CsERF113 and CsERF7 antagonistically modulate citrus fruit abscission by regulating CsSAMDC1/2/3-mediated spermidine...

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.