PLANT PHYSIOLOGY , Vol 108, Issue 4 1647-1656, Copyright © 1995 by American Society of Plant Biologists
|
DEVELOPMENT AND GROWTH REGULATION |
End-Product Control of Carbon Metabolism in Culture-Grown Sugar Beet Plants (Molecular and Physiological Evidence on Accelerated Leaf Development and Enhanced Gene Expression)
Y. Kovtun and J. Daie
329 Birge Hall, 430 Lincoln Drive, University of Wisconsin, Madison, Wisconsin 53706-1381
Sugar beet (Beta vulgaris L.) seedlings were grown on media containing 90
to 300 mM sucrose or glucose. Compared to controls, sugar-grown plants had
higher growth rate, photosynthesis, and leaf sugar levels. The steady-state
level of transcripts increased significantly for the small subunit of
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (rbcS) and the
cytosolic fructose-1,6-bisphosphatase and moderately for the Rubisco large
subunit (rbcL). The transcript level of sucrose phosphate synthase remained
unchanged. Fructose-1,6-bisphosphatase and Rubisco activities did not
change in the presence of sugars, but that of sucrose phosphate synthase
increased (44 and 90% under selective and nonselective assay conditions,
respectively). Accelerated leaf development was indicated by (a)
autoradiograms of leaves that showed that sucrose loading occurred earlier,
(b) export capacity that also occurred earlier but, after about 2 weeks,
differences were not detectable, and (c) sucrose synthase activity that
declined significantly. Several conclusions emerged: (a) response was
nonosmotic and gene and sugar specific, (b) sugars caused accelerated leaf
development and sink-to-source transition, (c) enhanced gene expression was
due to advanced leaf development, and (d) whereas Rubisco and cytosolic
fructose-1,6-bisphosphatase genes were sugar repressed in mature leaves of
greenhouse-grown plants, they were unaffected in mature, culture-grown
leaves. To our knowledge, these data provide the first evidence in higher
plants that, depending on the physiological/developmental context of
leaves, sugars lead to differential regulation of the same gene.
