END-PRODUCT CONTROL OF CARBON METABOLISM IN CULTURE-CROWN SUGAR-BEET PLANTS - MOLECULAR AND PHYSIOLOGICAL EVIDENCE ON ACCELERATED LEAF DEVELOPMENT AND ENHANCED GENE-EXPRESSION
Y. Kovtun et J. Daie, END-PRODUCT CONTROL OF CARBON METABOLISM IN CULTURE-CROWN SUGAR-BEET PLANTS - MOLECULAR AND PHYSIOLOGICAL EVIDENCE ON ACCELERATED LEAF DEVELOPMENT AND ENHANCED GENE-EXPRESSION, Plant physiology, 108(4), 1995, pp. 1647-1656
Sugar beet (Beta vulgaris L.) seedlings were grown on media containing
90 to 300 mM sucrose or glucose. Compared to controls, sugar-grown pl
ants had higher growth rate, photosynthesis, and leaf sugar levels. Th
e steady-state level of transcripts increased significantly for the sm
all subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisc
o) (rbcS) and the cytosolic fructose-1,6-bisphosphatase and moderately
for the Rubisco targe subunit (rbcL). The transcript level of sucrose
phosphate synthase remained unchanged. Fructose-1,6-bisphosphatase an
d Rubisco activities did not change in the presence of sugars, but tha
t of sucrose phosphate synthase increased (44 and 90% under selective
and nonselective assay conditions, respectively). Accelerated leaf dev
elopment was indicated by (a) autoradiograms of leaves that showed tha
t sucrose loading occurred earlier, (b) export capacity that also occu
rred earlier but, after about 2 weeks, differences were not detectable
, and (c) sucrose synthase activity that declined significantly. Sever
al 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 le
af development, and (d) whereas Rubisco and cytosolic fructose-1,6-bis
phosphatase 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.