Control of carbon partitioning and photosynthesis by the triose phosphate/phosphate translocator in transgenic tobacco plants (Nicotiana tabacum). II. Assessment of control coefficients of the triose phosphate/phosphate translocator

Transgenic tobacco (Nicotiana tabacum L.) plants with decreased and increas ed transport capacities of the chloroplast triose phosphate/phosphate trans locator (TPT) were used to study the control the TPT exerts on the flux of starch and sucrose biosynthesis, as well as CO2 assimilation, respiration a nd photosynthetic electron transport. For this purpose, tobacco lines with an antisense repression of the endogenous TPT (alpha TPT) and tobacco lines overexpressing a TPT gene from Flaveria trinervia (FtTPT) were used. In am bient CO2, there was no or little effect of altered TPT transport activitie s on either rates of photosynthetic electron transport and/or CO2 assimilat ion. However, in elevated CO2 (1500 mu l.l(-1)) and low O-2 (2%) the TPT ex erted strong control on the rate of CO2 assimilation (control coefficient f or the wild type; C-TPT(JA) = 0.30) in saturating light. Similarly, the inc orporation of C-14 into starch in high CO? was increased in tobacco plants with decreased TPT activity, but was reduced in plants overexpressing the T PT from F, trinervia. Thus, the TPT exerted negative control on the rate of starch biosynthesis with a C-TPT(JStarch) = -0.19 in the wild type estimat ed from a hyperbolic curve fitted to the data points. This was less than th e positive control strength on the rate of sucrose biosynthesis (C-TPT(JSuc ), = 0.35 in the wild type). Theoretically, the positive control exerted on sucrose biosynthesis should be numerically identical to the negative contr ol on starch biosynthesis unless additional metabolic pathways are affected . The rate of dark respiration showed some correlation with the TPT activit y in that it increased in FtTPT overexpressors, but decreased in alpha TPT plants with an apparent control coefficient of C-TPT(JRes) = 0.24. If the c ontrol on sucrose biosynthesis is referred to as "gain of carbon" (positive control) and the control on starch biosynthesis as well as dark respiratio n as a "loss of carbon" (negative control) for sucrose biosynthesis and sub sequent export, the sum of the control coefficients on dark respiration and starch biosynthesis would be numerically similar to the control coefficien t on the rate of sucrose biosynthesis. There was also some control on the r ate of photosynthetic electron transport, but only at high light and in ele vated CO2 combined with low O-2 The control coefficient for the rate of pho tosynthetic electron transport was C-TPT(JETR) = 0.16 in the wild type. Con trol coefficients were also calculated for plants with elevated and lowered TPT activity. Furthermore, the extent to which starch degradation/glucose utilisation compensates for the lack of triose phosphate export was assesse d. The TPT also exerted control on metabolite contents in air.