Effects of fusicoccin on the activity of a key pH-stat enzyme, PEP-carboxylase.

The phytotoxin fusicoccin (FC) causes rapid synthesis of malate in coleoptile tissues, presumably via phosphoenolpyruvate (PEP) carboxylase coupled with malate dehydrogenase. The possibility that FC directly affects PEP carboxylase in Avena sativa L. and Zea mays L. coleoptiles was studied and rejected. The activity of this enzyme is unaffected by FC whether FC is added in vitro or a pretreatment to the live material. FC does not change the sensitivity of the enzyme to bicarbonate or malate. The activity of FC, instead, appears to be indirect. The pH sensitivity of PEP carboxylase is such that its activity, and thus the rate of malate synthesis, may be enhanced by an increase in cytoplasmic pH accompanying FC-induced H(+) excretion. Since the enzyme is also particularily sensitive to bicarbonate levels, malate synthesis may also be enhanced by FC-induced uptake or generation of CO2.