DIFFERENTIAL GENE ACTIVATION AS A MODE OF ACTION OF PHYTOCHROME 730

Abstract— Phytochrome-induced photomorphogenesis in the mustard seedling ( Sinapis alba L.) which can be regarded as being representative of the dicotyledonous seedlings has been analysed. In the present paper a number of arguments are presented, including data on RNA and protein synthesis and on the effects of actinomycin D and puromycin, which support the hypothesis that the 'positive' photoresponses of the seedling can be explained by a differential gene activation through P,₃₀. 'Positive' photoresponses are those which are characterized by an initiation or an increase of biosynthetic or growth processes (e.g. biosynthesis of anthocyanin; growth of cotyledons). The lag-phase of this type of photoresponse is rather long, 'Negative' photoresponses are those which are characterized by an inhibition of growth processes or other physiological processes like translocation. Here the lag-phase is short. Inhibition of hypocotyl lengthening is a typical response of this sort. The concept of differential gene repression through P ₇₃₀ may serve as a working hypothesis to approach the causal analysis of phytochrome-induced 'negative' photoresponses.