Functional Analysis of the FT Homolog from Eustoma grandiflorum Reveals Its Role in Regulating A and C Functional MADS Box Genes to Control Floral Transition and Flower Formation

Abstract

High temperatures cause rosetting and the continued vegetative growth of Eustoma grandiflorum. Eustoma requires a period of cold treatment to promote flowering. Ortholog of flowering time gene FT (EgFT) was isolated and characterized from E. grandiflorum. The ectopic expression of EgFT significantly promotes flowering in transgenic E. grandiflorum even when grown at high (28–30 °C) temperature conditions without any cold treatment. In addition, a severe defect in stamen and carpel formation in which they are converted into sepal/petal-like structures were observed in transgenic E. grandiflorum that strongly expressed EgFT. This homeotic conversion was correlated with the upregulation of A (EgAP1/EgFUL) and E (EgSEP1/3) and downregulation of C (EgAG) functional MADS box genes in transgenic E. grandiflorum. In wild-type E. grandiflorum, EgFT mRNA was detected in leaves and was expressed higher in young than in mature flower buds. In flowers, EgFT mRNA was strongly expressed in sepals, moderately expressed in petal, and was almost undetectable in stamen and carpel. This expression pattern was very similar to that found for EgAP1 and completely inverse to that found for EgAG. These results indicated that EgFT is able to promote flowering and regulate sepal/petal formation by activating the A/E genes and suppressing the C gene in E. grandiflorum. 35S::EgFT also caused early flowering in transgenic Arabidopsis. However, the flower organ formation and the expression of the Arabidopsis AG gene were not affected. Thus, the ability for EgFT to regulate EgAG expression may represent a distinct mechanism in E. grandiflorum for the regulation of flower organ formation.