A Co-chaperonin Cpn20, Interacting With Mg-chelatase H Subunit, Negatively Regulates ABA Signal Transduction In Arabidopsis

The magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR) functions as acandidate receptor for ABA, which can positively regulate ABA signaling byantagonizing a group of WRKY-domain transcription repressors (WRKY40/WRKY18/WRKY60) in Arabidopsis.We screened a chloroplast co-chaperonin20(CPN20) as one of the interaction partnersof ABAR. Genetic and physiological studies showed that CPN20negatively regulatesABA signaling in the major ABA responses, which functions independently of itsco-chaperonin role, supported by the evidence that neither CPN10(1)/CPN10(2) norchaperonin60(CPN60) is involved in ABA signaling.The mechanism of how CPN20regulates ABAR-WRKY40mediated ABA signalingpathway was then studied. Downregulation of CPN20inhibits the hypo-sensitivity toABA in cch mutant and ABAR-RNAi transgenetic lines, while overexpressing ABAR incpn20mutant can not rescue its ABA-hypersensitivity phenotype. However,overexpressing CPN20in cch mutant can not rescue its ABA-hyposensitivity phenotypeeither, indicating that CPN20functions at the same node as, or downstream of ABAR.Meanwhile, wrky40mutant with overexpressed CPN20showed similar ABA-sensitivityto wrky40itself, suggesting that CPN20functions upstream of WRKY40. Furthermolecular and biochemistry analysis showed that expression of WRKY40is decreased incpn20mutant but increased in CPN20-overexpression lines as well as in cch mutant.ABA inhibition on expression of WRKY40in cch mutant is abolished, but enhanced incch mutant downregulating CPN20, which indicated that CPN20may positivelyregulate WRKY40expression by antagonizing ABAR. Moreover, while ABA weakensCPN20-ABAR interaction, CPN20can attenuate ABAR-WRKY40interaction bycompetitively binding to ABAR, suggesting another possible pathway where CPN20functions negatively in ABA signaling by antagnizing ABAR-WRKY40interaction.These findings provide a new insight into the role of co-chaperonins in the regulation of plant responses to environmental cues, and reveal that CPN20may function negativelyin ABA signaling by repressing WRKY40-mediated signaling downstream of ABAR orby antagonizing the ABAR-WRKY40-coupled signaling to de-repress a set of ABAresponsive genes.