Transition from C3 to proto-Kranz to C3-C4 intermediate type in the genus Chenopodium (Chenopodiaceae)

Optimization of preparation conditions for Salsola laricifolia protoplasts using response surface methodology and artificial neural network modeling

BACKGROUND

Salsola laricifolia is a typical C3-C4 typical desert plant, belonging to the family Amaranthaceae. An efficient single-cell system is crucial to study the gene function of this plant. In this study, we optimized the experimental conditions by using Box-Behnken experimental design and Response Surface Methodology (RSM)-Artificial Neural Network (ANN) model based on the previous studies.

RESULTS

Among the 17 experiment groups designed by Box-Behnken experimental design, the maximum yield (1.566 × 106/100 mg) and the maximum number of viable cells (1.367 × 106/100 mg) were obtained in group 12, and the maximum viability (90.81%) was obtained in group 5. Based on these results, both the RSM and ANN models were employed for evaluating the impact of experimental factors. By RSM model, cellulase R-10 content was the most influential factor on protoplast yield, followed by macerozyme R-10 content and mannitol concentration. For protoplast viability, the macerozyme R-10 content had the highest influence, followed by cellulase R-10 content and mannitol concentration. The RSM model performed better than the ANN model in predicting yield and viability. However, the ANN model showed significant improvement in predicting the number of viable cells. After comprehensive evaluation of the protoplast yield, the viability and number of viable cells, the optimal results was predicted by ANN yield model and tested. The amount of protoplast yield was 1.550 × 106/100 mg, with viability of 90.65% and the number of viable cells of 1.405 × 106/100 mg. The corresponding conditions were 1.98% cellulase R-10, 1.00% macerozyme R-10, and 0.50 mol L-1 mannitol. Using the obtained protoplasts, the reference genes (18SrRNA, β-actin and EF1-α) were screened for expression, and transformed with PEG-mediated pBI121-SaNADP-ME2-GFP plasmid vector. There was no significant difference in the expression of β-actin and EF1-α before and after treatment, suggesting that they can be used as internal reference genes in protoplast experiments. And SaNADP-ME2 localized in chloroplasts.

CONCLUSION

The current study validated and evaluated the effectiveness and results of RSM and ANN in optimizing the conditions for protoplast preparation using S. laricifolia as materials. These two methods can be used independently of experimental materials, making them suitable for isolating protoplasts from other plant materials. The selection of the number of viable cells as an evaluation index for protoplast experiments is based on its ability to consider both protoplast yield and viability. The findings of this study provide an efficient single-cell system for future genetic experiments in S. laricifolia and can serve as a reference method for preparing protoplasts from other materials.

Brassicaceae display variation in efficiency of photorespiratory carbon-recapturing mechanisms

Carbon-concentrating mechanisms enhance the carboxylase efficiency of Rubisco by providing supra-atmospheric concentrations of CO2 in its surroundings. Beside the C4 photosynthesis pathway, carbon concentration can also be achieved by the photorespiratory glycine shuttle which requires fewer and less complex modifications. Plants displaying CO2 compensation points between 10 ppm and 40 ppm are often considered to utilize such a photorespiratory shuttle and are termed 'C3-C4 intermediates'. In the present study, we perform a physiological, biochemical, and anatomical survey of a large number of Brassicaceae species to better understand the C3-C4 intermediate phenotype, including its basic components and its plasticity. Our phylogenetic analysis suggested that C3-C4 metabolism evolved up to five times independently in the Brassicaceae. The efficiency of the pathway showed considerable variation. Centripetal accumulation of organelles in the bundle sheath was consistently observed in all C3-C4-classified taxa, indicating a crucial role for anatomical features in CO2-concentrating pathways. Leaf metabolite patterns were strongly influenced by the individual species, but accumulation of photorespiratory shuttle metabolites glycine and serine was generally observed. Analysis of phosphoenolpyruvate carboxylase activities suggested that C4-like shuttles have not evolved in the investigated Brassicaceae. Convergent evolution of the photorespiratory shuttle indicates that it represents a distinct photosynthesis type that is beneficial in some environments.

Leaf anatomy explains the strength of C4 activity within the grass species Alloteropsis semialata

C₄ photosynthesis results from anatomical and biochemical characteristics that together concentrate CO₂ around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), increasing productivity in warm conditions. This complex trait evolved through the gradual accumulation of components, and particular species possess only some of these, resulting in weak C₄ activity. The consequences of adding C₄ components have been modelled and investigated through comparative approaches, but the intraspecific dynamics responsible for strengthening the C₄ pathway remain largely unexplored. Here, we evaluate the link between anatomical variation and C₄ activity, focusing on populations of the photosynthetically diverse grass Alloteropsis semialata that fix various proportions of carbon via the C₄ cycle. The carbon isotope ratios in these populations range from values typical of C₃ to those typical of C₄ plants. This variation is statistically explained by a combination of leaf anatomical traits linked to the preponderance of bundle sheath tissue. We hypothesize that increased investment in bundle sheath boosts the strength of the intercellular C₄ pump and shifts the balance of carbon acquisition towards the C₄ cycle. Carbon isotope ratios indicating a stronger C₄ pathway are associated with warmer, drier environments, suggesting that incremental anatomical alterations can lead to the emergence of C₄ physiology during local adaptation within metapopulations.

A scheme for C4 evolution derived from a comparative analysis of the closely related C3, C3-C4 intermediate, C4-like, and C4 species in the genus Flaveria

A comparative analysis of the genus Flaveria showed a C₄ evolutionary process in which the anatomical and metabolic features of C₄ photosynthesis were gradually acquired through C₃-C₄ intermediate stages. C₄ photosynthesis has been acquired in multiple lineages of angiosperms during evolution to suppress photorespiration. Crops that perform C₄ photosynthesis exhibit high rates of CO₂ assimilation and high grain production even under high-temperature in semiarid environments; therefore, engineering C₄ photosynthesis in C₃ plants is of great importance in the application field. The genus Flaveria contains a large number of C₃, C₃-C₄ intermediate, C₄-like, and C₄ species, making it a good model genus to study the evolution of C₄ photosynthesis, and these studies indicate the direction for C₄ engineering. C₄ photosynthesis was acquired gradually through the C₃-C₄ intermediate stage. First, a two-celled C₂ cycle called C₂ photosynthesis was acquired by localizing glycine decarboxylase activity in the mitochondria of bundle sheath cells. With the development of two-cell metabolism, anatomical features also changed. Next, the replacement of the two-celled C₂ cycle by the two-celled C₄ cycle was induced by the acquisition of cell-selective expression in addition to the upregulation of enzymes in the C₄ cycle during the C₃-C₄ intermediate stage. This was supported by an increase in cyclic electron transport activity in response to an increase in the ATP/NADPH demand for metabolism. Suppression of the C₃ cycle in mesophyll cells was induced after the functional establishment of the C₄ cycle, and optimization of electron transport by suppressing the activity of photosystem II also occurred during the final phase of C₄ evolution.

Meeting in the Middle: Lessons and Opportunities from Studying C3-C4 Intermediates

The discovery of C₃-C₄ intermediate species nearly 50 years ago opened up a new avenue for studying the evolution of photosynthetic pathways. Intermediate species exhibit anatomical, biochemical, and physiological traits that range from C₃ to C₄. A key feature of C₃-C₄ intermediates that utilize C₂ photosynthesis is the improvement in photosynthetic efficiency compared with C₃ species. Although the recruitment of some core enzymes is shared across lineages, there is significant variability in gene expression patterns, consistent with models that suggest numerous evolutionary paths from C₃ to C₄ photosynthesis. Despite the many evolutionary trajectories, the recruitment of glycine decarboxylase for C₂ photosynthesis is likely required. As technologies enable high-throughput genotyping and phenotyping, the discovery of new C₃-C₄ intermediates species will enrich comparisons between evolutionary lineages. The investigation of C₃-C₄ intermediate species will enhance our understanding of photosynthetic mechanisms and evolutionary processes and will potentially aid in crop improvement.

Proto Kranz-like leaf traits and cellular ionic regulation are associated with salinity tolerance in a halophytic wild rice

Species of wild rice (Oryza spp.) possess a wide range of stress tolerance traits that can be potentially utilized in breeding climate-resilient cultivated rice cultivars (Oryza sativa) thereby aiding global food security. In this study, we conducted a greenhouse trial to evaluate the salinity tolerance of six wild rice species, one cultivated rice cultivar (IR64) and one landrace (Pokkali) using a range of electrophysiological, imaging, and whole-plant physiological techniques. Three wild species (O. latifolia, O. officinalis and O. coarctata) were found to possess superior salinity stress tolerance. The underlying mechanisms, however, were strikingly different. Na+ accumulation in leaves of O. latifolia, O. officinalis and O. coarctata were significantly higher than the tolerant landrace, Pokkali. Na+ accumulation in mesophyll cells was only observed in O. coarctata, suggesting that O. officinalis and O. latifolia avoid Na+ accumulation in mesophyll by allocating Na+ to other parts of the leaf. The finding also suggests that O. coarctata might be able to employ Na+ as osmolyte without affecting its growth. Further study of Na+ allocation in leaves will be helpful to understand the mechanisms of Na+ accumulation in these species. In addition, O. coarctata showed Proto Kranz-like leaf anatomy (enlarged bundle sheath cells and lower numbers of mesophyll cells), and higher expression of C4-related genes (e.g., NADPME, PPDK) and was a clear outlier with respect to salinity tolerance among the studied wild and cultivated Oryza species. The unique phylogenetic relationship of O. coarctata with C4 grasses suggests the potential of this species for breeding rice with high photosynthetic rate under salinity stress in the future.

Effects of growth temperature and nitrogen nutrition on expression of C3-C4 intermediate traits in Chenopodium album

Proto-Kranz plants represent an initial phase in the evolution from C₃ to C₃-C₄ intermediate to C₄ plants. The ecological and adaptive aspects of C₃-C₄ plants would provide an important clue to understand the evolution of C₃-C₄ plants. We investigated whether growth temperature and nitrogen (N) nutrition influence the expression of C₃-C₄ traits in Chenopodium album (proto-Kranz) in comparison with Chenopodium quinoa (C₃). Plants were grown during 5 weeks at 20 or 30 °C under standard or low N supply levels (referred to as 20SN, 20LN, 30SN, and 30LN). Net photosynthetic rate and leaf N content were higher in 20SN and 30SN plants than in 20LN and 30LN plants of C. album but did not differ among growth conditions in C. quinoa. The CO₂ compensation point (Γ) of C. album was lowest in 30LN plants (36 µmol mol^-1), highest in 20SN plants (51 µmol mol^-1), and intermediate in 20LN and 30SN plants, whereas Γ of C. quinoa did not differ among the growth conditions (51-52 µmol mol^-1). The anatomical structure of leaves was not considerably affected by growth conditions in either species. However, ultrastructural observations in C. album showed that the number of mitochondria per mesophyll or bundle sheath (BS) cell was lower in 20LN and 30LN plants than in 20SN and 30SN plants. Immunohistochemical observations revealed that lower accumulation level of P-protein of glycine decarboxylase (GDC-P) in mesophyll mitochondria than in BS mitochondria is the major factor causing the decrease in Γ values in C. album plants grown under low N supply and high temperature. These results suggest that high growth temperature and low N supply lead to the expression of C₃-C₄ traits (the reduction of Γ) in the proto-Kranz plants of C. album through the regulation of GDC-P expression.

Gibberellic acid induces non-Kranz anatomy with C4-like biochemical traits in the amphibious sedge Eleocharis vivipara

Gibberellic acid induces photosynthetic tissues with non-Kranz anatomy and C₄-like biochemical traits in terrestrial-form plants of Eleocharis vivipara. This suggests that the structural and biochemical traits are independently regulated. The amphibious leafless sedge, Eleocharis vivipara Link, develops culms (photosynthetic organs) with C₄-like traits and Kranz anatomy under terrestrial conditions, and C₃ traits and non-Kranz anatomy under submerged conditions. The conversion from C₃ mode to C₄-like mode in E. vivipara is reportedly mediated by abscisic acid. Here, we investigated the effects of gibberellic acid (GA) on the differentiation of anatomical and photosynthetic traits because GA is involved in heterophylly in aquatic plants. When 100 µM GA was sprayed on terrestrial plants, the newly developed culms had non-Kranz anatomy in the basal part and Kranz-like anatomy in the upper part. In the basal part, the mesophyll cells were well developed, whereas the Kranz (bundle sheath) cells were reduced and contained few chloroplasts and mitochondria. Stomatal frequency was lower in the basal part than in the upper part. Nevertheless, these tissues had abundant accumulation and high activities of C₄ photosynthetic enzymes and had C₄-like δ¹³C values, as seen in the culms of the terrestrial form. When submerged plants were grown under water containing GA-biosynthesis inhibitors (uniconazole or paclobutrazol), the new culms had Kranz anatomy. The culms developed under paclobutrazol had the C₃ pattern of cellular accumulation of photosynthetic enzymes. These data suggest that GA induces production of photosynthetic tissues with non-Kranz anatomy in terrestrial plants of E. vivipara, without concomitant expression of C₃ biochemical traits. The data also suggest that the differentiation of C₄ structural and biochemical traits is regulated independently.

Transition From Proto-Kranz-Type Photosynthesis to HCO3 - Use Photosynthesis in the Amphibious Plant Hygrophila polysperma

Hygrophila polysperma is a heterophyllous amphibious plant. The growth of H. polysperma in submerged conditions is challenging due to the low CO2 environment, increased resistance to gas diffusion, and bicarbonate ion (HCO3 -) being the dominant dissolved inorganic carbon source. The submerged leaves of H. polysperma have significantly higher rates of underwater photosynthesis compared with the terrestrial leaves. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonate (DIDS), an anion exchanger protein inhibitor, and ethoxyzolamide (EZ), an inhibitor of internal carbonic anhydrase, repressed underwater photosynthesis by the submerged leaves. These results suggested that H. polysperma acclimates to the submerged condition by using HCO3 - for photosynthesis. H. polysperma transports HCO3 - into the leaf by a DIDS-sensitive HCO3 - transporter and converted to CO2 by carbonic anhydrase. Additionally, proteome analysis revealed that submerged leaves accumulated fewer proteins associated with C4 photosynthesis compared with terrestrial leaves. This finding suggested that H. polysperma is capable of C4 and C3 photosynthesis in the terrestrial and submerged leaves, respectively. The ratio of phosphoenol pyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the submerged leaves was less than that in the terrestrial leaves. Upon anatomical observation, the terrestrial leaves exhibited a phenotype similar to the Kranz anatomy found among C4 plants; however, chloroplasts in the bundle sheath cells were not located adjacent to the vascular bundles, and the typical Kranz anatomy was absent in submerged leaves. These results suggest that H. polysperma performs proto-Kranz type photosynthesis in a terrestrial environment and shifts from a proto-Kranz type in terrestrial leaves to a HCO3 - use photosynthesis in the submerged environments.

Elevated Atmospheric CO2 Concentration Improved C4 Xero-Halophyte Kochia prostrata Physiological Performance under Saline Conditions

A significant increase in atmospheric CO₂ concentration and associated climate aridization and soil salinity are factors affecting the growth, development, productivity, and stress responses of plants. In this study, the effect of ambient (400 ppm) and elevated (800 ppm) CO₂ concentrations were evaluated on the C₄ xero-halophyte Kochia prostrata treated with moderate salinity (200 mM NaCl) and polyethylene glycol (PEG)-induced osmotic stress. Our results indicated that plants grown at elevated CO₂ concentration had different responses to osmotic stress and salinity. The synergistic effect of elevated CO₂ and osmotic stress increased proline accumulation, but elevated CO₂ did not mitigate the negative effects of osmotic stress on dark respiration intensity and photosystem II (PSII) efficiency. This indicates a stressful state, which is accompanied by a decrease in the efficiency of light reactions of photosynthesis and significant dissipative respiratory losses, thereby resulting in growth inhibition. Plants grown at elevated CO₂ concentration and salinity showed high Na⁺ and proline contents, high water-use efficiency and time required to reach the maximum P700 oxidation level (PSI), and low dark respiration. Maintaining stable water balance, the efficient functioning of cyclic transport of PSI, and the reduction of dissipation costs contributed to an increase in dry shoot biomass (2-fold, compared with salinity at 400 ppm CO₂). The obtained experimental data and PCA showed that elevated CO₂ concentration improved the physiological parameters of K. prostrata under salinity.

Russ Monson and the evolution of C4 photosynthesis

Early in his career, Russ Monson produced a series of influential eco-physiological papers that helped lay the foundation for the study of C₄ plant evolution. Among the most important was a 1984 paper with Maurice Ku and Gerry Edwards that outlined the pathway for the evolutionary bridge from C₃ to C₄ photosynthesis. This model proposed C₄ photosynthesis arose out of a shuttle that imported photorespiratory metabolites into bundle sheath (BS) cells, where glycine decarboxylase cleaved off CO₂, allowing it to accumulate and be efficiently refixed by BS Rubisco. By the mid-1990's, Monson's research focus had shifted away from C₄ plants, save for one 2003 paper on C₃ versus C₄ stomatal control with Travis Huxman, and a series of critical reviews on C₄ evolution. These reviews heavily influenced the modern synthesis of C₄ evolutionary studies, which incorporates phylogenomic understanding with physiological, molecular, and structural characterizations of trait shifts in multiple evolutionary lineages. Subsequent research supported the Monson et al. model from 1984, by showing a glycine shuttle occurs in nearly all C₃-C₄ intermediate species identified. Monson also examined the physiological controls over the ecological distribution of C₃, C₃-C₄ intermediate, and C₄ photosynthesis, building our understanding of the fitness value of the intermediate and C₄ pathway in relevant microenvironments. By establishing the foundation for discoveries that followed, Russ Monson can rightly be considered a leading pioneer contributing to the evolutionary biology of C₄ photosynthesis.

Coleataenia prionitis, a C4-like species in the Poaceae

C₄-like plants represent the penultimate stage of evolution from C₃ to C₄ plants. Although Coleataenia prionitis (formerly Panicum prionitis) has been described as a C₄ plant, its leaf anatomy and gas exchange traits suggest that it may be a C₄-like plant. Here, we reexamined the leaf structure and biochemical and physiological traits of photosynthesis in this grass. The large vascular bundles were surrounded by two layers of bundle sheath (BS): a colorless outer BS and a chloroplast-rich inner BS. Small vascular bundles, which generally had a single BS layer with various vascular structures, also occurred throughout the mesophyll together with BS cells not associated with vascular tissue. The mesophyll cells did not show a radial arrangement typical of Kranz anatomy. These features suggest that the leaf anatomy of C. prionitis is on the evolutionary pathway to a complete C₄ Kranz type. Phosphoenolpyruvate carboxylase (PEPC) and pyruvate, Pi dikinase occurred in the mesophyll and outer BS. Glycine decarboxylase was confined to the inner BS. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) accumulated in the mesophyll and both BSs. C. prionitis had biochemical traits of NADP-malic enzyme type, whereas its gas exchange traits were close to those of C₄-like intermediate plants rather than C₄ plants. A gas exchange study with a PEPC inhibitor suggested that Rubisco in the mesophyll could fix atmospheric CO₂. These data demonstrate that C. prionitis is not a true C₄ plant but should be considered as a C₄-like plant.