An Arabidopsis thaliana knock‐out mutant of the chloroplast triose phosphate/phosphate translocator is severely compromised only when starch synthesis, but not starch mobilisation is abolished

Abstract

SummaryThe Arabidopsis thaliana tpt-1 mutant which is defective in the chloroplast triose phosphate/phosphate translocator (TPT) was isolated by reverse genetics. It contains a T-DNA insertion 24 bp upstream of the start ATG of the TPT gene. The mutant lacks TPT transcripts and triose phosphate (TP)-specific transport activities are reduced to below 5% of the wild type. Analyses of diurnal variations in the contents of starch, soluble sugars and phosphorylated intermediates combined with 14 CO 2 labelling studies showed, that the lack of TP export for cytosolic sucrose biosynthesis was almost fully compensated by both continuous accelerated starch turnover and export of neutral sugars from the stroma throughout the day. The utilisation of glucose 6-phosphate (generated from exported glucose) rather than TP for sucrose biosynthesis in the light bypasses the key regulatory step catalysed by cytosolic fructose 1,6-bisphosphatase. Despite its regulatory role in the feed-forward control of sucrose biosynthesis, variations in the fructose 2,6-bisphosphate content upon illumination were similar in the mutant and the wild type. Crosses of tpt-1 with mutants unable to mobilise starch (sex1) or to synthesise starch (adg1-1) revealed that growth and photosynthesis of the double mutants was severely impaired only when starch biosynthesis, but not its mobilisation, was affected. For tpt-1/sex1 combining a lack in the TPT with a deficiency in starch mobilisation, an additional compensatory mechanism emerged, i.e. the formation and (most likely) fast turnover of high molecular weight polysaccharides. Steady-state RNA levels and transport activities of other phosphate translocators capable of transporting TP remained unaffected in the mutants.

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.…”

Fast Retrograde Signaling in Response to High Light Involves Metabolite Export, MITOGEN-ACTIVATED PROTEIN KINASE6, and AP2/ERF Transcription Factors in Arabidopsis

“…Two different insertion mutants of the triose phosphate/ phosphate translocator (tpt1, Schneider et al, 2002;tpt2, Schmitz et al, 2012; see Supplemental Figure 4A for schematics of insertion site) lack the activity of the TPT. In contrast to the wild type, none of the investigated AP2/ERF-TFs showed strong upregulation of transcript in either tpt mutant at t = 10 min of L→H ( Figure 4; Supplemental Figure 3A).…”

“…Arabidopsis thaliana wild-type Columbia-0 (Col-0) and wild-type Wassilewskija plants and the mutants KatE2 (Oelze et al, 2012), erf6 (SAIL_1236_H11), mdh (Hameister et al, 2007), mpk6-2 (Müller et al, 2010), mpk6-3 (Bethke et al, 2009, stn7 (Pesaresi et al, 2009), tpt1 (Schneider et al, 2002), and tpt2 (Schmitz et al, 2012) were grown in soil culture (1:1:1 mixture of Frühsdorfer Erde Klocke P, perlite, and vermiculite) for 4.5 weeks under the following conditions: 10 h of light, 80 mmol quanta m 22 s 21 , 23°C, and 14 h darkness at 18°C; 55% relative humidity. Except for the KatE2 mutant, all mutants used in this study are T-DNA exon insertion mutants (Supplemental Figure 4).…”

Nighttime Sugar Starvation Orchestrates Gibberellin Biosynthesis and Plant Growth in Arabidopsis

“…During the night, starch is degraded through a circadian clock-regulated pathway (Graf et al, 2010) to fuel continued growth (Wiese et al, 2007). The synthesis of transitory starch is of great importance for growth and Arabidopsis mutants defective in either starch synthesis or degradation are dwarf (Caspar et al, 1985(Caspar et al, , 1991Lin et al, 1988aLin et al, , 1988bYu et al, 2001;Schneider et al, 2002;Niittylä et al, 2004;Stitt and Zeeman, 2012).…”