Tree Genetics and Molecular Breeding 2025, Vol.15, No.3, 108-116 http://genbreedpublisher.com/index.php/tgmb 109 and esters, such as linalool acetate and ethyl hexanoate, make the fragrance milder, more floral and sweet (Goh et al., 2022; Pan et al., 2023; Fan et al., 2024). Among different Citrus varieties, the types and proportions of these fragrance components vary greatly, which is also an important basis for us to distinguish varieties and create fragrances (Yu et al., 2018; Hu et al., 2024). 2.2 Physiological and ecological roles of volatile organic compounds (VOCs) The VOCs in Citrus fruits can not only make the fruits more fragrant but also help plants protect themselves. Volatile substances like terpenoids can prevent insect bites and reduce bacterial infections (Alquézar et al., 2017). Limonene can attract or drive away specific insects and may also affect the relationship between fruits and pathogens (Rodríguez et al., 2011). These volatile substances may also be involved in processes such as fruit ripening and seed propagation (Sharon-Asa et al., 2003). The variety of VOCs also enables Citrus to adapt to different environments and better cope with various challenges (Baccati et al., 2021). 2.3 Consumer preferences and market value of aromatic Citrus varieties Citrus varieties with rich and unique aromas are more popular and sell better. Sensory evaluation and metabolomics studies have found that special aroma components such as farnesene, octanal, and linalool are key to enhancing the aroma of Citrus and attracting consumers (Pan et al., 2023; Fan et al., 2024; Hu et al., 2024). The differences in aroma among different varieties not only affect people’s taste experience, but also provide a scientific basis for breeding and the development of high-quality products (Yu et al., 2018; Goh et al., 2022). 3 Biosynthetic Pathways of Aroma Compounds 3.1 Terpenoid biosynthesis: MVA and MEP pathways Among the aromas of Citrus, terpenoids are the most important type of substances. These compounds are mainly synthesized through two pathways. One is the mevalaronate pathway (MVA), which mainly occurs in the cytoplasm. The other one is the MEP pathway, which occurs in plastids. The MVA pathway is responsible for synthesizing sesquiterpenes, such as valencene, while the MEP pathway mainly synthesizes monoterpenes and diterpenes. The formation of valencene in Citrus fruits is closely related to a gene called Cstps1, which is expressed higher when the fruit is close to ripening, exactly in line with the accumulation time of aroma substances (Sharon-Asa et al., 2003; Alquézar et al., 2017). In addition, there are many kinds of terpene synthases (TPS) in Citrus fruits, which can synthesize various terpene substances, making the aroma of Citrus fruits more rich (Alquézar et al., 2017). 3.2 Fatty acid and amino acid-derived volatile pathways Citrus fruits produce a large number of volatile organic compounds derived from the metabolism of fatty acids and amino acids, which are synthesized through several different biological pathways. Fatty acids are converted into peroxides under the action of lipoxygenase (LOX), and then processed by alcohol dehydrogenase (ADH) and aldoesterase (AAT), they are transformed into alcohols and esters with more distinct aromas. These compounds make Citrus fruits smell rich and fresh (Figure 1) (Lu et al., 2024). Amino acids can be converted into some fragrant molecules through the action of enzymes such as transaminase, enriching the aroma of Citrus. 3.3 Enzyme families: TPS, LOX, ADH, AAT, and others Terpene synthase (TPS) can synthesize various terpenoids and is the basis of the source of Citrus aroma (Alquézar et al., 2017). Lipoxygenase (LOX) is the starting point of the fatty acid pathway, while ADH and AAT are the main enzymes for generating alcohols and esters (Aragüez and Fernández, 2013; Lu et al., 2024). Modifying enzymes such as UGT (glycosyltransferase) and OMT (O-methyltransferase) can modify aroma precursors, making aroma substances more stable and easier to release (Liao et al., 2023) The activity and expression of these enzymes can be affected by the fruit development stage, hormonal changes, and even gene regulation, ultimately determining the intensity and type of Citrus aroma (Sharon-Asa et al., 2003; Wang et al., 2024a).
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