Calcium antagonist interferes with auxin-regulated xyloglucan glycosyltransferase levels in pea membranes

Abstract

Incorporation of [ 3H]glucose into cell walls of auxin-treated stem segments of etiolated Pisum sativum seedlings was inhibited by over 50% during 2 h of treatment by the intracellular Ca 2+ antagonist 8-( N-N-diethylamino)-octyl-3,4,5-trimethoxybenzoate · HCl (TMB-8). A 2 h treatment of segments with TMB-8 also reduced by up to half the specific activities of two Golgi-localized polysaccharide synthases, glucan synthase I, which generates mainly 1,4-ß-linkages, and UDP-xylose; xyloglucan xylosyltransferase. Both of these enzymes appear to be required for xyloglucan biosynthesis. In contrast, the level of plasma membrane-localized glucan synthase II, which generates mainly 1,3-ß linkages, was reduced only slightly by TMB-8 treatment. TMB-8 failed to reduce the activity of these synthases substantially when applied directly in vitro to membrane preparations from untreated tissue. The presence of 1 mM CaCl 2 during the treatment of stem segments with TMB-8 nullified the TMB-8-induced inhibitions. The results are interpreted as indicating the potential involvement of a Ca 2+ messenger system in the stimulatory effect of auxin on the levels of xyloglucan glycosyltransferase.