The structure and phylogeny of MADS-box genes HAM91 of sunflower (Helianthus annuus) and CDM115 of chrysanthemum (Chrysanthemum morifolium) were characterized. It is shown that these genes encode MADS-domain transcription factors, which are orthologs of TM6 (Solanum lycopersicum) and APETALA3 (Arabidopsis thaliana), respectively. We obtained two types of transgenic tobacco plants (Nicotiana tabacum) with constitutive expression of HAM91 and CDM115 genes. Both types of plants flowered later than the control plants and formed more flowers and seed pods. The weight of seeds of 35S::CDM115 plants was significantly lower than in the control and 35S::HAM91 plants, which may indicate to a change in the identity of ovules in 35S::CDM115.