AN INTERCONNECTED PLASTIDOM IN ACETABULARIA - IMPLICATIONS FOR THE MECHANISM OF CHLOROPLAST MOTILITY

In the unicellular green alga Acetabularia, the vital fluorochrome 3,3 '-dihexyloxacarbocyanine (DiOC(6)) readily accumulates in chloroplasts and mitochondria at low concentrations, suboptimal for the visualizat ion of the endoplasmic reticulum (ER). These organelles align along mo tility tracks and partially obscure each other, resulting in the loss of image information in conventional fluorescence microscopy. However, superior imaging of organelles was achieved by confocal laser scannin g microscopy, which was particularly evident in areas where mitochondr ial profiles overlap with chloroplasts. In addition to the tubular mit ochondria, a new type of tubular membrane profiles was discovered in A cetabularia which connects the chloroplasts with each other. These tub ules may either form short bridges or may stretch over hundreds of mic rometers before connecting to the next chloroplast. Because staining i ntensity, size and overall shape of mitochondria and the connecting me mbrane tubules were very similar, pharmacological treatments have been applied to differentiate more clearly between the two compartments. I nhibitors of mitochondrial function are shown here to affect mitochond rial shape but not that of the chloroplast tubules. Finally, electron microscopic analysis of thin sectioned materials revealed long tubular emanations from the chloroplasts proving their plastidal origin. The function of these hitherto unknown plastidal membrane tubules is not k nown, but their behaviour suggests that they interact with the cytoske leton and effectively modify chloroplast behaviour.