Changes in proplastid organization promoted by an inhibitor of DNA-methyltransferase in dark-grown dividing Euglena gracilis cells

The dark-grown unicellular flagellate alga Euglena gracilis (Euglenophyceae) contains rudimentary proplastids, whose development can be stimulated in darkness by DNA-demethylating drugs. In this study, proplastid organization was examined in dark-grown cells treated with a non-competitive inhibitor of DNA-methyltransferase. With 10 U/ml inhibitor, the activating effect was observed in 15-20% of the cell population, with a peak after 20 h of exposure. Fluorescence, confocal laser scanning, and transmission electron microscopy revealed the conspicuous enlargement of the plastid system. The inner proplastid membranes consisted of several prothylakoids connected with enlarged and compact prolamellar bodies. A strong accumulation of protochlorophyll(ide) was shown with 77 K fluorescence emission spectra. Although the protochlorophyll(ide)-rich prolamellar bodies recalled a condition typical of etiolated higher plants, the absence of changes in protochlorophyll(ide) spectral forms and the lack of competence for building up crystalline membrane networks suggest that in Euglena the prolamellar body is simply a “generic” storage of membrane materials. The role of DNA methylation in light-triggered plastid development is discussed.