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

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, the proplastid organisation has been examined in dark-grown cells treated with a non-competitive inhibitor of the DNA-methyltransferase. With 10 U ml-1 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 showed 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 77K 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. However, the ability to form in darkness aggregates of tubules which can in case accommodate considerable amounts of protopigments may be the first step for the evolution of the highly ordered membrane networks described in the etioplasts of the Angiosperms.