The Mysterious Rescue of adg1-1/tpt-2 – an Arabidopsis thaliana Double Mutant Impaired in Acclimation to High Light – by Exogenously Supplied Sugars

Abstract

An Arabidopsis thaliana double mutant (adg1-1/tpt-2) defective in the day- and night-path of photoassimilate export from the chloroplast due to a knockout in the triose phosphate/phosphate translocator (TPT; tpt-2) and a lack of starch [mutation in ADP glucose pyrophosphorylase (AGPase); adg1-1] exhibits severe growth retardation, a decrease in the photosynthetic capacity, and a high chlorophyll fluorescence (HCF) phenotype under high light conditions. These phenotypes could be rescued when the plants were grown on sucrose (Suc) or glucose (Glc). Here we address the question whether Glc-sensing hexokinase1 (HXK1) defective in the Glc insensitive 2 (gin2-1) mutant is involved in the sugar-dependent rescue of adg1-1/tpt-2. Triple mutants defective in the TPT, AGPase, and HXK1 (adg1-1/tpt-2/gin2-1) were established as homozygous lines and grown together with Col-0 and Landsberg erecta (Ler) wild-type plants, gin2-1, the adg1-1/tpt-2 double mutant, and the adg1-1/tpt-2/gpt2-1 triple mutant [additionally defective in the glucose 6-phosphate/phosphate translocator 2 (GPT2)] on agar in the presence or absence of 50 mM of each Glc, Suc, or fructose (Fru). The growth phenotype of the double mutant and both triple mutants could be rescued to a similar extent only by Glc and Suc, but not by Fru. All three sugars were capable of rescuing the HCF and photosynthesis phenotype, irrespectively of the presence or absence of HXK1. Quantitative RT-PCR analyses of sugar-responsive genes revealed that plastidial HXK (pHXK) was up-regulated in adg1-1/tpt-2 plants grown on sugars, but showed no response in adg1-1/tpt-2/gin2-1. It appears likely that soluble sugars are directly taken up by the chloroplasts and enter further metabolism, which consumes ATP and NADPH from the photosynthetic light reaction and thereby rescues the photosynthesis phenotype of the double mutant. The implication of sugar turnover and probably signaling inside the chloroplasts for the concept of retrograde signaling is discussed.

The Combined Loss of Triose Phosphate and Xylulose 5-Phosphate/Phosphate Translocators Leads to Severe Growth Retardation and Impaired Photosynthesis in Arabidopsis thaliana tpt/xpt Double Mutants

“…To our great surprise tpt-1[2]/xpt-1 and the amiRNA: XPT tpt-2 plants exhibited pronounced growth and photosynthesis phenotypes ( Figures 2 , 3 ) similar to those described for adg1-1/tpt-1[2] . The latter has been extensively analyzed over the past decade ( Häusler et al, 2009 ; Heinrichs et al, 2012 ; Schmitz et al, 2012 , 2014 ; Häusler et al, 2014a ; Hölscher et al, 2016 ). Hence we included adg1-1/tpt-2 in most experiments as an additional control.…”

Extensive Posttranscriptional Regulation of Nuclear Gene Expression by Plastid Retrograde Signals

“…Over the past decades, substantial progress has been made with identifying signaling components and distinct pathways of retrograde signaling that regulate chloroplast biogenesis (biogenetic control) or control plastid homeostasis in response to environmental stimuli (operational control; Susek et al, 1993;Wagner et al, 2004;Koussevitzky et al, 2007;Pogson et al, 2008;Estavillo et al, 2011;Woodson et al, 2011;Ramel et al, 2012;Xiao et al, 2012). Various potential signaling molecules have been identified, including intermediates of tetrapyrrole biosynthesis (TPB; Strand et al, 2003;Woodson et al, 2011), carotenoid oxidation products (Ramel et al, 2012;Shumbe et al, 2014), isoprenoid precursors (Xiao et al, 2012), phosphoadenosines (Estavillo et al, 2011), hydrogen peroxide (Balazadeh et al, 2012;Maruta et al, 2012), and carbohydrate metabolites (Heinrichs et al, 2012;Vogel et al, 2014).…”

Chloroplasts Are Central Players in Sugar-Induced Leaf Growth

“…GPT1 is expressed throughout plant development (Niewiadomski et al, 2005). However, GPT2 expression is limited to certain tissues, such as senescing leaves, and is induced under different conditions, such as during relief of seed dormancy (Finch-Savage et al, 2007), during acclimatization to high light (Dyson et al, 2015), during Glc-induced senescence (Pourtau et al, 2006), in starch-free mutants (Kunz et al, 2010;Heinrichs et al, 2012), and when sugar levels increase (Price et al, 2004;Gonzali et al, 2006;Müller et al, 2007;Osuna et al, 2007).…”

“…Furthermore, in most studies, sugars are applied to cell suspension cultures, detached leaves, or liquid cultures (Price et al, 2004;Li et al, 2006;Müller et al, 2007;Osuna et al, 2007;Usadel et al, 2008;Kunz et al, 2014), highlighting the need for more targeted experimental designs to study the effect of Suc on organ growth such as leaves. In addition, most sugar-feeding experiments make use of high, nonphysiological Glc or Suc concentrations (Price et al, 2004;Gonzali et al, 2006;Li et al, 2006;Müller et al, 2007;Heinrichs et al, 2012).…”