Fig. 1.

RNA, protein, metabolites, or hormones might act as plastid signals and control the expression of nuclear genes at the level of transcription or translation. This model reflects the hypotheses developed on the basis of reduced amounts and activities of nuclear-encoded chloroplast proteins in barley mutants (Bradbeer and Börner 1978; Hagemann and Börner 1978; Bradbeer et al. 1979). That plastid signals affect nuclear gene expression already at the level of transcription was shown by studies on LHCP transcription and transcript levels in albinotic mutants and norflurazon-treated seedlings in the mid of the 1980s (Mayfield and Taylor 1984; Batschauer et al. 1986; Oelmüller and Mohr 1986)