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Thylakoid dismantling is one of the most relevant processes occurring when chloroplasts are converted to non-photosynthetically active plastids. The process is well characterised in senescing leaves, but other systems could present different features. In this study, thylakoid dismantling has been analysed in dividing cells of the unicellular alga, Euglena gracilis, cultured in darkness. Changes in photosynthetic pigments and in the abundance of LHC and PSII core proteins (D2 and CP43) showed that: (i) during the 0-24 h interval, the decline in LHCII was faster than that in the PSII core; (ii) during the 24-48 h interval, PSII and LHCII were strongly degraded to nearly the same extent; (iii) in the 48-72 h interval, the PSII core proteins declined markedly, while LHCII was maintained. These changes were accompanied by variations in room temperature fluorescence emission spectra recorded from single living cells with a microspectrofluorimeter (excitation, 436 nm; range 620-780 nm). Emission in the 700-715 nm range was proposed to derive from LHCI-II assemblages; changes in emission at 678 nm relative to PSII matched PSII core degradation phases. Overall, the results suggest that, in degreening E. gracilis, thylakoid dismantling is somewhat different from that associated with senescence, because of the early loss of LHCII. Moreover, it is proposed that, in this alga, disruption of the correct LHCI-II stoichiometry alters the energy transfer to photosystems and destabilises membrane appression leading to the thylakoid destacking observed using transmission electron microscopy.

Asynchronous cultures of wild-type Euglena gracilis were tested for their morphophysiological response to 10 mM MnSO4. Growth was only moderately slowed (15%), while oxygen evolution was never compromised. Inductively coupled plasma analyses indicated that the Mn cell content doubled with respect to controls, but no signs of localised accumulation were detected with X-ray microanalysis. Evident morphological alterations were found at the plastid level with transmission electron microscopy and confocal laser scanning microscopy. An increase in the plastid mass, accompanied by frequent aberrations of chloroplast shape and of the organisation of the thylakoid system, was observed. These aspects paralleled a decrease in the molar ratio of chlorophyll a to b and an increase in the fluorescence emission ratio of light-harvesting complex II to photosystem II, the latter evaluated by in vivo single-cell microspectrofluorimetry. These changes were observed between 24 and 72 h of treatment. However, the alterations in the pigment pattern and photosystem II fluorescence were no longer observed after 96 h of Mn exposure, notwithstanding the maintenance of the large plastid mass. The response of the photosynthetic apparatus probably allows the alga to limit the photooxidative damage linked to the inappropriately large peripheral antennae of photosystem II. On the whole, the resistance of Euglena gracilis to Mn may be due to an exclusion-tolerance mechanism since most Mn is excluded from the cell, and the small amount entering the organism is tolerated by means of morphophysiological adaptation strategies, mainly acting at the plastid level.

The acclimation response of the chloroplast was studied in the green marine microalga Koliella antarctica exposed to a simulated austral night of 90 d. On the basis of the micro- and submicroscopic aspects observed, the photosynthetic pigment patterns monitored spectrophotometrically, and the course of the assembly of the PSII chlorophyll-protein complexes evaluated microspectrofluorimetrically in vivo on single living cells, it was established that the alga tolerates the stress of light absence when cultured in the laboratory. During the treatment, the organism undergoes substantial structural and functional reorganization of the plastid, resulting in the formation of a chlorochromoplast-like structure, suitable for the storage, in different times and in specialized structures, of the products coming from the breakdown of the pre-existing plastid constituents. On the whole, the dark acclimation occurs in two different steps: a first acclimation, quickly realized, during the first 6 d and maintained up to 20 d of darkness, and a second acclimation period, starting from the 21st day, maintained up to the end of the experiment. [ABSTRACT FROM AUTHOR]

Rhodamine 123 (Rh 123), a relatively new mitochondrial marker, little used in the study of plant cells, was tested on excized leaves of Elodca canadensis Michx. and on suspension-cultured cells of Ranunculus serbicus Vis. In both preparations, the dye accumulated rapidly and selectively in the mitochondria whose number, morphology and cell distribution could be easily observed. In the presence of Rh 123, cytoplasmic movements could also be perceived and the spatial arrangement of the mitochondria with respect to that of the autofluorescent chloroplasts was studied in connection with a normal or altered cytoskeletal framework. The specific uptake of Rh 123 by the organelles seemed to be potential-dependent since it was influenced by cations, ionophores and inhibitors of electron transport. Short exposures to the stain were practically non-toxic, whereas prolonged treatments (6-20 h) provoked specific alterations in structure of the mitochondria. The data reported here indicate that Rh 123 may be an excellent vital stain to study the morphology, function and dynamics of the mitochondria in living plant cells. [ABSTRACT FROM AUTHOR]