Abstract
The protein kinase CK2 is a ubiquitous and highly conserved enzyme, the activity of which is vital for eukaryotic cells. We recently demonstrated that CK2 modulates salicylic acid (SA) homeostasis in Arabidopsis thaliana, and that functional interplay between CK2 and SA sustains transcriptional expression of PIN-FORMED (PIN) genes. In this work, we show that CK2 also plays a key role in the transcriptional regulation of PINOID (PID), an AGC protein kinase that modulates the apical/basal localization of auxin-efflux transporters. We show that PID transcription is up-regulated by auxin and by SA and that CK2 is involved in both pathways. On the one hand, CK2 activity is required for proteosome-dependent degradation of AXR3, a member of the AUX/IAA family of auxin transcriptional repressors that must be degraded to activate auxin-responsive gene expression. On the other hand, the role of CK2 in SA homeostasis and, indirectly, in SA-driven PID transcription, was confirmed by using Arabidopsis NahG transgenic plants, which cannot accumulate SA. In conclusion, our results evidence a role for CK2 as a functional link in the negative cross-talk between auxin- and SA-signaling.
Highlights
The phytohormone auxin plays a central role in plant development and in the plant’s responses to environmental stimuli such as gravitropism, phototropism and root plastic growth [1]
We recently reported that inhibition of CK2 activity in Arabidopsis plants had important effects on auxin signaling, on polar auxin transport (PAT) [32] and that PID was highly overexpressed in roots of CK2-defective plants
We have previously demonstrated that a dexamethasone (Dex)-inducible dominant-negative allele of CK2 (CK2mut) could be successfully used to deplete CK2 activity in Arabidopsis plants [31,32]
Summary
The phytohormone auxin plays a central role in plant development and in the plant’s responses to environmental stimuli such as gravitropism, phototropism and root plastic growth [1]. An important finding in the last decade was that auxin is not uniformly distributed within the plant tissues and that establishment of auxin maximum and minimum is a key step of organogenesis [2,3]. The asymmetric distribution of indoleacetic acid (IAA), the most abundant auxin in plants, relies mainly on cell-to-cell transport, called polar auxin transport (PAT) [3]. Several auxin carriers located at the plasma membrane have been identified to date. PLOS ONE | DOI:10.1371/journal.pone.0157168 June 8, 2016
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