In Arabidopsis, three receptors exist for the gibberellins (GA). Of the three, only a double loss-of-function mutant (atgid1a atgid1c) shows a dwarf phenotype. We show that no enzymatic activity exists in inflorescence stems of AtGID1b-GUS expression lines driven by AtGID1b promoter. The fact strongly suggests that atgid1a atgid1c lacks sufficient GA receptor for normal stem growth. On the other hand, we detected enzymatic activities of AtGID1c-GUS gene's product in stamens of the transformants, even though atgid1a atgid1b has short stamens and shows quite low fertility. Then we evaluated the affinity of the AtGID1-DELLA interaction by a competitive yeast three-hybrid system. It was elucidated that AtGID1c showed quite low affinities to major DELLA proteins in floral buds, which may be responsible for short stamens of atgid1a atgid1b. We propose that in a double loss-of-function mutant of GA receptors, the emergence of any phenotype(s) depends on the abundance of the remaining receptor and its preference to DELLA proteins existing at a target site.
