Compensation of decreased triose phosphate/phosphate translocator activity by accelerated starch turnover and glucose transport in transgenic tobacco
1998
Hausler, R.E. | Schlieben, N.H. | Schulz, B. | Flugge, U.I.
Tobacco (Nicotiana tabacum L.) plants were transformed with an antisense construct of the chloroplast triose phosphate/phosphate translocator (TPT). Three transformant lines of the T4 progeny, which showed a large decrease in the transcript level of the TPT were used for further biochemical and physiological characterisation. In all antisense lines tested, TPT transport activity was diminished by 50-70% compared with the wild type (WT). Despite this high reduction in the transport capacity, alphaTPT plants lacked anv visible phenotype. Hexokinase and alpha-amylase activities were increased in TPT plants compared with the WT, whereas activities of ribulose-1,5-bisphosphate carboxylase/oxygenase and ADP-glucose pyrophosphorylase (AGPase) were not affected. At the end of a 14-h light period, leaf starch contents in alphaTPT lines were similar to those of the WT and controls, indicating that a decrease in the TPT had no effect on starch accumulation. Sucrose contents were diminished by more than 50% in alphaTPT lines compared with control plants. The time course of starch accumulation revealed a transient increase in the starch content in a selected alphaTPT line after 6 h in the light, followed by a decrease towards the end of the light period. Labelling with 14C indicated that during the dark and light (late afternoon) periods starch is mobilised at higher rates in alphaTPT lines than in the controls. Glucose/fructose ratios at the end of the dark period were increased from 1.2 in control plants to 2 in alphaTPT lines indicating increased amylolytic starch degradation. Initial rates of [14C] glucose transport in isolated chloroplasts were increased by a factor of 2-3 in alphaTPT plants compared with the WT. Rates of CO2 assimilation were substantially diminished in the alphaTPT lines in high CO2 and low O2, but remained unaffected in ambient CO2. The rate of photosynthetic electron transport during the induction of photosynthesis in saturating CO2 exhibited pronounced oscillations only in WT and control plants. Oscillations were less pronounced in alphaTPT plants, indicating that phosphate limitation of photosynthesis is lowered in alphaTPT plants compared with the WT. It is proposed that photoassimilates are more readily directed into starch biosynthesis in alphaTPT plants. This is supported by determinations of 3-phosphoglycerate levels (an activator of AGPase) during the transition from dark to light in high CO2.
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