Publications

Chlorophylls are magnesium-tetrapyrrole molecules that play essential roles in photosynthesis. All chlorophylls have similar five-membered ring structures, with variations in the side chains and/or reduction states. Formyl group substitutions on the side chains of chlorophyll a result in the different absorption properties of chlorophyll b, chlorophyll d, and chlorophyll f. These formyl substitution derivatives exhibit different spectral shifts according to the formyl substitution position. Not only does the presence of various types of chlorophylls allow the photosynthetic organism to harvest sunlight at different wavelengths to enhance light energy input, but the pigment composition of oxygenic photosynthetic organisms also reflects the spectral properties on the surface of the Earth. Two major environmental influencing factors are light and oxygen levels, which may play central roles in the regulatory pathways leading to the different chlorophylls. I review the biochemical processes of chlorophyll biosynthesis and their regulatory mechanisms.

Climate change is resulting in increasing atmospheric [CO2], rising growth temperature (T), and greater frequency/severity of drought, with each factor having the potential to alter the respiratory metabolism of leaves. Here, the effects of elevated atmospheric [CO2], sustained warming, and drought on leaf dark respiration (R dark), and the short-term T response ofR dark were examined in Eucalyptus globulus. Comparisons were made using seedlings grown under different [CO2], T, and drought treatments. Using high resolution T–response curves of R dark measured over the 15–65 °C range, it was found that elevated [CO2], elevated growth T, and drought had little effect on rates of R dark measured at T <35 °C and that there was no interactive effect of [CO2], growth T, and drought on Tresponse of R dark. However, drought increased R dark at high leaf T typical of heatwave events (35–45 °C), and increased the measuring T at which maximal rates of R dark occurred (T max) by 8 °C (from 52 °C in well-watered plants to 60 °C in drought-treated plants). Leaf starch and soluble sugars decreased under drought and elevated growth T, respectively, but no effect was found under elevated [CO2]. Elevated [CO2] increased the Q10 of R dark (i.e. proportional rise in R dark per 10 °C) over the 15–35 °C range, while drought increased Q 10 values between 35 °C and 45 °C. Collectively, the study highlights the dynamic nature of the T dependence of R dark in plants experiencing future climate change scenarios, particularly with respect to drought and elevated [CO2].

This mini-review addresses advances in understanding the transmembrane topologies of two unrelated, single-subunit bicarbonate transporters from cyanobacteria, namely BicA and SbtA. BicA is a Na+-dependent bicarbonate transporter that belongs to the SulP/SLC26 family that is widespread in both eukaryotes and prokaryotes. Topology mapping of BicA via the phoA/lacZ fusion reporter method identified 12 transmembrane helices with an unresolved hydrophobic region just beyond helix 8. Re-interpreting this data in the light of a recent topology study on rat prestin leads to a consensus topology of 14 transmembrane domains with a 7+7 inverted repeat structure. SbtA is also a Na+-dependent bicarbonate transporter, but of considerably higher affinity (Km 2–5 μM versus >100 μM for BicA). Whilst SbtA is widespread in cyanobacteria and a few bacteria, it appears to be absent from eukaryotes. Topology mapping of SbtA via the phoA/lacZ fusion reporter method identified 10 transmembrane helices. The topology consists of a 5+5 inverted repeat, with the two repeats separated by a large intracellular loop. The unusual location of the N and C-termini outside the cell raises the possibility that SbtA forms a novel fold, not so far identified by structural and topological studies on transport proteins.

Du, Jiahui, Britta Förster, Loraine Rourke, Susan M Howitt, and G Dean Price. 2014. “Characterisation of cyanobacterial bicarbonate transporters in E. coli shows that SbtA homologs are functional in this heterologous expression system.”  PloS one 9 (12):e115905. Abstract Cyanobacterial HCO3– transporters BCT1, SbtA and BicA are important components of cyanobacterial CO2-concentration mechanisms. They also show […]

As the world’s population increases, the spectre of severe food shortages is growing, with the United Nations predicting1 that food production will need to double by 2050. It has been proposed that cyanobacteria — which obtain their energy from a highly efficient form of photosynthesis — might hold the key to increasing the yield of our most important crops and vegetables. On page 547 of this issue, Lin et al.2 report a major step towards realizing this possibility, finding that cyanobacteria can be used to improve photosynthesis in the leaves of crops.

Photosynthesis harnesses sunlight to convert carbon dioxide into simple sugars. Rubisco, the key enzyme for CO2 fixation into sugar, is inefficient because it cannot easily discriminate between oxygen and CO2 and so wastes energy by fixing O2. The enzyme evolved at a time when O2 levels in the atmosphere were much lower than they are today, and there was therefore little evolutionary pressure to select for an ability to discriminate between the two molecules. Photosynthetic organisms have evolved to circumvent the problem of rising atmospheric O2 levels in two ways: first, by making more of a slower-acting version of Rubisco with an improved ability to discriminate; or second, by using various ‘add-ons’, called CO2-concentrating mechanisms (CCMs), to elevate CO2 levels in the vicinity of the enzyme.

Most crops have adopted the first strategy, making Rubisco possibly the most abundant enzyme on Earth. This approach, however, results in a 30% reduction in photosynthetic efficiency through the associated O2 fixation. That can be partly ameliorated by raising CO2 levels around the leaf3 in a manner conceptually similar to adding a CCM. There is currently increased focus on the second strategy — if a CCM could be introduced into crops, it might turbocharge photosynthetic CO2 fixation. CCMs have evolved independently in cyanobacteria, microalgae and some plants (mostly those regarded by us as weeds). Although several types of CCM are being considered for introduction into crops, Lin and colleagues’ work focuses on the cyanobacterial CCM.

The carbon dioxide (CO2)-concentrating mechanism of cyanobacteria is characterized by the occurrence of Rubisco-containing microcompartments called carboxysomes within cells. The encapsulation of Rubisco allows for high-CO2 concentrations at the site of fixation, providing an advantage in low-CO2 environments. Cyanobacteria with Form-IA Rubisco contain α-carboxysomes, and cyanobacteria with Form-IB Rubisco contain β-carboxysomes. The two carboxysome types have arisen through convergent evolution, and α-cyanobacteria and β-cyanobacteria occupy different ecological niches. Here, we present, to our knowledge, the first direct comparison of the carboxysome function from α-cyanobacteria (Cyanobium spp. PCC7001) and β-cyanobacteria (Synechococcus spp. PCC7942) with similar inorganic carbon (Ci; as CO2 and HCO3−) transporter systems. Despite evolutionary and structural differences between α-carboxysomes and β-carboxysomes, we found that the two strains are remarkably similar in many physiological parameters, particularly the response of photosynthesis to light and external Ci and their modulation of internal ribulose-1,5-bisphosphate, phosphoglycerate, and Ci pools when grown under comparable conditions. In addition, the different Rubisco forms present in each carboxysome had almost identical kinetic parameters. The conclusions indicate that the possession of different carboxysome types does not significantly influence the physiological function of these species and that similar carboxysome function may be possessed by each carboxysome type. Interestingly, both carboxysome types showed a response to cytosolic Ci, which is of higher affinity than predicted by current models, being saturated by 5 to 15 mM Ci. This finding has bearing on the viability of transplanting functional carboxysomes into the C3 chloroplast.

CO2 fixation in higher plants is catalysed by Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) whose catalysis has a pervasive influence on the efficiency of photosynthesis and growth. Surprisingly, Rubisco is endowed with catalytic imperfections, which include confusion of substrate CO2 with O2 and a slow turnover rate. The confusion between CO2 and O2 has become exacerbated through the onset of oxygenic photosynthesis that has significantly reduced the earth’s atmospheric CO2/O2 ratio. To compensate for these imperfections in catalysis, the simpler evolutionary route for C3 plants is to make more Rubisco to maintain viable CO2 assimilation rates – despite the significant nitrogen and energy cost to the plant. In this issue of Plant, Cell & Environment, Galmes et al. (2014) presented Rubisco catalytic data from diverse C3 terrestrial plants from different phylogenetic lineages that have adapted to varying environmental conditions. Their findings help better understand how Rubisco function in C3 plants has adapted to atmospheric CO2 and O2 changes. Understanding such puzzles is a key to identifying solutions for improving Rubisco catalysis that, in coordination with other photosynthesis-enhancing approaches, are aimed at delivering a second ‘green revolution’ to increase productivity in globally important C3 grain crops such as wheat and rice (Evans & von Caemmerer 2011).

The reduction chemistry of molecular oxygen underpins the energy metabolism of multicellular organisms, liberating free energy needed to catalyze a plethora of enzymatic reactions. Measuring the isotope signatures of (16)O and (18)O during O2 reduction can provide insights into both kinetic and equilibrium isotope effects. However, current methods to measure O2 isotope signatures are time-consuming and disruptive. This paper describes the application of membrane inlet mass spectrometry to determine the oxygen isotope discrimination of a range of O2-consuming reactions, providing a rapid and convenient method for determining these values. A survey of oxygenase and oxidase reactions provides new insights into previously uncharacterized amino acid oxidase enzymes. Liquid and gas phase measurements show the ease of assays using this approach for purified enzymes, biological extracts and intact tissues.

Improving global yields of agricultural crops is a complex challenge with evidence indicating benefits in productivity are achieved by enhancing photosynthetic carbon assimilation. Towards improving rates of CO2 capture within leaf chloroplasts, this study shows the versatility of plastome transformation for expressing the Synechococcus PCC7002 BicA bicarbonate transporter within tobacco plastids. Fractionation of chloroplast membranes from transplastomic tobBicA lines showed that ~75% of the BicA localized to the thylakoid membranes and ~25% to the chloroplast envelope. BicA levels were highest in young emerging tobBicA leaves (0.12 μmol m–2, ≈7mg m–2) accounting for ~0.1% (w/w) of the leaf protein. In these leaves, the molar amount of BicA was 16-fold lower than the abundant thylakoid photosystem II D1 protein (~1.9 μmol m–2) which was comparable to the 9:1 molar ratio of D1:BicA measured in air-grown Synechococcus PCC7002 cells. The BicA produced had no discernible effect on chloroplast ultrastructure, photosynthetic CO2-assimilation rates, carbon isotope discrimination, or growth of the tobBicA plants, implying that the bicarbonate transporter had little or no activity. These findings demonstrate the utility of plastome transformation for targeting bicarbonate transporter proteins into the chloroplast membranes without impeding growth or plastid ultrastructure. This study establishes the span of experimental measurements required to verify heterologous bicarbonate transporter function and location in chloroplasts and underscores the need for more detailed understanding of BicA structure and function to identify solutions for enabling its activation and operation in leaf chloroplasts.