Characterization of the activity of a plastid-targeted green fluorescent protein in Arabidopsis

In Arabidopsis thaliana the PALE CRESS (PAC) gene product is required for both chloroplast and cell differentiation. Transgenic Arabidopsis plants expressing a translational fusion of the N-terminal part of the PAC protein harboring the complete plastid-targeting sequence and the green fluorescent protein (GFP) exhibit high GFP fluorescence. Detailed analyses based on confocal imaging of various tissues and cell types revealed that the PAC-GFP fusion protein accumulates in chloroplasts of mature stomatal guard cells. The GFP fluorescence within the guard cell chloroplasts is not evenly distributed and appears to be concentrated in suborganellar regions. GFP localization studies demonstrate that thin tubular projections emanating from chloroplasts and etioplasts often connect the organelles with each other. Furthermore, imaging of non-green and etiolated tissue further revealed that GFP fluorescence is present in proplastids, etioplasts, chromoplasts, and amyloplasts. Even photobleaching of carotenoid-free plastids does not affect PAC-GFP accumulation in the organelles of the guard cells indicating that the protein translocation machinery is functional in all types of plastids. The specific accumulation of GFP in guard cell chloroplasts, their tubular connections, the translocation of the precursor polypeptide into the different types of organelles, as well as the use of a plastid-targeted GFP protein as a versatile marker is discussed in the context of previously described observations.