What lies between: the evolution of stomatal traits on the road to C4 photosynthesis

Midday water use efficiency in sorghum is linked to faster stomatal closure rate, lower stomatal aperture and higher stomatal density

Most studies assume midday gas exchange measurements capture the leaf's daytime performance. However, stomatal conductance (gs ) and photosynthesis (An ) fluctuate diurnally due to endogenous and environmental rhythms, which can affect intrinsic water use efficiency (iWUE). Six Sorghum lines with contrasting stomatal anatomical traits were grown in environmentally controlled conditions, and leaf gas exchange was measured three times a day. Stomatal anatomy and kinetic responses to light transients were also measured. The highest An and gs and the lowest iWUE were observed at midday for most lines. Diurnally averaged iWUE correlated positively with morning and midday iWUE and negatively with the time taken for stomata to close after transition to low light intensity (kclose ). There was significant variation among sorghum lines for kclose , and smaller kclose correlated with lower gs and higher stomatal density (SD) across the lines. In turn, gs was negatively correlated with SD and regulated by the operational stomatal aperture regardless of stomatal size. Altogether, our data suggest a common physiology to improve iWUE in sorghum related to the control of water loss without impacting photosynthesis relying on higher SD, lower stomatal aperture and faster stomatal closing in response to low light intensity.

Anatomical drivers of stomatal conductance in sorghum lines with different leaf widths grown under different temperatures

Sustaining crop productivity and resilience in water-limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross-sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance (g_s ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned g_s variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well-watered conditions, but consume more water for growth and development, which is detrimental under water stress.

The evolution of stomatal traits along the trajectory toward C4 photosynthesis

C4 photosynthesis optimizes plant carbon and water relations, allowing high photosynthetic rates with low stomatal conductance. Stomata have long been considered a part of the C4 syndrome. However, it remains unclear how stomatal traits evolved along the path from C3 to C4. Here, we examined stomata in the Flaveria genus, a model used for C4 evolutionary study. Comparative, transgenic, and semi-in vitro experiments were performed to study the molecular basis that underlies the changes of stomatal traits in C4 evolution. The evolution from C3 to C4 species is accompanied by a gradual rather than an abrupt change in stomatal traits. The initial change appears near the Type I intermediate stage. Co-evolution of the photosynthetic pathway and stomatal traits is supported. On the road to C4, stomata tend to be fewer in number but larger in size and stomatal density dominates changes in anatomical maximum stomatal conductance (gsmax). Reduction of FSTOMAGEN expression underlies decreased gsmax in Flaveria and likely occurs in other C4 lineages. Decreased gsmax contributes to the increase in intrinsic water-use efficiency in C4 evolution. This work highlights the stomatal traits in the current C4 evolutionary model. Our study provides insights into the pattern, mechanism, and role of stomatal evolution along the road toward C4.

Physiological and Proteomic Analyses of Different Ecotypes of Reed (Phragmites communis) in Adaption to Natural Drought and Salinity

Drought and salinity are the two major abiotic stresses constraining the crop yield worldwide. Both of them trigger cellular dehydration and cause osmotic stress which leads to cytosolic and vacuolar volume reduction. However, whether plants share a similar tolerance mechanism in response to these two stresses under natural conditions has seldom been comparatively reported. There are three different ecotypes of reed within a 5 km2 region in the Badanjilin desert of Northwest China. Taking the typical swamp reed (SR) as a control, we performed a comparative study on the adaption mechanisms of the two terrestrial ecotypes: dune reed (DR) and heavy salt meadow reed (HSMR) by physiological and proteomic approaches coupled with bioinformatic analysis. The results showed that HSMR and DR have evolved C4-like photosynthetic and anatomical characteristics, such as the increased bundle sheath cells (BSCs) and chloroplasts in BSCs, higher density of veins, and lower density and aperture of stomata. In addition, the thylakoid membrane fluidity also plays an important role in their higher drought and salinity tolerance capability. The proteomic results further demonstrated that HSMR and DR facilitated the regulation of proteins associated with photosynthesis and energy metabolism, lipid metabolism, transcription and translation, and stress responses to well-adapt to the drought and salinity conditions. Overall, our results demonstrated that HSMR and DR shaped a similar adaption strategy from the structural and physiological levels to the molecular scale to ensure functionality in a harsh environment.

Photosynthetic, hydraulic and biomass properties in closely related C3 and C4 species

In plants, most water is absorbed by roots and transported through vascular conduits of xylem which evaporate from leaves during photosynthesis. As photosynthesis and transport processes are interconnected, it was hypothesized that any variation in water transport demand influencing water use efficiency (WUE), such as the evolution of C4 photosynthesis, should affect xylem structure and function. Several studies have provided evidence for this hypothesis, but none has comprehensively compared photosynthetic, hydraulic and biomass allocation properties between C3 and C4 species. In this study, photosynthetic, hydraulic and biomass properties in a closely related C3 Tarenaya hassleriana and a C4 Cleome gynandra are compared. Light response curves, measured at 30°C, showed that the C4 C. gynandra had almost twice greater net assimilation rates than the C3 T. hassleriana under each increasing irradiation level. On the contrary, transpiration rates and stomatal conductance were around twice as high in the C3 , leading to approximately 3.5 times higher WUE in the C4 compared with the C3 species. The C3 showed about 3.3 times higher hydraulic conductivity, 4.3 times greater specific conductivity and 2.6 times higher leaf-specific conductivity than the C4 species. The C3 produced more vessels per xylem area and larger vessels. All of these differences resulted in different biomass properties, where the C4 produced more biomass in general and had less root to shoot ratio than the C3 species. These results are in support of our previous findings that WUE, and any changes that affect WUE, contribute to xylem evolution in plants.

The evolutionary ecology of C4 plants

C4 photosynthesis is a physiological syndrome resulting from multiple anatomical and biochemical components, which function together to increase the CO2 concentration around Rubisco and reduce photorespiration. It evolved independently multiple times and C4 plants now dominate many biomes, especially in the tropics and subtropics. The C4 syndrome comes in many flavours, with numerous phenotypic realizations of C4 physiology and diverse ecological strategies. In this work, we analyse the events that happened in a C3 context and enabled C4 physiology in the descendants, those that generated the C4 physiology, and those that happened in a C4 background and opened novel ecological niches. Throughout the manuscript, we evaluate the biochemical and physiological evidence in a phylogenetic context, which demonstrates the importance of contingency in evolutionary trajectories and shows how these constrained the realized phenotype. We then discuss the physiological innovations that allowed C4 plants to escape these constraints for two important dimensions of the ecological niche--growth rates and distribution along climatic gradients. This review shows that a comprehensive understanding of C4 plant ecology can be achieved by accounting for evolutionary processes spread over millions of years, including the ancestral condition, functional convergence via independent evolutionary trajectories, and physiological diversification.

Increasing water use efficiency along the C3 to C4 evolutionary pathway: a stomatal optimization perspective

C4 photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO2 concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C3 to C4 photosynthesis at current (400 μmol mol(-1)) and low (280 μmol mol(-1)) atmospheric CO2 conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C3 to C4 species within a unified framework. Data on C3, three categories of C3-C4 intermediates, and C4 Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO2-concentrating mechanism (or C4 pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C3 to C4 transformation. Neither the marginal water use efficiency nor the C4 pump strength changed significantly from C3 to early C3-C4 intermediate stages, but both traits significantly increased between early C3-C4 intermediates and the C4-like intermediates with an operational C4 cycle. At low CO2, net photosynthetic rates showed continuous increases from a C3 state, across the intermediates and towards C4 photosynthesis, but only C4-like intermediates and C4 species (with an operational C4 cycle) had higher water use efficiencies than C3 Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4 Flaveria to improve their photosynthesis and water use efficiency compared with C3 species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related.

Strategies of ROS regulation and antioxidant defense during transition from C₃ to C₄ photosynthesis in the genus Flaveria under PEG-induced osmotic stress

In the present study, we aimed to elucidate how strategies of reactive oxygen species (ROS) regulation and the antioxidant defense system changed during transition from C₃ to C₄ photosynthesis, by using the model genus Flaveria, which contains species belonging to different steps in C₄ evolution. For this reason, four Flaveria species that have different carboxylation mechanisms, Flaveria robusta (C₃), Flaveria anomala (C₃-C₄), Flaveria brownii (C₄-like) and Flaveria bidentis (C₄), were used. Physiological (growth, relative water content (RWC), osmotic potential), and photosynthetical parameters (stomatal conductance (g(s)), assimilation rate (A), electron transport rate (ETR)), antioxidant defense enzymes (superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductases(GR)) and their isoenzymes, non-enzymatic antioxidant contents (ascorbate, glutathione), NADPH oxidase (NOX) activity, hydrogen peroxide (H₂O₂) content and lipid peroxidation levels (TBARS) were measured comparatively under polyethylene glycol (PEG 6000) induced osmotic stress. Under non-stressed conditions, there was a correlation only between CAT (decreasing), APX and GR (both increasing) and the type of carboxylation pathways through C₃ to C₄ in Flaveria species. However, they responded differently to PEG-induced osmotic stress in regards to antioxidant defense. The greatest increase in H₂O₂ and TBARS content was observed in C₃ F. robusta, while the least substantial increase was detected in C₄-like F. brownii and C₄ F. bidentis, suggesting that oxidative stress is more effectively countered in C₄-like and C₄ species. This was achieved by a better induced enzymatic defense in F. bidentis (increased SOD, CAT, POX, and APX activity) and non-enzymatic antioxidants in F. brownii. As a response to PEG-induced oxidative stress, changes in activities of isoenzymes and also isoenzymatic patterns were observed in all Flaveria species, which might be related to ROS produced in different compartments of cells.