Fig. 1. Auxin and gibberellin signalling pathways. (A) Under low auxin concentrations, the transcription of auxin-response genes via the ARFs is blocked by the Aux/IAA transcriptional repressor proteins. (B) In Arabidopsis, auxin is bound by the F-box protein TIR1, or by other AFB proteins that comprise the SCF^TIR1 complex (RBX1-Cullin-ASK1-TIR1 in the figure). The binding of auxin stimulates the interaction of Aux/IAAs with SCF^TIR1 and so promotes the ubiquitination of Aux/IAAs proteins, targeting them for destruction by the 26S proteosome and releasing the ARFs from their inhibitory chaperone proteins. (C,D) In rice, gibberellin (GA) signalling involves a similar process, whereby transcription of the gibberellin-response genes is regulated by the GA-dependent degradation of the DELLA protein SLR1. (C) Under low gibberellin concentrations, SLR1 represses gibberellin responses. (D) Under high gibberellin concentrations, GA binds to the GID1 protein directly and initiates a GA-dependent interaction with SLR1. The SCF^GID2 complex is then recruited to ubiquitinate SLR1, leading to its degradation. It is unclear whether GID1-GA induces stable conformational changes to SLR1 that lead to the recruitment of the SCF^GID2 complex, or whether the GA-GID1-SLR1 is targeted by SCF^GID2 as a whole. GA-TF, gibberellin-dependent transcription factors; +mod, unknown modification; NM, nuclear membrane; OsCUL1, Oryza sativa Cullin homologue 1; OsSKP15, Oryza sativa ASK1 homologue 15; +ub, ubiquitination.