Comparison of Three Phytochrome-mediated Processes in the Hypocotyl of Mustard

Abstract

Anthocyanin synthesis, hair formation and the synthesis of ascorbic acid oxidase are all phytochrome-mediated reactions occurring in the hypocotyl of mustard (Sinapis alba L.), controlled by phytochrome actually located in the hypocotyl. A comparison of these 3 reactions showed that in certain respects they differ greatly in their response to light. The ability of the seedling to respond to light by showing the 3 responses was strongly influenced by the state of development of the seedling. White light given very early after seed imbibition was unable to evoke any of the 3 reactions. By 50 h after imbibition, all systems were fully inducible by light. The addition of actinomycin D to a fully competent seedling coincident with illumination strongly inhibited the development of all 3 responses. In contrast, the addition of cordycepin at this time inhibited the synthesis of anthocyanin and ascorbic acid oxidase but had no effect on hair formation. Cycloheximide inhibited all 3 responses when given up to several hours after light. This suggests the necessity for RNA and protein synthesis for light-induced expression of these reactions, and that the RNA species involved in the 3 reactions may have differing degrees of polyadenylation. The lag period between the onset of light and the first display of the response was 3 h for anthocyanin and ascorbic acid oxidase synthesis, and about 5 h for hair formation. Amounts of light sufficient to give large increases in the levels of ascorbic acid oxidase and hair formation gave a much smaller increase in anthocyanin synthesis. Hair formation and ascorbic acid oxidase synthesis showed a much greater sensitivity to induction at early stages of seedling development than did anthocyanin synthesis. Following an inductive light period, anthocyanin synthesis was sensitive to far red light inhibition for a period twice as long as the other 2 reactions. The differences in the response of the 3 reactions to light suggest that the phytochrome-mediated reactions which control their development also differ.