Tree Genetics and Molecular Breeding 2025, Vol.15, No.3, 108-116 http://genbreedpublisher.com/index.php/tgmb 110 Figure 1 Synthetic pathways of terpenoids (a), phenylpropane (b), and fatty acid derivatives (c) related to the formation of aroma components (Adopted from Lu et al., 2024) Image caption: AACT, acetyl-CoA C-acetyltransferase; HMGCS, hydroxymethylglutaryl-CoA synthase; HMGCR, hydroxymethylglutaryl-CoA reductase; MVK, mevalonate kinase; PMK, phosphomevalonate kinase; MVD, mevalonate diphosphomevalonate decarboxylase; DXS, 1-deoxy-D-xylulose-5-phosphate synthase; DXR, 1-deoxy-D-xylulose-5-phosphate reductoisomerase; HDS, 4-hydroxy-3-methylbut-2-enyl-diphosphate synthase; HDR, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase; IDI, isopentenyl-diphosphate delta-isomerase; GPP, geranyl diphosphate synthase; GGPP, geranylgeranyl diphosphate synthase; DMAPP, dimethylallyl diphosphate; IPP, isopentenyl diphosphate; AADC, aromatic L-amino acid decarboxylase; PAL, phenylalaninammo-nialyase; EOMT, eugenol O-methyltransferase; PAR, phenylacetaldehyde reductase; 13-LOX, 13-lipoxygenase; 13-HPL, 13-hydroperoxidase lyase; ADH, ethanol dehydrogenase; AAT, alcohol acyltransferase (Adopted from Lu et al., 2024) 4 Genetic Regulation of Aroma Formation in Citrus 4.1 Key structural genes identified in Citrus genomes The fragrance of Citrus fruits mainly comes from some secondary metabolites called volatile terpenoids, which are synthesized within plants. Scientists have identified many genes related to terpene synthase (TPS) in the genome of Citrus fruits. In sweet oranges (Citrus sinensis), the TPS family has 95 loci, among which 55 can encode potentially functional TPS proteins. These proteins belong to several different subclasses, such as TPS-a, TPS-b, TPS-c, TPS-e/f and TPS-g (Alquézar et al., 2017). Early research by Sharon-Asa et al. (2003) found that the Cstps1 gene produces a terpene called valencene, which is a key component in the aroma of sweet oranges. This gene is particularly active when the fruit ripens and is influenced by ethylene. The strength of these gene expressions directly determines whether the aroma components of Citrus fruits are abundant or scarce and what types they are. 4.2 Transcriptional regulators: MYB, bHLH, and ERF families The synthesis process of the aroma of Citrus also cannot do without the regulation of transcription factors. Transcription factor families such as MYB, bHLH and ERF play a significant role throughout the regulatory process. The latest research shows that MYC5 in bHLH can regulate the formation of oil glands, which are important places for the accumulation of aroma components. Transcription factors such as LMI1 and DRNL can affect the expression of MYC5, thereby promoting the structural development of oil glands, such as sheath cell differentiation and cavity formation, so that flavor substances can accumulate (Wang et al., 2024a). In addition to these, MYB and ERF are also involved in the synthesis of many flavor precursor substances, such as terpenoids and propane phenols (Lu et al., 2024). 4.3 Epigenetic modifications and non-coding RNAs in aroma regulation In addition to genes and transcription factors, epigenetic mechanisms can also affect the aroma. Regulatory means such as DNA methylation and histone modification can alter the activity levels of certain genes, thereby affecting the synthesis of flavor substances (Lu et al., 2024). In addition, there are some non-coding RNAs, such as miRNAs and lncRNAs, which can affect the expression of transcription factors, sometimes by degrading these mrnas and sometimes by preventing them from being translated. This will also indirectly affect the expression levels of genes related to fragrance.
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