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Gibberellins (GAs), as one of the most important phytohormones, control different aspect of plant growth and development including seed germination, stem elongation and flower induction. Among more than a hundred and thirty GAs identified from plants, fungi and bacteria only a small number of them, such as GA1, GA3, GA4, GA5, GA6 and GA7, are thought to function as bioactive hormones. Therefore, many non-bioactive GAs exist in plants as precursors or deactivated metabolites. Recent biochemical, genetic and molecular studies have elucidated in detail the mechanism of GA perception and signal transduction in plants (fig. 11). In the first step the GA signal is perceived by the GA receptor – GID1 (GA insensitive dwarf 1), which is a soluble protein that is localized to both cytoplasm and nucleus. There is a single GID1 gene in rice (Oryza sativa), but three ortholoques in Arabidopsis thaliana (GID1a, GID1b, GID1c) with overlapping functions. The binding of bioactive GAs to GID1 promotes an interaction between GID1 and the DELLA-domain of DELLA proteins, which are main repressors of gibberellin pathway. The DELLA motif is essential for this interaction, because its deletion results in an inability to interact with GID1, despite the presence of GAs. Whereas rice has only one DELLA protein – SLR1 (slender rice 1), the Arabidopsis genome encodes five DELLAs – GAI (GA insensitive), RGA (repressor of GA1-3), RGL1, RGL2 and RGL3 (rga like 1/2/3). Structurally, DELLAs are a subgroup of proteins that belong to the GRAS (gai, rga, scarecrow) family of transcriptional regulators. The binding of DELLA by GA-GID1 enhances the affinity between DELLA and a specific SCF E3 ubiquitin-ligase complex involving the F-box proteins AtSLY1 and OsGID2 in Arabidopsis and rice, respectively. In turn, SCFSLY1/GID2 promotes the ubiquitinylation and subsequent destruction of DELLAs by the 26S proteasome. This situation is a key event in GA signaling and affect on transcription factors which regulate activity of target genes. The mechanism of hormonal signal transduction, presented in this paper has been described only in plants, however it can supposed, that this novel mechanism may also exist in other organisms.

The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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