About this Research Topic
Plants that biosynthesize secondary (specialized) metabolites have an enormous and still largely unexplored potential. Pigments such as carotenoids and anthocyanins, and volatiles including terpenoids, phenylpropanoids/benzenoids, and fatty acid derivatives, are important secondary/specialized metabolites in plants. They function to attract pollinators to flowers and seed-dispersing animals to fruits. Many also have important antioxidant or human health benefits.
Monoterpenes are the main volatiles in certain flowers (such as Lily) and fruits (Citrus spp.), and are biosynthesized mainly through the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway in the plastid. Carotenoids in plants are also biosynthesized via the MEP pathway. Whether these two pathways have common regulatory factors or have a competitive relationship is unknown. Similarly, anthocyanins and phenylalanine-derived volatiles, such as benzaldehyde, phenylacetaldehyde, and methyl salicylate, are derived from phenylalanine. Therefore, the regulation and interactions between the two pathways may share the same transcription factors. We know that volatiles and pigments accumulate and give color as fruits ripen; and a MYB-type transcription factor, SlMYB75 has been demonstrated to promote both anthocyanin accumulation and enhance volatile aroma production in tomato fruits. In other cases, the two pathways may have a competitive relationship. Some white flowers, for instance, are more fragrant than flowers with colors. The mechanisms regulating anthocyanin accumulation and responses to the environment in plants are established. However, little is known about the mechanism regulating accumulation of phenylalanine-derived volatiles, and there have been few studies on the detailed relationship between anthocyanins and phenylalanine-derived volatiles during plant evolution.
In addition, a considerable number of apparently carotenoid-derived compounds have been identified in nature, many of which play an important role as flavor and fragrance substances. Examples are the unusually potent norisoprenoid fragrances β-ionone and β-damascenone. Whereas detailed knowledge is available concerning the biosynthesis of carotenoids in plants, little is known about the metabolic pathways that lead to the formation of aroma compounds. As far as we know, CAROTENOID CLEAVAGE DIOXEGENASE 1 (CCD1) is involved in this pathway in many plants, such as grape, nectarine, tomato, petunia, crocus, and star fruit. Although CCD1 transcript level is known to exhibit a circadian rhythm in petunia flower, the regulatory mechanism is still unknown. Apart from their role in attraction of pollinators, the roles of carotenoid-derived volatiles in plant biotic or abiotic stress are still unclear.
For this Research Topic, Reviews and Original Research manuscripts dealing with the relationships between pigments and volatiles involved in reproduction or responses to the environment are welcomed. We particularly encourage the submission of manuscripts with a focus on the following topics:
• Metabolism and roles of apocarotenoids in plants under biotic and abiotic stress, and insights into the molecular basis behind such responses
• Role of apocarotenoids in crop domestication
• Roles and mechanisms of apocarotenoids in plant reproduction and evolution.
• Analysis of color and volatile accumulation in different plant germplasm resources, and insights into the molecular basis behind such variation.
Keywords: Anthocyanins, Carotenoids, Specialized metabolism, Secondary metabolism, Volatiles, Pigments
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