T-protein is present in large excess over the other proteins of the glycine cleavage system in leaves of Arabidopsis

Abstract

T-protein is present in large excess over the other proteins of the glycine cleavage system in leaves of Arabidopsis and therefore, exerts little control over the photorespiratory pathway. T-protein is the aminomethyltransferase of the glycine cleavage multienzyme system (GCS), also known as the glycine decarboxylase complex, and essential for photorespiration and one-carbon metabolism. Here, we studied what effects varying levels of the GCS T-protein would have on GCS activity, the operation of the photorespiratory pathway, photosynthesis, and plant growth. To this end, we examined Arabidopsis thaliana T-protein overexpression lines with up to threefold higher amounts of leaf T-protein as well as one knockdown mutant with about 5% residual leaf T-protein and one knockout mutant. Overexpression did not alter photosynthetic CO uptake and plant growth, and the knockout mutation was lethal even in the non-photorespiratory environment of air enriched to 1% CO. Unexpectedly in light of this very low T-protein content, however, the knockdown mutant was able to grow and propagate in normal air and displayed only some minor changes, such as a moderate glycine accumulation in combination with somewhat delayed growth. Neither overexpression nor the knockdown of T-protein altered the amounts of the other three GCS proteins, suggesting that the biosynthesis of the GCS proteins is not synchronized at this level. We also observed that the knockdown causes less T-protein mostly in leaf mesophyll cells, but not so much in the vasculature, and discuss this phenomenon in light of the dual involvement of the GCS and hence T-protein in plant metabolism. Collectively, this work shows that T-protein is present in large excess over the other proteins of the glycine cleavage system in leaves of Arabidopsis and therefore exerts little control over the photorespiratory pathway.

Redox-Regulation of Photorespiration through Mitochondrial Thioredoxin o1

“…As shown in Figure 4A, wild-type and trxo1 plants grew similarly (unchanged rosette diameter and leaf number) and were invariant from each other with regard to chlorophyll a fluorescence. In agreement with previous findings (Timm et al, 2018), gldt1 showed reduced growth (reduction in the rosette diameter), yellowish leaves, and lowered quotient of the variable versus the maximal fluorescence of PSII (F v /F m ) under photorespiratory conditions. Interestingly, the reduction of growth and PSII efficiency observed in the gldt1 mutant was further enhanced in the TxGT line (Fig.…”

“…Rationally, CO 2 concentrations were varied to distinguish between conditions with suppressed [high CO 2 , high carbon (HC), 1500 mL L 21 CO 2 , 9 h illumination] and active photorespiration after transferring the plants into normal air (390 mL L 21 CO 2 , 9 h illumination). For comparison, we included the GDC T-protein knockdown mutant gldt1, which is characterized by a ;70% reduction in total GDC activity (gldt1-1; Timm et al, 2018).…”

“…Next we hypothesized that if TRXo1 affects GDC activity, phenotypic and physiological alterations would be exacerbated in mutants that exhibit combined defects in TRXo1 and GDC expression under photorespiratory conditions. For this reason, trxo1 was crossed with the gldt1 knockdown mutant (5% residual GDC-T mRNA expression; Timm et al, 2018) to obtain the double mutant TxGT (for genotypic verification see Supplemental Fig. S1).…”

“…Hence, we investigated whether a mitochondrial thioredoxin (TRXo1) is involved in the potential redox regulation of photorespiratory reactions in this compartment. To this end, we compared the photosynthetic gas-exchange and the metabolic acclimation toward an altered CO 2 partial pressure of the transfer DNA (T-DNA) insertional line trxo1 (trxo1-1; Daloso et al, 2015) and a newly generated double mutant (TxGT), which is defective in TRXo1 expression and strongly reduced in the GDC T-protein (gldt1-1; Timm et al, 2018). Moreover, we performed in vitro lipoamide dehydrogenase (LPD) activity measurements using isolated mitochondria or recombinant proteins to provide direct experimental evidence for redox-regulation of GDC.…”

Overexpressing the H‐protein of the glycine cleavage system increases biomass yield in glasshouse and field‐grown transgenic tobacco plants

“…To achieve high expression levels, we used the CaMV 35S viral promoter which has been widely and successfully used in the past to drive high expression of the desired transgene and in the production of high yielding plants (Kohler et al ., ; Lefebvre et al ., ; Simkin et al ., ). Additionally, the ST‐LS1 promoter, previously used for the overexpression of GCS proteins in A. thaliana (Timm et al ., , , ), allowed us to achieve an intermediate level of overexpression in leaves. Somewhat unexpectedly however, the two sets of transgenic plants had contrasting growth phenotypes, and the transgenic plants with the larger increases in H‐protein had a severe slow growth phenotype.…”