Attenuation of cytokinin signaling via proteolysis of a Type-B response regulator

Abstract

Type-B response regulators (ARRs) are a group of transcription factors that are activated by cytokinin-initiated phosphorelays and regulate the expression of cytokinin-responsive genes. Recently, we reported that proteolysis of ARR2 in Arabidopsis is facilitated by cytokinins, resulting in attenuation of the signaling output of two-component circuitry. Interestingly, despite similarities in the primary structures and conserved receiver domains, the proteolytic properties of ARR2 are distinct from those of the other type-B ARRs. Using a gain-of-function mutant (ARR2 K90G ) resistant to protein degradation, we clearly demonstrated that increased levels of ARR2 affected cytokinin-mediated processes such as primary root growth, callus induction, hypocotyl elongation and leaf senescence. At the molecular level, expression of type-A ARRs was increased in transgenics expressing ARR2 K90G , resulting in enhanced cytokinin sensitivity. Here, we describe these findings and how they may be incorporated into the currently accepted model for the regulation of cytokinin signaling. In addition, we describe the proteomic approaches used to identify proteins that interact with ARR2. The putative roles of ARR2 proteolysis are also addressed with regard to other developmental processes. In conclusion, cytokinin-facilitated degradation of ARR2 must be appreciated as a post-translational event important for regulating cytokinin signaling intensity.Cytokinins are a group of phytohormones that mediate various developmental processes (e.g., seed germination, vasculature development, meristem maintenance, apical dominance and leaf senescence) and (a)biotic stress adaptation processes in plants. 1,2 The physiological effects of cytokinin responses are largely modulated by cytokinin levels and the activation of target signaling pathways.3 At the cellular level, cytokinins are recognized by CHASE-domain-containing histidine kinases located in the plasma membrane 4 and/or the endoplasmic reticulum (ER). [5][6][7] Upon cytokinin binding, type-B response regulators in the nucleus are activated via His-Asp multi-step phospho-relays similar to those observed in bacterial twocomponent systems (TCS). In Arabidopsis, 11 type-B response regulators (ARRs) have been identified and act as transcription factors with redundant function that participate in various developmental processes mediated by cytokinins. 8The output intensities of TCS signaling are primarily regulated by post-transcriptional and post-translational modifications of TCS components.3 For example, in Arabidopsis, the expression levels of AHKs (histidine kinase proteins), AHPs (histidine phosphotransfer proteins) and ARRs fluctuate in response to changing environmental conditions.3 At the protein level, sequential phosphorylations initiated by cytokinin receptors are essential for propagating cytokinin signals. However, the duration and intensity of these phosphorylation-propagated signals must be properly mediated by negative regulatory mechanisms. AHK4, a histidine kinase receptor...

Phytohormone-Based Regulation of Trichome Development

“…Overexpression of the CK biosynthesis-related gene isopentenyl transferase driven by the carpel-specific CRABS CLAW promoter of Arabidopsis produces numerous ectopic trichomes in hairless carpels ( Greenboim-Wainberg et al, 2005 ). ARR is a type-B response regulator that acts as a transcription factor in CK signaling ( Kim and Hwang, 2012 ). Specifically, ARR1, ARR2, ARR10 and ARR12 can interact with SPLs (SPL2, SPL9 and SPL10) to modulate shoot regenerative capacity ( Zhang T. et al, 2015 ).…”

Comparative physiological and transcriptomic analysis of two salt-tolerant soybean germplasms response to low phosphorus stress: role of phosphorus uptake and antioxidant capacity

“…ARR2 and CKXs had been identified as hub genes in this module. ARR2 acted as a transcription factor that was capable of reducing cytokinin signaling [ 59 ]. Meanwhile, cytokinin dehydrogenase played a crucial role as an enzyme responsible for degrading cytokinin [ 60 ].…”

New Insights into Multistep-Phosphorelay (MSP)/Two-Component System (TCS) Regulation: Are Plants and Bacteria That Different?

“…In the case of B-type ARRs, ARR2 was first reported as an example where RR proteolysis could regulate cytokinin signaling output [91,92]. In these studies, ARR2 degradation was revealed to be mediated by the 26S proteasome, as shown by experiments in Arabidopsis protoplasts were ARR2-hemagglutinin (HA) protein levels were measured in the presence or absence of the 26S proteasome inhibitor MG132 [91].…”