A Basic Helix-Loop-Helix Transcriptional Factor Regulates Flower Senescence by Mediating Ethylene Biosynthesis in Petunia

The bHLH transcription factors play important roles in plant growth and development. However, there was no report about the function of bHLH in flower senescence. In this study, a bHLH transcriptional factor, PhFBH4, was found to be dramatically up-regulated during flower senescence. Transcriptional level of PhFBH4 is induced by plant hormones and abiotic stress treatments. Ectopic expression of PhFBH4 influenced flower longevity in petunia. Silencing of PhFBH4 using virus-induced gene silencing and antisense approach extended flower longevity while transgenic petunia flowers with an over-expression construct showed a reduction in lifespan. Abundance of transcripts of senescence-related genes (SAG12, SAG29) was significantly changed in petunia PhFBH4 transgenic flowers. Furthermore, overexpression or silencing PhFBH4 increased or reduced, respectively, transcript abundances of important ethylene biosynthesis-related genes ACO1 and ACS1, therefore influencing ethylene production. We also found that PhFBH4 protein physically interacted with the G-box cis-element in the promoter of ACS1, suggesting that ACS1 was a direct target of PhFBH4 protein. In addition, ectopic expression of this gene altered plant development including plant height, internode length, and size of leaves and flowers, accompanying with alterations of transcript abundances in GA biosynthesis-related gene GA2OX3. Our results indicate PhFBH4 plays an important role in regulating plant development through modulating ethylene biosynthesis pathway.