Abstract
Massive changes in the transcriptome of Arabidopsis thaliana during onset and progression of leaf senescence imply a central role for transcription factors. While many transcription factors are themselves up- or down-regulated during senescence, the bZIP transcription factor G-box-binding factor 1 (GBF1/bZIP41) is constitutively expressed in Arabidopsis leaf tissue but at the same time triggers the onset of leaf senescence, suggesting posttranscriptional mechanisms for senescence-specific GBF1 activation. Here we show that GBF1 is phosphorylated by the threonine/serine CASEIN KINASE II (CKII) in vitro and that CKII phosphorylation had a negative effect on GBF1 DNA-binding to G-boxes of two direct target genes, CATALASE2 and RBSCS1a. Phosphorylation mimicry at three serine positions in the basic region of GBF1 also had a negative effect on DNA-binding. Kinase assays revealed that CKII phosphorylates at least one serine in the basic domain but has additional phosphorylation sites outside this domain. Two different ckII α subunit1 and one α subunit2 T-DNA insertion lines showed no visible senescence phenotype, but in all lines the expression of the senescence marker gene SAG12 was remarkably diminished. A model is presented suggesting that senescence-specific GBF1 activation might be achieved by lowering the phosphorylation of GBF1 by CKII.
Highlights
Onset and progression of leaf senescence is accompanied by a massive change in the transcriptome, implying an important role of transcription factors in this process [1,2,3,4]
The two transcription factor families, WRKY and NAC factors, which largely expanded in the plant kingdom are both overrepresented in the senescence transcriptome of Arabidopsis and several members of both families have already been characterized as playing important roles in senescence regulation in Arabidopsis and other plant species [5,6,7,8,9,10,11,12,13,14]
Since CASEIN KINASE II (CKII) are highly conserved among eukaryotes and subunits can complement each other [40], we verified GBF1 in vitro phosphorylation using mammalian CKII (Figure 1)
Summary
Onset and progression of leaf senescence is accompanied by a massive change in the transcriptome, implying an important role of transcription factors in this process [1,2,3,4]. The two transcription factor families, WRKY and NAC factors, which largely expanded in the plant kingdom are both overrepresented in the senescence transcriptome of Arabidopsis and several members of both families have already been characterized as playing important roles in senescence regulation in Arabidopsis and other plant species [5,6,7,8,9,10,11,12,13,14]. Members of both groups have been shown to react to elevated levels of reactive oxygen species (ROS), especially hydrogen peroxide at the transcriptional level, and are at the same time involved in regulating the intracellular concentrations of these molecules [7,8,9,10,11,14].
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