C2 photosynthesis:a promising route towards crop improvement?

C2 photosynthesis is a carbon concentrating mechanism that can increase net CO2 assimilation by capturing, concentrating, and re-assimilating CO2 released by photorespiration. Empirical and modelling studies indicate that C2 plants assimilate more carbon than C3 plants under high temperature, bright light, and low CO2 conditions. I argue that engineering C2 photosynthesis into C3 crops is a promising approach to improve photosynthetic performance under these, and temporally heterogeneous, environments and review the modifications that may re-create a C2 phenotype in C3 plants. While a C2 engineering program would encounter many of the same challenges faced by C4 engineering programs, the simpler leaf anatomical requirements make C2 engineering a feasible approach to improve crops in the medium term

Different water relations between flowering and leaf periods:a case study in flower-before-leaf-emergence Magnolia species

The differing water relations between flowers and leaves on a plant reflect the lack of co-ordination between reproductive and vegetative organs during the evolution of angiosperm species. The amount of water that flowers consume has been reported to vary across species, and compared with studies of leaves, accurate measurements of flower water relations at the branch level are lacking. Further, the mechanisms by which flowers regulate their hydraulic function and structure to maintain water balance remain unclear. To explore the ecophysiological basis underpinning the differences between flowers and leaves, we measured hydraulic and morphological traits and monitored sap flow in flowers and leaves from the same branches of two Magnoliaceae species that flower before leaf emergence (Magnolia denudata Desr. and Magnolia soulangeana Soul.-Bod.). Sap flux density (J(S)) of flowers was 22% and 55% of that predicted for leaves in M. denudata and M. soulangeana respectively. J(S) of flowers commenced before predawn and ceased early in the afternoon, reflecting their night-time flowering pattern and a dramatic decrease of J(S) with increasing vapour pressure deficit (D) under the high light of midday. Relative to leaves, tepals were thicker and more hydrated, and had bigger but scarcer stomata, leading to lower stomatal conductance (g(s)) and transpiration rate (E), less negative water potential ((tepal)) and lower hydraulic conductance. This study revealed different hydraulic patterns in the flowers and leaves of the two Magnolia species. Although flowers consumed less than half the water that leaves did, they used different strategies to maintain sufficiently high to sustain hydraulic safety. Magnolia flowers retained more hydrated tepals by exhibiting less water loss than leaves via lower hydraulic conductance. In contrast, Magnolia leaves maintained high transpiration rates through efficient stomatal responses to environmental changes compared with flowers

Bundle sheath chloroplast volume can house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling

C4 leaves confine Rubisco to bundle sheath cells. Thus, the size of bundle sheath compartments and the total volume of chloroplasts within them limit the space available for Rubisco. Rubisco activity limits photosynthesis at low temperatures. C3 plants counter this limitation by increasing leaf Rubisco content, yet few C4 species do the same. Because C3 plants usually outperform C4 plants in chilling environments, it has been suggested that there is insufficient chloroplast volume available in the bundle sheath of C4 leaves to allow such an increase in Rubisco at low temperatures. We investigated this potential limitation by measuring bundle sheath and mesophyll compartment volumes and chloroplast contents, as well as leaf thickness and inter-veinal distance, in three C4 Andropogoneae grasses: two crops (Zea mays and Saccharum officinarum) and a wild, chilling-tolerant grass (Miscanthus × giganteus). A wild C4 Paniceae grass (Alloteropsis semialata) was also included. Despite significant structural differences between species, there was no evidence of increased bundle sheath chloroplast volume per leaf area available to the chilling-tolerant species, relative to the chilling-sensitive ones. Maximal theoretical photosynthetic capacity of the leaf far exceeded the photosynthetic rates achieved even at low temperatures. C4 bundle sheath cells therefore have the chloroplast volume to house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling

Ecophysiological responses of two closely related Magnoliaceae genera to seasonal changes in subtropical China

Plants use a variety of hydraulic strategies to adapt to seasonal drought that differ by species and environmental conditions. The early-diverging Magnoliaceae family includes two closely related genera with contrasting leaf habits, Yulania (deciduous) andMichelia (evergreen), which naturally inhabit temperate and tropical regions, respectively. Here, we evaluate the hydraulic strategy of species from both genera that have beenex situ conserved in a subtropical region to determine how they respond to the novel cool-dry season climatic pattern. We measured ecophysiological traits in fiveMichelia and fiveYulania species conserved in the South China Botanical Garden in both wet and dry season conditions and monitored the whole-year sap flow for four of these species. We found that Magnoliaceae species that have beenex situ conserved in a subtropical climate did not suffer from excessive water stress due to the mild drought conditions of the dry season and the ecophysiological adjustments the species made to avoid this stress, which differed by leaf habit. Specifically, deciduous species completely shed their leaves during the dry season, while evergreen species decreased their turgor loss points, dry mass based photosynthetic rates, stomatal conductance and specific leaf areas (SLAs) compared to wet season measurements. In comparing the two distinct leaf habits during the wet season, the leathery-leaved evergreen species had higher leaf hydraulic conductance and leaf to sapwood area ratios than the papery-leaved deciduous species, while the deciduous species had greater hydraulic conductivity calculated on both a stem and leaf area basis, dry mass based photosynthetic rates, leaf nutrients, SLAs and stomatal sizes than the evergreen species. Interestingly, species from both genera maintained similar sap flow in the wet season. Both photosynthetically active radiation and vapour pressure deficit affected the diurnal patterns of sap flow in the wet season, while only vapour pressure deficit played a dominant role in the dry season. This study reveals contrasting hydraulic strategies inYulania andMichelia species under subtropical seasonal conditions, and suggests that these ecophysiological adjustments might be affected more by leaf habit than seasonality, thus reflecting the divergent evolution of the two closely related genera. Furthermore, we show that Magnoliaceae species that areex situ conserved in a subtropical climate are hydraulically sound, a finding that will inform future conservation efforts of this ancient family under the threat of climatic change

Anatomical constraints to C4 evolution: light harvesting capacity in the bundle sheath.

In C4 photosynthesis CO2 assimilation and reduction are typically coordinated across mesophyll (M) and bundle sheath (BS) cells, respectively. This system consequently requires sufficient light to reach BS to generate enough ATP to allow ribulose-1,5-bisphosphate (RuBP) regeneration in BS. Leaf anatomy influences BS light penetration and therefore constrains C4 cycle functionality. Using an absorption scattering model (coded in Excel, and freely downloadable) we simulate light penetration profiles and rates of ATP production in BS across the C3 , C3 -C4 and C4 anatomical continua. We present a trade-off for light absorption between BS pigment concentration and space allocation. C3 BS anatomy limits light absorption and benefits little from high pigment concentrations. Unpigmented BS extensions increase BS light penetration. C4 and C3 -C4 anatomies have the potential to generate sufficient ATP in the BS, whereas typical C3 anatomy does not, except some C3 taxa closely related to C4 groups. Insufficient volume of BS, relative to M, will hamper a C4 cycle via insufficient BS light absorption. Thus, BS ATP production and RuBP regeneration, coupled with increased BS investments, allow greater operational plasticity. We propose that larger BS in C3 lineages may be co-opted for C3 -C4 and C4 biochemistry requirements

Cell density and airspace patterning in the leaf can be manipulated to increase leaf photosynthetic capacity

The pattern of cell division, growth and separation during leaf development determines the pattern and volume of airspace in a leaf. The resulting balance of cellular material and airspace is expected to significantly influence the primary function of the leaf, photosynthesis, and yet the manner and degree to which cell division patterns affect airspace networks and photosynthesis remains largely unexplored. In this paper we investigate the relationship of cell size and patterning, airspace and photosynthesis by promoting and repressing the expression of cell cycle genes in the leaf mesophyll. Using microCT imaging to quantify leaf cellular architecture and fluorescence/gas exchange analysis to measure leaf function, we show that increased cell density in the mesophyll of Arabidopsis can be used to increase leaf photosynthetic capacity. Our analysis suggests that this occurs both by increasing tissue density (decreasing the relative volume of airspace) and by altering the pattern of airspace distribution within the leaf. Our results indicate that cell division patterns influence the photosynthetic performance of a leaf, and that it is possible to engineer improved photosynthesis via this approach

Contrasted histories of organelle and nuclear genomes underlying physiological diversification in a grass species: Intraspecific dispersal of C4 physiology

C 4 photosynthesis evolved multiple times independently in angiosperms, but most origins are relatively old so that the early events linked to photosynthetic diversification are blurred. The grass Alloteropsis semialata is an exception, as this species encompasses C 4 and non-C 4 populations. Using phylogenomics and population genomics, we infer the history of dispersal and secondary gene flow before, during and after photosynthetic divergence in A. semialata. We further analyse the genome composition of individuals with varied ploidy levels to establish the origins of polyploids in this species. Detailed organelle phylogenies indicate limited seed dispersal within the mountainous region of origin and the emergence of a C 4 lineage after dispersal to warmer areas of lower elevation. Nuclear genome analyses highlight repeated secondary gene flow. In particular, the nuclear genome associated with the C 4 phenotype was swept into a distantly related maternal lineage probably via unidirectional pollen flow. Multiple intraspecific allopolyploidy events mediated additional secondary genetic exchanges between photosynthetic types. Overall, our results show that limited dispersal and isolation allowed lineage divergence, with photosynthetic innovation happening after migration to new environments, and pollen-mediated gene flow led to the rapid spread of the derived C 4 physiology away from its region of origin.This study was funded by the European Research Council (grant no. ERC-2014-STG-638333), the Royal Society (grant no. RGF\EA\181050) and has benefited from ‘Investissements d'Avenir' grants managed by the Agence Nationale de la Recherche (CEBA, ref. ANR-10-LABX-25-01 and TULIP, ref. ANR-10-LABX-41). Edinburgh Genomics, which contributed to the sequencing, is partly supported through core grants from the NERC (grant no. R8/H10/ 56), MRC (grant no. MR/K001744/1) and BBSRC (grant no. BB/ J004243/1). P.A.C. is funded by a Royal Society University Research Fellowship (grant no. URF\R\180022).Abstract 1. Introduction 2. Materials and methods (a) Sampling, sequencing and data filtering (b) Genome sizing and carbon isotope analyses (c) Assembly of organelle genomes and molecular dating (d) Phylogenetic analyses of the nuclear genome (e) Genetic structure (f) Genome composition 3. Results (a) Genome sizes (b) Time-calibrated organelle phylogenies (c) Nuclear phylogeny (d) Population structure and genome composition 4. Discussion (a) Limited seed dispersal in the region of origin (b) Widespread pollen flow and sweep of the C4 nuclear genome (c) Recurrent hybridization and polyploidization 5. Concluding remarks Data accessibility Authors' contributions Competing interests Funding Acknowledgements Footnote

Stomatal Function Requires Pectin De-methyl-esterification of the Guard Cell Wall

Stomatal opening and closure depends on changes in turgor pressure acting within guard cells to alter cell shape. The extent of these shape changes is limited by the mechanical properties of the cells, which will be largely dependent on the structure of the cell walls. Although it has long been observed that guard cells are anisotropic due to differential thickening and the orientation of cellulose microfibrils, our understanding of the composition of the cell wall that allows them to undergo repeated swelling and deflation remains surprisingly poor. Here, we show that the walls of guard cells are rich in unesterified pectins. We identify a pectin methylesterase gene, PME6, which is highly expressed in guard cells and required for stomatal function. pme6-1 mutant guard cells have walls enriched in methyl-esterified pectin and show a decreased dynamic range in response to triggers of stomatal opening/closure, including elevated osmoticum, suggesting that abrogation of stomatal function reflects a mechanical change in the guard cell wall. Altered stomatal function leads to increased conductance and evaporative cooling, as well as decreased plant growth. The growth defect of the pme6-1 mutant is rescued by maintaining the plants in elevated CO2, substantiating gas exchange analyses, indicating that the mutant stomata can bestow an improved assimilation rate. Restoration of PME6 rescues guard cell wall pectin methyl-esterification status, stomatal function, and plant growth. Our results establish a link between gene expression in guard cells and their cell wall properties, with a corresponding effect on stomatal function and plant physiology

Bundle sheath chloroplast volume can house sufficient Rubisco to avoid limiting C4 photosynthesis during chilling

C4 leaves confine Rubisco to bundle-sheath cells. Thus, the size of bundle-sheath compartments, and total volume of chloroplasts within them, limits space available for Rubisco. Rubisco activity limits photosynthesis at low temperatures. C3 plants counter this limitation by increasing leaf Rubisco content, yet few C4 species do the same. Because C3 plants usually outperform C4 plants in chilling environments, it has been suggested that there is insufficient chloroplast volume available in the bundle-sheath of C4 leaves to allow such an increase in Rubisco at low temperatures. We investigated this potential limitation by measuring bundle-sheath and mesophyll compartment volumes and chloroplast contents, as well as leaf thickness and inter-veinal distance in three C4Andropogoneae grasses: two crops (Zea mays, Saccharum officinarum) and a wild, chilling-tolerant grass (Miscanthus x giganteus). A wild C4Paniceae grass (Alloteropsis semialata) was also included. Despite significant structural differences between species, there was no evidence of increased bundle-sheath chloroplast volume per leaf area available to the chilling-tolerant species, relative to the chilling-sensitive ones. Maximal theoretical photosynthetic capacity of the leaf far exceeded the photosynthetic rates achieved even at low temperatures. C4 bundle-sheath cells therefore house more than enough chloroplasts to avoid Rubisco limitation to photosynthesis during chilling

The NANOGrav Nine-year Data Set:Mass and Geometric Measurements of Binary Millisecond Pulsars

We analyze 24 binary radio pulsars in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) nine-year data set. We make 14 significant measurements of the Shapiro delay, including new detections in four pulsar-binary systems (PSRs J0613−0200, J2017+0603, J2302+4442, and J2317+1439), and derive estimates of the binary-component masses and orbital inclination for these MSP-binary systems. We find a wide range of binary pulsar masses, with values as low as ${m}_{{\rm{p}}}={1.18}_{-0.09}^{+0.10}\,{M}_{\odot }$ for PSR J1918−0642 and as high as ${m}_{{\rm{p}}}={1.928}_{-0.017}^{+0.017}\,{M}_{\odot }$ for PSR J1614−2230 (both 68.3% credibility). We make an improved measurement of the Shapiro timing delay in the PSR J1918−0642 and J2043+1711 systems, measuring the pulsar mass in the latter system to be ${m}_{{\rm{p}}}={1.41}_{-0.18}^{+0.21}\,{M}_{\odot }$ (68.3% credibility) for the first time. We measure secular variations of one or more orbital elements in many systems, and use these measurements to further constrain our estimates of the pulsar and companion masses whenever possible. In particular, we used the observed Shapiro delay and periastron advance due to relativistic gravity in the PSR J1903+0327 system to derive a pulsar mass of ${m}_{{\rm{p}}}={1.65}_{-0.02}^{+0.02}\,{M}_{\odot }$ (68.3% credibility). We discuss the implications that our mass measurements have on the overall neutron-star mass distribution, and on the "mass/orbital-period" correlation due to extended mass transfer