Abstract
We have previously identified proteins in poplar which belong to an osmosensing (OS) signaling pathway, called a multistep phosphorelay (MSP). The MSP comprises histidine-aspartate kinases (HK), which act as membrane receptors; histidine phosphotransfer (HPt) proteins, which act as phosphorelay proteins; and response regulators (RR), some of which act as transcription factors. In this study, we identified the HK proteins homologous to the Arabidopsis cytokinin (CK) receptors, which are first partners in the poplar cytokinin MSP, and focused on specificity of these two MSPs (CK and OS), which seem to share the same pool of HPt proteins. Firstly, we isolated five CK HKs from poplar which are homologous to Arabidopsis AHK2, AHK3, and AHK4, namely, HK2, HK3a, HK3b, HK4a, HK4b. These HKs were shown to be functional kinases, as observed in a functional complementation of a yeast HK deleted strain. Moreover, one of these HKs, HK4a, was shown to have kinase activity dependent on the presence of CK. Exhaustive interaction tests between these five CK HKs and the 10 HPts characterized in poplar were performed using two-hybrid and BiFC experiments. The resulting partnership was compared to that previously identified between putative osmosensors HK1a/1b and HPt proteins. Finally, in planta coexpression analysis of genes encoding these potential partners revealed that almost all HPts are coexpressed with CK HKs in four different poplar organs. Overall, these results allowed us to unravel the common and specific partnerships existing between OS and CK MSP in Populus.
Highlights
It has been known for a long time that the phytohormone cytokinin (CK) is involved in numerous physiological processes crucial to plant growth and development [1,2,3,4]
In the context of via increasing periods, it isto highly important to understand how plants are able to control CK homeostasis via the CK signaling pathway to preserve growth while tolerating drought. This notion leads to the hypothesis of a Yin-Yang relationship between CK homeostasis and plant acclimation/adaption to drought [14]
An OS signaling pathway has been identified in Arabidopsis and is controlled by the AHK1 histidine-aspartate kinase [17,18,19]
Summary
It has been known for a long time that the phytohormone cytokinin (CK) is involved in numerous physiological processes crucial to plant growth and development [1,2,3,4]. In Arabidopsis thaliana, the OS and CK pathways belong to a multistep phosphorelay (MSP) system derived from the bacterial two-component system [16] In such a system, the membrane-bound histidine-aspartate kinase (HK) receptor senses its environment and autophosphorylates upon signal activation. Regarding the CK signaling pathway, three HKs containing the CK-binding domain (CHASE domain) have been isolated and characterized (reviewed in [4,20]) These CK HKs, namely, AHK2, AHK3, and AHK4 (CRE1), have been well studied and shown to interact with the four authentic HPt proteins, AHP1, AHP2, AHP3, and AHP5 [21]. It has been shown in this plant that AHK1 can only interact with one HPt protein, AHP2 [22], leading to the conclusion that AHP2 is able to interact with either
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