Biochemical characterization of predicted Precambrian RuBisCO

The antiquity and global abundance of the enzyme, RuBisCO, attests to the crucial and longstanding role it has played in the biogeochemical cycles of Earth over billions of years. The counterproductive oxygenase activity of RuBisCO has persisted over billions of years of evolution, despite its competition with the carboxylase activity necessary for carbon fixation, yet hypotheses regarding the selective pressures governing RuBisCO evolution have been limited to speculation. Here we report the resurrection and biochemical characterization of ancestral RuBisCOs, dating back to over one billion years ago (Gyr ago). Our findings provide an ancient point of reference revealing divergent evolutionary paths taken by eukaryotic homologues towards improved specificity for CO2, versus the evolutionary emphasis on increased rates of carboxylation observed in bacterial homologues. Consistent with these distinctions, in vivo analysis reveals the propensity of ancestral RuBisCO to be encapsulated into modern-day carboxysomes, bacterial organelles central to the cyanobacterial CO2 concentrating mechanism

Increasing metabolic potential: C-fixation

Due to a growing world population, crop yields must increase to meet rising demand. Crop plants also require adaptation to optimise performance in the changing environments caused by climate change. Improving photosynthetic carbon fixation is a promising, albeit technically challenging, strategy whose potential has only just begun to be considered in breeding programs. Rubisco, a fundamental enzyme of carbon fixation, is extremely inefficient and many strategies to improve photosynthesis focus on overcoming the limitations of this enzyme, either by improving Rubisco activity and regulation or by improving the supply of substrates. Although progress is being made, the need to tailor solutions for each crop and their respective environments has been highlighted. Even so, continuing research will be required to achieve these objectives and to grow crops more sustainably in the future

Dissecting wheat grain yield drivers in a mapping population in the UK

Improving crop yields arises as a solution to ensure food security in the future scenarios of growing world population, changes in food consumption patterns, climate change and limitation on resources allocated to agriculture. Defining traits that can be reliable cornerstones to yield improvement and understanding their interaction and influence on yield formation is an important part of ensuring the success of breeding programs for high yield. Traits that can drive yield increases, such as light interception and conversion efficiency, carbon assimilation and allocation, were intensively phenotyped in a double-haploid wheat mapping population grown under field conditions in the UK. Traits were analysed for their correlation to yield, genetic variation and broad-sense heritability. Canopy cover and reflectance, biomass production and allocation to stems and leaves, as well as flag leaf photosynthesis at a range of light levels measured pre- and post-anthesis correlated with plant productivity and contributed to explain different strategies of wheat lines to attain high grain yields. This research mapped multiple traits related to light conversion into biomass. The findings highlight the need to phenotype traits throughout the growing season and support the approach of targeting photosynthesis and its components as traits for breeding high yielding wheat

Rubisco catalytic properties of wild and domesticated relatives provide scope for improving wheat photosynthesis

Rubisco is a major target for improving crop photosynthesis and yield, yet natural diversity in catalytic properties of this enzyme is poorly understood. Rubisco from 25 genotypes of the Triticeae tribe, including wild relatives of bread wheat (Triticum aestivum), were surveyed to identify superior enzymes for improving photosynthesis in this crop. In vitro Rubisco carboxylation velocity (V c), Michaelis–Menten constants for CO2 (K c) and O2 (K o) and specificity factor (S c/o) were measured at 25 and 35 °C. V c and K c correlated positively, while V c and S c/o were inversely related. Rubisco large subunit genes (rbcL) were sequenced, and predicted corresponding amino acid differences analysed in relation to the corresponding catalytic properties. The effect of replacing native wheat Rubisco with counterparts from closely related species was analysed by modelling the response of photosynthesis to varying CO2 concentrations. The model predicted that two Rubisco enzymes would increase photosynthetic performance at 25 °C while only one of these also increased photosynthesis at 35 °C. Thus, under otherwise identical conditions, catalytic variation in the Rubiscos analysed is predicted to improve photosynthetic rates at physiological CO2 concentrations. Naturally occurring Rubiscos with superior properties amongst the Triticeae tribe can be exploited to improve wheat photosynthesis and crop productivity

Overexpression of ca1pase decreases Rubisco abundance and grain yield in wheat

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyses the fixation of CO2 into organic compounds that are used for plant growth and production of agricultural products. Specific sugar-phosphate derivatives bind tightly to the active sites of Rubisco, locking the enzyme in a catalytically inactive conformation. 2-carboxy-D-arabinitol-1-phosphate phosphatase (CA1Pase) dephosphorylates such tight-binding inhibitors, contributing towards maintaining Rubisco activity. The hypothesis that overexpressing ca1pase would decrease the abundance of inhibitors, thereby increasing the activity of Rubisco and enhancing photosynthetic performance and productivity was investigated in wheat. Plants of four independent wheat transgenic lines showed up to 30-fold increases in ca1pase expression compared to wild-type (WT). Plants overexpressing ca1pase had lower quantities of Rubisco tight-binding inhibitors and higher Rubisco activation states than WT, however the amount of Rubisco active sites decreased by 17-60% in the four transgenic lines compared to WT. The lower Rubisco content in plants overexpressing ca1pase resulted in lower initial and total carboxylating activities measured in flag leaves at the end of the vegetative stage, and lower aboveground biomass and grain yield measured in fully mature plants. Hence, contrary to what would be expected from our theory, ca1pase overexpression caused decreased Rubisco content and compromised wheat grain yields. These results support a possible role for Rubisco inhibitors in protecting the enzyme and maintaining an adequate content of Rubisco active sites available to support carboxylation rates in planta

Structural and functional analyses of Rubisco from arctic diatom species reveal unusual posttranslational modifications

The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Natural diversity in the catalytic properties of Rubisco indicates possibilities for improvement. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO2 sequestration in polar cold waters. Until now, the biochemical properties and three-dimensional structures of Rubisco from diatoms were unknown. Here, diatoms from Arctic waters were collected, cultivated and analyzed for their CO2 fixing capability. We characterized the kinetic properties of five, and determined the crystal structures of four Rubiscos selected for their high CO2-fixing efficiency. The DNA sequences of the rbcL and rbcS genes of the selected diatoms were similar, reflecting their close phylogenetic relationship. The Vmax and KM for the oxygenase and carboxylase activities at 25°C and the specificity factors (Sc /o) at 15, 25 and 35°C, were determined. The Sc/o values were high, approaching those of mono- and dicot plants, thus exhibiting good selectivity for CO2 relative to O2. Structurally, diatom Rubiscos belong to Form I C/D, containing small subunits characterised by a short βA-βB loop and a carboxy-terminal extension that forms a β- hairpin structure (βE-βF loop). Of note, the diatom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxy-proline, betahydroxyleucine, hydroxylated, and nitrosylated cysteine, mono-, and di-hydroxylated lysine, and tri-methylated lysine. Our studies suggest adaptation toward achieving efficient CO2-fixation in Arctic diatom Rubiscos

Structural and functional analyses of Rubisco from arctic diatom species reveal unusual posttranslational modifications

The catalytic performance of the major CO2-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Natural diversity in the catalytic properties of Rubisco indicates possibilities for improvement. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO2 sequestration in polar cold waters. Until now, the biochemical properties and three-dimensional structures of Rubisco from diatoms were unknown. Here, diatoms from Arctic waters were collected, cultivated and analyzed for their CO2 fixing capability. We characterized the kinetic properties of five, and determined the crystal structures of four Rubiscos selected for their high CO2-fixing efficiency. The DNA sequences of the rbcL and rbcS genes of the selected diatoms were similar, reflecting their close phylogenetic relationship. The Vmax and KM for the oxygenase and carboxylase activities at 25°C and the specificity factors (Sc/o) at 15, 25 and 35°C, were determined. The Sc/o values were high, approaching those of mono- and dicot plants, thus exhibiting good selectivity for CO2 relative to O2 Structurally, diatom Rubiscos belong to Form I C/D, containing small subunits characterised by a short βA-βB loop and a carboxy-terminal extension that forms a β-hairpin structure (βE-βF loop). Of note, the diatom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxy-proline, betahydroxyleucine, hydroxylated, and nitrosylated cysteine, mono-, and di-hydroxylated lysine, and tri-methylated lysine. Our studies suggest adaptation toward achieving efficient CO2-fixation in Arctic diatom Rubiscos