Biology subjects

Coe, R. A.

Publications and source records attributed to Coe, R. A..

3 recordsLinked to original sources

bioRxiv2020

Targeted knockdown of ribulose-1, 5-bisphosphate carboxylase-oxygenase in rice mesophyll cells impact on photosynthesis and growth

We generated antisense constructs targeting two of the five Rubisco small subunit genes (OsRBCS2 and 4) which account for between 30-40% of the RBCS transcript abundance in leaf blades. The constructs were driven by a maize phosphoenolpyruvate carboxylase (PEPC) promoter known to have enriched expression in mesophyll cells (MCs). In the resulting lines leaf Rubisco protein content was reduced by between 30-50% and CO2 assimilation rate was limited under photorespiratory and non-photorespiratory conditions. A relationship between Rubisco protein content and CO2 assimilation rate was found. This was associated with a significant reduction in dry biomass accumulation and grain yield of between 37 to 70%. In addition to serving as a resource for reducing Rubisco accumulation in a cell-preferential manner, these lines allow us to characterize gene function and isoform specific suppression on photosynthesis and growth. Our results suggest that the knockdown of multiple genes is required to completely reduce Rubisco accumulation in MCs.

plant biology↗

bioRxiv2020

A partial C4 photosynthetic biochemical pathway in rice

Introduction of a C4 photosynthetic pathway into C3 rice (Oryza sativa) requires installation of a biochemical pump that concentrates CO2 at the site of carboxylation in modified bundle sheath cells. To investigate the feasibility of this, we generated a quadruple line that simultaneously expresses four of the core C4 photosynthetic enzymes from the NADP-malic enzyme subtype, phosphoenolpyruvate carboxylase (ZmPEPC), NADP-malate dehydrogenase (ZmNADP-MDH), NADP-malic enzyme (ZmNADP-ME) and pyruvate phosphate dikinase (ZmPPDK), in a cell-specific manner. This led to enhanced enzyme activity but was largely neutral in its effects on photosynthetic rate and growth. Measurements of the flux of 13CO2 through photosynthetic metabolism revealed a significant increase in the incorporation of 13C into malate, consistent with increased fixation of 13CO2 via PEP carboxylase in lines expressing the maize PEPC enzyme. We also showed 13C labelling of aspartate indicating additional 13CO2 fixation into oxaloacetate by PEPC and conversion to aspartate by the endogenous aspartate aminotransferase activity. However, there were no significant differences in labelling of 3-phosphoglycerate (3PGA) or phosphoenolpyruvate (PEP) indicating limited carbon flux through C4 enzymes into the Calvin-Benson cycle. Crossing the quadruple line with a line with reduced glycine decarboxylase H-protein (OsGDCH) abundance led to a photosynthetic phenotype characteristic of the reduced OsGDCH line and higher labelling of malate, aspartate and citrate. While Kranz anatomy or other anatomical modifications have not yet been installed in these plants to enable a fully functional C4 cycle, these results demonstrate for the first-time flux through the carboxylation phase of C4 metabolism in transgenic rice containing the key metabolic steps in the C4 pathway.

plant biology↗

bioRxiv2020

A role for neutral variation in the evolution of C4 photosynthesis

Convergent trait evolution is a recurrent phenomenon in all domains of the tree of life. While some convergent traits are caused by simple sequence changes, many are associated with extensive changes to the sequence and regulation of large cohorts of genes. It is unknown how organisms traverse this expansive genotype space to assemble such complex convergent phenotypes. C4 photosynthesis is a paradigm of large-scale phenotypic convergence. Conceptual and mathematical models propose that C4 photosynthesis evolved from ancestral C3 photosynthesis through sequential adaptive changes. These adaptive changes could have been rapidly assembled if modifications to the activity and abundance of enzymes of the C4 cycle was neutral in C3 plants. This neutrality would enable populations of C3 plants to maintain genotypes with expression levels of C4 enzymes analogous to those in C4 species and thus enable rapid assembly of a functional C4 cycle from naturally occurring genotypes given shared environmental selection. Here we show that there is substantial natural variation in expression of genes encoding C4 cycle enzymes between natural accessions of the C3 plant Arabidopsis thaliana. We further show through targeted transgenic experiments in the C3 crop Oryza sativa, that high expression of the majority of C4 cycle enzymes in rice is neutral with respect to growth, development, biomass and photosynthesis. Thus, substantial variation in the abundance and activity of C4 cycle enzymes is permissible within the limits of operation of C3 photosynthesis and the emergence of component parts of this complex convergent trait can be facilitated by neutral variation.

plant biology↗