capable of performing the hydroxylation step. One functional copy of F3H and one pseudogene have so far been identified in I. nil (Hoshino et al., 1997a; S. Iida, personal communication).
Another hydroxylase, dihydroflavanol 3′ hydroxylase (F3′H), hydroxylates the 3′ position of the dihydroflavonol produced by F3H. This results in the eventual production of the red/magenta cyanidin. F3′H has been characterized in both I. purpurea and I. nil (Morita et al., 1999) (S. Iida, personal communication).* Yet another hydroxylase, dihydroflavonol 3′5′-hydroxylase (F3′5′H), hydroxylates the 3′ and 5′ position of the dihydroflavonol produced by F3H. This product ultimately leads to the production of the blue/purple delphinidin. F3′5′H has not yet been characterized in Ipomoea.
The next step in the flavonoid pathway is dihydroflavonol reductase (DFR) which reduces dihydroflavonols to leucoanthocyanidins. In I. purpurea, DFR is a small gene family consisting of at least three tandemly arranged copies (DFR-A, -B, and -C) (Inagaki et al., 1999). DFR-B has been identified as the gene responsible for anthocyanin production in the floral limb based on work from I. nil in which a transposon disrupts the DFR-B gene, resulting in a sectoring phenotype and loss of pigment (Inagaki et al., 1994; see below). The function of the two other DFR genes in I. purpurea is not known, nor is it known whether they are capable of performing the reductase reaction.
Anthocyanidin synthase (ANS) encodes a dioxygenase and appears to be single copy in I. purpurea. UDP-glucose flavonol 3-0-glucosyl transferase glycosylates anthocyanidins and flavonols on the 3 position. This gene appears to be single copy in I. purpurea. Rhamnosyl transferase adds rhamnosyl to glucose to form rutinoside. This gene is as yet uncharacterized in I. purpurea.
A wide variety of mobile elements (Table 2) have been identified in the Ipomoea genome, largely because of work from the laboratory of Shigeru Iida at the National Institute for Basic Biology in Okasaki, Japan. Some of these mobile element insertions cause phenotypic changes, including those responsible for several flower color variants (Table 3). Much of this work has concentrated on the Japanese morning glory (I. nil), where the rich history of morning glory genetics in Japan has provided an extensive research foundation. Considerable work has also been done both in
Morita, Y., Hoshino, A., Tanaka, Y. Kusumi, T., Saito, N. & Iida, S. (1999) Plant Cell Physiol. 40, Suppl., 124 (abstr.).