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

Abstract

The physiological properties of transgenic tobacco plants (Nicotiana tabacum L.) with decreased or increased transport capacities of the chloroplast triose phosphate/phosphate translocator (TPT) were compared in order to investigate the extent to which the TPT controls metabolic fluxes in wild-type tobacco. For this purpose, tobacco lines with an antisense repression of the endogenous TPT (alphaTPT) and tobacco lines overexpressing the TPT gene isolated from the C4 plant Flaveria trinervia (FtTPT) were used. The F. trinervia TPT expressed in yeast cells exhibited transport characteristics identical to the TPT from C3 plants. Neither antisense TPT plants nor FtTPT overexpressors showed a phenotype when grown in a greenhouse in air. Contents of starch and soluble sugars in upper source leaves were similar in TPT underexpressors and FtTPT overexpressors compared to the wild type at the end of the photoperiod. The FtTPT overexpressors incorporated more 14CO2 in sucrose than the wild type, indicating that the TPT limits sucrose biosynthesis in the wild type. There were only small effects on labelling of amino acids and organic acids. The mobilisation of starch was enhanced in alphaTPT lines but decreased in FtTPT overexpressors compared to the wild type. Enzymes involved in starch mobilisation or utilisation, such as alpha-amylase or hexokinase were increased in alphaTPT plants and, in the case of amylases, decreased in FtTPT overexpressors. Moreover, alpha-amylase activity exhibited a pronounced diurnal variation in alphaTPT lines with a maximum activity after 8 h in the light. These changes in starch hydrolytic activities were confirmed by activity staining of native gels. Activities of glucan phosphorylases were unaffected by either a decrease or an increase in TPT activity. There were also effects of TPT activities on steady-state levels of phosphorylated intermediates as well as total amino acids and malate. In air, there was no or little effect of altered TPT transport activity on either rates of photosynthetic electron transport and/or CO2 assimilation. However, in elevated CO2 (1500 microl x l(-1)) and low O2 (2%) the rate of CO2 assimilation was decreased in the alphaTPT lines and was slightly higher in FtTPT lines. This shows that the TPT limits maximum rates of photosynthesis in the wild type.

Maltose is the major form of carbon exported from the chloroplast at night

“…The potato antisense TPT plants showed only a slight reduction in height, and tuber yield was unchanged compared to WT (Riesmeier et al 1993). However, CO 2 assimilation rates in transgenic tobacco plants with antisense repression of TPT were reduced by about 40% under nonphotorespiratory conditions in comparison to the WT (Ha¨usler et al 2000a(Ha¨usler et al , 2000b. This shows that maximal photosynthetic rates require a fully operational TPT.…”

Carbon Balance and Circadian Regulation of Hydrolytic and Phosphorolytic Breakdown of Transitory Starch

“…Starch breakdown in the light under photorespiratory conditions provides evidence that circadian or light/dark control of the rate of starch degradation can be overridden by internal signals within the chloroplast in response to carbon deficit. Starch breakdown in the light was also found in plants lacking the triose phosphate transporter grown in high light (Häusler et al, 2000;Walters et al, 2004). Presumably, starch breakdown in the light helps plants overcome a Figure 4.…”

Antisense inhibition of sorbitol synthesis leads to up-regulation of starch synthesis without altering CO2 assimilation in apple leaves

“…2004;Schleucher et al 1998;Weise et al 2004) to provide carbon for subsequent sucrose synthesis and export. Studies on transgenic plants or mutants with decreased TPT activity (Ha¨usler et al 1998(Ha¨usler et al , 2000Heineke et al 1994;Riesmeier et al 1993;Schneider et al 2002), cytoFBPase activity (Sharkey et al 1992;Strand et al 2000;Zrenner et al 1996), phosphoglucose isomerase (Neuhaus et al 1989), or SPS activity (Krause 1994) have shown that when sucrose synthesis is decreased, starch synthesis is up-regulated via the increased PGA/Pi ratio resulting from accumulation of hexose phosphates in the cytosol. As a result, photosynthetic carbon partitioning to starch is increased without altering photosynthesis at ambient CO 2 .…”