In plants cytokinins stimulate plastid differentiation and related processes like chlorophyll and carotenoid biosynthesis. It might well be that some of the "chlorophyll" mutants affected in their pigment content are due to reduction in endogenous cytokinin content. If so they should respond to exogenous hormone subministration by greening. Accordingly the response of several chlorophyll mutants was tested by applying a few drops of N6(benzyl) adenine (BA). Out of 25 mutants screened five (lw1, l15, lc4, lc5, l11) seemed to give some positive response. They were thus further analyzed. BA treatment in this case was done in two ways: 1) by germinating seeds in flasks containing 1 µM BA on a rotary shaker for 48 hrs and 2) by applying a 20 µl drop of 100 µM BA on the scutellar node when the shoot was about 1 cm long. The effect of the treatment was measured by establishing the pigment content of the first two leaves of young seedlings (third leaf not yet expanded). The only mutant showing a significant increase in pigments following cytokinin treatment is l11. In the mutant treated with BA the PEPcarboxylase activity is restored to the level of the untreated control, thus suggesting that BA treatment leads to a restoration of photosynthetic functions. Correlated to this repair of BA on pigment levels and enzyme activity is the effect of this cytokinin on the mesophyll plastid development as determined from ultrastructural studies.
C. Tonnelli, C. Salvi, G. Gavazzi, G. Marziani, G. Rossi and C. Longo
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