Largely different contents of terpenoids in beef red-flesh tangerine and its wild type

Abstract

Background: Niurouhong (Citrus reticulata Blanco. Niurouhong) (NRH) is a spontaneous beef-red flesh mutant with distinctive flavor compared with its wild type orange-red flesh Zhuhongju (ZHJ). To illustrate the biochemical mechanism of its special flesh color and flavor, fruits at commercial mature stage were used to profile the volatiles in the flavedo and determine the levels of carotenoids, limonoid aglycones and phytohormones in the juice sacs in two seasons.

Results: Our results showed the content of total volatile terpenoids in NRH was 1.27-fold that in ZHJ. The components of volatiles were found to be common between the two tangerines. This result indicates that the distinctive flavor of NRH might not be derived from the presence/absence of specific volatiles; instead, it was derived from the altered concentrations or balance of α-citral, β-citral, 2-cyclohexen-1-one, (S)-3-methyl-6-(1-methylethenyl) and n-hexadecanoic acid. Analyses of the contents of total and specific carotenoids indicated that the beef-red color of NRH flesh might be largely attributed to the over accumulation of β-cryptoxanthin and β-carotene. However, lower ABA level was found in NRH than in ZHJ, reflecting a possible feedback regulation of ABA biosynthesis on carotenogenesis and the balance in the metabolism among terpenoids.

Conclusions: Collectively, our study suggested that the MEP pathway was enhanced in NRH tangerine. However, a certain unknown co-regulatory mechanism might be present in the metabolism pathway of secondary metabolites (especially terpenoids) in beef-red flesh mutant. Our study provides new insights into the regulatory network of terpenoid metabolism and mutation mechanism of red-fleshed citrus.

Keywords: Carotenoids; Citrus reticulate Blanco; Limonoid aglycones; Niurouhong; Phytohormone; Terpenoids; Volatile.

Fig. 1

Mature fruit of NRH tangerine (a) and its wild type ZHJ tangerine (b)

Fig. 2

Classes and proportions of volatile compounds in fruit flavedo of NRH (a) and ZHJ (b)

Fig. 3

Volatiles, carotenoids, imonoid aglycones and phytohormones in metabolic network of terpenoids [1, 5]. Note: MVA, mevalonate; MEP, 2-C-methyl-D-erythritol 4-phosphate; IPP, isopentenyl diphosphate; IPPI, isopentenyl diphosphate Delta-isomerase; FPS, farnesyl diphosphatesynthase; GGPS, geranylgeranyl diphosphate synthase; SPS, solanesyl diphosphate synthase; DMAPP, dimethylallyl diphosphate; IPT, isopentenyl transferase; FPP, farnesyl diphosphate; SS, sesquiterpenoid synthase; SQS, squalene synthase; GGPP, geranylgeranyl diphosphate; DS, diterpenoid synthase; PPP, polyprenyl diphosphate; GPS, geranyl diphosphate synthase; PPS, polyprenyl diphosphate synthase; GPP, geranyl diphosphate; SPP, solanesyl diphosphate; MS, monoterpenoid synthase; PSY, phytoene synthase; HST, homogentisate solanesyl transferase; CCD, carotenoid cleavage dioxygenase; NXS, neoxanthin synthase; NCED, 9-cis-epoxycarotenoid dioxygenase. Solid lines indicate a single enzymatic step, and dashed lines indicate several steps. Terpenoids investigated in this study are shown in red font