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

Xingshuai Ma,Ye Yuan,Minglei Zhao,Qian Wu,Caiqin Li,Zidi He,Jianguo Li

doi:10.1038/s41438-021-00540-z

Xingshuai Ma, Ye Yuan + Show 5 more

Open Access

https://doi.org/10.1038/s41438-021-00540-z

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Abstract

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.

Highlights

Abscission in plants, which occurs in a specific region called the abscission zone (AZ), is a physiological phenomenon and complex process
We found that the ET-induced expression levels of LcACS1/4 and LcACO2/3 were dramatically repressed by ET + EBR treatment in the fruitlet abscission zone (FAZ) (Fig. 1c–f), which is consistent with ET production under the ET and ET + EBR treatments
We found that LcBZR1/2 transcripts did not change in either the control or ET-treated FAZ tissues, whereas their expression levels in the FAZ were significantly increased after the combination of ET and EBR treatments (Fig. 3a), indicating that LcBZR1/2 are involved in

Summary

Introduction

Abscission in plants, which occurs in a specific region called the abscission zone (AZ), is a physiological phenomenon and complex process. Abscission, in an evolutionary context, is a highly significant process that causes seed and fruit dispersal and eliminates infected or damaged organs. Organ abscission is initiated by many internal and external cues, among which plant hormones have been revealed to play major roles in regulating the initiation of abscission[2,3]. Ethylene (ET) and auxin are considered major regulators, and they function in an antagonistic manner to regulate abscission[4,5]. The effect of ABA and cytokinins on plant organ abscission seems to be mediated by auxin or ET rather than a direct effect[4,6,7,8,9].

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