PLANT PHYSIOLOGY , Vol 111, Issue 4 1021-1029, Copyright © 1996 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Sucrose Phosphate Synthase Expression at the Cell and Tissue Level Is Coordinated with Sucrose Sink-to-Source Transitions in Maize Leaf
W. H. Cheng, K. H. Im and P. S. Chourey
Program in Plant Molecular and Cellular Biology (W.-H.C., P.S.C.), Departments of Plant Pathology (K.H.I., P.S.C.) and Agronomy (P.S.C.), University of Florida and United States Department of Agriculture-Agricultural Research Service (P.S.C.), Gainesville, Florida 32611-0680
Immunohistological analyses for sucrose phosphate synthase (SPS) show that
the protein is localized in both bundle-sheath cells (BS) and mesophyll
cells (M) in maize (Zea mays) leaves. In young leaves, SPS protein was
predominantly in the BS, whereas mature leaves showed nearly equal levels
of signal in both BS and M. A cell-type-specific response was also seen in
light and dark treatments. Dark treatments led to reduced signal in M;
however, little or no change was detected in BS. We suggest that SPS in BS
is engaged in sucrose biosynthesis by both photoassimilatory and starch
turnover reactions in maize leaves. In addition, we suggest that the enzyme
in BS may play a major role in the early biosynthesis of sucrose in young
leaves. These cell-specific changes in expression in situ were in agreement
with the estimates of extractable enzyme activity from isolated BS and M of
mature leaves (R. Ohsugi, S.C. Huber [1987] Plant Physiol 84: 1096-1101).
In contrast, western blot analyses did not show any significant changes in
the levels of SPS protein in either young or mature leaves subsequent to
similar dark treatments. It is interesting that the northern blot analyses
indicate that the steady-state levels of SPS transcripts were markedly
reduced after dark treatments of > 12 h. Overall, our results indicate
that Sps gene expression in maize leaf is modulated at multiple levels of
controls by both developmental and environmental factors.
