Multifaceted Signaling Networks Mediated by Abscisic Acid Insensitive 4

Abstract

Although ABSCISIC ACID INSENSITIVE 4 (ABI4) was initially demonstrated as a key positive regulator in the phytohormone abscisic acid (ABA) signaling cascade, multiple studies have now shown that it is actually involved in the regulation of several other cascades, including diverse phytohormone biogenesis and signaling pathways, various developmental processes (such as seed dormancy and germination, seedling establishment, and root development), disease resistance and lipid metabolism. Consistent with its versatile biological functions, ABI4 either activates or represses transcription of its target genes. The upstream regulators of ABI4 at both the transcription and post-transcription levels have also been documented in recent years. Consequently, a complicated network consisting of the direct target genes and upstream regulators of ABI4, through which ABI4 participates in several phytohormone crosstalk networks, has been generated. In this review, we summarize current understanding of the sophisticated ABI4-mediated molecular networks, mainly focusing on diverse phytohormone (including ABA, gibberellin, cytokinin, ethylene, auxin, and jasmonic acid) crosstalks. We also discuss the potential mechanisms through which ABI4 receives the ABA signal, focusing on protein phosphorylation modification events.

Keywords: ABI4; crosstalk; phytohormones; target genes; transcription factor.

Figure 1

Controversial View on ABI4 Involvement in Plastid to Nucleus Retrograde Signaling. Recent evidence reported by Kacprzak et al. (2019) shows that ABI4 is not involved in retrograde signaling via GUN1 as previously understood. Because the expression of ABI4 increases under treatment with chloroplast inhibitors, we postulate the existence of an unknown signaling factor beyond ABI4 that regulates expression of retrograde genes independently of GUN1; the unidentified factor is indicated by a dashed line.

Figure 2

Transcriptional and Translational Regulators of ABI4. (A) Transcriptional level regulation involving transcription factors regulating ABI4 transcript levels (positively and negatively) in different tissues or developmental stages. (B) The known factors involved in degradation of ABI4 protein; also highlighted is the unknown factor that is possibly directly involved in the ubiquitination of ABI4 and enhances its turnover. X indicates the unidentified subunit of E3 ligase directly participating in the degradation of the ABI4 protein.

Figure 3

Model Illustrating Crosstalk between ABI4 and Phytohormones Involving the Direct Targets of Transcription Factor ABI4 Participating in Diverse Signaling Pathways. The cis-binding elements targeted by ABI4 are highlighted in black font. GA, unknown factors involved in ABI4-mediated gibberellic acid signaling are highlighted by a dashed line; JA, direct targets in the jasmonic signaling cascade that are targeted independently of ascorbate levels are highlighted; IAA, possible factors involved in auxin-mediated ABI4 signaling downstream of APX6 and ROS are proposed; ET, unidentified ethylene biosynthesis genes, in particular ACO enzymes, regulated by ABI4 are highlighted; CK, antagonistic effect of cytokinin on seed germination regulated by ABI4 is highlighted.