title: Light activates the degradation of PIL5 protein to promote seed germination through gibberellin in Arabidopsis.
abstract: Angiosperm seeds integrate various environmental signals, such as water availability and light conditions, to make a proper decision to germinate. Once the optimal conditions are sensed, gibberellin (GA) is synthesized, triggering germination. Among environmental signals, light conditions are perceived by phytochromes. However, it is not well understood how phytochromes regulate GA biosynthesis. Here we investigated whether phytochromes regulate GA biosynthesis through PIL5, a phytochrome-interacting bHLH protein, in Arabidopsis. We found that pil5 seed germination was inhibited by paclobutrazol, the ga1 mutation was epistatic to the pil5 mutation, and the inhibitory effect of PIL5 overexpression on seed germination could be rescued by exogenous GA, collectively indicating that PIL5 regulates seed germination negatively through GA. Expression analysis revealed that PIL5 repressed the expression of GA biosynthetic genes (GA3ox1 and GA3ox2), and activated the expression of a GA catabolic gene (GA2ox) in both PHYA- and PHYB-dependent germination assays. Consistent with these gene-expression patterns, the amount of bioactive GA was higher in the pil5 mutant and lower in the PIL5 overexpression line. Lastly, we showed that red and far-red light signals trigger PIL5 protein degradation through the 26S proteasome, thus releasing the inhibition of bioactive GA biosynthesis by PIL5. Taken together, our data indicate that phytochromes promote seed germination by degrading PIL5, which leads to increased GA biosynthesis and decreased GA degradation.
authors.affiliation: Department of Biological Sciences, KAIST, Daejeon 305-701, Korea.
authors.forename: Eunkyoo
authors.initials: E
authors.lastname: Oh
authors.forename: Shinjiro
authors.initials: S
authors.lastname: Yamaguchi
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kamiya
authors.forename: Gabyong
authors.initials: G
authors.lastname: Bae
authors.forename: Won-Il
authors.initials: WI
authors.lastname: Chung
authors.forename: Giltsu
authors.initials: G
authors.lastname: Choi
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000502:physiology* / Q000528:radiation effects; D051792:Basic Helix-Loop-Helix Proteins / Q000502:physiology* / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant; D018525:Germination / Q000502:physiology* / Q000528:radiation effects; D005875:Gibberellins / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D008027:Light; D010834:Phytochrome / Q000502:physiology; D012639:Seeds / Q000502:physiology* / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D005875:Gibberellins; D010834:Phytochrome; C508068:At2g20180 protein, Arabidopsis