High Levels of Malic Enzyme Activities in Vicia faba L. Epidermal Tissue

Abstract

Specific activities of NADP-malic enzyme, NAD-malic enzyme, phosphoenolpyruvate carboxykinase and pyruvate, orthophosphate dikinase in various cells of V. faba L. leaflets were determined [to learn the pathway by which malate is metabolized during stomatal closure]. Expressed on dry weight, chlorophyll or protein basis, the averages for NADP- and NAD-malic enzyme specific activities were higher in guard cells than in photosynthetic parenchyma cells. Malic enzyme-specific activities were also high in epidermal cells. Phosphoenolpyruvate carboxykinase activity was not detected in Vicia leaf extracts or guard cells; the assay techniques were validated by mixed Vicia-Brachiaria leaf extraction and assays on nanogram samples of Brachiaria bundlesheath cells. Apparently guard cell malate depletion is by decarboxylation to pyruvate in the epidermal layer, but how the various epidermal cells interact remains obscure. Pyruvate, orthophosphate dikinase activity could not be demonstrated unequivocally in Vicia leaf extracts, Vicia guard cell protoplast extracts, or in Vicia guard cells. The assay techniques were validated by mixed Vicia-Kochia leaf extraction and assays on nanogram samples of Kochia mesophyll cells. How (or if) pyruvate is phosphorylated by epidermal tissue for entry into gluconeogenesis is unknown.