Cytokinin and Light Quality Interact at the Molecular Level in the Chloroplast-Mutant PC22 of the Moss Physcomitrella

A new test for quantification of cytokinin-sensitivity in mass cultures was established with a wild-type strain of the moss Physcomitrella patens, two differentiation mutants (PC22 and P24) derived from it, and their somatic hybrid PC22(+)P24. The influence of exogenous cytokinin on total and plastid proteins as well as on steady state levels of rbcL-transcripts were analysed in the cytokinin-sensitive chloroplast mutant PC22 under two different light qualities. Cytokinin promotes a transient expression of plastid polypeptides and an increase of rbcL-transcript levels only under white light conditions. No molecular effect of cytokinin was observed when cultures were illuminated with enhanced blue light. Restriction enzyme- and Southern-analyses did not indicate RFLP's in the ptDNAs of the four genotypes but revealed methylation of the ptDNA in the case of the chloroplast mutant PC22. This DNA modification seemed to be restricted to the area around the rbcL-gene of this mutant. This ptDNA region was cloned as a 5.5 kb BglII-fragment of the wild-type plastid DNA.