APETALA2/ethylene responsive factor in fruit ripening: Roles, interactions and expression regulation

Abstract

Insects and animals are attracted to, and feed on ripe fruit, thereby promoting seed dispersal. As a vital vitamin and nutrient source, fruit make up an indispensable and enjoyable component of the human diet. Fruit ripening involves a series of physiological and biochemical changes in, among others, pigmentation, chlorophyll (Chl) degradation, texture, sugar accumulation, and flavor. Growing evidence indicates that the coordinated and ordered trait changes during fruit ripening depend on a complex regulatory network consisting of transcription factors, co-regulators, hormonal signals, and epigenetic modifications. As one of the predominant transcription factor families in plants and a downstream component of ethylene signaling, more and more studies are showing that APETALA2/ethylene responsive factor (AP2/ERF) family transcription factors act as critical regulators in fruit ripening. In this review, we focus on the regulatory mechanisms of AP2/ERFs in fruit ripening, and in particular the recent results on their target genes and co-regulators. We summarize and discuss the role of AP2/ERFs in the formation of key fruit-ripening attributes, the enactment of their regulatory mechanisms by interaction with other proteins, their role in the orchestration of phytohormone-signaling networks, and the epigenetic modifications associated with their gene expression. Our aim is to provide a multidimensional perspective on the regulatory mechanisms of AP2/ERFs in fruit ripening, and a reference for understanding and furthering research on the roles of AP2/ERF in fruit ripening.

Keywords: AP2/ERF; epigenetic regulation; fruit ripening; phytohormone; protein interaction; transcription regulation.

FIGURE 1

Model of ethylene signaling. CTR1 is a serine/threonine protein kinase that directly phosphorylates and inhibits EIN2 in the absence of ethylene. EIN2 contains multiple transmembrane domains at its N terminus and a cytoplasmic C terminus. In the presence of ethylene, the EIN2 C-terminal domain is cleaved and translocated into the nucleus. ETHYLENE INSENSITIVE3 BINDING F-BOX (EBF) proteins are responsible for targeting and degrading EIN3/EIL. EIN2 binds to and leads to the degradation of mRNAs encoding EBF1 and EBF2. In addition, EIN2 can regulate EIN3-dependent transcription. The accumulation and stabilization of EIN3/EIL lead to numerous transcriptional changes. Arrows represent activation, T-bars indicate repression.

FIGURE 2

The AP2/ERF superfamily acts as a critical regulator in fruit ripening. Target genes are in rectangles. Protein–protein interactions or co-regulators are in ovals. Red arrows represent expression regulation by AP2/ERF.