Enzyme Chnages During Lignogenesis in Pea Shoots Induced by Illumination

When the epicotyls of etiolated 7 d pea seedlings were illuminated, there was a rapid rise in the lignin content of the apical shoot (consisting of the third internode with its terminal bud), to give a 10-fold increase after 24 h; there was no change in the unilluminated controls. About 80% of the lignin in the apical shoot was found in the stem internode, both before and after illumination. Over this period, the activities of phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate: CoA ligase increased co-ordinately up to five times those of the dark controls over 12 h, when lignification was most rapid, and then declined to about half their maximum activity. All three enzymes showed the same 1.5 h lag period before increasing. By comparison, no increases were observed in the later enzymes of lignin biosynthesis, namely S-adenosylmethionine: caffeate O-methyltransferase, cinnamoyl-CoA reductase, NADP+-cinnamyl alcohol dehydrogenase, and cell-wall peroxidase, but the proportions of these enzymes in buds and stem internodes were close to the distribution of lignin between them, both before and after illumination. Two forms of 4-coumarate: CoA ligase were found; despite their different activities and substrate specificities, both forms showed substantial changes. The results suggest that lignogenesis is initiated by an increase in the activities of the three enzymes of general phenylpropanoid metabolism in cells already containing enzymes catalysing the later stages of lignin synthesis; they are discussed in relation to biochemical and anatomical differentiation within plant organs generally.