Identification of Hub Genes and Physiological Effects of Overexpressing the Photosynthesis-Related Gene Soly720 in Tomato under High-CO2 Conditions

Changes in the atmospheric CO2 concentration influence plant growth and development by affecting the morphological structure and photosynthetic performance. Despite evidence for the macro-effects of elevated CO2 concentrations on plant morphology and yield in tomato, the gene regulatory network and key genes related to cross-regulation have not been reported. To identify the hub genes and metabolic pathways involved in the response of tomato to CO2 enrichment, weighted gene co-expression network analysis was conducted using gene expression profiles obtained by RNA sequencing. The role of the photosynthesis-related gene Soly720 (Solyc01g007720) in CO2-enriched tomato plants was explored. Tomato plants responded to CO2 enrichment primarily through RNA-related pathways and the metabolism of amino acids, fatty acids, and carbohydrates. The hub genes in co-expression networks were associated with plant growth and development, including cellular components and photosynthesis. Compared to wild-type plants, transgenic plants overexpressing the Soly720 gene exhibited 13.4%, 5.5%, 8.9%, and 4.1% increases in plant height, stem diameter, leaf length, and leaf width, respectively, under high-CO2 conditions. The morphological improvements in transgenic plants were accompanied by enhancement of photosynthetic performance in terms of chlorophyll contents, photosynthetic characteristics, and key enzyme activities. This study elucidates the response network of tomato to CO2 enrichment and demonstrates the regulatory role of Soly720 in photosynthesis under high-CO2 conditions.


Introduction
The carbon cycle between Earth's atmosphere, land surface, and water constitutes a dynamically balanced global carbon exchange system.Terrestrial plants have been regarded as a fixation and absorption pool of carbon dioxide (CO 2 ).For crop plants, the elevation of CO 2 concentration in the environment is equivalent to gas fertilizer application, which can enhance plant biomass accumulation and yield formation [1].When atmospheric CO 2 concentrations are progressively exceeded, the CO 2 absorption capacity of plants is compromised and limited.In this case, it is crucial to understand the adaptability of plants to rapidly changing climatic conditions.Therefore, substantial research has been conducted to characterize the response of CO 2 -enriched plants and the effects of CO 2 enrichment on plant growth, morphology, and physiology [2,3].
Tomato (Solanum lycopersicum L., Solanaceae) is one of the most popular vegetable crops cultivated in greenhouses, especially across north and south China.Tomato fruit is rich in nutrients (e.g., vitamin C, sugars, proteins, organic acids, minerals) and bioactive substances (e.g., carotenoids, flavonoids, phenolic acids), which can impart health benefits to the human body [4,5].Tomato provides a model plant for the study of physiological and biochemical mechanisms in Solanaceae vegetables.The completion of tomato genome sequencing and accumulation of gene annotation information have offered opportunities for exploring photosynthesis and carbohydrate metabolism in tomato and other Solanaceae vegetables [6].
It has been found that applying CO 2 to tomato plants in the low-temperature season leads to increased plant growth rate, fruit setting rate, and yield [7].At high temperatures, elevated CO 2 concentrations contribute to plant photosynthesis by reducing the functional constraints on energy flux, electron transport, and redox homeostasis in tomato leaves [8].As CO 2 is the direct substrate of plant photosynthesis, changes in the CO 2 concentration inevitably affect the photosynthetic performance of crops, which in turn influences their vegetative and reproductive growth.CO 2 enrichment plays a positive role in leaf net photosynthesis, plant biomass accumulation, and crop quality improvement.At the flowering stage, dry matter accumulation in tomato roots, stems, and leaves is promoted under CO 2 enrichment [9].CO 2 enrichment also affects the accumulation and distribution of carbohydrates in tomato and other plants [10,11], with enhanced activity of photosynthetic enzymes, such as ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and sedoheptulose-1,7-bisphosphatase (SBPase) [12,13].
In addition to direct response, there are other pathways through which cross-regulation and -response occur in plants under high-CO 2 conditions.For example, the melatonin content in tomato leaves is increased in a high-CO 2 environment, and endogenous melatonin levels affect the accumulation of sugars and starches [14].Abscisic acid-deficient mutant plants show lower leaf stomatal conductance in a high-CO 2 environment than in normal conditions [15].At the molecular level, high CO 2 concentrations affect gene expression in grape, maize, and rice [16][17][18].Co-overexpression of two chloroplast glyceraldehyde-3phosphate dehydrogenase (GAPDH) genes, GAPA and GAPB, in rice enhances the activity of GAPDH, leading to increased CO 2 assimilation rate in transgenic plants under high-CO 2 conditions [19].Silencing of the chloroplast vesiculation gene in rice enables plants to maintain photorespiration under elevated CO 2 conditions [20].Therefore, chloroplast-related genes play vital roles in regulating plant response to CO 2 enrichment.
In our previous study, high expression of the chloroplast-related gene Soly720 (Solyc01g007720, Log 2 FC = 4.24) was observed in tomato plants under high-CO 2 conditions [21].However, little is known about the regulatory role of Soly720 and whether there are other cross-regulatory networks and pathways in CO 2 -enriched tomato plants.In this study, the cross-response mechanisms of tomato to CO 2 enrichment were explored by constructing the weighted gene co-expression networks.Through the analysis of gene expression patterns in modules with high relevance in the co-expression network, key genes and metabolic pathways were identified in response to CO 2 enrichment in tomato.Furthermore, through a comprehensive analysis of transgenic plants, the effects of overexpressing Soly720 on tomato plants were revealed based on morphological, photosynthetic, and enzymatical observations.The molecular mechanisms of tomato plant response to CO 2 enrichment were deciphered, which could be helpful for breeding new crop varieties suitable for cultivation in high-CO 2 environments.

Construction of Weighted Gene Co-Expression Networks under CO 2 Enrichment
To investigate the gene regulatory network of tomato under high-CO 2 conditions, weighted gene co-expression network analysis (WGCNA) was conducted to identify modules associated with CO 2 enrichment.The input genes were divided into seven modules based on their expression patterns and labeled with different colors: MEbrown, MEblack, MEturquoise, MEblue, MEyellow, MEgreen, and MEred (Figure 1).Among them, MEblack (normal control: r = 0.80; CO 2 enrichment: r = −0.80)and MEturquoise (normal control: r = 0.99; CO 2 enrichment: r = −0.99)showed significant positive or negative correlation with CO 2 concentration.This suggests that members of the MEblack and MEturquoise modules were well representative of genes in response to CO 2 enrichment (Figure 2).

Comprehensive Analysis of Hub Genes in Co-Expression Networks
To further investigate gene functions in the networks, functional annotation and functional enrichment analysis of the hub genes were performed based on the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases.In the MEblack module (Figure 4A, Supplemental Table S1), two genes were enriched in endocytosis (ko04144: Soly100860, Soly84690), steroid biosynthesis (ko00100: Soly110290, Soly111830), pyrimidine metabolism (ko00240: Soly89970, Soly47630), and plant-pathogen interaction (ko04626: Soly104530, Soly112250).A total of four genes were enriched in RNA-related pathways: Soly98180 and Soly6470 in RNA transport (ko03013), and Soly73650 and Soly47360 in mRNA surveillance (ko03015).
There were four genes associated with fatty acid metabolism in the MEturquoise module: Soly5750 and Soly6540 in linoleic acid and alpha-linolenic acid metabolism (ko00591, ko00592); Soly90410 in fatty acid metabolism (ko01212) and fatty acid elongation (ko00062); and Soly10860 in sphingolipid metabolism (ko00600).Additionally, three genes related to carbon metabolism were identified: Soly104000 and Soly108800 in carbon metabolism (ko01200); Soly112290 in porphyrin and chlorophyll metabolism (ko00860); and Soly104000 in one carbon pool by folate (ko00670).Furthermore, there were two genes related to sugar metabolism: Soly90410 in pentose and glucuronate interconversions (ko00040) as well as fructose and mannose metabolism (ko00051); and Soly74030 in starch and sucrose metabolism (ko00500) (Table S3).

Generation of Transgenic Plants
Based on previous research and gene functional annotations, the Soly720 gene was selected to study its function in plant growth and development of tomato.The full-length coding sequence of Soly720 was inserted into the pCAMBIA1305.1 vector containing the 35S promoter of cauliflower mosaic virus to construct a 35S::Soly720 overexpression vector.Transgenic plants overexpressing Soly720 were obtained using Agrobacterium-mediated transformation (Figure 5A).Quantitative real-time PCR (qPCR) analysis revealed that compared to wild-type (WT) plants, the gene expression level of Soly720 in transgenic plants was 1.44 and 1.71 times higher under natural and CO 2 -enriched conditions, respectively (Figure 5B).

Effects of Constitutive Overexpression of Soly720 on Plant Growth
To assess the effect of overexpressing the Soly720 gene on plant growth, the morphological parameters of transgenic tomato were measured.Under natural and CO 2 -enriched conditions, plant height, stem diameter, leaf length, and leaf width of T1-generation transgenic plants were all higher than those of WT (Figure 6).For example, under natural conditions, the average plant height of transgenic plants increased by 35.5%, 30.0%, 29.6%, 8.0%, 11.4%, and 5.9% compared to that of WT plants at days 5, 10, 15, 20, 25, and 30, respectively.Less prominent increases occurred in the corresponding average plant height of transgenic plants in the high-CO 2 environment (13.4%, 15.5%, 20.0%, 5.0%, 1.2%, and 2.8%, respectively; Figure 6A).
Compared to that of WT plants, the average stem diameter of transgenic plants increased by 5.5-12.0%under high-CO 2 conditions during the experimental period (Figure 6B).The average leaf length of transgenic plants also increased by 4.2-0.4% under natural conditions, with a further increase of 8.9-0.8%under high-CO 2 conditions (Figure 6C).A similar pattern was observed in the average leaf width in transgenic plants, which increased by 5.0-1.6%under natural conditions and by 4.1-1.4% under high-CO 2 conditions (Figure 6D).

Effects of Constitutive Overexpression of Soly720 Gene on Photosynthetic Characteristics
The effects of overexpressing the Soly720 on transgenic tomato plants were further investigated by measuring leaf photosynthetic parameters.Under high-CO 2 conditions, the net photosynthetic rate in transgenic plants was 1.26 times higher than that of WT plants (p < 0.05; Figure 7A).Compared to WT plants, the intercellular CO 2 concentration in transgenic plants was 2.12 and 1.18 times higher under natural and high-CO 2 conditions, respectively (p < 0.05; Figure 7B).In contrast, the transpiration rate (Figure 7C) and stomatal conductance (Figure 7D) in transgenic plants were significantly lower than those in WT plants, by 36% and 47% under natural conditions and by 31% and 21% under CO 2 -rich conditions, respectively.6B).The average leaf length of transgenic plants also increased by 4.2-0.4% under natural conditions, with a further increase of 8.9-0.8%under high-CO2 conditions (Figure 6C).A similar pattern was observed in the average leaf width in transgenic plants, which increased by 5.0-1.6%under natural conditions and by 4.1-1.4% under high-CO2 conditions (Figure 6D).

Effects of Constitutive Overexpression of Soly720 Gene on Photosynthetic Characteristics
The effects of overexpressing the Soly720 on transgenic tomato plants were further investigated by measuring leaf photosynthetic parameters.Under high-CO2 conditions, the net photosynthetic rate in transgenic plants was 1.26 times higher than that of WT plants (p < 0.05; Figure 7A).Compared to WT plants, the intercellular CO2 concentration in transgenic plants was 2.12 and 1.18 times higher under natural and high-CO2 conditions, respectively (p < 0.05; Figure 7B).In contrast, the transpiration rate (Figure 7C) and stomatal conductance (Figure 7D) in transgenic plants were significantly lower than those in WT plants, by 36% and 47% under natural conditions and by 31% and 21% under CO2rich conditions, respectively.The activities of key photosynthetic enzymes were also measured in tomato plants.The RuBisCO activity followed the following order: transgenic plants under CO 2 -enriched conditions> WT plants under CO 2 -enriched conditions> transgenic plants under natural conditions> WT plants under natural conditions (Figure 8A).The fructose-1, 6-bisphosphate aldolase (FBAase) and SBPase activities were significantly higher in transgenic plants than in WT plants, but no response to CO 2 enrichment was observed (Figure 8B,D).The transketolase (TK) activity was also enhanced in transgenic plants compared with WT plants and responded positively to CO 2 enrichment (Figure 8C).Moreover, the chlorophyll a, chlorophyll b, and total chlorophyll contents in tomato leaves were measured, which mirrored the pattern of TK activity (Figure 9).conditions> WT plants under natural conditions (Figure 8A).The fructose-1, 6-bisphosphate aldolase (FBAase) and SBPase activities were significantly higher in transgenic plants than in WT plants, but no response to CO2 enrichment was observed (Figure 8B,D).The transketolase (TK) activity was also enhanced in transgenic plants compared with WT plants and responded positively to CO2 enrichment (Figure 8C).Moreover, the chlorophyll a, chlorophyll b, and total chlorophyll contents in tomato leaves were measured, which mirrored the pattern of TK activity (Figure 9).

Discussion
The elevation of atmospheric CO 2 concentration has a fertilization effect on plant growth and development by affecting photosynthesis.Thus, high CO 2 concentration is conducive to increasing plant biomass and the yield of crop product organs in greenhouses [22][23][24][25].However, carbon uptake by plants is constrained as global temperatures and atmospheric CO 2 concentrations continue to rise.The complicated strategies of CO 2enriched plants to utilize carbon sources have not been sufficiently studied.Here, we constructed gene co-expression networks in tomato plants under high-CO 2 conditions using WGCNA.Then, we explored the hub genes and metabolic response pathways in the gene regulatory network of tomato in response to CO 2 enrichment.In two highly correlated network modules (MEblack and MEturquoise), a number of genes were enriched in multiple RNA-related and amino acid metabolism pathways.This provides evidence that CO 2 enrichment accelerates protein renewal in tomato plants.
It has been reported that CO 2 -enriched plants have increased membrane stability under high-temperature stress [26].Fatty acids are the main chemical components of the phospholipid bilayer that maintains the stability of cell membranes.In the co-expression network of the MEturquoise module, a few genes were found enriched in fatty acid metabolism pathways.This indicates that tomato plants regulate cell membrane stability by modulating fatty acid metabolism under CO 2 enrichment.Furthermore, the concentration of CO 2 in the ambient environment has a direct impact on the levels of glucose, fructose, and overall reducing carbohydrates in plants [27].Genes related to carbohydrate metabolism pathways were also identified in the co-expression network of the MEturquoise module, indicating that CO 2 enrichment affects carbohydrate metabolism in tomato.Carbohydrate plays multiple roles in plants by regulating fruit quality and plant resistance.
To demonstrate the regulatory role of the chloroplast cell component-related gene Soly720 in CO 2 -enriched tomato, we compared the morphological parameters of Soly720transgenic and WT plants.Compared to WT plants, transgenic plants showed higher plant height, leaf length, leaf width, and stem diameter (by 2.8%, 0.8%, 6.4%, and 11.7%, respectively) under CO 2 enrichment over the 30-day experimental period.The results suggest that overexpression of the Soly720 gene promotes plant growth from a morphological perspective and enhances the growth performance of tomato under CO 2 enrichment.Therefore, Soly720 is likely to be a key gene involved in the regulation of plant growth and response mechanisms to CO 2 enrichment.
Chlorophylls and carotenoids are part of the light harvesting system that captures light energy and drives photosynthetic electron transport [28].Therefore, we measured the chlorophyll content and net photosynthetic rate in Soly720-transgenic tomato plants to better understand the potential gene function.The chlorophyll content of transgenic plants was higher than that of WT plants and increased in response to CO 2 enrichment.Transgenic plants also exhibited a higher net photosynthetic rate under CO 2 -enriched conditions compared to the normal control.These findings indicate that the Soly720 gene is directly involved in the regulation of photosynthesis and plays a greater role in the high-CO 2 environment.
Leaf stomata are vital for plant communication with the external environment and serve as the primary passages for CO 2 and water movement in and out of plants.Liang et al. [15] found that the stomatal conductance of plants in a high-CO 2 environment was lower than that of plants grown under normal conditions.Moreover, drought-stressed plants in the high-CO 2 environment maintained better water status and higher water use efficiency at both leaf and whole plant levels.In this study, Soly720-transgenic tomato plants exhibited a reduction in stomatal conductance compared to WT plants under both normal and CO 2 -enriched conditions.This provides molecular evidence that the Soly720 gene plays a role in regulating stomatal conductance among tomato plants.According to Liang et al. [15], stomatal conductance is regulated by abscisic acid and ethylene biosynthesis pathways in CO 2 -enriched plants.It would be interesting to verify whether similar hormonal regulatory pathways exit in Soly720-transgenic tomato plants.
Elevated concentrations of CO 2 mainly result in the acceleration of RuBisCO carboxylation in C3 plants while inhibiting oxygenase reaction.This process ultimately enhances net photosynthesis in plants [29].SBPase, a key enzyme in the Calvin cycle, is mainly responsible for the regeneration of CO 2 molecular receptor RuBP.Thus, SBPase plays a vital role in the circulation of carbon sources in the Calvin cycle [30].FBAase and TK, which are also essential enzymes in the Calvin cycle, affect CO 2 assimilation in plant leaves and the flow of carbon [31,32].The enzyme activities of RuBisCO, SBPase, TK, and FBAase were all enhanced in transgenic tomato plants than in WT plants under different CO 2 conditions.Accordingly, the Soly720 gene participates in the Calvin cycle and the regulation of carbon flow in tomato.The findings indicate that Soly720 is of significance in controlling photosynthesis in tomato plants under high-CO 2 conditions.In addition, chlorophyll-related genes have been reported to play important roles in plant response to environmental stresses [33].In tomato, Cai et al. [32] found that the expression level of the chlorophyll aldolase gene, SlFBA4, changes significantly under low-temperature stress and that MDA content is reduced in SlFBA4 gene overexpression transgenic tomato plants.Chloroplasts are essential for their function in plants, and by regulating chloroplast-related genes they also help to repair damaged photosystems under conditions of high temperature and intense light stress [34].Therefore, the study of chloroplast genes could help to improve the adaption of tomato plants in response to environmental changes.At present, the relevant role of the Soly720 gene has not been reported, which will need further experiments to explore.

WGCNA and Identification of Hub Genes
WGCNA was performed using the R package v1.68 [35] to construct the gene coexpression network of tomato in response to CO 2 enrichment.The input data were gene expression profiles obtained by RNA sequencing in a pervious study [21].Network construction and module identification were carried out using the pickSoftThreshold function with default parameters.The co-expression network was constructed using the top 100 hub genes in each selected network module and visualized using Cytoscape v3.9.1 [36].GO and KEGG enrichment analyses were performed as previously described [37].

Plasmid Construction and Plant Transformation
The full-length coding sequence of the Soly720 gene (Solyc01g007720) was amplified from tomato leaf cDNA by PCR with appropriate primers (forward primer: 5 -ATGGATAGTTCAAT GTGCTCAT-3 and reverse primer: 5 -TTATTTTCCCATTGAGGCCGAA-3 ).The amplified sequence was inserted into the SMAI and SPEI sites of binary vector pCAMBIA1300.Transgenic 'Micro-Tom' tomato plants were obtained by Agrobacterium tumefacien-mediated transformation with strain EHA105 [38]).

Detection of Transgenic Plants
DNA was extracted from leaves of T0-generation transgenic plants using the DNA secure plant kit (Tiangen Biotech, Beijing, China).The plant expression vector resistance gene Hygromycin B (HygB) was used for PCR detection with the primers HygB-F (5 -TAGCGAGAGCCTGACCTATT-3 ) and HygB-R (5 -GATGTTGGCGACCTCGTATT-3 ).Overexpression of the target gene Soly720 in the T0 generation of transgenic plants was verified by PCR using a primer pair for the hygromycin resistance gene (HYG).Seeds collected from 13 transgenic lines and WT plants were germinated overnight in culture dishes at 28 • C in the dark.Seedlings were then grown in a growth chamber for 16 days (25 • C) and 8 h (18 • C).When two true leaves developed, the T1-generation transgenic seedlings were transferred into a standard glasshouse in the experimental field of Shanxi Agricultural University (Jinzhong, China).

Plant Physiological Measurements
The content of chlorophyll was determined by colorimetry after acetone extraction.The net photosynthetic rate, transpiration rate, intercellular CO 2 concentration, and stomatal conductance were measured using a LI-6400 portable photosynthesometer (LI-COR Biosciences, Lincoln, NE, USA).The activities of key photosynthetic enzymes were assayed using the Solarbio kit (Beijing Solarbio Science & Technology Co., Ltd., Beijing, China).Briefly, leaf samples (0.1 g each) were homogenized with 1 mL of extraction buffer in ice and then centrifuged at 10,000× g and 4 • C for 10 min; the supernatants were collected to measure RuBisCO activity by reading the absorbance at the wavelength of 340 nm at 20 s and 5 min 20 s.For FBAase, leaf extracts were centrifuged at 8000× g and 4 • C for 10 min, and the supernatants were collected to read the absorbance at the wavelength of 340 nm at 10 s and 310 s after 5 min of water bath at 25 • C. Enzyme-linked immunosorbent assay was used to measure TK and SBPase activities at the wavelength of 450 nm, after 10 min of centrifugation of leaf extracts at 3000 and 5000× g, respectively.

Conclusions
This study investigated the comprehensive cross-response mechanisms of greenhouse tomato to CO 2 enrichment.WGCNA results showed that tomato plants are rich in amino acid metabolism in high-CO 2 environments and maintain the stability of cellular structure through fatty acid metabolism pathways.The hub genes in co-expression networks were mainly involved in plant growth and development, such as cellular components and photosynthesis.Under CO 2 -enriched conditions, transgenic plants overexpressing the photosynthesis-related gene Soly720 showed enhanced growth performance and carbon assimilation ability based on morphological, photosynthetic, and enzymatical observations.Thus, Soly720 plays a vital role in the response of tomato plants to high CO 2 conditions.The results of this study could be useful for the cultivation of new tomato varieties in the context of global climate change.

Figure 1 .
Figure 1.Cluster dendrogram from the weighted co-expression gene network analysis (WGCNA) of tomato plants under natural and high-CO2 conditions.Different colors represent various network modules based on their expression patterns.

Figure 2 .
Figure 2. Correlations between network modules and CO2 concentrations in tomato plants.Data represent the correlation coefficients, with corresponding p values in brackets.The color gradient indicates the strength of the correlation.

Figure 1 .
Figure 1.Cluster dendrogram from the weighted co-expression gene network analysis (WGCNA) of tomato plants under natural and high-CO 2 conditions.Different colors represent various network modules based on their expression patterns.

Figure 1 .
Figure 1.Cluster dendrogram from the weighted co-expression gene network analysis (WGCNA) of tomato plants under natural and high-CO2 conditions.Different colors represent various network modules based on their expression patterns.

Figure 2 .
Figure 2. Correlations between network modules and CO2 concentrations in tomato plants.Data represent the correlation coefficients, with corresponding p values in brackets.The color gradient indicates the strength of the correlation.

Figure 2 .
Figure 2. Correlations between network modules and CO 2 concentrations in tomato plants.Data represent the correlation coefficients, with corresponding p values in brackets.The color gradient indicates the strength of the correlation.

Figure 3 .
Figure 3. Co-expression network of the top 100 hub genes from the (A) MEblack and (B) MEturquoise modules.The size and color of nodes represent the connectivity of genes, and the diamonds represent the top 15 hub genes in the module.

Figure 3 .
Figure 3. Co-expression network of the top 100 hub genes from the (A) MEblack and (B) MEturquoise modules.The size and color of nodes represent the connectivity of genes, and the diamonds represent the top 15 hub genes in the module.

Figure 4 .
Figure 4. Functional annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment of hub genes in the (A) MEblack and (B) MEturquoise modules.

Figure 4 .
Figure 4. Functional annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment of hub genes in the (A) MEblack and (B) MEturquoise modules.

Figure 5 .
Figure 5. Generation of Soly720-transgenic tomato plants.(A) Agrobacterium-mediated tomato transformation.(B) Relative gene expression level of Soly720 in transgenic (OE-Soly720) and wild-type (WT) plants.Data are the means ± standard deviations of three biological replicates.Different letters above the error bars indicate significant differences among the group means according to Tukey's test (p < 0.05).*, p < 0.05; ***, p < 0.001; NS, not significant.

Figure 6 .
Figure 6.Morphological parameters of wild-type (WT) and Soly720-transgenic (OE-Soly720) tomato plants under natural and CO 2 -enriched conditions.(A) Average plant height; (B) average stem diameter; (C) average leaf length; (D) average leaf width.Different letters above the error bars indicate significant differences among the group means according to Tukey's test (p < 0.05).Data are the means ± standard deviations of three biological replicates.

Figure 9 .
Figure 9. Contents of chlorophyll a, chlorophyll b, and total chlorophyll in leaves of wild-type (WT) and Soly720-transgenic (OE-Soly720) tomato plants under different CO 2 conditions.Data are the means ± standard deviations (n = 3).Different letters above the error bars indicate significant differences among the group means according to Tukey's test (p < 0.05).***, p < 0.001.