ABSTRACT
Signaling mediated by reactive oxygen species (ROS) has emerged as a key component of plants' responses to environmental stress. The ROS-regulated transcription factor ZAT12 was revealed as a negative regulator of iron (Fe) deficiency responses through its direct interaction with the bHLH protein FIT. In the epidermis of the early root differentiation zone, ZAT12 stability depended on the presence of the ZAT12 EAR motif. It was concluded that ZAT12 may be the target of 2 alternative degradation pathways. Here, we present a model aiming to explain the regulatory mechanisms by which ZAT12 could be targeted for degradation and to predict the types of potential regulators involved. In addition to an E3 ubiquitin ligase, we predict 2 critical regulatory factors, namely a protein interacting with the ZAT12 EAR motif and a ROS-responsive regulatory protein.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/metabolism , Protein Stability , Transcription Factors/metabolism , Amino Acid Motifs , Arabidopsis/physiology , Arabidopsis Proteins/chemistry , Hydrogen Peroxide/metabolism , Models, Biological , Proteolysis , Reactive Oxygen Species/metabolism , Signal Transduction , Stress, Physiological , Transcription Factors/chemistryABSTRACT
Plants grown under iron (Fe)-deficient conditions induce a set of genes that enhance the efficiency of Fe uptake by the roots. In Arabidopsis (Arabidopsis thaliana), the central regulator of this response is the basic helix-loop-helix transcription factor FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT). FIT activity is regulated by protein-protein interactions, which also serve to integrate external signals that stimulate and possibly inhibit Fe uptake. In the search of signaling components regulating FIT function, we identified ZINC FINGER OF ARABIDOPSIS THALIANA12 (ZAT12), an abiotic stress-induced transcription factor. ZAT12 interacted with FIT, dependent on the presence of the ethylene-responsive element-binding factor-associated amphiphilic repression motif. ZAT12 protein was found expressed in the root early differentiation zone, where its abundance was modulated in a root layer-specific manner. In the absence of ZAT12, FIT expression was upregulated, suggesting a negative effect of ZAT12 on Fe uptake. Consistently, zat12 loss-of-function mutants had higher Fe content than the wild type at sufficient Fe. We found that under Fe deficiency, hydrogen peroxide (H2O2) levels were enhanced in a FIT-dependent manner. FIT protein, in turn, was stabilized by H2O2 but only in the presence of ZAT12, showing that H2O2 serves as a signal for Fe deficiency responses. We propose that oxidative stress-induced ZAT12 functions as a negative regulator of Fe acquisition. A model where H2O2 mediates the negative regulation of plant responses to prolonged stress might be applicable to a variety of stress conditions.
