The Interaction of the Arabidopsis Response Regulator ARR18 with bZIP63 Mediates the Regulation of PROLINE DEHYDROGENASE Expression

Abstract

As the first and rate-limiting enzyme of proline degradation, PROLINE DEHYDROGENASE1 (PDH1) is tightly regulated during plant stress responses, including induction under hypoosmolarity and repression under water deficit. The plant receptor histidine kinases AHKs, elements of the two-component system (TCS) in Arabidopsis thaliana, are proposed to function in water stress responses by regulating different stress-responsive genes. However, little information is available concerning AHK phosphorelay-mediated downstream signaling. Here we show that the Arabidopsis type-B response regulator 18 (ARR18) functions as a positive osmotic stress response regulator in Arabidopsis seeds and affects the activity of the PDH1 promoter, known to be controlled by C-group bZIP transcription factors. Moreover, direct physical interaction of ARR18 with bZIP63 was identified and shown to be dependent on phosphorylation of the conserved aspartate residue in the ARR18 receiver domain. We further show that bZIP63 itself functions as a negative regulator of seed germination upon osmotic stress. Using reporter gene assays in protoplasts, we demonstrated that ARR18 interaction negatively interferes with the transcriptional activity of bZIP63 on the PDH1 promoter. Our findings provide new insight into the function of ARR18 and bZIP63 as antagonistic regulators of gene expression in Arabidopsis.

Insights into B-type RR members as signaling partners acting downstream of HPt partners of HK1 in the osmotic stress response in Populus

“…Pairwise protein sequence comparison revealed that PtRR19 and PtRR22 shared a good sequence identity with ARR12 (42.56%) and ARR10 (42.42%) respectively, linking up PtRR18 to ARR18 as plausible orthologs. In Arabidopsis, it has been reported an increase of ARR18 transcripts in leaves when the plants were submitted to an osmotic stress applied for six hours by a PEG 6000 treatment (Veerabagu et al, 2014). Based on these results, another attempt to isolate the RR18 in the Dorskamp genotype was conducted in experimental conditions which were reproduced according to this previous study.…”

“…Similarly, here, the interactions of the newly isolated RR18 with the three HPts were validated, consistent with the ability of its homologous protein in Arabidopsis, ARR18, to establish intermolecular contacts notably with AHP1 , and protein sharing high sequence similarities with poplar HPt2, 7 and 9 (H ericourt et al, 2013). The conditions of its isolation, the interactions observed with downstream interacting HPt proteins of HK1 and the highlighted role of its homologous protein in Arabidopsis (Veerabagu et al, 2014) prompted us to study the regulation of RR18 expression in poplar cuttings submitted to osmotic stress during a six hours time-course. Although the levels of RR18 transcripts in different organs in control condition are relatively identical, they were increased in the leaves after 40 min of osmotic stress and so it went on until the end of the kinetic.…”

“…In the middle of the sequence, there is a DNA-binding domain related to the MYB domain family, which in plants, is designated as the GARP domain (Riechmann et al, 2000) and finally, at the carboxy-terminal end, there is a transactivation sequence. Because of its particular architecture, a presumed function of transcription factor has been assigned to Btype RRs and further demonstrated in planta for some Arabidopsis RRs (Hwang and Sheen, 2001;Sakai et al, 2001;Veerabagu et al, 2014).…”

“…However, the studies which deal with signaling partners acting downstream of this HK have not fully explored the details yet. Indeed, only three AHPs (AHP2, 3 and 5) (Nishiyama et al, 2013) and one B-type RR, the ARR18 (Veerabagu et al, 2014), seem to be negative or positive regulators of drought stress responses respectively.…”

“…One of the three missing RRs, the poplar RR18 presents high sequence similarities with ARR18 considered as an actor in the osmosensing signaling pathway in Arabidopsis (Veerabagu et al, 2014). The authors concluded that ARR18 could be a major transcriptional factor in seed germination under osmotic stress via interactions with AHP2 downstream of AHK1.…”