REDUCTION OF THE CYTOSOLIC FRUCTOSE-1,6-BISPHOSPHATASE IN TRANSGENIC POTATO PLANTS LIMITS PHOTOSYNTHETIC SUCROSE BIOSYNTHESIS WITH NO IMPACT ON PLANT-GROWTH AND TUBER YIELD
R. Zrenner et al., REDUCTION OF THE CYTOSOLIC FRUCTOSE-1,6-BISPHOSPHATASE IN TRANSGENIC POTATO PLANTS LIMITS PHOTOSYNTHETIC SUCROSE BIOSYNTHESIS WITH NO IMPACT ON PLANT-GROWTH AND TUBER YIELD, Plant journal, 9(5), 1996, pp. 671-681
Sucrose produced in source leaves is the predominant carbon source for
developing sink tissues in most higher plants. Consequently the rate
of sucrose synthesis is likely to be important for sink development an
d final crop yield. Two sucrose biosynthetic enzymes are believed to p
ossess regulatory properties with respect to the rate of sucrose synth
esis: (i) cytosolic FBPase and (ii) sucrose phosphate synthase. To stu
dy the impact of reduced photosynthetic sucrose biosynthesis on plant
growth and crop yield a cDNA clone encoding cytosolic FBPase was isola
ted from a potato leaf cDNA library and used for antisense experiments
in transgenic potato plants. The cDNA clone cy-F1, containing an open
reading frame of 1020 bp highly homologous (85%) to other known seque
nces of plant cytosolic FBPases, was cloned in reversed orientation be
tween the 35S CaMV promoter and the octopine synthase polyadenylation
signal. Out of 75 independent transformants five transgenic lines havi
ng 9 to 55 % of the wild-type FBPase activity were chosen for further
analysis. A 45% reduction of the cytosolic FBPase activity did not cau
se any measurable change in metabolite concentrations, growth behaviou
r or photosynthetic parameters of the transgenic plants. Inhibition of
cytosolic FBPase activity below 20% of the wild-type activity led to
an accumulation of 3-PGA, triose-phosphates and fructose-1,6-bisphosph
ate in source leaves. This resulted in a reduced light-saturated rate
of assimilation measured via gas exchange and a decreased photosynthet
ic rate under conditions of the leaf disc electrode with saturating li
ght and CO2. Measuring photosynthetic carbon fluxes by labelling leaf
discs with (CO2)-C-14 revealed a 53-65% reduction of sucrose synthesis
whereas starch synthesis decreased only by 18-24%. The flux into the
anionic and cationic fraction was not altered. Despite these changes s
teady-state sucrose concentrations were not effected in source leaves
from transgenic plants. Starch accumulated by more than a factor of 3
compared with wild-type leaves and was degraded during the night. This
provides strong evidence for the hypothesis that hexoses and/or hexos
ephosphates are exported out of the chloroplasts, thereby circumventin
g the limitation of sucrose biosynthesis caused by the inhibition of c
ytosolic FBPase in the dark. Accordingly, plant growth and potato tube
r yield remained unaltered. From these data it can be concluded that a
reduced photosynthetic sucrose biosynthetic capacity can be efficient
ly compensated without any reduction in crop yield under greenhouse or
growth chamber conditions by changing carbon export strategy. Whether
the same holds true for field conditions remains to be elucidated.