Ferricyanide Reduction by Guard Cell Protoplasts

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Abstract
Guard cell protoplasts of Commelina communis L. reduced exogenous ferricyanide at pH values lower than 5·0; upon addition of NADH, reduction of ferricyanide by guard cell protoplasts was stimulated over the pH range 4·0 to 9·0 with two peaks of activity at pH 5·0 and between pH 8·0 and pH 9·0. Calcium chloride (1·0 mol m−3) and MgCl2 (1·0 mol m−3) increased the NADH-stimulated reduction of ferricyanide. Superoxide dismutase and cyanide had little effect on the NADH-stimulated reduction of ferricyanide by guard cell protoplasts, but, salicylhydroxamic acid completely inhibited this activity. The NADH-stimulated reduction of ferricyanide also occurred in the cell-free supernatant. Horseradish peroxidase did not reduce ferricyanide in the absence of NADH over a broad range of pH (4·0 to 9·0). However, in the presence of NADH, horseradish peroxidase reduced ferricyanide over the pH range 5·0 to 9·0 with maximal activity at pH 8·0. The NADH-stimulated reduction of ferricyanide by horseradish peroxidase showed similar properties to those observed with guard cell protoplasts. Mannitol, superoxide dismutase, and cyanide did not inhibit the NADH-stimulated reduction of ferricyanide by horseradish peroxidase; SHAM, however, completely inhibited the reduction of ferricyanide by horseradish peroxidase. Catalase inhibited the NADH-stimulated reduction of ferricyanide by horseradish peroxidase by 20%, while absence of oxygen in the assay medium stimulated this activity over 60%. We propose that the reduction of ferricyanide in the presence of NADH by guard cell protoplasts, can be explained in terms of peroxidase activity associated with the plasma membrane and secreted to the extracellular medium. However, the capacity of guard cell protoplasts to reduce ferricyanide at acid pH values where little peroxidase activity occurs may indicate the presence of a plasma membrane redox system in guard cells of C. communis.

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