Abstract
SummaryPhotorespiration is essential for C3 plants, enabling oxygenic photosynthesis through the scavenging of 2‐phosphoglycolate. Previous studies have demonstrated that overexpression of the L‐ and H‐proteins of the photorespiratory glycine cleavage system results in an increase in photosynthesis and growth in Arabidopsis thaliana. Here, we present evidence that under controlled environment conditions an increase in biomass is evident in tobacco plants overexpressing the H‐protein. Importantly, the work in this paper provides a clear demonstration of the potential of this manipulation in tobacco grown in field conditions, in two separate seasons. We also demonstrate the importance of targeted overexpression of the H‐protein using the leaf‐specific promoter ST‐LS1. Although increases in the H‐protein driven by this promoter have a positive impact on biomass, higher levels of overexpression of this protein driven by the constitutive CaMV 35S promoter result in a reduction in the growth of the plants. Furthermore in these constitutive overexpressor plants, carbon allocation between soluble carbohydrates and starch is altered, as is the protein lipoylation of the enzymes pyruvate dehydrogenase and alpha‐ketoglutarate complexes. Our data provide a clear demonstration of the positive effects of overexpression of the H‐protein to improve yield under field conditions.
Highlights
The photorespiratory pathway is a major route for the flow of carbon in the biosphere and is intrinsically linked to photosynthetic carbon assimilation through the Calvin–Benson–Bassham (CB) cycle
The first contained the leaf-specific and light-regulated, ST-LS1, promoter from potato (Stockhaus et al, 1989; Timm et al, 2012) and the second contained the cauliflower mosaic virus (CaMV) 35S constitutive promoter known to produce high levels of expression. These promoters were used to drive the transcription of the full-length A. thaliana H-protein cDNA (AT2G35370) and contained the commonly used HSP terminator sequence (Nagaya et al, 2010) (Figure S1)
Successful overexpression of the H-protein in leaf tissue was confirmed in experimental plants by immunoblotting, and a threefold to sevenfold increase in Hprotein for 35S::H OX compared to the ST-LS1::H OX plants was detected (Figures 1, S2 and S3a)
Summary
The photorespiratory pathway is a major route for the flow of carbon in the biosphere and is intrinsically linked to photosynthetic carbon assimilation through the Calvin–Benson–Bassham (CB) cycle. Facilitating photorespiratory carbon flow through overexpression of subunits of the glycine cleavage system (GCS) appears to be another viable approach for improving photorespiration [reviewed in Timm et al (2016)]. The H-protein has no catalytic activity, it acts as a substrate for the P-, T- and L-proteins and has been demonstrated, in vitro, to be capable of enhancing the activity of the GCS (Hasse et al, 2009) Overexpression of this protein in A. thaliana has led to increases in biomass, maximum relative electron transport rate, light saturation point and net CO2 uptake (Timm et al, 2012). Overexpression of the L-protein has been shown to increase plant growth and improve photosynthesis by facilitating photorespiratory carbon flow through the GCS (Timm et al, 2015).
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