Cytokinin and abscisic acid in regulation of chloroplast protein gene expression and photosynthetic activity

The effect of cytokinin 6-benzylaminopurine (BAP) and abscisic acid (ABA) on biogenesis of thylakoid membranes of chloroplasts and on the functional activity of pigment-protein complexes in excised lupine yellow cotyledons (Lupinus luteus L.) were studied. Three groups of thylakoid polypeptides which responded differentially to BAP and ABA were distinguished: (i) cytochrome b 559, subunit IV of the cytochrome complex, and the 33 kDa polypeptide of the oxygen-evolving system were induced by BAP in the dark and their synthesis was activated in the light; (ii) accumulation of the P700 chlorophyll a apoprotein of photosystem I and 43 kDa chlorophyll α apoprotein of photosystem II is light-dependent, but it was activated by cytokinin and inhibited by ABA after illumination; (iii) the β-subunit of the ATP synthase and cytochrome b 6 were already found in the etiolated material and a comparatively weak effect of phytohormones on accumulation of these polypeptides was observed. It was found that phytohormones had negligible effects on the transcript levels of 15 different chloroplast genes. Comparison of mRNA and corresponding protein accumulation in BAP- and ABA-treated cotyledons showed clearly the post-transcriptional regulation of chloroplast gene expression. At the same time, BAP increased significantly accumulation of mRNA of nuclear encoded genes for chloroplast proteins (data not shown) and the activity of nuclear RNA polymerase II. This supports the hypothesis of transcriptional regulation of nuclear-coding genes for chloroplast proteins under cytokinin effect. BAP- and ABA-induced changes in thylakoid membrane polypeptide accumulation correlated well with the phytohormonal effects on the functional activity of both photosystems and on the photosynthetic CO2 assimilation. These results show the biological importance of hormonal regulation of chloroplast biogenesis.