Triose phosphate formed in the PCR cycle is transported from chloroplasts to
cytosol by a phosphate transporter located in the inner membrane of the envelope.
It is then used as the carbon source for sucrose synthesis (Flü ge, 1998). Sucrose
formed in the mesophyll cells is transferred to phloem companion cells symplastically
and through the apoplastic space. The final uploading of sucrose into
companion cells against the steep concentration gradient of sucrose is conducted
by a sucrose transporter coupled to ATP hydrolysis (Weise et al.
Transgenic tobacco plants overexpressing the phosphate transporter have
been created. Sucrose synthesis is promoted in the absence of significant increases
in photosynthesis (Häusler et al.
, 2000). Sucrose phosphate synthase (SPS) is an
important regulatory enzyme in sucrose synthesis in the cytosol of mesophyll cells
(Huber and Huber, 1996). Overexpression of the gene for SPS has been attempted
with various plants, but the effects of the transgene on productivity varied
between experiments (Galtier et al.
, 1993; Lunn et al.
, 2003). Although more carbon
was directed to sucrose in the transformants than in the wild type, photosynthesis
was not enhanced in a reproducible manner. There are four family members for
the sucrose transporter (SUT1–4) (Weise et al.
, 2000). Since repression of SUT1
gave rise to severe morphological changes, it has been deduced that the transporter
participated in sucrose uploading into the phloem (Riesmeier et al.
Potato transformants expressing SUT1 under control of the Cauliflower mosaic virus
35S promoter showed lower sucrose level in leaves than wild type (Leggewie et al.
2003). However, no changes in either photosynthesis, starch content, or tuber