Transcriptomic and metabolomic analyses provide insight into the volatile compounds of citrus leaves and flowers

Abstract

Background: Previous reports have mainly focused on the volatiles in citrus fruits, and there have been few reports about the volatiles in citrus leaves and flowers. However, citrus leaves and flowers are also rich in volatile compounds with unique aromas. Here, to investigate the volatiles in citrus leaves and flowers, volatile profiling was performed on leaves from 62 germplasms and flowers from 25 germplasms. Results: In total, 196 and 82 volatile compounds were identified from leaves of 62 citrus germplasms and flowers of 25 citrus germplasms, respectively. The dominant volatile terpenoids were more diverse in citrus leaves than in peels. A total of 34 volatile terpenoids were commonly detected in the leaves of at least 20 germplasms, among which 31 were overaccumulated in the leaves of wild or semiwild germplasms. This result was consistent with the high expression levels of five genes and one key gene of the mevalonate and 2-C-methyl-D-erythritol-4-phosphate (MEP) biosynthetic pathways, respectively, as well as the low expression levels of geranylgeranyl diphosphate synthase of the MEP pathway, relative to the levels in cultivars. Fully open flowers showed increased levels of four terpene alcohols and a decrease in sabinene content compared with balloon-stage flowers, especially in sweet orange. A monoterpene synthase gene was identified and functionally characterized as a sabinene synthase in vitro. Conclusions: Collectively, our results suggest that 31 important terpenoids are abundant in wild or semiwild citrus germplasms, possibly because of a negative effect of domestication on the volatiles in citrus leaves. The sweet smell of fully open flowers may be attributed to increased levels of four terpene alcohols. In addition, a sabinene synthase gene was identified by combined transcriptomic and metabolomic analyses.

Domestication of Crop Metabolomes: Desired and Unintended Consequences

“…Although many studies have been carried out in cereals, a wide range of non-grass studies have been performed, providing insight into the metabolic changes that occurred during domestication of a range of other species including tomato [54,98], lettuce [99], tea [100], watermelon [101], soybean [102], strawberry (Fragaria × ananassa) [103], and citrus [104]. The evaluation of the tomato metabolome was carried out at a previously unprecedented level in a large-scale multiomic study including 610 tomato accessions, extending work on a previous population assembled to gain insight into the processes of tomato domestication and improvement [105].…”

“…Furthermore, quinate and chlorogenic acid levels were strongly reduced upon domestication of lettuce, probably as a consequence of the desire to reduce bitterness. Similarly, melon domestication was characterized by negative selection of cucurbitacins and flavonoids but also by elevated levels of sugars, carotenoids, and citrate:malate ratios [107], soybean domestication by negative selection of isoflavones [102], strawberry by shifts in ripening regulated metabolites [103], and citrus by negative selection on terpenoids [104]. By contrast, a pedigree and metabolite analysis in tea, which has a more complex domestication history, revealed virtually no long-term artificial directional selection for flavor-related metabolites [100].…”

Chemical Variability of Peel and Leaf Essential Oils in the Citrus Subgenus Papeda (Swingle) and Few Relatives

“…In a recent study, Zhang et al [22] reported the chemical composition of ten accessions of C. ichangensis. Five of ten oil samples exhibited the two aforementioned ocimenes as major components, in addition to linalyl acetate in four accessions and α-pinene in another one, whereas two of ten oil samples were dominated by sabinene, in addition to with γ-terpinene and limonene.…”

“…Data concerning the chemical composition of peel and leaf oils from Citrus classified as Papeda are scarce; the literature is mainly focused on cultivated hybrids such as Yuzu [18,19]. Leaf oil composition was also reported in C. ichangensis [20][21][22]. C. hystrix is also well described in the literature [21][22][23][24], while C. macroptera leaf oil was described by Huang et al [19] and Waikedre et al [24].…”

Full-Length Transcriptome Sequences by A Combination of Sequencing Platforms Applied to Isoflavonoid and Triterpenoid Saponin Biosynthesis of Astragalus Mongholicus Bunge

“…Each technological change and advancement in sequencing technology has a profound impact on transcriptome research as a means to quickly, accurately, and e ciently interpret gene-level information from medicinal plants. Because of the high-throughput, sensitivity, accuracy, and low-cost advantages of NGS, it is widely used as a technical tool to reveal more information about the genes of medicinal plants [32][33][34][35]. Currently, transcriptome studies of A. mongholicus are mainly based on NGS [28,30,36].…”