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Metabolism and Enzymology

Diacylglycerol Levels Unchanged during Auxin-Stimulated Growth of Excised Hypocotyl Segments of Soybean ¹

Department of Medicinal Chemistry and Pharmacognosy, Purdue University, West Lafayette, Indiana, Biology Section, Botanical Institute, Johannes Gutenberg University, Mainz, Federal Republic of Germany

Diacylglycerol contents of excised soybean (Glycine max L.) hypocotyl segments, incubated for 4 hours in the presence or absence of a growth promoting concentration of 2,4-dichlorophenoxyacetic acid (2,4-D) were monitored by three different methods as a sensitive measure of the action in vivo of C-type phospholipases. By all three methods, steady state levels of diacylglycerols representing about 3% of the total lipids or about 7% of the neutral lipids, depending on method of assay, declined 18% over 4 hours of incubation as determined by extraction of total lipids and analysis by thin layer chromatography and densitometry. The average decline with 2,4-D-treated segments was less but the difference from controls was not significant. In those experiments where a small effect of 2,4-D was noted, the fraction showing an elevated diacylglycerol level in response to 2,4-D, after separation into membrane and supernatant fractions, was the supernatant and not the membranes. Results were confirmed from analyses of total fatty acids in each of the major lipid fractions and from diacylglycerol assays by conversion into phosphatidic acid upon incubation with [-³²P]ATP and purified diacylglycerol phosphokinase from Escherichia coli. In the presence of 2,4-D, the diacylglycerol content of the membranes was unchanged compared to membranes from control segments. As with the densitometric method, the small 2,4-D induced increase in diacylglycerols, when observed, was insignificant and in the supernatant. The only membrane-associated lipid fraction consistently showing a response to 2,4-D was the fraction containing sterols esterified with fatty acids. Either total microsomes or purified plasma membranes when incubated for 10 to 20 minutes with 1 micromolar 2,4-D showed no accelerated formation of diacylglycerols compared to membranes not incubated. The results do not support operation during auxin growth of the animal paradigm where diacylglycerol activation of C-type protein kinases occurs in response to activated phospholipase C breakdown of phosphoinositides.