METABOLIC INVOLVEMENT IN ACID-MEDIATED EXTENSION GROWTH OF MAIZE COLEOPTILES

The acid-growth response (AGR) induced by acidic buffer (pH 4) in abra ded maize (Zea mays L.) coleoptile segments can be completely inhibite d within a few minutes by inhibitors of the haemoprotein function (KCN , Na-azide) or ionophores collapsing the proton gradients across membr anes (carbonyl cyanide m-chlorophenyl hydrazone, monensin). These subs tances also interfere with the acid-mediated increase in cell-wall ext ensibility measured with a constant-load extensiometer in vivo in turg id or non-turgid segments, but have no effect on the extensibility of the cell walls measured in vitro with frozen/thawed segments. The inhi bitors do not cause an alkalinization of the apoplastic solution or a decrease in the osmotic pressure of the cell sap of acid-treated segme nts. In contrast, inhibitors of ATP synthesis (N,N'-dicyclohexylcarbod iimide, diethylstilbestrol), which arrest auxin-mediated growth in a s imilar way to KCN, have no effect on AGR. Removal of O-2 inhibits grow th at pH 4 by about 25%; the anoxia-insensitive part of the AGR can be fully inhibited by azide. Diminishing the membrane potential with val inomycin has no effect on AGR. It is concluded that the AGR is control led by protoplastic functions, possibly localized in the plasma membra ne, which are lost when the cells are killed. The isolated cell wall m ay not represent a sufficient model system for the biochemical mechani sm of AGR.