Red light-transmittance bagging promotes carotenoid accumulation of grapefruit during ripening

Carotenoids are crucial pigments in plants, contributing to color, photosynthesis, and human health. Grapefruit, an economically significant citrus fruit, is particularly rich in carotenoids, making it an ideal model for studying carotenoid accumulation. Light is a key environmental factor influencing secondary metabolite production, including carotenoids. This study aimed to investigate the impact of light quality on carotenoid metabolism in grapefruit during ripening using light-transmitting bags that modified the spectral composition of light reaching the fruit. Understanding the mechanisms behind light's influence on carotenoid biosynthesis can lead to strategies for improving fruit quality and nutritional value. The research question focuses on how different light qualities (red, blue, white) affect carotenoid accumulation compared to natural sunlight exposure, and aims to elucidate the underlying molecular mechanisms. This is important because increased carotenoid content in grapefruit improves its commercial value and nutritional benefits for consumers, and improving the production of high-quality grapefruit is of significant economic importance, especially in China, where grapefruit production is substantial. The study employs a controlled experimental approach to isolate the effects of light quality on carotenoid production, thus contributing valuable insights into the complex interplay between light and secondary metabolism in plants.

Extensive research exists on carotenoid biosynthesis, a pathway involving the condensation of geranylgeranyl diphosphate (GGPP) to produce phytoene, followed by desaturation, isomerization, and cyclization reactions to yield various carotenoids. Many genes, including those encoding enzymes like phytoene synthase (PSY), phytoene desaturase (PDS), ζ-carotene desaturase (ZDS), and lycopene cyclase (LYC), are involved. Transcription factors (TFs) also play a regulatory role, but fewer TFs involved in carotenoid metabolism have been identified compared to structural genes. Previous studies have shown varied effects of light on carotenoid accumulation in different fruits. Some studies reported reduced carotenoid content under light deprivation, while others showed enhanced accumulation with light irradiation, highlighting the complexity of light's role and its dependence on factors like light quality, intensity, and plant species. This study builds upon existing knowledge by investigating the effects of specific light qualities (red, blue, and white) on carotenoid metabolism in grapefruit using a controlled bagging technique, offering a more nuanced understanding of light's influence than previous studies.

The experiment used 'Houyat' grapefruit trees under identical conditions. Fruits were harvested at 184 and 220 days after blossom (DAB). Four treatments were implemented: a control group (CK) with no bagging, and treatments with white (WL), blue (BL), and red (RL) light-transmitting bags. Basic physiological parameters (TSS, TA, color index) were measured. Carotenoids were extracted and identified using HPLC. RNA was extracted, and RNA-seq was performed to analyze gene expression at both ripening stages (184 and 220 DAB) for each treatment. Differentially expressed genes (DEGs) were identified, and weighted gene co-expression network analysis (WGCNA) was used to identify modules correlated with carotenoid metabolism. Candidate transcription factors (TFs) were identified and analyzed phylogenetically. qRT-PCR was used to verify the expression levels of candidate structural genes and TFs. Three biological replicates were used for all analyses, with statistical analyses conducted using IBM SPSS Statistics software and Origin 2018.

Red light (RL) transmittance treatment significantly increased total carotenoid content by 62% compared to the control group. Specific carotenoids such as β-carotene, lycopene, and ζ-carotene also showed increased levels under RL treatment. WGCNA identified the ‘blue’ module, containing 4832 genes, as significantly positively correlated with carotenoid content (correlation coefficient of 0.69, p = 0.0002), while the ‘turquoise’ module showed a negative correlation. Transcriptome analysis revealed the involvement of several TFs, including bHLH128, NAC2-like/21/72, MYB-like, AGL11/AGL61, ERF023/062, WRKY20, and SBP-like-17/73 in regulating carotenoid metabolism in response to RL. These TFs regulated the expression of carotogenic genes like GGPPS2, PDS, Z-ISO, ZDS/Z2, CRTISO3, CYP97A, CHYB, ZE2P, and CCD1-2. The heatmap analysis visually confirmed the differential expression of these genes across the different light treatments. qRT-PCR results validated the RNA-seq data, supporting the involvement of the identified genes and TFs in RL-mediated carotenoid accumulation. Blue and white light treatments had no significant effect on carotenoid accumulation.

The findings demonstrate the significant role of red light in promoting carotenoid accumulation in grapefruit during ripening. The identified TFs and their regulatory networks provide valuable insight into the molecular mechanisms underlying this effect. The upregulation of specific carotogenic genes under red light treatment corroborates with previous studies on light's influence on carotenoid biosynthesis. The contrasting effects of different light qualities highlight the importance of spectral composition in regulating carotenoid metabolism. This study offers a deeper understanding of light signaling pathways involved in carotenoid biosynthesis and presents a potential strategy for manipulating light conditions to improve fruit quality and enhance nutritional value. The results provide a strong foundation for further research targeting specific TFs and genes to optimize carotenoid production in citrus fruits.

This study shows that red light-transmittance bagging significantly enhances carotenoid accumulation in grapefruit during ripening. The identification of key transcription factors and their regulatory networks sheds light on the underlying molecular mechanisms. These findings suggest a practical approach to improve fruit quality and nutritional value by manipulating light conditions during cultivation. Future research could focus on further characterizing the identified TFs and their interaction networks, and exploring the potential for manipulating these pathways to achieve even greater improvements in carotenoid content and composition.

This study focused on a single grapefruit cultivar ('Houyat'). The findings might not be generalizable to all grapefruit varieties or other citrus species. The study primarily focused on the peel; further investigation is needed to understand the impact of light treatments on carotenoid accumulation in other parts of the fruit. While the qRT-PCR results validated the RNA-seq data, additional experiments, such as protein expression analysis, could strengthen the findings.