Guard-cell-targeted overexpression of Arabidopsis \u3ci\u3eHexokinase 1\u3c/i\u3e can improve water use efficiency in field-grown tobacco plants

Water deficit currently acts as one of the largest limiting factors for agricultural productivity worldwide. Additionally, limitation by water scarcity is projected to continue in the future with the further onset of effects of global climate change. As a result, it is critical to develop or breed for crops that have increased water use efficiency and that are more capable of coping with water scarce conditions. However, increased intrinsic water use efficiency (iWUE) typically brings a trade-off with CO2 assimilation as all gas exchange is mediated by stomata, through which CO2 enters the leaf while water vapor exits. Previously, promising results were shown using guard-cell-targeted overexpression of hexokinase to increase iWUE without incurring a penalty in photosynthetic rates or biomass production. Here, two homozygous transgenic tobacco (Nicotiana tabacum) lines expressing Arabidopsis Hexokinase 1 (AtHXK1) constitutively (35SHXK2 and 35SHXK5) and a line that had guard-cell-targeted overexpression of AtHXK1 (GCHXK2) were evaluated relative to wild type for traits related to photosynthesis and yield. In this study, iWUE was significantly higher in GCHXK2 compared with wild type without negatively impacting CO2 assimilation, although results were dependent upon leaf age and proximity of precipitation event to gas exchange measurement

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

In this study, four tobacco transformants with the overexpression of inorganic carbon transporter B (ictB) were screened for photosynthetic performance relative to 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 photosystem II (ΦPSII), and mesophyll conductance (gm). Additionally, all plants were harvested to compare differences in above-ground biomass. Overall, transformants did not perform better than 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

Characterizing natural genetic variation in photosynthetic efficiency across 44 accessions and five subpopulations of oryza sativa

With an ever-growing human population and a finite amount of arable land, the ability to produce higher yields with fewer inputs has become increasingly imperative. Improving photosynthetic efficiency at the leaf level, especially leaf CO2 uptake (Asat) is an approach to not only bettering crop yields, but also optimizing water and nutrient use efficiency. As the world’s second most cultivated crop, improving photosynthesis in Oryza sativa would have positive implications for millions worldwide who are dependent on rice for their economic livelihoods and the majority of their dietary calories. In this study, 44 accessions representing all five rice subpopulations from the larger Rice Diversity Panel 1 (RDP1) were phenotyped for photosynthetic efficiency at the International Rice Research Institute (IRRI) in Los Baños, Philippines. Phenotyping of both physiological and biochemical traits allowed for an in depth understanding of overall photosynthetic activity at the genotype and subpopulation levels. This study found that there are significant differences for individual components of photosynthetic efficiency between the subpopulations of rice. Differences between subpopulations were found at the biochemical level for carboxylation efficiency, maximum rate of carboxylation (Vcmax) maximum electron transport rate (Jmax), and triose-phosphate use (TPU) limitations – among 25 other traits. For example, subpopulation tropical japonica had the highest values for both Vcmax and Jmax, and demonstrated 11.9% and 19% higher rates for both traits respectively when compared with the lowest performing subpopulations. These differences were more pronounced at the accession level when genotypes were compared against the control IR64, confirming a wealth of natural variation that might be exploited to improve photosynthetic efficiency in cultivated rice. Examining existing natural genetic variation allows superior genotypes and traits to be identified, aiding in targeted plant improvement through mapping in the future

Assessing heritability of biochemical limitations in photosynthesis can help elucidate new targets for improvement

Understanding heritability has historically been critical to the incorporation of traits of interest into plant breeding programmes for improvement. In our previous study, we assessed the heritability of several photosynthetic traits in indica rice, including biochemical limitations of photosynthesis. Here, we discuss that even without directly evaluating sink limitation, there is still great value in this study for understanding how photosynthetic traits in rice could be incorporated into a breeding programme and which traits could be given priority. Additionally, we assert that limitation by Jmax will likely be more relevant than TPU limitation in the immediate future and could be a valuable factor for selection. However, we agree with Fabre and Dingkuhn (2022, https://doi.org/10.1111/pbr.13000) that sink limitation should be further examined within the context of plant improvement in the future

Variation in photosynthetic induction between rice accessions and its potential for improving productivity

Photosynthetic induction describes the transient increase in leaf CO2 uptake with an increase in light. During induction, efficiency is lower than at steady state. Under field conditions of fluctuating light, this lower efficiency during induction may cost > 20% of potential crop assimilation. Accelerating induction would boost photosynthetic and resource-use efficiencies. Variation between rice accessions and potential for accelerating induction was analysed by gas exchange. Induction during shade to sun transitions of 14 accessions representing five subpopulations from the 3000 Rice Genome Project Panel (3K RGP) was analysed. Differences of 109% occurred in the CO2 fixed during the first 300 s of induction, 117% in the half-time to completion of induction, and 65% in intrinsic water-use efficiency during induction, between the highest and lowest performing accessions. Induction in three accessions with contrasting responses (AUS 278, NCS 771 A and IR64-21) was compared for a range of [CO2] to analyse limitations. This showed in vivo capacity for carboxylation at Rubisco (Vc,max), and not stomata, as the primary limitation to induction, with significant differences between accessions. Variation in nonsteady-state efficiency greatly exceeded that at steady state, suggesting a new and more promising opportunity for selection of greater crop photosynthetic efficiency in this key food crop. © 2020 The Authors. New Phytologist © 2020 New Phytologist Trus

Guard-cell-targeted overexpression of Arabidopsis Hexokinase 1 can improve water use efficiency in field-grown tobacco plants

Water deficit currently acts as one of the largest limiting factors for agricultural productivity worldwide. Additionally, limitation by water scarcity is projected to continue in the future with the further onset of effects of global climate change. As a result, it is critical to develop or breed for crops that have increased water use efficiency and that are more capable of coping with water scarce conditions. However, increased intrinsic water use efficiency (iWUE) typically brings a trade-off with CO2 assimilation as all gas exchange is mediated by stomata, through which CO2 enters the leaf while water vapor exits. Previously, promising results were shown using guard-cell-targeted overexpression of hexokinase to increase iWUE without incurring a penalty in photosynthetic rates or biomass production. Here, two homozygous transgenic tobacco (Nicotiana tabacum) lines expressing Arabidopsis Hexokinase 1 (AtHXK1) constitutively (35SHXK2 and 35SHXK5) and a line that had guard-cell-targeted overexpression of AtHXK1 (GCHXK2) were evaluated relative to wild type for traits related to photosynthesis and yield. In this study, iWUE was significantly higher in GCHXK2 compared with wild type without negatively impacting CO2 assimilation, although results were dependent upon leaf age and proximity of precipitation event to gas exchange measurement

Guard-cell-targeted overexpression of Arabidopsis Hexokinase 1 can improve water use efficiency in field-grown tobacco plants.

Water deficit currently acts as one of the largest limiting factors for agricultural productivity worldwide. Additionally, limitation by water scarcity is projected to continue in the future with the further onset of effects of global climate change. As a result, it is critical to develop or breed for crops that have increased water use efficiency and that are more capable of coping with water scarce conditions. However, increased intrinsic water use efficiency (iWUE) typically brings a trade-off with CO2 assimilation as all gas exchange is mediated by stomata, through which CO2 enters the leaf while water vapor exits. Previously, promising results were shown using guard-cell-targeted overexpression of hexokinase to increase iWUE without incurring a penalty in photosynthetic rates or biomass production. Here, two homozygous transgenic tobacco (Nicotiana tabacum) lines expressing Arabidopsis Hexokinase 1 (AtHXK1) constitutively (35SHXK2 and 35SHXK5) and a line that had guard-cell-targeted overexpression of AtHXK1 (GCHXK2) were evaluated relative to wild type for traits related to photosynthesis and yield. In this study, iWUE was significantly higher in GCHXK2 compared with wild type without negatively impacting CO2 assimilation, although results were dependent upon leaf age and proximity of precipitation event to gas exchange measurement.This work was supported by the project Realizing Increased Photosynthetic Efficiency (RIPE), that is funded by the Bill & Melinda Gates Foundation, Foundation for Food and Agriculture Research (FFAR), and the UK Foreign Commonwealth and Development Office under grant number OPP1172157