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

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.