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The molecular and enzymatic basis of bitter/non‐bitter flavor of citrus fruit: evolution of branch‐forming rhamnosyltransferases under domestication
Author(s) -
Frydman Ahuva,
Liberman Raya,
Huhman David V.,
CarmeliWeissberg Mira,
SapirMir Maya,
Ophir Ron,
W. Sumner Lloyd,
Eyal Yoram
Publication year - 2013
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1111/tpj.12030
Subject(s) - flavanone , flavonols , flavones , biology , flavor , botany , domestication , citrus × sinensis , food science , flavonoid , biochemistry , orange (colour) , genetics , antioxidant
Summary Domestication and breeding of citrus species/varieties for flavor and other characteristics, based on the ancestral species pummelo, mandarin and citron, has been an ongoing process for thousands of years. Bitterness, a desirable flavor characteristic in the fruit of some citrus species (pummelo and grapefruit) and undesirable in others (oranges and mandarins), has been under positive or negative selection during the breeding process of new species/varieties. Bitterness in citrus fruit is determined by the composition of branched‐chain flavanone glycosides, the predominant flavonoids in citrus. The flavor‐determining biosynthetic step is catalyzed by two branch‐forming rhamnosyltransferases that utilize flavanone‐7‐ O‐ glucose as substrate. The 1,2‐rhamnosytransferase (encoded by C m1,2 R ha T ) leads to the bitter flavanone‐7‐ O‐ neohesperidosides whereas the 1,6‐rhamnosytransferase leads to the tastelessflavanone‐7‐ O‐ rutinosides. Here, we describe the functional characterization of C s1,6 R ha T , a 1,6‐rhamnosyltransferase‐encoding gene directing biosynthesis of the tasteless flavanone rutinosides common to the non‐bitter citrus species. C s1,6 R ha T was found to be a substrate‐promiscuous enzyme catalyzing branched‐chain rhamnosylation of flavonoids glucosylated at positions 3 or 7. In vivo substrates include flavanones, flavones, flavonols and anthocyanins. C s1,6 R ha T enzyme levels were shown to peak in young fruit and leaves, and gradually subside during development. Phylogenetic analysis of C m1,2 R ha T and C s1,6 R ha T demonstrated that they both belong to the branch‐forming glycosyltransferase cluster, but are distantly related and probably originated separately before speciation of the citrus genome. Genomic data from citrus, supported by a study of C s1,6 R ha T protein levels in various citrus species, suggest that inheritance, expression levels and mutations of branch‐forming rhamnosyltransferases underlie the development of bitter or non‐bitter species/varieties under domestication.