Plastids develop into a variety of forms in different tissues of the vascular plant, reflecting the variety of important functions these organelles perform. Plastids replicate by division and appear in textbooks as discrete, autonomous bodies. However, sporadic reports in the literature have described thin filamentous projections of the plastid envelope which sometimes connect with other plastids. These structures, rarely observable under the light microscope, have received little attention, and their very existence has sometimes been dismissed as an artifact. This project capitalizes on the observation that when green fluorescent protein (GFP) is expressed as a stromal protein in plastids, the structures are readily detected. Preliminary experiments have demonstrated that GFP can move through the tubular interconnections between plastids. This raises the possibility that these stroma-containing structures, denoted "stromules", permit exchange of other, functionally significant molecules. The general aim of this project is to characterize the stromules further and to probe their as yet unknown role in plastid function and plant cell biology. A thorough characterization of the stromules with regard to abundance, size, and degree of interconnections in different cell types during their cell cycle and development and under environmental stress will be carried out in the model plant Arabidopsis by light and electron microscopy. Transgenic plants will be produced that encode either GFP or blue fluorescent protein in their plastid genomes for future studies that will be undertaken to monitor the degree of transfer of different classes of proteins between plastids in hybrid cells. The interaction in vivo of stromules with other subcellular structures will also be examined.
