Arabidopsis thaliana E3 ligase AIRP4 is involved in GA synthesis.

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) in Arabidopsis thaliana. After proAtAIRP4::GUS transgenic lines were treated with GA, the GUS activity decreased in hypocotyls. Increased hypocotyl elongation in response to GA 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. The levels of endogenous GA 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. Thus, AtAIRP4 plays a negative role in GA-mediated hypocotyl elongation and root growth, and it inhibits the synthesis of endogenous biologically active GA to some extent.