MOLECULAR-CLONING AND EXPRESSION OF 4-COUMARATE - COENZYME A LIGASE, AN ENZYME INVOLVED IN THE RESISTANCE RESPONSE OF SOYBEAN (GLYCINE-MAX L) AGAINST PATHOGEN ATTACK

We have isolated three classes of cDNAs that probably encode three 4-c oumarate:coenzyme A ligase (4CL) isoenzymes in soybean (Glycine max L. ). The deduced amino acid sequences reveal several regions of extended sequence identity among 4CLs of all plants analyzed to dale. The sequ ences of two of these regions are consistent with a domain structure p roposed for a group of enzymes catalyzing the ATP-dependent covalent b inding of AMP to their substrates during the reaction sequence. By usi ng two cDNA fragments that do not cross-hybridize under the conditions used, we demonstrate that 4CL in soybean is very likely encoded by a small gene family. Members of this family are differentially expressed in soybean cell cultures treated with beta-glucan elicitors of Phytop hthora megasperma f. sp. glycinea or in soybean roots infected with ei ther an incompatible or compatible race of the fungus. These results a re in agreement with our previous observation that elicitor treatment of soybean cells caused a preferential enhancement in the activity lev el of one of the 4CL isoenzymes. In soybean, 4CL isoenzymes possessing different substrate affinities for substituted cinnamic acids, and sh owing differential regulation to environmental stress, may play a pivo tal role in distributing substituted cinnamate intermediates at a bran ch point of general phenylpropanoid metabolism into subsequent specifi c pathways.