Histochemical Approach to Properties of Vicia faba Guard Cell Phosphoenolpyruvate Carboxylase

Abstract

Properties of phosphoenolpyruvate carboxylase in guard cells dissectedfrom frozen-dried Vicia faba L. leaflets were studied using quantitativehistochemical techniques. Control experiments with palisade cellsand whole leaflet extract proved that the single cell approach wasvalid. Most characteristics of enzyme activity in guard cells wereidentical to those in the leaflet extract. The activities were highlydependent on temperature, with maximum activity at 25 to 35 C. Half-maximumactivity (with 1 millimolar phosphoenolpyruvate [PEP]) was observedat 0.1 millimolar Mg(2+). Two-hundred millimolar NaCl inhibited thereaction by 50%. With frozen-dried leaflet extract, the apparentK(m(PEP)) was 0.15 millimolar at pH 7.7; with guard cells, the valueswere 1.49, 0.5 to 0.8, and 0.24 millimolar in three successive experiments.Additional experiments showed that apparent K(m(PEP)) of guard cellactivity from plants within a single growth lot was reproducibleand did not change during stomatal opening. Mixed extract experimentsproved that soluble compounds were not responsible for the differenceobserved between leaflet and guard cell activities. The differencesin apparent K(m(PEP)) of guard cell activity could not be unambiguouslyinterpreted. The physiological implications of the properties ofthis enzyme in guard cells are discussed.