acclimated root tips had enhanced ADH activity compared with unacclimated root tips, through induction of Adh2. Despite this, enzyme activity was still only about 5% that of acclimated, wild-type root tips and about half that of unacclimated, wild-type root tips. During anoxia, acclimated Adh1- root tips showed a higher rate of anaerobic respiration and ethanol production, greater concentrations of ATP and total adenylates, and a greater adenylate energy charge compared with unacclimated root tips. These results suggest that although enhanced ADH activity may have raised fermentation rates in acclimated Adh1- tissues and thereby contributed to energy metabolism and viability, the high levels of ADH activity inducible in acclimated, wild-type maize root tips appear to be in excess of that required to increase rates of fermentation.