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Metabolism and Enzymology

Involvement of Macromolecule Biosynthesis in Auxin and Fusicoccin Enhancement of -Glucan Synthase Activity in Pea ¹

Department of Biological Sciences, Stanford University, Stanford, California 94305

In pea stem segments whose cuticle has been made permeable by abrading it, actinomycin D (ActD) and 80S ribosomal protein synthesis inhibitors such as cycloheximide (CHI) inhibit enhancement by indole 3-acetic acid (IAA) of the activity of the cell wall biosynthetic enzyme, glucan synthase I (GS). This supersedes earlier, negative results with inhibitors, obtained with segments having an intact cuticle, which prevents adequate inhibitor uptake. Since these inhibitors also block IAA-stimulated H⁺ extrusion, which according to earlier results is involved in the GS response, the significance of these inhibitions would be ambiguous without additional evidence. ActD does not inhibit fusicoccin (FC) enhancement of GS activity, which indicates existence of a post-transcriptional control mechanism for GS, but does not preclude involvement of transcription in the GS response to IAA. Although protein synthesis inhibitors such as CHI do not block FC-stimulated H⁺ extrusion, they do inhibit FC enhancement of GS activity, indicating an involvement of protein synthesis in the GS response to FC, and presumably also to IAA. However, protein synthesis inhibitors (but not ActD) by themselves paradoxically elevate GS activity, less strongly than IAA does but resembling the IAA enhancement in several characteristics. These results suggest that IAA may enhance GS activity at least in part by inhibiting the synthesis or action of a labile repressor of the transcription of, or a labile destabilizer of, mRNA for GS or some polypeptide that enhances GS activity. However, resemblances between the IAA and FC effects on GS suggest that IAA also has a posttranscriptional GS-enhancing action like that of FC. Lipid biosynthesis may be involved in this aspect of the response since both IAA and FC enhancements of GS activity are inhibited by cerulenin.