Ethylene exerts a divergent role in modulating plant organ abscission. Spermidine (Spd) shares a common precursor for de novo biosynthesis with ethylene, but the antagonistic role between them in the regulation of plant organ abscission remains largely elusive. Here, RNA-sequencing of abscission zones from ethylene-treated sweet orange (Citrus sinensis L.) fruit led to the identification of two ethylene-responsive factors (ERFs), CsERF113 and CsERF7. CsERF113 functioned in promoting fruit abscission and suppressed Spd accumulation, whereas CsERF7 exhibited an opposite trend. CsERF113 and CsERF7 were verified to act as a transcriptional repressor or an activator of CsSAMDC1/2/3 encoding S-adenosylmethionine decarboxylase (SAMDC), an enzyme responsible for Spd biosynthesis, by binding directly to their promoters in an ethylene-dependent manner. Moreover, CsSAMDC1/2/3 functioned to inhibit fruit abscission by promoting Spd synthesis. Interestingly, CsERF7 and CsERF113 transcriptionally repressed each other to modulate the ethylene-mediated downregulation of CsSAMDC1/2/3, forming an antagonistic regulatory circuit that could be negatively orchestrated by exogenous Spd. Taken together, our results uncover that CsERF113 and CsERF7 integrate the ethylene signal to antagonistically regulate CsSAMDC1/2/3-mediated synthesis of Spd and fruit abscission. This study provides important insights into understanding the molecular mechanisms by which ethylene and Spd antagonize to modulate fruit abscission.
Chu et al. (Wed,) studied this question.