Functions of BOIs in Arabidopsis development

Abstract

RING-containing proteins regulate various stages of plant development such as seed germination, vegetative phase transition, and flowering. Among the 469 predicted RING proteins in Arabidopsis, many of them have been shown to function as single subunit E3 ubiquitin ligases or as components of E3 ubiquitin ligase complexes. Monoubiquitylations or polyubiquitylations by E3 ubiquitin ligases are markers for protein degradation, protein activation, transcriptional regulation, DNA repair, or intracellular sorting. Several RING proteins also participate in protein-protein interactions in which their molecular functions are still unclear. Here, I studied four Arabidopsis RING domain proteins which have been previously identified as BO-TRYTIS SUSCEPTIBLE1 INTERACTOR (BOI), BOI-RELATED GENE1 (BRG1), BRG2, and BRG3 (collec-tively referred to as BOIs). A boi quadruple mutant (boi brg1 brg2 brg3) showed enhanced seed germination frequency in the presence of paclobutrazol (PAC), precocious vegetative development, and early flowering. On the contrary, the BOI-overexpressing lines displayed reduced germination frequency on the PAC-containing me-dium, late juvenile-to-adult phase transition, and late flowering. Phenotypes of the boi quadruple mutant and BOI-overexpressing lines suggest that BOIs act as repressors in Arabidopsis development. Overexpression of a RINGless BOI did not exhibit the phenotypes observed in the overexpressing lines of full-length BOI, indicating that the RING motif is required for the function of BOI. In yeast two-hybrid, in vitro pull-down and in vivo co-immunoprecipitation assays, BOIs was shown to interact with DELLAs, key negative regulators in the phyto-hormonal gibberellin (GA) signaling. Together with DELLAs, BOIs inhibited a subset of GA-responses such as seed germination, chlorophyll accumulation, vegetative phase transition and flowering; however, BOIs did not regulate DELLA-mediated hypocotyl and root elongation. Further studies with a ga...