Point mutations in Arabidopsis Cullin1 reveal its essential role in jasmonate response

The SKP1-Cullin/Cdc53-F-box protein ubiquitin ligases (SCF) target many important regulatory proteins for degradation and play vital roles in diverse cellular processes. In Arabidopsis there are 11 Cullin members (AtCUL). AtCUL1 was demonstrated to assemble into SCF complexes containing COI1, an F-box protein required for response to jasmonates (JA) that regulate plant fertility and defense responses. It is not clear whether other Cullins also associate with COI1 to form SCF complexes, thus, it is unknown whether AtCUL1, or another Cullin that assembles into SCF(COI1) (even perhaps two or more functionally redundant Cullins), plays a major role in JA signaling. We present genetic and physiological data to directly demonstrate that AtCUL1 is necessary for normal JA responses. The homozygous AtCUL1 mutants axr6-1 and axr6-2, the heterozygous mutants axr6/AXR6, and transgenic plants expressing mutant AtCUL1 proteins containing a single amino acid substitution from phenylalanine-111 to valine, all exhibit reduced responses to JA. We also demonstrate that ax6 enhances the effect of coi1 on JA responses, implying a genetic interaction between COI1 and AtCUL1 in JA signaling. Furthermore, we show that the point mutations in AtCUL1 affect the assembly of COI1 into SCF, thus attenuating SCF(COI1) formation.