The Banana MaLBD (LATERAL ORGAN BOUNDARIES DOMAIN) Transcription Factors Regulate EXPANSIN Expression and Are Involved in Fruit Ripening

The LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (LBD) proteins are plant-specific transcriptional factors (TFs) functioning in the growth and development of Arabidopsis and other plant species. However, the involvement of LBD TFs in regulating the ripening of economically important fruits is largely unknown. In the present study, four full-length LBD genes, designated as MaLBD1-MaLBD4, were isolated and characterized from banana fruit. Expressions of MaLBD1-MaLBD4 in fruit with three ripening characteristics revealed that MaLBD1/2/3 were ethylene-inducible, and their transcript levels obviously increased during fruit ripening, while MaLBD4 changed slightly. Moreover, the activities of MaLBD1/2/3 promoters were activated after ethylene treatment, further supporting their involvement in fruit ripening. Subcellular localization showed that MaLBD1/2/3 were nuclear proteins, and a transactivation assay in protoplasts demonstrated that MaLBD1/2/3 had transactivation activity. More importantly, a transient expression assay further indicated that MaLBD1/2/3 were transcriptional activators that regulated ripening-related MaEXP1/2 expressions by directly binding to MaEXP1/2 promoters. These results suggest that MaLBDs are involved in regulating fruit ripening, in part by transcriptional activation of the EXPANSIN expression related to cell wall modification. Taken together, our findings provide some new information about the functions of LBD TFs involved in the regulation of fruit ripening.