Abstract
To investigate gibberellin (GA) signaling using the rice (Oryza sativa) GA receptor GIBBERELLIN-INSENSITIVE DWARF1 (GID1) mutant gid1-8, we isolated a suppressor mutant, Suppressor of gid1-1 (Sgd-1). Sgd-1 is an intragenic mutant containing the original gid1-8 mutation (L45F) and an additional amino acid substitution (P99S) in the loop region. GID1(P99S) interacts with the rice DELLA protein SLENDER RICE1 (SLR1), even in the absence of GA. Substitution of the 99th Pro with other amino acids revealed that substitution with Ala (P99A) caused the highest level of GA-independent interaction. Physicochemical analysis using surface plasmon resonance revealed that GID1(P99A) has smaller K(a) (association) and K(d) (dissociation) values for GA(4) than does wild-type GID1. This suggests that the GID1(P99A) lid is at least partially closed, resulting in both GA-independent and GA-hypersensitive interactions with SLR1. One of the three Arabidopsis thaliana GID1s, At GID1b, can also interact with DELLA proteins in the absence of GA, so we investigated whether GA-independent interaction of At GID1b depends on a mechanism similar to that of rice GID1(P99A). Substitution of the loop region or a few amino acids of At GID1b with those of At GID1a diminished its GA-independent interaction with GAI while maintaining the GA-dependent interaction. Soybean (Glycine max) and Brassica napus also have GID1s similar to At GID1b, indicating that these unique GID1s occur in various dicots and may have important functions in these plants.
Highlights
Gibberellins (GAs) are a large family of tetracyclic diterpenoid plant hormones that induce a wide range of plant growth responses, including seed germination, stem elongation, leaf expansion, flowering, and pollen maturation (Thomas et al, 2005; Aya et al, 2009)
We found that soybean (Glycine max) and Brassica napus have similar unique GIBBERELLIN-INSENSITIVE DWARF1 (GID1) that have GA-independent interaction activity with DELLA, indicating that these GID1s occur in various kinds of dicot species and might have crucial roles in some developmental process
We focus on one mutant, Suppressor for gid1-1 (Sgd-1), which showed the clearest phenotype among the five mutants
Summary
Gibberellins (GAs) are a large family of tetracyclic diterpenoid plant hormones that induce a wide range of plant growth responses, including seed germination, stem elongation, leaf expansion, flowering, and pollen maturation (Thomas et al, 2005; Aya et al, 2009). The molecular system involving GID1, DELLA, and F-box proteins is accepted as a basic concept of GA perception, we are still far from the complete picture as to how GAs modulate plant growth, and many questions still remain to be resolved. It is still unknown why GID1–DELLA protein interaction induces the interaction with an F-box protein, GID2/SLY1, and the following degradation of the DELLA protein. To answer these remaining but important questions on the GID1–DELLA–dependent GA perception system, we attempted to isolate and characterize suppressor
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