The expression of lactate dehydrogenase in Zea mays seedlings under hypoxic and anoxic conditions

Abstract

The ability of root cells to survive flooded (low 02) conditions may depend, in part, on energy produced anaerobically during fermentation to ethanol or lactic acid. The Davies-Roberts hypothesis predicts only a transient formation of lactic acid in anoxic maize root tips, catalyzed by lactate dehydrogenase (LDH), prior to sustained ethanolic fermentation. The initial lactic acid production may contribute to cytoplasmic acidosis. However, barley roots, when deprived of oxygen, exhibit a long term induction of LDH activity, which might indicate an alternative role for LDH in oxygen-depleted tissues. LDH enzymatic activity, message levels and lactic acid accumulation in response to hypoxic environments were examined in a variety of maize root tissues (primary root axis, nodal root axis, primary root tip, and the nodal root tip) from seedlings of different ages. In the 'open system', in which the roots were sparged with N2 while shoots remained exposed to air, Ldh1 transcripts and enzymatic activity were induced in the root axes. In contrast, root tips had high basal (aerobic) levels of LDH expression which did not increase further under 02-deprivation. There was less accumulation of intracellular lactic acid in the root tips during anoxia following 24 h of hypoxia. However, total lactic acid production increased in the root axes, corresponding with the increase in LDH expression. Experiments with pre-emergent seedlings of different ages from the 'closed system', in which the entire seedling was maintained in the same gaseous atmosphere, revealed different LDH expression patterns in response to 02 deficit, depending on age. Intracellular accumulation of lactate during the first hour of anoxia was several times higher in younger seedlings, and regardless of treatment, the initial rate of lactic acid export into the medium remained the same. These results suggest that lactic acid fermentation during anoxia may be regulated by the relative expression levels of PDC and ADH. Furthermore, the increased expression of LDH in the root axes, which are more resistant to long term anoxia than root tips, may indicate LDH contributes to energy metabolism and cell survival in these more mature zones.