ABI4 and its role in chloroplast retrograde communication

Abstract

The acquisition of plastids is a landmark event in plant evolution. The proper functionality of these organelles depends on strict and continuous communication between the plastids and the nucleus to precisely adjust gene expression in response to the organelle’s requirements. Signals originating from the plastids impact the expression of a variety of nuclear genes, and this retrograde communication is essential to couple the nuclear expression of plastid-localized products with organelle gene expression and, ultimately, functionality. Major advances have been made in this field over the past few years with the characterization of independent retrograde signaling pathways and the identification of some of their components. One such factor is the nuclear transcriptional factor ABI4 (ABA-INSENTIVE 4). ABI4, together with the plastid PPR GUN1 protein, has been proposed to function as a node of convergence for multiple plastid retrograde signaling pathways. ABI4 is conserved among plants and also plays important roles in various critical developmental and metabolic processes. ABI4 is a versatile regulator that positively and negatively modulates the expression of many genes, including other transcriptional factors. However, its mode of action during plastid retrograde signaling is not fully understood. In this review, we describe the current evidence that supports the participation of ABI4 in different retrograde communication pathways. ABI4 is regulated at the transcriptional and post-transcriptional level. A known regulator of ABI4 includes the PTM transcription factor, which moves from the chloroplast to the nucleus. This transcription factor is a candidate for the transmission of retrograde signals between the plastid and ABI4.

Plastid osmotic stress influences cell differentiation at the plant shoot apex

“…However, the application of pharmacological inhibitors of plastid development or function was used to demonstrate that leaf adaxial/abaxial patterning is regulated by plastid protein translation in a GUN1-dependent pathway (Tameshige et al, 2013), a pathway that is required to facilitate the switch from leaf cell proliferation to expansion and differentiation (Andriankaja et al, 2012). Taken together with the results reported here, these data support previously proposed models wherein a variety of plastid dysfunctions are communicated to leaf development through similar or overlapping pathways that include GUN1 (Koussevitzky et al, 2007;Leon et al, 2012). ABI4 may function with GUN1 in this retrograde signaling pathway, or the reduction in msl2 msl3 callus production in the abi4-1 mutant background may result from indirect effects on sugar signaling or ABA biosynthesis.…”

“…One retrograde signaling pathway that may serve to connect plastid signals to shoot development is the GENOMES UNCOUPLED (GUN1)/ABA-INSENSITIVE 4 (ABI4) pathway (Fernandez and Strand, 2008;Leon et al, 2012). ABI4 is a nuclear transcription factor involved in many plant developmental pathways, including the response to ABA, sugar signaling, and mitochondrial retrograde signaling (Leon et al, 2012).…”

“…ABI4 is a nuclear transcription factor involved in many plant developmental pathways, including the response to ABA, sugar signaling, and mitochondrial retrograde signaling (Leon et al, 2012). GUN1 is a plastid protein of unclear molecular function that is thought to act with ABI4 in at least two retrograde signaling pathways: one that coordinates plastid and nuclear gene expression during development, and one that respond to defects in chlorophyll biosynthesis (Cottage et al, 2010;Koussevitzky et al, 2007;Sun et al, 2011).…”

Retrograde signaling between plastid and nucleus: A review

“…The immunoblot analysis of PTM with anti-C-PTM revealed that PTM functions as a stress sensor in the chloroplasts' outer envelope membrane, which after a proteolytic mechanism is activated by GUN1. In this reaction, the N-terminal part of the protein (N-PTM) is released and moves toward the nucleus as a retrograde signal, where it induces an ABI4 transcription factor (León et al, 2013). The proteolytic cleavage of PTM occurs in response to the retrograde signals while the N-PTM release to the nucleus is dependent on the plastid signals (Sun et al, 2011).…”

Transcription Factors Involved in ABA Signaling

“…However, later studies showed that it plays diverse roles in sugar signaling, stress response, lipid mobilization, chloroplast retrograde signaling, root development, and biosynthesis of ABA and GA (Leon et al 2012;Shu et al 2013;Wind et al 2013). ABI4 belongs to the DREB/CBF subfamily of the AP2/ERF superfamily (Sakuma et al 2002), and its binding sites have been determined in vitro employing maize ABI4 (Niu et al 2002).…”