Protein phosphorylation is a major molecular switch involved in the regulation of stomatal opening and closure. Previous research defined interaction between MAP kinase 12 and Raf‐like kinase HT1 as a required step for stomatal movements caused by changes in CO2concentration. However, whether MPK12 kinase activity is required for regulation of CO2‐induced stomatal responses warrants in‐depth investigation. We apply genetic, biochemical, and structural modeling approaches to examining the noncatalytic role of MPK12 in guard cell CO2signaling that relies on allosteric inhibition of HT1. We show that CO2/HCO3−‐enhanced MPK12 interaction with HT1 is independent of its kinase activity. By analyzing gas exchange of plant lines expressing various kinase‐dead and constitutively active versions of MPK12 in a plant line where These data add to the model that MPK12 kinase‐activity‐independent interaction with HT1 functions as a molecular switch by which guard cells sense changes in atmospheric CO2concentration.
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Summary MPK12 is deleted, we confirmed that CO2‐dependent stomatal responses rely on MPK12's ability to bind to HT1, but not its kinase activity. We also demonstrate that purified MPK12 and HT1 proteins form a heterodimer in the presence of CO2/HCO3−and present structural modeling that explains the MPK12:HT1 interaction interface. -
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Abstract Plant steroid hormones brassinosteroids (
BR s) regulate plant growth and development at many different levels. Recent research has revealed that stress‐responsive NAC (petunia NAM and Arabidopsis ATAF1, ATAF2, and CUC2) transcription factorRD 26 is regulated byBR signaling and antagonizesBES 1 in the interaction between growth and drought stress signaling. However, the upstream signaling transduction components that activateRD 26 during drought are still unknown. Here, we demonstrate that the function ofRD 26 is modulated byGSK 3‐like kinaseBIN 2 and protein phosphatase 2CABI 1. We show thatABI 1, a negative regulator inabscisic acid (ABA) signaling, dephosphorylates and destabilizesBIN 2 to inhibitBIN 2 kinase activity.RD 26 protein is stabilized byABA and dehydration in aBIN 2‐dependent manner.BIN 2 directly interacts and phosphorylatesRD 26in vitro andin vivo .BIN 2 phosphorylation ofRD 26 is required forRD 26 transcriptional activation on drought‐responsive genes.RD 26 overexpression suppressed the brassinazole (BRZ) insensitivity ofBIN 2 triple mutantbin2 bil1 bil2 , andBIN 2 function is required for the drought tolerance ofRD 26 overexpression plants. Taken together, our data suggest a drought signaling mechanism in which drought stress relievesABI 1 inhibition ofBIN 2, allowingBIN 2 activation. Sequentially,BIN 2 phosphorylates and stabilizesRD 26 to promote drought stress response.
