Altered sucrose synthesis in rice plants with reduced activity of fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase

Fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase (F2KP) is a bifunctional enzyme involved in regulating photosynthetic carbon metabolism by modulating the cellular level of fructose-2,6-bisphosphate (Fru-2,6-P2) in plants. Rice appears to have two F2KP genes — OsF2KP1 and OsF2KP2 — on chromosomes 5 and 3, respectively. RT-PCR analysis showed that both genes are primarily expressed in the leaves, with transcripts somewhat less detectable in the flowers, roots, and seeds. To elucidate the functions of these OsF2KPs, we generated a Tos17 insertion mutant of OsF2KP1 and an RNAi mutant of OsF2KP2. In mutant plants, expression of the corresponding OsF2KPs was inhibited mainly in the leaves. At the end of day, sucrose contents had increased in the leaves of mutant plants while both fructose-6-phosphate 2-kinase activities and Fru-2,6-P2 levels had declined. Our results suggest that the two OsF2KPs are involved in the regulation of sucrose synthesis by controlling the levels of Fru-2,6-P2 during the daytime.