Papaya CpERF9 acts as a transcriptional repressor of cell-wall-modifying genes CpPME1/2 and CpPG5 involved in fruit ripening.

KEY MESSAGE: CpERF9 controls papaya fruit ripening through transcriptional repression of cell-wall-modifying genes CpPME1/2 and CpPG5 by directly binding to their promoters. Papaya fruit ripening is an intricate and highly coordinated developmental process which is controlled by the action of ethylene and expression of numerous ethylene-responsive genes. Ethylene response factors (ERFs) representing the last regulators of ethylene-signaling pathway determine the specificities of ethylene response. However, knowledge concerning the transcriptional controlling mechanism of ERF-mediated papaya fruit ripening is limited. In the present work, a gene-encoding AP2/ERF protein with two ERF-associated amphiphilic repression (EAR) motifs, named CpERF9, was characterized from papaya fruit. CpERF9 was found to localize in nucleus, and possess transcriptional repression ability. CpERF9 expression steadily decreased during papaya fruit ripening, while several genes encoding pectin methylesterases (PMEs) and polygalacturonases (PGs), such as CpPME1/2 and CpPG5, were gradually increased, paralleling the decline of fruit firmness. Electrophoretic mobility shift assay (EMSA) demonstrated a specific binding of CpERF9 to promoters of CpPME1/2 and CpPG5, via the GCC-box motif. Transient expression of CpERF9 in tobacco repressed CpPME1/2 and CpPG5 promoter activities, which was depended on two EAR motifs of CpERF9 protein. Taken together, these findings suggest that papaya CpERF9 may act as a transcriptional repressor of several cell-wall modifying genes, such as CpPME1/2 and CpPG5, via directly binding to their promoters.