Identification and characterization of Arabidopsis gibberellin receptors


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Three gibberellin (GA) receptor genes (AtGID1a, AtGID1b and AtGID1c), each an ortholog of the rice GA receptor gene (OsGID1), were cloned from Arabidopsis, and the characteristics of their recombinant proteins were examined. The GA-binding activities of the three recombinant proteins were confirmed by an in vitro assay. Biochemical analyses revealed similar ligand selectivity among the recombinants, and all recombinants showed higher affinity to GA4 than to other GAs. AtGID1b was unique in its binding affinity to GA4 and in its pH dependence when compared with the other two, by only showing binding in a narrow pH range (pH 6.4–7.5) with 10-fold higher affinity (apparent Kd for GA4 = 3 × 10−8 m) than AtGID1a and AtGID1c. A two-hybrid yeast system only showed in vivo interaction in the presence of GA4 between each AtGID1 and the Arabidopsis DELLA proteins (AtDELLAs), negative regulators of GA signaling. For this interaction with AtDELLAs, AtGID1b required only one-tenth of the amount of GA4 that was necessary for interaction between the other AtGID1s and AtDELLAs, reflecting its lower Kd value. AtDELLA boosted the GA-binding activity of AtGID1 in vitro, which suggests the formation of a complex between AtDELLA and AtGID1–GA that binds AtGID1 to GA more tightly. The expression of each AtGID1 clone in the rice gid1-1 mutant rescued the GA-insensitive dwarf phenotype. These results demonstrate that all three AtGID1s functioned as GA receptors in Arabidopsis.