THE H-SUCROSE COTRANSPORTER NTSUT1 IS ESSENTIAL FOR SUGAR EXPORT FROMTOBACCO-LEAVES()

In many species translocation of sucrose from the mesophyll to the phl oem is carrier mediated. A sucrose/H+-symporter cDNA, NtSUT1, was isol ated from tobacco (Nicotiana tabacum) and shown to be highly expressed in mature leaves and at low levels in other tissues, including floral organs. To study the in vivo function of NtSUT1, tobacco plants were transformed with a SUT1 antisense construct under control of the cauli flower mosaic virus 35S promoter. Upon maturation, leaves of transform ants expressing reduced amounts of SUT1 mRNA curled downward, and stro ngly affected plants developed chloroses and necroses that led to deat h. The leaves exhibited impaired ability to export recently fixed (CO2 )-C-14 and were unable to export transient starch during extended peri ods of darkness. As a consequence, soluble carbohydrates accumulated a nd photosynthesis was reduced. Autoradiographs of leaves show a hetero genous pattern of CO2 fixation even after a 24-h chase. The C-14 patte rn does not change with time, suggesting that movement of photosynthat e between mesophyll cells may also be impaired. The affected lines sho w a reduction in the development of the root system and delayed or imp aired flowering. Taken together, the effects observed in a seed plant (tobacco) demonstrate the importance of SUT1 for sucrose loading into the phloem via an apoplastic route and possibly for intermesophyll tra nsport as well.