Can increased leaf photosynthesis be converted into higher crop mass production? A simulation study for rice using the crop model GECROS

Carboxysome encapsulation of the CO2-fixing enzyme Rubisco in tobacco chloroplasts

A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating mechanisms (CCMs) into plant chloroplasts. Cyanobacterial CCMs enable relatively rapid CO2 fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key α-carboxysome structural proteins. This minimal gene set produces carboxysomes, which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO2. This result demonstrates the formation of α-carboxysomes from a reduced gene set, informing the step-wise construction of fully functional α-carboxysomes in chloroplasts.

Overexpression of VaWRKY14 increases drought tolerance in Arabidopsis by modulating the expression of stress-related genes

Overexpression of VaWRKY14 increases drought tolerance in Arabidopsis by modulating the expression of stress-related genes, including COR15A, COR15B, COR413, KIN2, and RD29A. The WRKY family is one of a largest transcription factors in plants, and it is a key component of multiple stress responses. In this study, the drought- and cold-induced WRKY family gene VaWRKY14 was isolated and characterized. Phylogenetic analysis indicated that VaWRKY14 belongs to the WRKY IIa subfamily, of which several members participate in biotic and abiotic stress responses in plants. Fluorescence observation from Arabidopsis mesophyll protoplasts transformed with the VaWRKY14::eGFP fusion vector suggested that VaWRKY14 was localized in the nucleus. The VaWRKY14 in yeast cells did not display any transcriptional activity. The expression of VaWRKY14 could be induced by exogenous phytohormones, including salicylic acid (SA) and abscisic acid (ABA). Overexpression of VaWRKY14 enhanced the drought tolerance of transgenic Arabidopsis. Compared with wild-type Arabidopsis, the VaWRKY14-OE lines exhibited higher water content and antioxidant enzyme activities in leaves after drought treatment. RNA sequencing analysis revealed that several stress-related genes, including COR15A, COR15B, COR413, KIN2, and RD29A, were upregulated in transgenic plants relative to their expression in wild-type Arabidopsis under normal conditions. Several genes (3 upregulated and 49 down-regulated) modulated by VaWRKY14 were also affected by drought stress in wild-type plants. These data suggest that VaWRKY14 responds to drought and cold stresses and that drought tolerance may be enhanced by regulating the expression of stress-related genes in Arabidopsis.

Water- and Nitrogen-Use Efficiencies of Hemp (Cannabis sativa L.) Based on Whole-Canopy Measurements and Modeling

Interest in hemp (Cannabis sativa L.) as a crop for the biobased economy is growing worldwide because hemp produces a high and valuable biomass while requiring low inputs. To understand the physiological basis of hemp's resource-use efficiency, canopy gas exchange was assessed using a chamber technique on canopies exposed to a range of nitrogen (N) and water levels. Since canopy transpiration and carbon assimilation were very sensitive to variations in microclimate among canopy chambers, observations were adjusted for microclimatic differences using a physiological canopy model, with leaf-level parameters estimated for hemp from our previous study. Canopy photosynthetic water-use efficiency (PWUEc), defined as the ratio of gross canopy photosynthesis to canopy transpiration, ranged from 4.0 mmol CO2 (mol H2O)-1 to 7.5 mmol CO2 (mol H2O)-1. Canopy photosynthetic nitrogen-use efficiency (PNUEc), the ratio of the gross canopy photosynthesis to canopy leaf-N content, ranged from 0.3 mol CO2 d-1 (g N)-1 to 0.7 mol CO2 d-1 (g N)-1. The effect of N-input levels on PWUEc and PNUEc was largely determined by the N effect on canopy size or leaf area index (LAI), whereas the effect of water-input levels differed between short- and long-term stresses. The effect of short-term water stress was reflected by stomatal regulation. The long-term stress increased leaf senescence, decreased LAI but retained total canopy N content; however, the increased average leaf-N could not compensate for the lost LAI, leading to a decreased PNUEc. Although hemp is known as a resource-use efficient crop, its final biomass yield and nitrogen use efficiency may be restricted by water limitation during growth. Our results also suggest that crop models should take stress-induced senescence into account in addition to stomatal effects if crops experience a prolonged water stress during growth.

The energy budget in C4 photosynthesis: insights from a cell-type-specific electron transport model

Extra ATP required in C₄ photosynthesis for the CO₂ -concentrating mechanism probably comes from cyclic electron transport (CET). As metabolic ATP : NADPH requirements in mesophyll (M) and bundle-sheath (BS) cells differ among C₄ subtypes, the subtypes may differ in the extent to which CET operates in these cells. We present an analytical model for cell-type-specific CET and linear electron transport. Modelled NADPH and ATP production were compared with requirements. For malic-enzyme (ME) subtypes, c. 50% of electron flux is CET, occurring predominantly in BS cells for standard NADP-ME species, but in a ratio of c. 6 : 4 in BS : M cells for NAD-ME species. Some C₄ acids follow a secondary decarboxylation route, which is obligatory, in the form of 'aspartate-malate', for the NADP-ME subtype, but facultative, in the form of phosphoenolpyruvate-carboxykinase (PEP-CK), for the NAD-ME subtype. The percentage for secondary decarboxylation is c. 25% and that for 3-phosphoglycerate reduction in BS cells is c. 40%; but these values vary with species. The 'pure' PEP-CK type is unrealistic because its is impossible to fulfil ATP : NADPH requirements in BS cells. The standard PEP-CK subtype requires negligible CET, and thus has the highest intrinsic quantum yields and deserves further studies in the context of improving canopy productivity.

Increasing metabolic potential: C-fixation

Due to the growing world population, crop yields must increase to meet the rising demand. Crop plants also require adaptation to optimize 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 programmes. Ribulose-1,5-bisphosphate carboxylase/oxygenase (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.