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The phytohormone abscisic acid (ABA) regulates plant growth and development and stress resistance through the ABA receptor PYLs. To date, no interaction between CPK and PYL has been reported, even in Arabidopsis and rice. In this study, we found that MdCPK4 from Malus domestica (Md for short) interacts with two MdPYLs, MdPYL2/12, in the nucleus and the cytoplasm in vivo and phosphorylates the latter in vitro as well. Compared with the wild type (WT), the MdCPK4- or MdPYL2/12 -overexpressing Arabidopsis lines showed more sensitivity to ABA, and therefore stronger drought resistance. The ABA-related genes (ABF1 , ABF2 , ABF4 , RD29A and SnRK2.2) were significantly upregulated in the overexpressing (OE) lines after ABA treatment. These results indicate that MdCPK4 and MdPYL2/12 act as positive regulators in response to ABA-mediated drought resistance in apple. Our results reveal the relationship between MdCPK4 and MdPYL2/12 in ABA signaling, which will further enrich the molecular mechanism of drought resistance in plants. [ABSTRACT FROM AUTHOR]

Arabidopsis abscisic acid ABA-Insensitive RING Proteins (AtAIRP1–4) are RING E3s that play significant roles in ABA-signaling pathways. However, it is still unclear whether they have other functions. Here, AtAIRP4 was determined to play a role in response to gibberellin A3 (GA 3) in Arabidopsis thaliana. After proAtAIRP4::GUS transgenic lines were treated with GA 3 , the GUS activity decreased in hypocotyls. Increased hypocotyl elongation in response to GA 3 seen in WT was not observed in the AtAIRP4 -overexpression lines, whereas AtAIRP4 -overexpression lines were hypersensitive to Paclobutrazol (PAC, an inhibitor of GA biosynthesis) during the seed germination stage. Additionally, AtAIRP4 -overexpressing lines showed the lowest level of primary root elongation in the presence of GA 3. The levels of endogenous GA 3 in 35S::AtAIRP4 lines were lower than those in wild-type. In addition, among the plants, the mRNA levels of the GA synthetic gene GIBBERELLIN 20-OXIDASE1 (GA20ox1) was the lowest in overexpressing line. However, the expression of the response gene DELLA RGA-LIKE3 (RGL3) was the highest in overexpressing lines after treatment with GA 3. Thus, AtAIRP4 plays a negative role in GA-mediated hypocotyl elongation and root growth, and it inhibits the synthesis of endogenous biologically active GA 3 to some extent. [ABSTRACT FROM AUTHOR]