Ipomoea is the largest genus in the family Convolvulaceae. Ipomoea nil (Japanese morning glory) has been utilized as a model plant to study the genetic basis of floricultural traits, with over 1,500 mutant lines. In the present study, we have utilized second- and third-generation-sequencing platforms, and have reported a draft genome of I. nil with a scaffold N50 of 2.88 Mb (contig N50 of 1.87 Mb), covering 98% of the 750 Mb genome. Scaffolds covering 91.42% of the assembly are anchored to 15 pseudo-chromosomes. The draft genome has enabled the identification and cataloguing of the Tpn1 family transposons, known as the major mutagen of I. nil, and analysing the dwarf gene, CONTRACTED, located on the genetic map published in 1956. Comparative genomics has suggested that a whole genome duplication in Convolvulaceae, distinct from the recent Solanaceae event, has occurred after the divergence of the two sister families.

SummaryThe transcriptional regulators for anthocyanin pigmentation include proteins containing R2R3-MYB domains, bHLH domains and conserved WD40 repeats, and their interactions determine the set of genes to be expressed. Spontaneous ivory seed (ivs) mutants of Ipomoea purpurea displaying pale pigmented flowers and ivory seeds are caused by insertions of DNA transposons into the bHLH2 gene that encodes a bHLH transcriptional regulator. A partial reduction in the expression of all structural genes encoding enzymes for anthocyanin biosynthesis was observed in the young flower buds of these ivs mutants. The DFR-B and ANS transcripts were completely abolished in the ivs seed coats, whereas the early biosynthetic genes for flavonol biosynthesis remained active. The production and accumulation of both proanthocyanidin and phytomelanin pigments in the ivory seed coats were drastically reduced. Moreover, the unbranched trichomes in the ivory seeds were smaller in size and fewer in number than those in the wild-type dark-brown seeds, and the surface of the epidermis without trichomes in the dark-brown seeds looked rougher, due to the protruding tangential walls, than that in the ivory seeds. Although the I. purpurea bHLH2 gene is the most closely related to the petunia AN1 gene, whose mutation is known to confer white flowers and to be deficient in acidification of their vacuoles, the vacuolar alkalization in the epidermal flower limbs of I. purpurea ivs mutants appears to occur normally. These results are discussed with regard to the function of bHLH transcriptional regulators controlling flower and seed pigmentation as well as other epidermal traits.

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These authors contributed equally to this work. SummaryWhile the wild-type morning glory (Ipomoea tricolor) displays bright-blue¯owers and dark-brown seeds, its spontaneous mutant, Blue Star, carrying the mutable ivory seed-variegated (ivs-v) allele, exhibits pale-bluē owers with a few ®ne blue spots and ivory seeds with tiny dark-brown spots. The mutable allele is caused by an intragenic tandem duplication of 3.3 kbp within a gene for transcriptional activator containing a basic helix-loop-helix (bHLH) DNA-binding motif. Each of the tandem repeats is¯anked by a 3-bp sequence AAT, indicating that the 3-bp microhomology is used to generate the tandem duplication. The transcripts in the pale-blue¯ower buds of the mutant contain an internal 583-bp tandem duplication that results in the production of a truncated polypeptide lacking the bHLH domain. The mRNA accumulation of most of the structural genes encoding enzymes for anthocyanin biosynthesis in the¯ower buds of the mutant was signi®cantly reduced. The transcripts identical to the wild-type mRNAs for the transcriptional activator were present abundantly in blue spots of the variegated¯owers, whereas the transcripts containing the 583-bp tandem duplication were predominant in the pale-blue background of the same¯owers. The¯ower and seed variegations studied here are likely to be caused by somatic homologous recombination between an intragenic tandem duplication in the gene encoding a bHLH transcriptional activator for anthocyanin biosynthesis, whereas various¯ower variegations are reported to be caused by excision of DNA transposons inserted into pigmentation genes.