| Summary: | Flowers with more petals are of more ornamental value. It is well known that <i>AGAMOUS</i> (<i>AG</i>) is the core member of the C-class gene which plays an essential role in double flower formation and identification of stamens and carpels in <i>Arabidopsis</i> <i>thaliana</i>. We searched C-class genes in the genome of the carnation, and found two <i>AG</i> orthologs (<i>DcaAGa</i>, <i>DcaAGb</i>). Phylogenetic analysis showed that the two genes were closely related to the <i>euAG</i> subclade. Then we searched the genomes of other Caryophyllales plants (<i>Beta vulgaris</i>, <i>Spinacia oleracea</i>, <i>Chenopodium quinoa</i>) for C-class genes, and found that their C-class genes all belonged to the <i>euAG</i> subclade. Semi-quantitative PCR (sq-PCR) analysis indicated that the expression of <i>DcaAG</i> genes in the single flower phenotype was higher than that in the double flower phenotype. Quantitative real-time RT-PCR (qRT-PCR) analysis showed that the expressions of <i>DcaAG</i> genes in the flower bud were significantly different from those in the root, stem, and leaf between the single and double flower phenotype carnations, and that <i>DcaAG</i> genes were specifically expressed in the stamen and carpel of carnation. Moreover, the expression of other floral organ identity genes (<i>AP1</i> and <i>AP2</i>, <i>PI</i> and <i>AP3</i>, <i>SEP1</i> and <i>SEP3</i> corresponding to the A-, B-, and E-class of genes, respectively) showed no significant difference in all floral organs between the single and double flower phenotype carnations, suggesting that C-class (<i>DcaAG</i>) genes might play an important role in the double flower phenotype in carnation. Petal loss or decrease, precocious flowering, silique shortening, and seed sterility were observed in <i>35S::DcaAGa</i> and <i>35S::DcaAGb</i> transgenic <i>Arabidopsis</i> plants. All these results show that <i>DcaAG</i> genes might affect the petal number negatively and have a specific function in stamen and carpel development in carnation.
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