Transgenic insertion of the cyanobacterial membrane protein ictB increases grain yield in Zea mays through increased photosynthesis and carbohydrate production

Understanding source-sink interactions: Progress in model plants and translational research to crops

Agriculture is facing a massive increase in demand per hectare as a result of an ever-expanding population and environmental deterioration. While we have learned much about how environmental conditions and diseases impact crop yield, until recently considerably less was known concerning endogenous factors, including within-plant nutrient allocation. In this review, we discuss studies of source-sink interactions covering both fundamental research in model systems under controlled growth conditions and how the findings are being translated to crop plants in the field. In this respect we detail efforts aimed at improving and/or combining C₃, C₄, and CAM modes of photosynthesis, altering the chloroplastic electron transport chain, modulating photorespiration, adopting bacterial/algal carbon-concentrating mechanisms, and enhancing nitrogen- and water-use efficiencies. Moreover, we discuss how modulating TCA cycle activities and primary metabolism can result in increased rates of photosynthesis and outline the opportunities that evaluating natural variation in photosynthesis may afford. Although source, transport, and sink functions are all covered in this review, we focus on discussing source functions because the majority of research has been conducted in this field. Nevertheless, considerable recent evidence, alongside the evidence from classical studies, demonstrates that both transport and sink functions are also incredibly important determinants of yield. We thus describe recent evidence supporting this notion and suggest that future strategies for yield improvement should focus on combining improvements in each of these steps to approach yield optimization.

α-cyanobacteria possessing form IA RuBisCO globally dominate aquatic habitats

RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) is one the most abundant enzymes on Earth. Virtually all food webs depend on its activity to supply fixed carbon. In aerobic environments, RuBisCO struggles to distinguish efficiently between CO2 and O2. To compensate, organisms have evolved convergent solutions to concentrate CO2 around the active site. The genetic engineering of such inorganic carbon concentrating mechanisms (CCMs) into plants could help facilitate future global food security for humankind. In bacteria, the carboxysome represents one such CCM component, of which two independent forms exist: α and β. Cyanobacteria are important players in the planet's carbon cycle and the vast majority of the phylum possess a β-carboxysome, including most cyanobacteria used as laboratory models. The exceptions are the exclusively marine Prochlorococcus and Synechococcus that numerically dominate open ocean systems. However, the reason why marine systems favor an α-form is currently unknown. Here, we report the genomes of 58 cyanobacteria, closely related to marine Synechococcus that were isolated from freshwater lakes across the globe. We find all these isolates possess α-carboxysomes accompanied by a form 1A RuBisCO. Moreover, we demonstrate α-cyanobacteria dominate freshwater lakes worldwide. Hence, the paradigm of a separation in carboxysome type across the salinity divide does not hold true, and instead the α-form dominates all aquatic systems. We thus question the relevance of β-cyanobacteria as models for aquatic systems at large and pose a hypothesis for the reason for the success of the α-form in nature.

Integration of mRNA and microRNA analysis reveals the molecular mechanisms underlying drought stress tolerance in maize (Zea mays L.)

Drought is among the most serious environmental issue globally, and seriously affects the development, growth, and yield of crops. Maize (Zea mays L.), an important crop and industrial raw material, is planted on a large scale worldwide and drought can lead to large-scale reductions in maize corn production; however, few studies have focused on the maize root system mechanisms underlying drought resistance. In this study, miRNA-mRNA analysis was performed to deeply analyze the molecular mechanisms involved in drought response in the maize root system under drought stress. Furthermore, preliminary investigation of the biological function of miR408a in the maize root system was also conducted. The morphological, physiological, and transcriptomic changes in the maize variety "M8186" at the seedling stage under 12% PEG 6000 drought treatment (0, 7, and 24 h) were analyzed. With prolonged drought stress, seedlings gradually withered, the root system grew significantly, and abscisic acid, brassinolide, lignin, glutathione, and trehalose content in the root system gradually increased. Furthermore, peroxidase activity increased, while gibberellic acid and jasmonic acid gradually decreased. Moreover, 32 differentially expressed miRNAs (DEMIRs), namely, 25 known miRNAs and 7 new miRNAs, and 3,765 differentially expressed mRNAs (DEMRs), were identified in maize root under drought stress by miRNA-seq and mRNA-seq analysis, respectively. Through combined miRNA-mRNA analysis, 16 miRNA-target gene pairs, comprising 9 DEMIRs and 15 DEMRs, were obtained. In addition, four metabolic pathways, namely, "plant hormone signal transduction", "phenylpropane biosynthesis", "glutathione metabolism", and "starch and sucrose metabolism", were predicted to have important roles in the response of the maize root system to drought. MiRNA and mRNA expression results were verified by real-time quantitative PCR. Finally, miR408a was selected for functional analysis and demonstrated to be a negative regulator of drought response, mainly through regulation of reactive oxygen species accumulation in the maize root system. This study helps to elaborate the regulatory response mechanisms of the maize root system under drought stress and predicts the biological functions of candidate miRNAs and mRNAs, providing strategies for subsequent mining for, and biological breeding to select for, drought-responsive genes in the maize root system.

Field-grown ictB tobacco transformants show no difference in photosynthetic efficiency for biomass relative to the wild type

In this study, four tobacco transformants overexpressing the inorganic carbon transporter B gene (ictB) were screened for photosynthetic performance relative to the wild type (WT) in field-based conditions. The WT and transgenic tobacco plants were evaluated for photosynthetic performance to determine the maximum rate of carboxylation (Vc, max), maximum rate of electron transport (Jmax), the photosynthetic compensation point (Γ*), quantum yield of PSII (ΦPSII), and mesophyll conductance (gm). Additionally, all plants were harvested to compare differences in above-ground biomass. Overall, transformants did not perform better than the WT on photosynthesis-, biomass-, and leaf composition-related traits. This is in contrast to previous studies that have suggested significant increases in photosynthesis and yield with the overexpression of ictB, although not widely evaluated under field conditions. These findings suggest that the benefit of ictB is not universal and may only be seen under certain growth conditions. While there is certainly still potential benefit to utilizing ictB in the future, further effort must be concentrated on understanding the underlying function of the gene and in which environmental conditions it offers the greatest benefit to crop performance. As it stands at present, it is possible that ictB overexpression may be largely favorable in controlled environments, such as greenhouses.

First person – Sophie Johnson

First Person is a series of interviews with the first authors of a selection of papers published in Biology Open, helping early-career researchers promote themselves alongside their papers. Sophie Johnson is first author on ‘A Year at the Forefront of Engineering Photosynthesis’, published in BiO. Sophie is a PhD student in Jane Langdale's lab in the Department of Plant Sciences, University of Oxford, investigating plant development and improving photosynthesis.

A Year at the Forefront of Engineering Photosynthesis

Multiple proof-of-principle experiments and successful field trials have demonstrated that engineering photosynthesis is a viable strategy for improving crop yields. Advances to engineering technologies have accelerated efforts to improve photosynthesis, generating a large volume of published literature: this Review therefore aims to highlight the most promising results from the period February 2021 to January 2022. Recent research has demonstrated the importance of understanding the impact of changing climates on photosynthesis to ensure that proposed engineering strategies are resilient to climate change. Encouragingly, there have been several reports of strategies that have benefits at temperatures higher than current ambient conditions. There has also been success in engineering synthetic bypass pathways, providing support for the feasibility of a synthetic biology approach. Continued developments in all areas of engineering photosynthesis will be necessary for sustainably securing sufficient crop yields for the future. This article has an associated First Person interview with the first author of the paper.