Chloroplasts affect the leaf response to cytokinin

The effect of cytokinin on green and white leaves of the mutant line albostrians of barley was studied. Chloroplast development is completely blocked in white leaves of albostrians barley due to a nuclear mutation causing a lack of plastid ribosomes. We found white leaves to contain distinctly less abscisic acid than green leaves. In contrast, cytokinin (zeatin, zeatinriboside) content was higher in white vs. green leaves. Both white and green leaves contain a cytokinin-binding protein (CBP) of 67 kDa. CBP from white and green leaves, together with trans-zeatin (10−7 mol/L), activated RNA synthesis in an in vitro transcription elongation system containing chromatin associated with RNA polymerase I isolated from wild-type barley leaves. In spite of the high cytokinin content and the presence of a protein showing properties of a cytokinin receptor, white leaves differed markedly from green ones in their response to added cytokinin. Cytokinin promoted stomatal opening in both types of leaves, though white leaves proved to be less sensitive. During senescence of detached leaf segments, protein degradation in white leaves occurred much more rapidly and was less retarded by cytokinin than in green leaves. Cytokinin enhanced the incorporation of methionine into protein in green leaves, but did so to a much lower degree in white leaves. Therefore, we conclude that unimpaired chloroplast development and/or chloroplast gene expression is required for normal leaf responses to cytokinin.