Arabidopsis co-chaperonin CPN20 antagonizes Mg-chelatase H subunit to derepress ABA-responsive WRKY40 transcription repressor

Our previous study demonstrated that a chloroplast co-chaperonin 20 （CPN20）, one of the interaction partners of the magne- sium-protoporphyrin IX chelatase H subunit （CHLH/ABAR）, negatively regulates ABA signaling at the same node with ABAR but upstream of WRKY40 transcription repressor in Arabidopsis thaliana. In the present experiment, we showed that ABA directly inhibits the ABAR-CPN20 interaction, and also represses expression of CPN20, which depends on ABAR. CPN20 inhibits ABAR-WRKY40 interaction by competitively binding to ABAR. ABAR downregulates, but CPN20 upregu- lates, WRKY40 expression. The cpn20-1 mutation induces downregulation of WRKY40, and suppresses the upregulated level of WRKY40 due to the cch mutation in the ABAR gene. ABA-induced repressive effect of the WRKY40 gene is strengthened by downregulation of CPN20 but reduced by upregulation of CPN20. Together with our previously reported genetic data, we pro- vide evidence that CPN20 functions through antagonizing the ABAR-WRKY40 coupled pathway, and ABA relieves this pathway of repression by inhibiting the ABAR-CPN20 interaction to activate ABAR-WRKY40 interaction.