Compensation of decreased triose phosphate/phosphate translocator activity by accelerated starch turnover and glucose transport in transgenic tobacco

Abstract

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, alpha TPT plants lacked any visible phenotype. Hexokinase and alpha-amylase activities were increased in alpha 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 alpha TPT 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 alpha TPT lines compared with control plants. The time course of starch accumulation revealed a transient increase in the starch content in a selected alpha TPT 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 alpha TPT 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 alpha TPT lines indicating increased amylolytic starch degradation. Initial rates of [14C] glucose transport in isolated chloroplasts were increased by a factor of 2-3 in alpha TPT plants compared with the WT. Rates of CO2 assimilation were substantially diminished in the alpha TPT 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 alpha TPT plants, indicating that phosphate limitation of photosynthesis is lowered in alpha TPT plants compared with the WT. It is proposed that photoassimilates are more readily directed into starch biosynthesis in alpha TPT plants. This is supported by determinations of 3-phosphoglycerate levels (an activator of AGPase) during the transition from dark to light in high CO2.

Differential expression and phylogenetic analysis suggest specialization of plastid-localized members of the PHT4 phosphate transporter family for photosynthetic and heterotrophic tissues

“…40 Surprisingly, plants with reduced levels of TPT have no substantial growth phenotype or reduction in photosynthetic capacity when grown under ambient conditions. [41][42][43][44][45][46] Such plants do, however, exhibit increased rates of starch turnover and export of neutral sugars to compensate for the defect in carbon allocation, which suggests that redundant or compensatory mechanisms also exist for the coupled defect in Pi import. PHT4;4 and PHT2;1 are candidates for this activity.…”

Detection of Sugar Accumulation and Expression Levels of Correlative Key Enzymes in Winter Wheat (Triticum aestivum) at Low Temperatures

“…TPT is the major transporter in photosynthetic plastids as carbon fluxes, and it affects the rates of intraplastid starch biosynthesis and mobilization, and sucrose biosynthesis in the cytosol. 19,20) PK is a key regulatory enzyme in glycolysis, and is usually regulated under stressful conditions. [21][22][23] However, most studies have reported only a few genes for one or two metabolic reactions, which fails to explain the overall landscape of carbohydrate metabolic flows in plants.…”

Leaf Starch Turnover Occurs in Long Days and in Falling Light at the End of the Day

“…The simplifying assumption that leaf starch degradation is low or absent during the light period is supported by pulse-chase experiments with 14 CO 2 , which failed to detect significant starch degradation in standard light periods in pea (Pisum sativum; Kruger et al, 1983), pepper (Capsicum annuum; Grange, 1984), wild-type Arabidopsis (Arabidopsis thaliana; Walters et al, 2004;Zeeman et al, 2002), and tobacco (Nicotiana tabacum; Häusler et al, 1998) leaves (limitations of this approach are discussed in Supplemental Text S1). However, starch degradation may occur during long periods in continuous light, in some mutants with chloroplast export deficiencies, and in response to acute abiotic stress.…”

“…Arabidopsis mutants with deficiencies in starch degradation accumulate higher levels of starch in constant light than wild-type plants, implying that starch levels in these conditions are determined by simultaneous synthesis and degradation (Caspar et al, 1991;Baslam et al, 2017). In triose phosphate transporter mutants of Arabidopsis and tobacco, the block in export of carbon from the chloroplast in the light is bypassed through starch turnover and export of starch degradation products (Häusler et al, 1998;Walters et al, 2004). Imposition of strong osmotic stress or photorespiratory conditions (zero CO 2 and elevated O 2 ) also rapidly induces starch degradation in the light during normal photoperiods (Weise et al, 2006;Thalmann et al, 2016).…”