Rosa hybrid RhERF1 and RhERF4 mediate ethylene and auxin-regulated petal abscission by influencing pectin degradation

Authors: GAO, YUERONG - China Agricultural University; LIU, YANG - China Agricultural University; LIANG, YUE - China Agricultural University; LU, JINGYUN - China Agricultural University; JIANG, CHUYAN - China Agricultural University; FEI, ZHANGJUN - US Department Of Agriculture (USDA); Jiang, Cai-Zhong; MA, CHAO - China Agricultural University; GAO, JUNPING - China Agricultural University

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/13/2019
Publication Date: 6/26/2019
Citation: Gao, Y., Liu, Y., Liang, Y., Lu, J., Jiang, C., Fei, Z., Jiang, C., Ma, C., Gao, J. 2019. Rosa hybrid RhERF1 and RhERF4 mediate ethylene and auxin-regulated petal abscission by influencing pectin degradation. Plant Journal. 99(6):1159-1171. https://doi.org/10.1111/tpj.14412.
DOI: https://doi.org/10.1111/tpj.14412

Interpretive Summary: Abscission is a highly programmed developmental process in which plants shed senescent, injured, infected or dispensable organs, such as leaves, flowers and fruits. The process involves cell separation that occurs in a specialized tissue, the abscission zone (AZ), which is located at a predetermined site of the organ that is ultimately to be shed. The initiation of organ abscission is triggered by developmental and environmental cues, and phytohormones have been demonstrated to play critical roles in regulating abscission initiation. In general, ethylene is considered to be a major regulator that accelerates organ abscission, while auxin inhibits abscission, in an ethylene-antagonistic manner. The rate of abscission is regulated by the endogenous balance between ethylene and auxin levels in the AZ tissue, and abscission occurs when auxin depletion render AZ sensitive to ethylene. To date, regulatory genes involved in ethylene and auxin signal transduction pathways have been shown to be involved in the regulation of abscission. However, the mechanisms of ethylene and auxin crosstalk involved in organ abscission are still unknown. Organ shedding involves the breakdown of the primary cell wall and degradation of pectic polysaccharides in the middle lamella regions in the AZ. This facilitates cell-cell separation and detachment of the abscising organ from the parent plant. The middle lamella is rich in pectin, and the levels and chemical modification of pectins are critical for regulating adhesion of two adjacent cells. Pectic polysaccharides broadly comprise galacturonic acid-rich polymers and can be divided into three major types: homogalacturonan (HG), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II). HG has a backbone of 1,4-linked a-D-galacturonosyluronic acid residues that can be methyl- or acetyl-esterified. RG-I comprises interspersed a-D-galacturonosyl residues and rhamnosyl residues, with side-chains of galactosyl and arabinosyl residues, referred to galactans and arabinans, respectively. RG-II is less abundant than HG and RG-I, and has a complex composition. Degradation of pectic polysaccharides during abscission occurs as a result of the activity of multiple pectin-metabolizing enzymes, and several genes encoding such enzymes have been demonstrated to have a function in abscission, including POLYGALACTURONASE (PG), PECTINMETHYLESTERASE (PME), PECTATE LYASE, and ß-GALACTOSIDASE (BGLA). However, the precise mechanisms of cell separation and their control are not well understood. The AP2/ERF (APETALA2/Ethylene-Responsive Factor) family is one of the largest transcription factor families in the plant kingdom. AP2/ERF members containing a single AP2/ERF DNA binding domain can be classified into two subfamilies, the ERF and DREB (Dehydration Responsive Element Binding) subfamilies. ERF members recognize GCC-box cis-acting elements (AGCCGCC) in the promoters of ethylene-responsive genes, and function in the transcriptional regulation of diverse biological processes related to growth and development, as well as in stress responses. In A. thaliana, silencing of three ERF members, SHINE1/2/3, resulted in earlier flower organ abscission, while in tomato, silencing of SlERF52 delayed pedicel abscission, and SlERF52 was shown to regulate genes encoding abscission-associated enzymes and AZ-specific transcription factors. Taken together, these results suggest that the ERF family may play a pivotal role in the regulation of abscission. The timing of petal shedding in rose (Rosa hybrid) is a major factor in determining the longevity of the rose flower, and is important for its economic value. Here, we identified two ethylene response factors from rose, RhERF1 and RhERF4, which play a role in petal abscission. The expression of RhERF1 and RhERF4 is regulated by ethylene and auxin, respectively. Silencing of RhERF1 or Rh

Technical Abstract: The timing of plant organ abscission is regulated by the balance of two hormones, ethylene and auxin, while the mechanism of organ shedding depends on the loss of middle lamella pectin in the abscission zone (AZ). However, the mechanisms involved in sensing the balance of auxin and ethylene, and that affect pectin degradation during abscission, are not known. In this study, we identified two ethylene response factors in rose (Rosa hybrid), RhERF1 and RhERF4, which play a role in petal abscission. The expression of RhERF1 and RhERF4 is regulated by ethylene and auxin, respectively. Silencing of RhERF1 or RhERF4 was observed to accelerate petal abscission, and reduce pectin levels in the rose petal AZ. Global expression analysis and real time (RT)-PCR assays revealed that RhERF1 and RhERF4 regulate genes encoding pectin metabolizing enzymes. RhERF1 and RhERF4 were shown to bind to the promoter of the ß-GALACTOSIDASE 1 (RhBGLA1) gene, and silencing of RhBGLA1 delayed petal abscission. We conclude that during petal abscission, RhERF1 and RhERF4 integrate and coordinate ethylene and auxin signals to modulate pectin metabolism in part by regulating RhBGLA1 expression.