Plant Disease and the Regulation of Enzymes Involved in Lignification

Abstract

The mechanism underlying the increase of activity of the 3 O-methyltransferases of tobacco after infection by tobacco mosaic virus (TMV) was investigated with a density-labeling method. The 3 O-methyltransferases from healthy or TMV-infected leaves fed with H2O or 2H2O were purified by ion-exchange chromatography and their mean buoyant densities were calculated from their respective distribution profiles after centrifugation to equilibrium on RbCl gradients. Densities were corrected with respect to the mean buoyant density of a radioactive density marker prepared from tobacco leaves floated on a solution containing L-[3H]leucine and selected on a preparative gradient for its density close to those of the O-methyltransferases. The introduction of 2H into the pool of amino acids from which the enzymic proteins were synthesized was monitored. By measurement of the labeling of .beta.-galactosidase synthesized by bacteria from the plant amino acids, it was shown that infection did not alter the rate of labeling of the pool of amino acids. The buoyant-density values of the 3 O-methyltransferases were determined, and density-labeled enzymes from healthy and infected materials were compared. The largest density shifts from the native enzyme were measured for O-methyltransferases from infected leaves. The increase in activity of the 3 enzymes after infection is due to the stimulation of the rate of de novo synthesis of enzyme proteins.