STUDY ON THE DEGRADATION OF THE REPRESSOR OF GA SIGNALING

赤霉素（Gibberellins, GAs）是一大类四环二萜植物激素，可诱导多种植物生长反应，包括种子萌发、茎伸长、叶片膨胀、花粉成熟和诱导开花。最近对GA不敏感和GA敏感突变体的研究表明，GA反应是通过推定的转录调节因子DELLA蛋白的降解以GA依赖的方式发生的。然而，GA感知的分子机制尚未明确。在这里，我们报道了一个新的砷不敏感水稻矮突变体gid1^1的分离和特性。GID1基因编码一种与激素敏感脂肪酶相似的未知蛋白，我们观察到GID1- gfp信号在细胞核中的优先定位。重组GST-GID1仅对具有生物活性的GAs具有高亲和力，而对应于三个gid1等位基因的突变GST-GID1没有ga结合亲和力。估计GA_4的解离常数约为10^<-7> M，足以说明GA依赖于茎伸长。此外，GID1在酵母细胞中以ga依赖的方式与水稻DELLA蛋白SLR1结合。GID1过表达导致ga超敏表型。综上所述，这些结果表明GID1是水稻中介导GA信号的可溶性受体。

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, pollen maturation and induction of flowering. Recent works using GA insensitive and GA hypersensitive mutants revealed that GA response is occurred by the degradation of DELLA protein, a putative transcription regulator, in GA dependent manner. However, molecular mechanisms of GA perception have not been clarified yet.Here we report isolation and characterization of a new GA-insensitive dwarf mutant of rice, gid1^1. The GID1 gene encodes an unknown protein with similarity to the hormone-sensitive lipases, and we observed preferential localization of the GID1-GFP signal in nuclei. Recombinant GST-GID1 had a high affinity only for biologically active GAs, whereas mutated GST-GID1 corresponding to three gid1 alleles had no GA-binding affinity. The dissociation constant for GA_4 was estimated to be around 10^<-7> M, enough to account for the GA dependency on shoot elongation. Moreover, GID1 bound to SLR1, a rice DELLA protein, in a GA-dependent manner in yeast cells. GID1 overexpression resulted in a GA-hypersensitive phenotype. Taken together, these results indicate that GID1 is a soluble receptor mediating GA signalling in rice.