A Kinase–Phosphatase–Transcription Factor Module Regulates Adventitious Root Emergence in Arabidopsis Root–Hypocotyl Junctions

Abstract

Adventitious roots form from non-root tissues as part of normal development or in response to stress or wounding. The root primordia form in the source tissue, and during emergence the adventitious roots penetrate the inner cell layers and the epidermis; however, the mechanisms underlying this emergence remain largely unexplored. Here, we report that a regulatory module composed of the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), the MAP kinases MPK3 and MPK6, and the phosphatase PP2C12 plays an important role in the emergence of junction adventitious roots (J-ARs) from the root-hypocotyl junctions in Arabidopsis thaliana. ABI4 negatively regulates J-AR emergence, preventing the accumulation of reactive oxygen species and death of epidermal cells, which would otherwise facilitate J-AR emergence. Phosphorylation by MPK3/MPK6 activates ABI4 and dephosphorylation by PP2C12 inactivates ABI4. MPK3/MPK6 also directly phosphorylate and inactivate PP2C12 during J-AR emergence. We propose that this ''double-check'' mechanism increases the robustness of MAP kinase signaling and finely regulates the local programmed cell death required for J-AR emergence.

Phosphorylation of Serine114 of the transcription factor ABSCISIC ACID INSENSITIVE 4 is essential for activity

“…Although the relevance of each phosphosite was not tested individually, a mutant form of ABI4 with all 3 residues converted to Ala was unable to bind an LHCB promoter in vitro or repress LHCB expression in transgenic plants. More recently, phosphorylation of ABI4 by MPK3 and MPK6 was found to repress production of adventitious roots at the root-hypocotyl junction, in part by inhibiting accumulation of reactive oxygen species and programmed cell death in overlying cell layers (Bai et al, 2020). Furthermore, these MPKs phosphorylate PP2C12, which dephosphorylates ABI4, thereby inactivating PP2C12 and stabilizing ABI4 in an activated state.…”

“…Previous studies of ABI4 phosphorylation focused on its role in retrograde signaling controlling light-regulated gene expression (Guo et al, 2016) and inhibiting adventitious root formation (Bai et al, 2020). Guo et al demonstrated Ca 2+ -dependent scaffolding of MAPK cascades resulting in activation of MPK3 and MPK6, which in turn activated ABI4 to repress the photosynthesis-associated nuclear gene LHCB.…”

Protein Persulfidation in Plants: Function and Mechanism

“…However, Cys180 and Cys283 mutations had no effect on ABA and H 2 S signals, indicating that Cys250 residues of ABI4 might be a key factor in ABI4 response to ABA signaling. According to Bai et al, ABI4 was regulated by the MAPK signaling cascade in the process of A. thaliana adventitious root formation [75]. Mitogen-Activated Protein Kinase Kinase Kinase 18 (MAPKKK18), a member of the MEKK family, was confirmed to play a role in ABA signaling in A. thaliana [76].…”

Anchor Root Development: A World within Worlds

“…In notable contrast to LRs, ANR emergence is constitutively repressed in Arabidopsis and requires specific signaling to activate both cell wall softening and local programmed cell death (PCD). In their study, Bai et al (2020) investigated the mechanism by which ANRs emerge after the observation of a significant increase in ANR numbers in abi4 mutants. ABI4 is a member of the AP2/ERF family of transcription factors, which are strongly conserved among land plants and play a key role in abscisic acid (ABA)-dependent transcriptional regulation.…”

“…ANR primordia activation may represent a generic mechanism to compensate for the loss of the growth potential of the primary root (PR), as it is enhanced by mutations affecting PR growth, by whole-root excision or by nitrogen deficiency (Jia et al, 2019). A recent paper by Bai et al (2020) provides direct evidence that a regulatory module involving the AP2/ERF transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), the mitogen-activated protein kinases (MAPKs) MPK3 and MPK6, and the phosphatase PP2C12 controls ANR emergence in Arabidopsis.…”