Phloem unloading was studied in potato plants in real time during the early
stages of tuberization using carboxyfluorescein (CF) as a phloem-mobile tr
acer, and the unloading pattern was compared with autoradiography of tubers
that had transported C-14 assimilates. In stolons undergoing extension gro
wth, apoplastic phloem unloading predominated. However, during the first vi
sible signs of tuberization, a transition occurred from apoplastic to sympl
astic transport, and both CF and C-14 assimilates subsequently followed ide
ntical patterns of phloem unloading. It is suggested that the switch to sym
plastic sucrose unloading may be responsible for the upregulation of severa
l genes involved in sucrose metabolism. A detailed analysis of sugar levels
and C-14 sugar partitioning in tuberizing stolons revealed a distinct diff
erence between the apical region of the tuber and the subapical region. Ana
lysis of invertase activity in nontuberizing and tuberizing stolons reveale
d a marked decline in soluble invertase in the subapical region of swelling
stolons, consistent with the switch from apoplastic to symplastic unloadin
g. However, cell wall-hound invertase activity remained high in the apical
1 to 2 mm of tuberizing stolons. Histochemical analysis of potato lines tra
nsformed with the promoter of an apoplastic invertase gene (invGE) linked t
o a reporter gene also revealed discrete gene expression in the apical bud
region. Evidence is presented that the apical and lateral tuber buds functi
on as isolated domains with respect to sucrose unloading and metabolism.