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Plastids at the end of their tether
The green fluorescent protein (GFP) from the jellyfish, Aequorea victoria, is proving its worth as a label that can be detected non-invasively in living cells. It can provide information on sub-cellular localizations of proteins, on the spatial distribution of expression of particular genes, on the activity of gene promoters and enhancers, and on the development and physiology of organelles. Thus, the targeting of GFP to plastids has revealed dynamic filamentous structures called stromules that grow out from plastids and may provide a linkage between plastids. At other times, stromules may retract. The physiological significance of stromules is not understood but they clearly illustrate how the development of new tools leads to new understanding, even in areas previously well-researched. It is to plastids that Kevin Pyke and Caroline Howells at Nottingham University, UK (pp. 559-566) have applied GFP imaging. They investigated the chloroplasts in tomato leaf epidermis, the very variable plastids in leaf trichomes and the chromoplasts in tomato fruit. It is on the latter that we focus here. In green fruit, the pericarp cells contain numerous functional chloroplasts that do not extrude stromules. As ripening progresses, the photosynthetic pigments and proteins are lost, lycopene (the red carotenoid pigment) is accumulated and stromules begin to appear as string-like extensions from the plastids. At maturity, the deposition of lycopene crystals may be extensive enough to deform the plastids, while the development of stromules is truly spectacular. These now form long strings or even networks, associated with clusters of chromoplasts, and form interconnections between individual chromoplasts. Many of the stromules have a beaded appearance and the presence of GFP in these beads suggests that they represent foci of material being transported within the stromules. The authors suggest that plastids may coordinate their activities via networked communication. GFP has thus opened up a whole new area for research.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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