A Novel “Petaloid” Mutant of Torenia (<i>Torenia fournieri</i> Lind. ex Fourn.) Bears Double Flowers through Insertion of the DNA Transposon <i>Ttf1</i> into a C-class Floral Homeotic Gene

Abstract

A double-flowered torenia (Torenia fournieri Lind. ex Fourn.) mutant, "Petaloid", was obtained from selfed progeny of the "Flecked" mutant, in which the transposition of the DNA transposon Ttf1 is active. A normal torenia flower has a synsepalous calyx consisting of 5 sepals, a synpetalous corolla consisting of 5 petals, 4 distinct stamens, and a syncarpous pistil consisting of 2 carpels. In contrast, a flower of the "Petaloid" mutant has 4 distinct petals converted from stamens, whereas the calyx, corolla, and pistil remain unchanged. The double-flower trait of the "Petaloid" mutant was unstable; some or all of the 4 petals converted from stamens frequently reverted to stamens. Furthermore, most S 1 plants obtained from self-pollination of the somatic revertant flower bore only normal single flowers. In petals converted from stamens, expression of the C-class floral homeotic gene T. fournieri FARINELLI (TfFAR) was almost completely inhibited. This inhibition was caused by insertion of Ttf1 into the 2nd intron of TfFAR, whereas reversion of converted petals to stamens was caused by excision of Ttf1 from TfFAR. The clear correspondence of the TfFAR genotype to the floral phenotype suggested that homozygous TfFAR alleles with the Ttf1 insertion caused the mutant phenotype. In contrast, TfFAR was moderately expressed in the pistil of the "Petaloid" mutant, leaving the pistil unchanged. We succeeded in inactivating Ttf1 transposition by cross-pollination between mutant and normal-type plants to genetically separate the transposon Ttf1 from the unidentified factor activating its transposition, which made the "Petaloid" mutation more stable.

A Versatile and Highly Reliable Method of Identifying Genes Responsible for Mutations Caused by Highly Active Transposable Elements

“…This study used three Torenia lines: "Flecked" (Nishijima et al 2013), "Petaloid" (Nishijima et al 2016), and S4 plants of a "Flecked" revertant (abbreviated as "rFlecked"). "Petaloid" was isolated from the selfed progeny of "Flecked" (Nishijima et al 2016).…”

“…"Petaloid" was isolated from the selfed progeny of "Flecked" (Nishijima et al 2016). Genomic sites of Ttf1 integration in "rFlecked" were expected to be different from those in "Flecked" and "Petaloid" due to active Ttf1 transposition spanning four generations of reproduction.…”

“…In "Flecked," highly active transposition of Ttf1 induces petals that are densely spotted or fully colored in violet , as well as causing new mutations. One such new mutation, the double-flowered mutant "Petaloid" in which stamens have been converted into petals, has been reported (Nishijima et al 2016). In this mutant, Ttf1 is integrated in the class C floral homeotic gene TfFAR.…”

“…Mutations of this type are also frequently observed in selfed descendants of Torenia "Flecked" (e.g., Nishijima 2016). Since this makes the selection of "true" revertants difficult, it may be unreasonable to identify the target gene based on loss of the band in some revertant-type plants.…”

“…In the present study, we applied this method to two Torenia mutants in which the genes responsible for the mutations are already known -"Flecked" (described above) and an imperfect penetrance mutant "Petaloid" (Nishijima et al 2016) -in order to assess the effectiveness of the method with mutations caused by a highly active transposon and also with mutations with imperfect penetrance.…”