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Abscission in plants is tightly controlled by multiple phytohormones and the expression of various genes. However, whether the plant hormone brassinosteroids (BRs) are involved in this process is largely unknown. Here, we found that exogenous application of BRs reduced the ethylene-induced fruitlet abscission of litchi due to lower ethylene (ET) production and suppressed the expression of the ethylene biosynthetic genes  LcACS1 / 4  and  LcACO2 / 3  in the fruitlet abscission zone (FAZ). Two genes that encode the BR core signaling components brassinazole resistant (BZR) proteins, namely,  LcBZR1  and  LcBZR2 , were characterized. LcBZR1/2 were localized to the nucleus and acted as transcription repressors. Interestingly, the  LcBZR1 / 2  transcript levels were not changed during ET-induced fruitlet abscission, while their expression levels were significantly increased after BR application. Moreover, gel shift and transient expression assays indicated that LcBZR1/2 could suppress the transcription of  LcACS1 / 4  and  LcACO2 / 3  by specifically binding to their promoters. Importantly, ectopic expression of  LcBZR1 / 2  in  Arabidopsis  significantly delayed floral organ abscission and suppressed ethylene biosynthesis. Collectively, our results suggest that BRs suppress ET-induced fruitlet abscission through LcBZR1/2-controlled expression of genes related to ethylene biosynthesis in litchi. In addition, similar results were observed in the  Arabidopsis  gain-of-function mutant  bzr1-1D , which showed delayed floral organ abscission in parallel with lower expression of  ACS / ACO  genes and reduced ethylene production, suggesting that the mechanism of BZR-controlled organ abscission via regulation of ethylene biosynthesis might be conserved in  Arabidopsis .

horticulture research

Brassinosteroids suppress ethylene-induced fruitlet abscission through LcBZR1/2-mediated transcriptional repression of LcACS1/4 and LcACO2/3 in litchi