Carotenoids are the principal pigments in the loquat. Although the metabolic pathway of plant carotenoids has been extensively investigated, few studies have been explored the regulatory mechanisms of loquat carotenoids because knowledge of the loquat genome is incomplete. The chromoplast-specific lycopene β-cyclase gene (CYC-B) could catalyze cyclization of lycopene to β-carotene. In this study, the differential accumulation patterns of loquat with different colors were analyzed and virus-induced gene silencing (VIGS) was utilized in order to verify CYC-B gene function. Using a cloning strategy of homologous genes, a CYC-B gene orthologue was successfully identified from the loquat. At a later stage of maturation, CYC-B gene expression and carotenoids concentrations in the 'Dawuxing' variety were higher than in 'Chuannong 1-5-9 , possibly leading to the difference in pulp coloration of loquat. Interference of CYC-B gene expression in the loquat demonstrated clear visual changes. The green color in negative control fruits became yellow, while TRV2-CYC-B silenced fruits remained green. CYC-B gene expression and total carotenoid content in the pulp decreased by 32.5% and 44.1%, respectively. Furthermore, multiple key genes in the carotenoid metabolic pathway synergistically responded to downregulation of CYC-B gene expression. In summary, we provide direct evidences that CYC-B gene is involved in carotenoid accumulation and coloration in the loquat.They participate in the assembly of photosystems and perform a significant role in light capture, whereby they absorb light in a range of the blue spectrum broader than that achieved in chlorophyll. The excited carotenoids undergo excitation energy transfer to chlorophyll [8,9]. Carotenoids protect the photosynthetic apparatus from photooxidative damage caused by excess light energy through the quenching of both singlet and triplet state chlorophylls [9,10]. Carotenoids are conducive to produce scents and flavors that attract insects and animals for pollination and seed dispersal [11]. They are also significant precursors for the biosynthesis of metabolites, such as vitamin A, strigolactone, and abscisic acid [12][13][14][15][16]. Furthermore, carotenoids are thought to protect the human body from oxidative stress by removing free radicals, which can prevent cancer, cardiovascular diseases, and other chronic diseases [17][18][19].Sites for the biosynthesis and accumulation of plant carotenoids are plastids, containing chloroplasts and chromoplasts [20]. The plant carotenoid biosynthesis and cleavage pathway is shown in Supplementary Figure S1. Geranylgeranyl diphosphate (GGPP), a precursor of carotenoid metabolism, is catalyzed to produce phytoene by phytoene synthase (PSY). Phytoene is the first carotenoid molecule and undergoes desaturation and isomerization to form lycopene. This process is catalyzed by desaturases (phytoene desaturase, PDS; ζ-carotene desaturase, ZDS) and isomerases (carotene isomerase, CRTISO; 15-cis-ζ-carotene isomerase, ZISO). Cyclization of lycop...

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To better understand the fruit flesh coloration mechanism of peach (Prunus persica), the composition and accumulation of carotenoids were compared, the expression profile of key genes involved in carotenoid biosynthetic and catabolic pathways was performed, and the differentially expressed genes were identified using "Piqiutao" (white fruit flesh) and its mutant yellow "Piqiutao" at different fruit development stages. The results showed that the total carotenoid content in yellow "Piqiutao" was remarkably higher than that of "Piqiutao," and the accumulation of β-cryptoxanthin, α-carotene, and β-carotene was significantly different, which was most likely caused by the differential expression of CCD4. Therefore, CCD4 may be an essential gene that causes the yellow fruit flesh of yellow "Piqiutao." However, the coding region sequence of CCD4 was entirely identical, and the intron was inserted by a retrotransposon in "Piqiutao" and its mutant, indicating that the expression difference was not caused by the sequence mutation and retrotransposon insertion.

In this study, third-generation full-length (FL) transcriptome sequencing was performed of loquat using single-molecule real-time(SMRT) sequencing from the pooled cDNA of embryos of young loquat fruit under different low temperatures (three biological replicates for treatments of 1˚C,-1˚C, and-3˚C, for 12 h or 24 h) and the control group(three biological replicates for treatments of room temperature), Illumina sequencing was used to correct FL transcriptome sequences. A total of 3 PacBio Iso-Seq libraries (1-2 kb, 2-3 kb and 3-6 kb) and 21 Illumina transcriptome libraries were constructed, a total of 13.41 Gb of clean reads were generated, which included 215,636 reads of insert (ROIs) and 121,654 FL, non-chimaric (FLNC) reads. Transcript clustering analysis of the FLNC reads revealed 76,586 consensus isoforms, and a total of 12,520 high-quality transcript sequences corrected with non-FL sequences were used for subsequent analysis. After the redundant reads were removed, 38,435 transcripts were obtained. A total of 27,905 coding DNA sequences (CDSs) were identified, and 407 long non-coding RNAs (lncRNAs) were ultimately predicted. Additionally, 24,832 simple sequence repeats (SSRs) were identified, and a total of 1,295 alternative splicing (AS) events were predicted. Furthermore, 37,993 transcripts were annotated in eight functional databases. This is the first study to perform SMRT sequencing of the FL transcriptome of loquat. The obtained transcriptomic data are conducive for further exploration of the mechanism of loquat freezing injury and thus serve as an important theoretical basis for generating new loquat material and for identifying new ways to improve loquat cold resistance.

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