MODE OF COACTION BETWEEN UV‐A AND LIGHT ABSORBED BY PHYTOCHROME IN CONTROL OF APPEARANCE OF RIBULOSE‐1.5‐BISPHOSPHATE CARBOXYLASE IN THE SHOOT OF MILO (Sorghum vulgare Pers.)

Abstract

Abstract— In shoots of milo (Sorghum vulgare Pers.) appearance of ribulosebisphosphate carboxylase (RuBPCase) and of translatable mRNA for its small subunit is stimulated strongly by red light (R, operating through phytochrome) and UV‐A light (UV‐A). Ultraviolet‐A is more effective than R.The mode of coaction between phytochrome and light absorbed by the blue/UV‐A light photoreceptor (‘cryptochrome’) was analyzed in detail in case of enzyme appearance. Fluence rate dependencies, lagphases and the time course of the response are compatible with the view that UV‐A intensifies a process which is occurring in R alone albeit at a lower rate.With both light qualities the light effect is fully reversible by far‐red light up to 1 h. This means that during this period only phytochrome (Pfr) controls the terminal response, i.e. the actual appearance of RuBPCase. During this 1 h period after the onset of light UV‐A or R have no effect on the level of translatable mRNA for the small subunit of RuBPCase indicating that it requires more than 1 h for the light signal to affect gene expression.When R and UV‐A are given longer onset of escape from full reversibility is observed at the same time for both light qualities in the case of RuBPCase appearance. The extent of the reversible response is greater after UV‐A pretreatment than after a R pretreatment.It is argued that the data are consistent with the concept that phytochrome (Pfr) controls the terminal photoresponse, in the present case appearance of RuBPCase, while light absorbed via cryptochrome leads to an increase in responsiveness of the RuBPCase producing machinery towards Pfr.