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Title: The role of two F-box proteins, SLEEPY1 and SNEEZY, in arabidopsis GA signaling

item ARIIZUMI, TOHRU - Washington State University
item LAWRENCE, PAULRAJ - Washington State University
item Steber, Camille

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2010
Publication Date: 2/1/2011
Citation: Ariizumi, T., Lawrence, P.K., Steber, C.M. 2011. The role of two F-box proteins, SLEEPY1 and SNEEZY, in arabidopsis GA signaling. Plant Physiology. 115:765-775.

Interpretive Summary: This study examines the relative roles of two similar genes, SLEEPY1 (SLY1) and SNEEZY (SNE) in transmitting the GA hormone signal. The F-box gene SLY1 was first identified as a positive regulator of gibberellin (GA) signaling such that mutations in SLY1 cause all of the defects you'd expect if a plant cannot sense the GA hormone including dwarfism, reduced fertility, delayed flowering, and increased seed dormancy. These sly1 defects can be rescued by a gene that has similar sequence, SNEEZY (SNE)/SLEEPY2 (SLY2). SLEEPY1 is known to stimulate plant germination, growth, and fertility by triggering the destruction of DELLA proteins which otherwise block these aspects of plant growth and development. This study determined that SNEEZY can also cause DELLA destruction, but not as well as SLEEY1. The parts of the SLEEPY1 and SNEEZY genes/proteins that are needed for function were identified. Finally, it was shown that SLEEPY1 is the more important regulator, but SNEEZY may have some unique roles. SLEEPY1 protein was shown to directly bind to DELLA and the GA receptor proteins giving a clearer knowledge of its role in GA hormone signlaing.

Technical Abstract: The F-box gene SLY1 is a positive regulator of gibberellin (GA) signaling and loss of SLY1 results in GA-insensitive phenotypes including dwarfism, reduced fertility, delayed flowering, and increased seed dormancy. These sly1 phenotypes can be partially rescued by overexpression of the SLY1 homolog, SNEEZY (SNE)/SLEEPY2 (SLY2) suggesting that SNE can functionally replace SLY1. GA responses are repressed by DELLA proteins, and the function of SLY1 as the F-box subunit of an SCF E3 ubiqitin ligase is to bind, ubiquitinate, and target DELLA proteins for destruction. SCFSLY1 mediated DELLA destruction occurs when DELLA is bound by the GA receptor GA-INSENSITIVE DWARF1 (GID1) in complex with GA hormone. If SNE can replace SLY1, then SNE would form an SCF complex, physically interact with and target DELLA for destruction. HA-SLY1 and HA-SNE/SLY2 translational fusions were expressed in sly1-10 lines under control of the constitutive 35S promoter. Both HA-SLY1 and HA-SNE interacted with the CUL1 (cullin) subunit of the SCF complex in co-immunoprecipitation assays, and this interaction required the F-box domain. Both HA-SLY1 and HA-SNE co-immunoprecipitated with the DELLA RGA, and this interaction required the C-terminal domain. Whereas HA-SLY1 overexpression resulted in decreased expression of both DELLA RGA and RGL2, HA-SNE caused a decrease in DELLA RGA but not in RGL2 protein levels. This suggests that one reason HA-SLY1 is able to effect a stronger rescue of sly1-10 phenotypes than HA-SNE is because SLY1 is able to interact with a broader spectrum of DELLA proteins. SLY1, the predominant F-box gene in Arabidopsis GA signaling, is expressed at higher levels than SNE in all tissues examined. The HA-SLY1 protein was found to co-immunopreciptiate with the GA receptor FLAG-GID1b supporting the model that SLY1 regulates DELLA through interaction with the DELLA-GID1 complex.