Summary Cytosolic ABA Receptor Kinases (CARKs) play a pivotal role in abscisic acid (ABA)‐dependent pathway in response to dehydration, but their regulatory mechanism in ABA signaling remains unexplored. In this study, we showed that CARK4/5 of CARK family physically interacted with ABA receptors (RCARs/PYR1/PYLs), including RCAR3, RCAR11–RCAR14, while CARK2/7/11 only interacted with RCAR11–RCAR14, but not RCAR3. It indicates that the members in CARK family function redundantly and differentially in ABA signaling. RCAR12 can form heterodimer with RCAR3 in vitro and in vivo. Moreover, the members of CARK family can form homodimer or heterodimer in a kinase activity dependent manner. ITC (isothermal titration calorimetry) analysis demonstrated that the phosphorylation of RCAR12 by CARK1 enhanced the ABA binding affinity. The phosphor‐mimic RCAR12T105D significantly displayed ABA‐induced inhibition of the phosphatase ABI1 (ABA insensitive 1) activity, leading to upregulation of ABA‐responsive genes RD29A and RD29B in cark157:RCAR12T105D transgenic plants, which exhibited ABA hypersensitive phenotype. The transcription factor ABI5 (ABA insensitive 5) activates the transcriptions of CARK1 and CARK3 by binding to ABA‐response elements (ABREs) of their promoters. Collectively, our data imply that the dimeric CARKs phosphorylate homodimer or heterodimer ABA receptors, leading to monomerization for triggering ABA responses in Arabidopsis.
The phytohormone abscisic acid (ABA) plays an important role in the ability of plants to cope with drought stress. As core members of the ABA signaling pathway, protein phosphatase type 2Cs (PP2Cs) have been reported in many species. However, the functions of MdPP2Cs in apple (Malus domestica) are unclear. In this study, we identified two PP2C-encoding genes, MdPP2C24/37, with conserved PP2C catalytic domains, using sequence alignment. The nucleus-located MdPP2C24/37 genes were induced by ABA or mannitol in apple. Genetic analysis revealed that overexpression of MdPP2C24/37 in Arabidopsis thaliana led to plant insensitivity to ABA or mannitol treatment, in terms of inhibiting seed germination and overall seedling establishment. The expression of stress marker genes was upregulated in MdPP2C24/37 transgenic lines. At the same time, MdPP2C24/37 transgenic lines displayed inhibited ABA-mediated stomatal closure, which led to higher water loss rates. Moreover, when exposed to drought stress, chlorophyll levels decreased and MDA and H2O2 levels accumulated in the MdPP2C24/37 transgenic lines. Further, MdPP2C24/37 interacted with MdPYL2/12 in vitro and vivo. The results indicate that MdPP2C24/37 act as negative regulators in response to ABA-mediated drought resistance.
As a drought-tolerant crop, Tartary buckwheat survives under adverse environmental conditions, including drought stress. Proanthocyanidins (PAs) and anthocyanins are flavonoid compounds, and they participate in the regulation of resistance to both biotic and abiotic stresses by triggering genes’ biosynthesis of flavonoids. In this study, a basic leucine zipper, basic leucine zipper 85 (FtbZIP85), which was predominantly expressed in seeds, was isolated from Tartary buckwheat. Our study shows that the expressions of FtDFR, FtbZIP85 and FtSnRK2.6 were tissue-specific and located in both the nucleus and the cytosol. FtbZIP85 could positively regulate PA biosynthesis by binding to the ABA-responsive element (ABRE) in the promoter of dihydroflavonol 4-reductase (FtDFR), which is a key enzyme in the phenylpropanoid biosynthetic pathway. Additionally, FtbZIP85 was also involved in the regulation of PA biosynthesis via interactions with FtSnRK2.6 but not with FtSnRK2.2/2.3. This study reveals that FtbZIP85 is a positive regulator of PA biosynthesis in TB.
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