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Transcriptome study on the effect of leaf-fruit ratio on molecular regulation mechanism of Camellia oleifera

BMC Plant Biology volume 25, Article number: 153 (2025) Cite this article

Abstract

Background

Camellia oleifera Abel. is one of the four major woody oil species whose seeds produce high-grade edible oil. In recent years, the planting area of Camellia oleifera is increasing in China. However, in the process of cultivation, due to the high fruit load, the Camellia oleifera tree has small fruit and poor quality. In previous studies, we explored the optimal leaf-fruit ratio. In this study, the changes of molecular regulation mechanism of Camellia oleifera under different leaf-fruit ratios were revealed by combining physiological indexes with transcriptome data.

Result

The physiological results showed that the content of MDA and starch in leaves increased significantly with the decrease in the leaf-to-fruit ratio. The results of transcriptome showed that there was a close relationship between leaf-fruit ratio and phenylpropanoid biosynthesis pathway. With the decrease of leaf-fruit ratio, the expression of genes related to lignin and flavonoid biosynthesis increased significantly, which promoted the synthesis of lignin and flavonoid.

Conclusions

Combining physiological indicators and transcriptomics, we demonstrated that leaf-fruit ratio can significantly affect the normal growth of plants. When the fruit load is too high, the fruit as a ‘sink’ will consume a large amount of nutrients in the plant body and promote the biosynthesis of lignin and flavonoids in the body. The results provide a more reliable scientific basis for the study of cultivation and management of Camellia oleifera.

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Background

Camellia oleifera is a dioecious cross-pollination plant with a large number of flower buds, a high pollination rate, and a high fruit load in the early stage. In actual production, the high fruit load often leads to insufficient nutrient supply of Camellia oleifera trees, and the quality of fruit decreased. At the same time, there is a serious ‘fruit abscission’ phenomenon during fruit development, and even a common ‘big and small year’ phenomenon in production, that is called ‘alternating growth phenomenon’ [1, 2].

The study found that the main reason for the phenomenon of ‘big and small year’ is that the fruit load on the tree is too high, and it will lead to the destruction of the normal ‘sink-source’ relationship balance, and form a significant inhibitory effect on the development of flower buds [3]. At this time, immature fruits will continue to develop, and flower buds will stop developing and eventually fall due to insufficient nutrients after a period of development [4, 5]. It can be seen that maintaining an appropriate fruit load is an important strategy to ensure the sustainable and healthy growth of plants. The ‘sink-source’ relationship refers to the source and sink that have a corresponding relationship in the supply and demand relationship of assimilates. The sink-source ratio is an important indicator to measure the relationship between sink and source, and different plants correspond to different sink-source ratio [6]. Studies have shown that the fruit is the strongest ‘sink’ on the tree, while the leaves can produce nutrients through photosynthesis and are one of the ‘sources’ on the plant [7].

The sink-source ratio, also known as, the leaf-fruit ratio, is closely related to the yield of plants [8]. It has been studied in a variety of plants. It has been found that too high or too low ratio of sink to source will affect the growth and development of plants. When the sink-source ratio is too high, it means that the fruit load is too high, and the excessive fruit load requires a large amount of nutrients. At this time, the leaves as the ' source ' cannot provide sufficient nutrients, resulting in a lack of nutrients in the plant. The fruit is small, the quality is poor, and even the fruit falls off [9]. When the sink-source ratio is too low, there are fewer fruits and more leaves. When the temperature of the leaves is too high during the day, transpiration consumes a large amount of water. At the same time, photosynthesis cannot be performed at night, and a large amount of nutrients will be consumed. However, the process of nutrient transport from the source leaves to the fruit sink is not absolute, but a dynamic balance. When the leaves consume too mach nutrients and the photosynthetic products cannot meet the needs of the leaves, the main body in the sink-source relationship will change. The source leaves will be converted into a sink, and the nutrients in the fruits will be converted into nutrients to the leaves and other tissue parts. Eventually, the fruit development is hindered, the fruit yield is reduced, and the quality is reduced [10, 11].

The effect of leaf-fruit ratio on the growth of Camellia oleifera is very significant (Fig. 1). Because the optimal leaf-fruit ratio suitable for Camellia oleifera is not determined, there are serious phenomena of ‘flower and fruit abscission’ and ‘big and small years’ in the planting industry of Camellia oleifera. Some researchers have proposed the tending measures of ‘flower and fruit thinning’ [11, 12]. The team also took the 5-year-old ‘Huaxin’ Camellia oleifera tree as the research object, studied the effects of different leaf-fruit ratios on its growth and fruiting, and finally selected the leaf-fruit ratio suitable for the growth of Camellia oleifera with the best economic traits, which provided a scientific reference for the development of Camellia oleifera planting industry [13]. At the same time, during the experiment, we found that with the decrease in the leaf-fruit ratio, the yellowing phenomenon of Camellia oleifera leaves was also serious, and the number of new shoots also decreased. It is very important to master the leaf-fruit ratio for the growth and molecular regulation mechanism exploration of Camellia oleifera. This study aims to explore the changes in molecular regulatory mechanisms of Camellia oleifera under different leaf-fruit ratios by combining physiological indicators and transcriptome analysis methods, so as to provide reference for the planting and tending management of Camellia oleifera.

Fig. 1
figure 1

Growth status of Camellia oleifera under different fruit load. A Camellia oleifera tree body without fruit in September; B Camellia oleifera tree body with medium leaf fruit ratio in September; C Camellia oleifera tree body with high leaf fruit ratio in September

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Results

The growth state of Camellia oleifera under different leaf-fruit ratios

This study found that the leaf-fruit ratio had a significant effect on the growth of Camellia oleifera (Fig. 2). The growth of trees under CK and LFL treatment was the most vigorous, and new shoots were grown in spring, summer and autumn, and even a few trees under CK treatment grew winter shoots (Fig. 2, A3, B3); the MFL and HFL treatment of Camellia oleifera trees generally only spring shoots, no summer shoots and autumn shoots (Fig. 2, C3, D3), the overall growth is far weaker than CK and LFL.

Fig. 2
figure 2

The growth status of Camellia oleifera young plants and leaves at various stages of growth under different fruit loads. (A1) Saplings without fruit (CK) in June; (A2) leaf without fruit (CK) in June; (A3) saplings without fruit (CK) in October at the fruit-picking stage; (A4) leaf without fruit (CK) in October; (B1) saplings with a low fruit load (LFL) in June; (B2) leaf with an LFL in June; (B3) saplings with an LFL in October at the picking stage; (B4) leaf with an LFL in October; (C1) saplings with a medium fruit load (MFL) in June; (C2) leaf with an MFL in June; (C3) saplings with an MFL in October at the picking 240 stage; (C4) leaf with an MFL in October; (D1) saplings with a high fruit load (HFL) in June; (D2) leaf with an HFL in June; (D3) saplings with an HFL in October at the picking stage; (D4)leaf with an HFL in October

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In addition, by observing the growth of Camellia oleifera trees under different fruit-setting rates in June and October, it can be seen that the leaf-fruit ratio had a significant effect on the growth of Camellia oleifera trees and leaves in both the early stage of fruit development (June) and the mature stage of fruit (October). The growth of trees under CK treatment was in good condition and was almost no impact (Fig. 2, A1-A4); the growth status of trees under LFL treatment was affected by fruit development and seasonal changes, and the leaves were slightly darkened, but it was not significant (Fig. 2, B1-B4). Under the MFL and HFL treatments, with the decrease in leaf-fruit ratio, the load of trees also increased, in addition to the smaller fruit, the yellowing of leaves was also more serious (Fig. 2, C1-C4, D1-D4). These phenomena indicate that too high fruit setting rate will excessively consume the nutrients of Camellia oleifera trees and have a serious inhibitory effect on their growth.

Effect of leaf-fruit ratio on physiological indexes of Camellia oleifera

Because of the extremely significant effect of leaf-fruit ratio on Camellia oleifera leaves, we measured some physiological indexes of Camellia oleifera leaves. By observing the anatomical structure of Camellia oleifera leaves under different treatments, it was found that there was no significant change in the thickness of leaves under different treatments (Fig. 3A), but the chloroplast content in Camellia oleifera leaves under HFL treatment was significantly lower than that under other treatments (Fig. 3B). Further analysis of chlorophyll content in leaves, compared with CK, MFL, and HFL treatments decreased by 4.23% and 5.82%, respectively, and the difference was significant (P < 0.05).

Fig. 3
figure 3

Determination of physiological indexes of Camellia oleifera leaves under different leaf-fruit ratio. A Distribution of chloroplasts in leaves; B Chlorophyll content in leaves; C Starch content in leaves; D Nitrogen content in leaves; E MDA content in leaves; F Determination of POD activity in leaves

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At the same time, with the decrease in leaf-fruit ratio, the content of each substance in plant leaves was significantly affected. The starch content increased with the decrease in leaf-fruit ratio, the starch content in leaves under LFL, MFL, and HFL treatments was 9.21%, 4.58%, and 14.34% higher than that of CK, respectively, and the difference was significant (P < 0.05) (Fig. 3C). The nitrogen content in leaves decreased gradually with the decrease in leaf-fruit ratio. Compared with CK, the nitrogen content in leaves under LFL, MFL, and HFL treatments decreased by 8.71%, 4.02%, and 17.07%, respectively (Fig. 3D). The content of MDA in leaves was the lowest under CK treatment, and the content of MDA in leaves was higher than that of CK under the other three treatments. The content of MDA in leaves reached the highest under the HFL treatment and was significantly higher than that of other treatments (Fig. 3E). The activity of POD increased first and then decreased with the decrease in leaf-fruit ratio. The POD activity in leaves under LFL, MFL, and HFL treatments was 70.80%, 40.66%, and 24.35% higher than that of CK, respectively (Fig. 3F). In summary, fruiting will have a load on the growth of Camellia oleifera tree, and a suitable leaf-fruit ratio can promote the normal growth of Camellia oleifera tree. When the leaf-fruit ratio is too low, it will have a stress effect on Camellia oleifera tree, which will eventually have a negative impact on the growth of tree and the development of its fruits.

Transcriptome analysis of DEGs under different leaf-fruit ratios

The leaves of Camellia oleifera trees under CK, LFL, MFL and HFL treatments were collected for transcriptome analysis to clarify the molecular regulatory mechanisms that may change during the growth of Camellia oleifera leaves under different treatments. The four samples were divided into three combinations of LFL_vs_CK, MFL_vs_CK and HFL_vs_CK for pairwise analysis. As shown in Table 2, among these comparisons, the HFL_vs_CK group had the largest number of DEGs, with 1415, of which 1064 were up-regulated genes and 351 were down-regulated genes. There were only 135 DEGs (122 up-regulated genes and 13 down-regulated genes) in the LFL_vs_CK group. The comparative analysis identified 46 up-regulated genes and 7 down-regulated genes in all three pairs of comparison, i.e., LFL_vs_CK, MFL_vs_CK and HFL_vs_CK (Table 1, Fig. 4A, B).

Table 1 The number of differential genes under different leaf-fruit ratios
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Fig. 4
figure 4

Veen plots of DEGs between different treatment groups. A Up-regulated DEGs between different treatment groups; B Down-regulated DEGs between different treatment groups

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Functional annotation of DEGs under different treatments

Through Gene ontology (GO) functional and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses, we can screen out the biological functions and metabolic pathways that are significantly related to the functions of DEGs. In this study, we performed GO and KEGG analyses of the DEGs in the three treatments of LFL_vs_CK (Fig. 5A), MFL_vs_CK (Fig. 5B) and HFL_vs_CK (Fig. 5C), respectively. The results of GO analysis showed that the subclasses of DEGs in different treatment groups significantly enriched in the category of biological processes gradually decreased with the decrease of leaf-fruit ratios. On the contrary, the subclasses concentrated in the two categories of cell components and molecular functions gradually increased. DEGs in the HFL_vs_CK treatment group were annotated into 56 categories, biological processes (BP, 21 subcategories), cellular components (CC, 20 subcategories), and molecular functions (MF, 15 subcategories). Moreover, in the category of biological process, a large number of DEGs were enriched in three GO terms: single-organism process, carbohydrate metabolic process and response to stimulus (Fig. 5B); in the category of cellular component, a large number of DEGs were mainly enriched in the three GO terms of intrinsic component of membrane, membrane part and membrane; in the category of molecular function, a large number of DEGs were mainly enriched in three GO terms: catalytic activity, cation binding and metal ion binding (Fig. 5C).

Fig. 5
figure 5

GO and KEGG enrichment results of DEGs between different treatment groups. A, B, C GO enrichment of DEGs in different treatment groups; D, E, F KEGG enrichment of DEGs in different treatment groups

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To further study the biological functions of DEGs, we performed pathway enrichment analysis based on the KEGG database, and selected the top 20 pathways with the highest enrichment of DEGs in each treatment group for visualization (Fig. 5D, E, F). The analysis results showed that in the three treatment groups, DEGs were significantly enriched in the Plant-pathogen interaction pathway. At the same time, with the decrease in leaf-fruit ratios, the number of enriched DEGs also increased. In the HFL_vs_CK treatment group, the first three pathways with significant enrichment of differential genes were ‘Plant-pathogen interaction’, ‘Phenylpropanoid biosynthesis’ and ‘MAPK signaling pathway-plant’ (Fig. 5F).

Analysis of transcription factors involved in regulation

The transcription factors involved in the regulation of the physiological mechanism of Camellia oleifera trees were analyzed, and the first 11 transcription factors with abundance from high to low were visualized. The analysis showed that the number of Unigenes annotated to the WRKY transcription factor family was the largest, followed by AP2/ERF (Fig. 6A). Also, the expression level of the WRKY transcription factor in HFL treatment was significantly higher than those in other treatments (Fig. 6B). The WRKY and AP2/ERF transcription factor families are important families that regulate plant growth, development, and defense. The WRKY transcription factor family plays an important role in the regulation of plant drought and cold stress, while the AP2/ERF transcription factor family is involved in a variety of plant physiological processes, such as plant morphogenesis, stress response mechanism, hormone transduction, and metabolite regulation. Therefore, through the prediction results of transcription factors, it can be seen that when the leaf-fruit ratio of Camellia oleifera trees reaches 9: 1, excessive fruit load will stress the growth of trees and affect the normal growth of them.

Fig. 6
figure 6

Analysis of transcription factor expression. A Transcription factors involved in regulating the growth of Camellia oleifera under different leaf-fruit ratios; B The expression abundance of transcription factors under different treatments

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DEGs involved in lignin synthesis

Lignin is the main component of cell wall, which can enhance the ability of plants to resist biotic and abiotic stresses. Combined with the results of GO enrichment and KEGG enrichment analysis, it was found that the expression of related genes in the lignin synthesis pathway also changed significantly with the change of leaf-fruit ratios. After analysis, we identified 21 DEGs encoding PAL, 4CL, CCR, CAD, HCT, C3’H, COMT and PER/LAC (Fig. 7A).

Fig. 7
figure 7

The expression pattern of DEGs related to lignin synthesis. A Lignin biosynthesis pathway and gene expression pattern involved in regulation [14]; B The expression changes of differential genes under different treatments

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In-depth analysis of the expression patterns of these 21 genes in different treatment groups showed that except for two genes encoding 4CL and two genes encoding PER/LAC, the expression patterns of the remaining genes were up-regulated. Among the 17 DEGs up-regulated in the expression pattern, the expression levels of six DEGs, one encoding CCR, two encoding CAD, one encoding PER, one encoding PAL, and one encoding HCT, increased with the decrease of leaf-fruit ratios. The expression levels of these 11 DEGs encoding 4CL, 2 encoding CAD, 4 encoding PER, 2 encoding COMT, 1 encoding HCT, and 1 encoding C3’H decreased first and then increased with the decrease of leaf-fruit ratios (Fig. 7B). Although the expression patterns of these 17 up-regulated genes were slightly different, in general, when the leaf-fruit ratios reached HFL, the expression levels of the genes were significantly increased, which promoted the synthesis of lignin. Combined with the results of physiological indicators, it is indicated that the fruit load under HFL treatment is too high for 5-year-old Camellia oleifera trees, which causes stress on the growth of Camellia oleifera trees.

DEGs involved in the flavonoid biosynthesis pathway

Flavonoids and lignin are important products in the phenylpropanoid biosynthesis pathway. They are important stimulating metabolites that can regulate plant growth and development and enhance plant resistance to stress. It was found that the regulation patterns of the flavonoid biosynthesis pathway and lignin synthesis pathway changed with the decrease of leaf-fruit ratios of Camellia oleifera. We analyzed and visualized the DEGs expression profiles (Fig. 8), Caffeoyl CoA is a precursor for the synthesis of lignin and flavonoids. Five DEGs were identified in the pathway from caffeoyl CoA to proanthocyanidin synthesis. the CAFF, DFR, and LAR genes all regulated Catechin and Cyanidin, where CAFF was up-regulated with decreasing leaf-fruit ratio, while DFR was the opposite. the ANR gene indirectly affected anthocyanin synthesis. Similarly, the gene F3GT1, which regulates Pelargonidin and Delphinidin, was up-regulated in the CK treatment and down-regulated in the HFL treatment.

Fig. 8
figure 8

The expression pattern of DEGs involved in the regulation of flavonoid biosynthesis

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WGCNA analysis

Weighted gene co-expression network analysis (WGCNA) was performed using FPKM values of 8,132 genes (FPKM > 1 for all sequenced sites) and leaf phenotypes of each treatment to further identify genes associated with leaf yellowing. Genes with the same expression pattern were clustered into the same module to generate a clustering dendrogram (Fig. 9A). A total of 28 co-expression modules (M1-M28) were identified. Among these modules, MEdarkolivegreen and MEblue had relatively high numbers of genes, 919 and 741, respectively, and the MEblue module was highly correlated (positively correlated), regular, and consistent with the variation in leaf yellowing (Fig. 9B).

Fig. 9
figure 9

Construction of co-expression modules by WGCNA based on RNA-seq data and leaf-fruit ratio. A Clustering dendrogram of the genes, with dissimilarity based on the topological overlap, together with the assigned module colors; B Correlation between samples and modules; C The network of crucial co-expressed genes induced by leaf-fruit ratio

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The co-expression network of the above genes (including flavonoid biosynthesis and carotenoid biosynthesis) was exported and visualized to reveal the key genes involved in leaf phenotype (color) under different leaf-fruit ratio treatments (Fig. 9C). The co-expression network contained 38 interlinked genes, of which PAL, OMT1, GSVIVT-1, CAD1, CYP98A2, and At4g20840 were the most highly connected pivotal genes. These nine key genes all encode flavonoid biosynthesis; therefore, these genes may be critical for the variable leaf phenotypes.

Discussion

Camellia oil has been widely used in cooking and health care and has a very broad market prospect [15]. However, Camellia oleifera is affected by various factors such as temperature, moisture, and tending management measures during the planting process, and there are problems such as low yield and poor quality of tea seeds [16, 17]. Controlling a reasonable leaf-fruit ratio is an important part of plant tending management. Its effect on plant growth and yield has been confirmed in many plants, such as peach [6], apple [18] and mango [19]. In previous studies, our team screened out the best leaf-fruit ratio suitable for the growth and fruiting of Camellia oleifera, and found that there were significant differences in physiological indexes of Camellia oleifera trees under different treatments. However, at this stage, the effect of the leaf-fruit ratio on the growth mechanism of Camellia oleifera was still unclear. Therefore, we measured the physiological indexes of Camellia oleifera leaves and analyzed the transcriptome of Camellia oleifera leaves under different treatments. The results of the current analysis focused on the effect of leaf-fruit ratio on the synthesis of lignin and flavonoids.

During the experiment, we observed that with the change in leaf-fruit ratio, the growth status of Camellia oleifera trees showed significant differences. The specific performance was as follows: with the decrease in leaf-fruit ratio, the branch density of Camellia oleifera trees also decreased, and the number of new shoots also decreased significantly (Fig. 2). This feature also exists in the study of peach, apple, persimmon, and other grain tree species. Ning Ding et al. found that with the decrease in crop load, fruit quality and annual branch number increased significantly when studying the effect of crop load on apple fruit quality [20]. This is consistent with the results of this study, which fully shows that too high fruit load will cause stress on plants and affect the normal growth of plants. In addition, some researchers have conducted in-depth research on the inhibition mechanism of fruit load on branch growth and finally found that the inhibition of fruit load on flower bud branch growth is mediated by auxin and sugar signals [21].

The nutrients required for fruit growth and development mainly come from carbon assimilates produced by the photosynthesis of leaves, and the sink-source ratio can not only affect the photosynthesis of plants, but also have a significant impact on photosynthetic products [22,23,24]. Studies have shown that the sink-source ratio can greatly regulate the photosynthetic efficiency of plants [25]. Under the high leaf-fruit ratio in mango, a large amount of starch and non-structural carbohydrates (NSC) is accumulated in the leaves, thus affecting normal photosynthesis [26]. The situation in apple is the same as that in mango, appropriate increase in fruit load can increase the photosynthetic rate of leaves [27]. Yui Ozawa et al. studied the role of non-structural carbohydrates in soybean and kidney bean and found that the imbalance between sink and source may lead to changes in the balance of sugar-phosphate transporters in chloroplast membranes, which can promote the accumulation of starch in chloroplasts [28].

Combined with the existing research, the data obtained in this study were analyzed. It was found that with the decrease in leaf-fruit ratio, the content of starch showed an increasing trend, which was consistent with the research results of Yui Ozawa et al., and contrary to the situation in apple and mango. In view of this result, we guess that with the decrease in the leaf-fruit ratio of Camellia oleifera, the photosynthesis in the leaves of Camellia oleifera is affected, resulting in the accumulation of starch in the leaves. However, excessive starch accumulation may lead to the formation of large particles, which will eventually damage the normal function of chloroplasts, and at the same time make the leaves chlorotic and appear serious yellowing [29, 30]. We will conduct in-depth research on this issue in subsequent experiments to understand the effect of leaf-fruit ratio on photosynthesis and synthesis and accumulation of photosynthetic products in Camellia oleifera leaves.

Phenylpropanoids can promote the regulation of biotic and abiotic stresses in plants [31]. In this study, a large number of DEGs were enriched in the phenylpropanoid biosynthesis pathway, and most of the DEGs were related to the biosynthesis of lignin and flavonoids. Lignin and flavonoids are polymers formed by phenylpropanoid monomers [32]. Lignin is one of the main components of plant cell walls, while flavonoids are the main precursors of anthocyanin synthesis [33, 34]. Both of them not only play a key role in the growth and development of plants but also have an important impact on regulating the adaptability of plants to biotic and abiotic stresses [35]. It has been verified in many plants such as rice [36] and tea plants [37]. When plants are faced with stress, the expression of genes regulating lignin and flavonoid synthesis will increase, which is consistent with the analysis results of this study. With the decrease of leaf-fruit ratio, the expression of genes related to lignin and flavonoid synthesis in Camellia oleifera leaves will also increase.

In terms of lignin synthesis, PAL, 4CL, CCR, CAD, CCoAOMT, COMT and other genes are the key genes regulating lignin synthesis. The expression levels of CCoAOMT1, 4CL1, 4CL2, COMT, PAL1, PAL2, and AtPrx52 genes were significantly increased when Arabidopsis thaliana was exposed to high salt stress [38]. In this study, the expression levels of PAL, 4CL, CCR, CAD, CCoAOMT, and COMT genes in Camellia oleifera leaves were also significantly increased under high leaf-fruit ratio. However, for plants, lignin content is not the higher the better. Excessive lignin content will lead to corkification symptoms and inhibit the normal growth of plant [39]. It has been proved that lignin content can also affect the photosynthesis of plants. Lignin is the main material for the synthesis of plant secondary cell wall, and plant cell wall has the functions of protecting, supporting and restricting the growth of plant cells. With the increase of lignin content, plant cell wall is gradually hardened, the growth of mesophyll cells is limited, and then the photosynthesis of plants will be limited [40, 41].

In the published papers, it is pointed out that leaf-fruit ratio can not only affect the photosynthesis of plants, but also affect the quality and thickness of plant cell wall [42, 43]. Lignin is one of the main components of plant cell wall [44]. In this study, with the decrease in leaf-fruit ratio, almost all genes related to lignin synthesis are in an up-regulated expression pattern, which promotes the synthesis of lignin. We suspect that with the decrease in leaf-fruit ratio, the growth of Camellia oleifera is obviously stressed. In order to alleviate the stress caused by the imbalance of sink-source on plant growth, plants promote the biosynthesis of lignin and flavonoids and increase the thickness of cell wall. In turn, it leads to a decrease in photosynthetic rate and a large accumulation of starch, which eventually leads to obvious yellowing of leaves. This conjecture has attracted our close attention, which is also our team's upcoming research topic. We will conduct in-depth research and analysis on this conjecture.

In this study, through transcriptomics analysis, it was found that the leaf-fruit ratio was closely related to the phenylpropanoid biosynthesis pathway. Among them, genes related to lignin and flavonoid biosynthesis were significantly regulated, which promoted the biosynthesis of lignin and flavonoids, indicating that excessive fruit load would cause stress on the growth and fruiting of Camellia oleifera. In addition, during the experiment, we found that the leaves of Camellia oleifera showed serious yellowing with the decrease of leaf-fruit ratio, while the increase of lignin content and excessive accumulation of starch would damage the chloroplasts in plants and cause yellowing of leaves. Therefore, in the future, we will further study the relationship between leaf-fruit ratio and yellowing of leaves, and provide a more sufficient scientific basis for the cultivation and management of Camellia oleifera.

Conclusion

Combined with the results of physiological index determination and transcriptomic analysis, we demonstrated that the leaf-fruit ratio can significantly affect the normal growth of plants. When the fruit load was too high, the fruit as a ‘sink’ would consume a large amount of nutrients in the plant body and promote the biosynthesis of lignin and flavonoids in the body.

Materials and methods

Plant materials and treatments

In February 2019, 120 healthy 5-year-old Camellia oleifera ‘Huaxin’ were selected and transplanted to the roof of the tree building of Central South University of Forestry and Technology (28° 10’N; 113° 23’E). After 1 year of cultivation, 100 Camellia oleifera trees with consistent growth status, and no pest and disease infection were selected. In November 2020, Camellia oleifera ‘Huajin’ was used as the male parent to pollinate these cultivated Camellia oleifera trees ‘Huaxin ‘When the young fruit grows, the leaf-fruit ratio of Camellia oleifera trees is controlled according to four treatments: no fruit (CK), high leaf-fruit ratio (LFL), medium leaf fruit ratio (MFL) and low leaf fruit ratio (HFL) (Table 2), with 25 trees in each treatment.

Table 2 Different treatment methods
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In November 2021, the C. oleifera trees with strong and consistent growth were selected for sampling. The highest branch of C. oleifera was selected during sampling, and the third leaf was removed from top to bottom. Five trees were randomly selected from each treatment.. At the same time, the leaves were collected and stored at-80 °C, and the RNA was extracted for transcriptome sequencing. Three biological replicates were set up in all treatments.

Physiological index determination

The changes of chlorophyll content in the leaves of Camellia oleifera under different treatments were determined by spectrophotometry. Three trees were randomly selected from each treatment, and the leaves in the middle and upper positions of Camellia oleifera were collected. Three leaves were collected from each tree, and the leaves were cut into 0.5 * 0.5 cm square pieces. The chlorophyll in the leaves was extracted by soaking the leaves in the dark environment of an acetone-ethanol mixed solution for 24 h. The spectrophotometer wavelengths were set at 633 and 625 nm to detect chlorophyll a and chlorophyll b. The total chlorophyll content was calculated as: Chl (a + b) = Chl a + Chl b [45].

The method of observing chloroplast morphology refers to the experimental method of Feng Qianqian et al. The fresh leaves were cut into 2 * 2 * 2 mm3, and they were put into the frozen section machine (CM1950; Leica; Germany) after embedding and then used laser confocal microscope (TCSSP8; Leica; Germany) for observation.

Weigh 1 g of fresh leaves, grind and crush them, and use the corresponding kit to determine their physiological indicators.

The total nitrogen content was determined by Kjeldahl method [46].

The content of soluble sugar (SS) was determined by the anthrone colorimetric method. On this basis, the soluble sugar and starch in the sample were separated by acid hydrolysis technology, and glucose was obtained by decomposition. Using glucose content, continue to use the anthrone colorimetric method, and calculate the starch content [47].

The malondialdehyde (MDA) content and peroxidase (POD) content were measured and calculated by ELISA detection kit method and peroxidase (POD) ELISA detection kit method, respectively.

RNA extraction, sequencing and assembly

Total RNA was extracted from leaves under different treatments by plant RNA extraction kit (DP441, Tiangen, Beijing). The quality and concentration of RNA were determined by Agilent 2100 biological analyzer (United States) and NanoDrop 2000 ultraviolet–visible spectrophotometer (Thermo Fisher, United States). The QiaQuick PCR kit (Qiagen, Germany) was used to purify, construct RNA library, and sequence by Illumina NovaSeq high-throughput sequencing platform. After the assembly was completed, Bowtie [48] compared the sequenced Reads with the Unigene library, and based on the comparison results, combined with RSEM [49] to estimate the expression level. The FPKM value was used to represent the expression abundance of the corresponding Unigene.

Gene function annotation and analysis

DESeq2 (1.6.3) was used to analyze the differential expression between two biological replicates. In the process of differential expression analysis, the Benjamini–Hochberg method, which is recognized as an effective method, was used to correct the significant p-value obtained by the original hypothesis test. Finally, the corrected p-value, FDR (False Discovery Rate), was used as a key indicator for screening differentially expressed genes (DEGs). The FDR < 0.01 and the difference multiple FC (Fold Change) greater than and equal to 2 were used as the screening criteria to screen out the DEGs between the samples treated with different leaf-fruit ratios and the CK samples. The functional annotations of GO and KEGG were performed on the DEGs between different comparison groups through the BMKCloud platform.

WGCNA analysis

In this study, the gene co-expression network and visualization were constructed by using BMKCloud platform and Cytoscape 3.9.1 software to explore the hub genes involved in regulating the growth of Camellia oleifera under different leaf-fruit ratios. All samples were screened with FPKM > 1 as the standard to construct a co-expression network module. The minimum module size was 30 and the height was 0.25, which was used to merge similar transcripts. The intrinsic gene values of each module were calculated and used to test the correlation with each physiological index.

Data availability

No datasets were generated or analysed during the current study.

Abbreviations

PAL:

Phenylalanine ammonia-lyase activity

4CL:

4-coumaric acid: coenzyme A ligase

CCR:

Cinnamoyl-CoA reductase

CAD:

Cinnamic acid–ethanol dehydrogenase

HCT:

Shikimic acid hydroxycinnamoyl transferase

C3’H:

P-coumaroyl shikimic acid 3’-hydroxylase

COMT:

Caffeic acid O-methyltransferase

PER/LAC:

Peroxidase

CAFF:

Caffeic acid 3-O-methyltransferase

DFR:

Bifunctional dihydroflavonol 4-reductase/flavanone 4-reductase

LAR:

Colorless anthocyanin reductase

ANR:

Anthocyanin reductase-flavan-3-ol-forming

F3GT1:

Anthocyanin 3-O-galactosyltransferase

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Funding

This work was supported by the science and technology innovation Program of Hunan Province (2022RC1155) and Hunan Province key research and development project “Research and Demonstration of Pathways and Mechanisms for Enhancing Carbon Sequestration in Mountain, Water, Forest, Field, Lake, Grassland, and Desert Systems”(2023SK2055).

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Author notes

  1. Xiaoyan Zhang and Hui Zhang contributed equally to this work.

Authors and Affiliations

  1. Key Laboratory of Cultivation and Protection for Non-Wood Forest Trees, Ministry of Education, Central South University of Forestry and Technology, Changsha, Hunan, 410004, China

    Xiaoyan Zhang, Hui Zhang, Zhanying Gu, Lingli Wu, Xiaofeng Tan & Ze Li

  2. Forestry Bureau of Xiangxi Tujia and Miao Autonomous Prefecture, Jishou, Hunan, 416000, China

    Xiuzhong Wu

  3. Camellia Oil Tree Research Institute of Central South University of Forestry and Technology, Changsha, Hunan, 410004, China

    Xiaoyan Zhang, Hui Zhang, Zhanying Gu, Lingli Wu, Xiaofeng Tan & Ze Li

  4. Lutou National Station for Scientific Observation and Research of Forest Ecosystem in Hunan Province, Yueyang, Hunan, 414000, China

    Xiaoyan Zhang, Hui Zhang, Zhanying Gu, Lingli Wu, Xiaofeng Tan & Ze Li

  5. The Belt and Road International Union Research Center for Tropical Arid Non-wood Forest in Hunan Province, Changsha, Hunan, 410004, China

    Xiaoyan Zhang, Hui Zhang, Zhanying Gu, Lingli Wu, Xiaofeng Tan & Ze Li

Authors
  1. Xiaoyan Zhang
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  2. Hui Zhang
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  3. Xiuzhong Wu
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  4. Zhanying Gu
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  5. Lingli Wu
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  7. Ze Li
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Contributions

Ze Li, Hui Zhang and Xiaoyan Zhang formulated and designed the experiments; Ze Li, Lingli Wu and Xiaoyan Zhang performed the experiments; Hui Zhang and Xiaoyan Zhang analysed the data as well as wrote the paper; Xiuzhong Wu and Zhanying Gu helped to perform the experiments and provide experimental materials; Ze Li and Xiaofeng Tan revised and proofread the paper. All authors have read and agreed to the published version of the manuscript.

Corresponding author

Correspondence to Ze Li.

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The plant samples used in this study were grown and collected at Central South University of Forestry and Technology in Hunan, China. The samples were collected in strict accordance with the relevant regulations.

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Zhang, X., Zhang, H., Wu, X. et al. Transcriptome study on the effect of leaf-fruit ratio on molecular regulation mechanism of Camellia oleifera. BMC Plant Biol 25, 153 (2025). https://doi.org/10.1186/s12870-025-06105-9

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BMC Plant Biology

ISSN: 1471-2229

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