Control of carbon partitioning and photosynthesis by the triose phosphate/phosphate translocator in transgenic tobacco plants (Nicotiana tabacum L.).I. Comparative physiological analysis of tobacco plants with antisense repression and overexpression of the triose phosphate/phosphate translocator

The physiological properties of transgenic tobacco plants (Nicotiana tabacu m L.) with decreased or increased transport capacities of the chloroplast t riose phosphate/phosphate translocator (TPT) were compared in order to inve stigate the extent to which the TPT controls metabolic fluxes in wild-type tobacco. For this purpose, tobacco lines with an antisense repression of th e endogenous TPT (alpha TPT) and tobacco lines overexpressing the TPT gene isolated from the C-4 plant Flaveria trinervia (FtTPT) were used. The F. tr inervia TPT expressed in yeast cells exhibited transport characteristics id entical to the TPT from C-3 plants. Neither antisense TPT plants nor FtTPT overexpressors showed a phenotype when grown in a greenhouse in air. Conten ts of starch and soluble sugars in upper source leaves were similar in TPT underexpressors and FtTPT overexpressors compared to the wild type at the e nd of the photoperiod. The FtTPT overexpressors incorporated more (CO2)-C-1 4 in sucrose than the wild type, indicating that the TPT limits sucrose bio synthesis in the wild type. There were only small effects on labelling of a mino acids and organic acids. The mobilisation of starch was enhanced in al pha TPT lines but decreased in FtTPT overexpressors compared to the wild ty pe. Enzymes involved in starch mobilisation or utilisation, such as alpha-a mylase or hexokinase were increased in alpha TPT plants and, in the case of amylases, decreased in FtTPT overexpressors. Moreover, alpha-amylase activ ity exhibited a pronounced diurnal Variation in alpha TPT lines with a maxi mum activity after 8 h in the light. These changes in starch hydrolytic act ivities were confirmed by activity staining of native gels. Activities of g lucan phosphorylases were unaffected by either a decrease or an increase in TPT activity. There were also effects of TPT activities on steady-state le vels of phosphorylated intermediates as well as total amino acids and malat e. In air, there was no or little effect of altered TPT transport activity on either rates of photosynthetic electron transport and/or CO2 assimilatio n. However, in elevated CO2 (1500 mu l.1(-1)) and low O-2 (2%) the rate of CO2 assimilation was decreased in the alpha TPT lines and was slightly high er in FtTPT lines. This shows that the TPT limits maximum rates of photosyn thesis in the wild type.