Cover Image

PAPERBACK
$19.95



View/Hide Left Panel

TABLE 1. Estimated copy number of flavonoid genes in the I. purpurea genome

Genes

Number of known family members

CHS

6

CHI

1

F3H

2

F3′H

1

F3′5′H

?

DFR

3

ANS

1

UF3GT

1

RT

?

UF3GT, UDP-glucose flavonol 3-0-glucosyl transferase; RT, rhamnosyltransferase.

code enzymes that produce bisnoryangonin but not naringenin chalcone (H. Noguchi, personal communication). All of the Ipomoea CHS genes (except the pseudogene) are expressed, but each displays differential regulatory and developmental control (Durbin et al., 2000; Johzuka-Hisatomi et al., 1999). CHS-D is the most abundantly expressed transcript and is now known to be the one CHS gene solely responsible for anthocyanin production in the floral limb (Durbin et al., 2000; Habu et al., 1998). As discussed in greater detail below, the identification of CHS-D as the A/a locus followed the identification of a transposon (Habu et al., 1998) in CHS-D, which accounted for the sectoring phenotype and loss of pigment. The CHS-E gene is also expressed in the limb but at a much lower level than CHS-D (Durbin et al., 2000; Johzuka-Hisatomi et al., 1999). CHS-E is now believed to be responsible for pigment production in the tube of the flower (Johzuka-Hisatomi et al., 1999).

The next step in the pathway is encoded by chalcone isomerase (CHI), which catalyzes the isomerization of the naringenin chalcone product of CHS to the flavanone naringenin. In the absence of functional CHI, spontaneous isomerization may still occur in vivo because this is known to occur in vitro under physiological conditions (Miles and Main, 1985). CHI appears to be a single copy gene in I. purpurea, as multiple screenings of the genomic library have yielded only a single gene (unpublished data).

Flavanone 3β-hydroxylase (F3H) follows CHI in the pathway and hydroxylates flavonones at the 3 position to form dihydroflavonols, which are required for the synthesis of anthocyanidins and flavonols. F3H consists of at least two copies in the I. purpurea genome, both of which are expressed (McCaig, 1998). It is not known at this time whether one of the F3H genes is solely responsible for anthocyanin production in the limb (as is the case for CHS-D). Also, because they were both identified as F3H on the basis of nucleotide similarity, it is not known whether both are even



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement