Tandemly arranged chalcone synthase A genes contribute to the spatially regulated expression of siRNA and the natural bicolor floral phenotype in Petunia hybrida

Abstract

SUMMARYThe natural bicolor floral traits of the horticultural petunia (Petunia hybrida) cultivars Picotee and Star are caused by the spatial repression of the chalcone synthase A (CHS-A) gene, which encodes an anthocyanin biosynthetic enzyme. Here we show that Picotee and Star petunias carry the same short interfering RNA (siRNA)-producing locus, consisting of two intact CHS-A copies, PhCHS-A1 and PhCHS-A2, in a tandem headto-tail orientation. The precursor CHS mRNAs are transcribed from the two CHS-A copies throughout the bicolored petals, but the mature CHS mRNAs are not found in the white tissues. An analysis of small RNAs revealed the accumulation of siRNAs of 21 nucleotides that originated from the exon 2 region of both CHS-A copies. This accumulation is closely correlated with the disappearance of the CHS mRNAs, indicating that the bicolor floral phenotype is caused by the spatially regulated post-transcriptional silencing of both CHS-A genes. Linkage between the tandemly arranged CHS-A allele and the bicolor floral trait indicates that the CHS-A allele is a necessary factor to confer the trait. We suppose that the spatially regulated production of siRNAs in Picotee and Star flowers is triggered by another putative regulatory locus, and that the silencing mechanism in this case may be different from other known mechanisms of post-transcriptional gene silencing in plants. A sequence analysis of wild Petunia species indicated that these tandem CHS-A genes originated from Petunia integrifolia and/or Petunia inflata, the parental species of P. hybrida, as a result of a chromosomal rearrangement rather than a gene duplication event.

Comprehensive analyses of anthocyanin and related compounds to understand flower color change in ion-beam mutants of cyclamen (Cyclamen spp.) and carnation (Dianthus caryophyllus)

“…Inhibition of enzyme activity involved in anthocyanin biosynthesis causes a decrease directly in anthocyanin concentration. In the white marginal tissue of a Petunia picotee ower, expression of chalcone synthase, an anthocyanin biosynthetic enzyme, was specifically repressed by endogenously operating post-transcriptional gene silencing (Saito et al 2006;Morita et al 2012). In addition to decreasing the anthocyanin concentration, a signi cant increase in the concentrations of caffeoyl glucosides and coumaroyl glucoside were found in white ower tissue compared with colored flower tissue (Saito et al 2006;Saito et al 2007).…”

A reverse genetics approach identifies novel mutants in light responses and anthocyanin metabolism in petunia

“…The fertility of homozygous lines is reduced in comparison to wild-type plants (Table 1). Extensive gene redundancy in the CHS family in petunia (Koes et al 2005;Morita et al 2012) may be responsible for the lack of visible phenotype in the identified line. A novel mutant allele of the gene coding for the R2R3 MYB protein PH4 in whiteflowered plants was also identified (Fig.…”

A Research Strategy to Understand the Mechanisms that Govern Flower Color Pattern Formation

“…2F). The color pattern-formation mechanisms have been studied in restricted species, such as Petunia (Hosokawa et al 2013, Koseki et al 2005, Morita et al 2012, Saito et al 2006, Saito et al 2007, van Houwelingen et al 1998, Dianthus (Itoh et al 2002, Momose et al 2013, Pharbitis (Iida et al 2004, Inagaki et al 1994, Camellia (Tateishi et al 2010), and Dahlia (Ohno et al 2011). In these species, the chemical structures and biosynthetic genes of anthocyanins have been well studied.…”

“…There are also exogenous factors such as viral infection and endogenous factors such as transposon and PTGS, which cause flower color pattern formation. The latter are probably the simplest research objects when studying differentiation mechanisms In this paper, I will present a general procedure to elucidate the mechanisms responsible for flower color pattern formation, referring to my own studies on the marginal picotee pattern of Petunia flowers to explain the idea in detail (Morita et al 2012, Saito et al 2006, Saito et al 2007. While pigments popularly distributed in plants are anthocyanins and carotenoids, those comprising flower color pattern are mainly anthocyanins or betalains.…”