Abstract
BackgroundIn plants, the multistep phosphorelay signaling pathway mediates responses to environmental factors and plant hormones. This system is composed of three successive partners: hybrid Histidine-aspartate Kinases (HKs), Histidine-containing Phosphotransfer proteins (HPts), and Response Regulators (RRs). Among the third partners, B-type RR family members are the final output elements of the pathway; they act as transcription factors and clearly play a pivotal role in the early response to cytokinin in Arabidopsis. While interactions studies between partners belonging to the multistep phosphorelay system are mainly focused on protagonists involved in cytokinin or ethylene pathways, very few reports are available concerning partners of osmotic stress signaling pathway.ResultsIn Populus, we identified eight B-type RR proteins, RR12-16, 19, 21 and 22 in the Dorskamp genotype. To assess HPt/B-type RR interactions and consequently determine potential third partners in the osmosensing multistep phosphorelay system, we performed global yeast two-hybrid (Y2H) assays in combination with Bimolecular Fluorescence Complementation (BiFC) assays in plant cells. We found that all B-type RRs are able to interact with HPt predominant partners (HPt2, 7 and 9) of HK1, which is putatively involved in the osmosensing pathway. However, different profiles of interaction are observed depending on the studied HPt. HPt/RR interactions displayed a nuclear localization, while the nuclear and cytosolic localization of HPt and nuclear localization of RR proteins were validated. Although the nuclear localization of HPt/RR interaction was expected, this work constitutes the first evidence of such an interaction in plants. Furthermore, the pertinence of this partnership is reinforced by highlighting a co-expression of B-type RR transcripts and the other partners (HK1 and HPts) belonging to a potential osmosensing pathway.ConclusionBased on the interaction studies between identified B-type RR and HPt proteins, and the co-expression analysis of transcripts of these potential partners in poplar organs, our results favor the model that RR12, 13, 14, 16 and 19 are able to interact with the main partners of HK1, HPt2, 7 and 9, and this HPt/RR interaction occurs within the nucleus. On the whole, the five B-type RRs of interest could be third protagonists putatively involved in the osmosensing signaling pathway in Populus.
Highlights
In plants, the multistep phosphorelay signaling pathway mediates responses to environmental factors and plant hormones
In Arabidopsis thaliana, multistep phosphorelay members similar to the one found in yeast are involved in signaling pathways: Arabidopsis histidine kinases (AHKs), Arabidopsis histidine-phosphotransfer proteins (AHPs) and Arabidopsis response regulators (ARR) [3]
Urao et al found that the osmosensor receptor, AHK1, is only able to interact with one Histidine-containing Phosphotransfer proteins (HPts) protein AHP2 [15]
Summary
The multistep phosphorelay signaling pathway mediates responses to environmental factors and plant hormones. The resulting intracellular signal transduction relies notably on phosphorylation events, which are mediated by multistep phosphorelay signaling This system involves three components: a hybrid Histidine-aspartate Kinase (HK) receptor, Histidinecontaining Phosphotransfer proteins (HPt) and Response Regulators (RR). Within the eleven members of B-type RR family identified in Arabidopsis (ARR1, 2, 10, 11, 12, 13, 14, 18, 19, 20, 21), some interacting partners of AHP2 [16,17,28] have been shown to be associated with cytokinin signal transduction [29,30,31,32] This signal is relayed from membrane to nucleus where these RRs function as transcription factors that operate in the last step of the primary cytokinin response pathway. A recent study led us to propose HPt2, 7 and 9 as specific partners of HK1 in the Populus osmosensing signaling pathway [40]
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