C. Harn et al., EXPRESSION DYNAMICS OF GENES ENCODING KEY CARBON METABOLISM ENZYMES DURING SINK TO SOURCE TRANSITION OF DEVELOPING LEAVES, Plant and Cell Physiology, 34(7), 1993, pp. 1045-1053
During ontogeny, leaves undergo transition from net importers of photo
assimilates (sink) to net exporters (source). Results of broadly-based
experiments are presented showing metabolic, physiological and molecu
lar changes which occur during developmental transition of sugarbeet (
Beta vulgaris L.) leaves. Changes in the expression of genes encoding
key enzymes of carbon metabolism; the cytosolic fructose-1,6-bisphosph
atase (FBPase), sucrose-phosphate synthase (SPS) and ribulose-1,5-bisp
hosphatase (Rubisco) as well as several related metabolic parameters w
ere monitored during the course of leaf development. Six leaf stages w
ere used according to their sink/source status (leaf 1, youngest) as w
ell as a single ''transitional'' leaf in which the base, middle and th
e tip function as sink, transitional and source, respectively. In this
leaf, FBPase and Rubisco transcripts increased as source status was a
ttained in each section. Photosynthetic rates and chlorophyll content
increased as leaves expanded, reaching their maximum in leaves 3, 4 an
d 5, but declined in leaf 6. Significant changes were also observed in
leaf sucrose, glucose and fructose. Sucrose was highest in leaves 3,
4 and 5. Leaves 1, 2, and 3 had high levels of hexose. In general, FBP
ase, SPS and Rubisco activity, protein and transcript levels were low
in immature leaves, reached maximal levels in leaves 3, 4 and/or 5 and
declined or remained stable in leaf 6. While transcripts for FBPase,
SPS and Rubisco changed as transition progressed, that of the cytosoli
c aldolase, (a non-regulatory enzyme in sucrose pathway) did not. The
coordinated pattern of gene expression correlated highly with leaf pho
tosynthetic rates and sucrose content indicating molecular coordinatio
n of sucrose synthesis and photosynthesis. The data provide first evid
ence on the regulation of expression of genes encoding the two regulat
ory enzymes of sucrose biosynthesis in developing leaves and on the mo
lecular coordination of some of the regulatory enzymes of carbon metab
olism during leaf transition.