Reduction of mRNA Level for two Nuclear Encoded LightRegulated Genes in the Barley Mutant albostrians is not Correlated with Phytochrome Content and Activity

Abstract

Summary White leaves of the barley mutant albostrians contain undifferentiated plastids which lack ribosomes. Itwas previously shown that this plastid defect leads to extremely low amounts of certain nuclear-gene encoded proteins including rbcS and cab. Here we show that white leaves of dark-grown seedlings contain no detectable cab and rbcS mRNA. Their quantity increases by illumination maximally to the level reached by the dark-grown wild-type. The phytochrome mRNA is present in white leaves of dark-grown mutant seedlings with approximately 45% of its content in etiolated wild-type leaves. Corresponding amounts of phytochrome (somewhat more than 50% of wild-type) were found in etiolated mutant leaves. The phytochrome is photoreversible as revealed by spectroscopic measurements and analysis of the cleavage products obtained from partial proteolysis of Pr and Pfr. The phytochrome-mediated light induction of the cab and rbcS genes and the light-dependent down-regulation of the phytochrome gene seem to work in mutant leaves. We conclude that the extreme low levels of cab and rbcS mRNA in mutant leaves are due to the lack of (a) plastid factor(s) in cells containing mutant plastids rather than to a phytochrome deficiency. The effect of the chloroplast (plastid) on nuclear gene expression is observed already in dark-grown seedlings and seems to be independent of phytochrome-mediated effects of light on transcription of the genes studied.