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 magnesium-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 upregulates, 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 provide 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.