Double flower formation in Tricyrtis macranthopsis is related to low expression of AGAMOUS ortholog gene

Complex flowers containing large numbers of petals are especially prized as ornamental plants. Double-flowered Tricyrtis macranthopsis, in which stamens and carpels are converted to petaloid organs, is related to AGAMOUS (AG) gene expression. So we isolated AGAMOUS gene from T. macranthopsis, named as TrimAG. Since TrimAG expression was limited to whorls 3 and 4 and ectopic expression of TrimAG in Arabidopsis thaliana caused the same phenotype as overexpression of other AG orthologs in transgenic Arabidopsis, this gene has similar function as its AG orthologs. In the double-flowered T. macranthopsis cultivar, the level of TrimAG expression was reduced in the two inner reproductive whorls (stamens and carpels) and caused reproductive organ replacement with petaloid tepals and floral indeterminacy. Southern blot analysis of genomic DNA indicated that there were no significant differences between the double-flowered cultivar and wild-type plants. Analysis of promoter and AG intron II sequences did not identify any critically mutated sequences in the doubled-flowered cultivar. These results suggest that reduced TrimAG expression in whorls 3 and 4 might be caused by mutation in one of the genes in regulatory network which controls AGAMOUS expression or transcriptional silencing by methylation of TrimAG promoter/intron sequences.