PLANT PHYSIOLOGY , Vol 110, Issue 4 1089-1096, Copyright © 1996 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Subcellular Visualization of Gene Transcripts Encoding Key Proteins of the Chlorophyll Accumulation Process in Developing Chloroplasts
J. L. Marrison, PHD. Schunmann, H. J. Ougham and R. M. Leech
Department of Biology, University of York, P.O. Box 373, York, Y01 5YW, United Kingdom (J.L.M., R.M.L.)
The coordination of the synthesis of chlorophyll (Chl) and light-harvesting
Chl proteins was determined by observing the sequence of appearance of the
specific mRNAs for the nuclear genes CHLH, Por, and Lhcb1*2 (AB180). CHLH
encodes a magnesium protoporphyrin chelatase subunit that is involved in
the first committed step in Chl biosynthesis; Por encodes
protochlorophyllide oxidoreductase, which catalyzes the penultimate and
only light-dependent step in Chl biosynthesis; and Lhcb1*2 encodes
light-harvesting Chl a/b binding protein of the type-1 light-harvesting
complex of photosystem II. Using digoxigenin-labeled antisense and sense
RNA probes and a highly sensitive in situ hybridization technique, we have
visualized the first appearance of the specific mRNAs in postmitotic
mesophyll cells of developing 7-d-old wheat leaves (Triticum aestivum cv
Maris dove). The transcripts for CHLH and POR are detectable in the
youngest (18 h postmitotic) leaf tissue containing dividing cells;
light-harvesting complex of photosystem II transcripts appear 12 h later.
This is consistent with a requirement for accumulation of Chl before
synthesis of Chl a/b binding protein can proceed at a high rate. All of the
transcripts are most abundant in mesophyll cells. In the first leaf the POR
message is initially restricted to the palisade, but 12 h later it is also
present in the spongy mesophyll cells. All three transcripts aggregated
around the surface of the chloroplasts, suggesting that translation may
occur preferentially in the vicinity of the target organelle for the
primary translation products.