Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs, mediate karrikin and strigolactone signaling through direct binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetected or weak transcriptional repression activities, but can still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of smax1 smxl2 mutant. SMAX1 and SMXL2 directly interacted with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 and enhanced the protein stability of PIF4 and PIF5 by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were further identified by treatment with GR24^ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24^ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1 that is independent of the EAR motif had a global effect on gene expression. Taken together, these results reveal that the non-transcriptional regulatory activities of SMAX1 and SMXL2 mediates the karrikin-regulated seedling response to red light.

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs, mediate karrikin and strigolactone signaling through direct binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetected or weak transcriptional repression activities, but can still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of smax1 smxl2 mutant. SMAX1 and SMXL2 directly interacted with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 and enhanced the protein stability of PIF4 and PIF5 by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were further identified by treatment with GR24^ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24^ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1 that is independent of the EAR motif had a global effect on gene expression. Taken together, these results reveal that the non-transcriptional regulatory activities of SMAX1 and SMXL2 mediates the karrikin-regulated seedling response to red light.

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs, mediate karrikin and strigolactone signaling through direct binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetected or weak transcriptional repression activities, but can still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of smax1 smxl2 mutant. SMAX1 and SMXL2 directly interacted with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 and enhanced the protein stability of PIF4 and PIF5 by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were further identified by treatment with GR24^ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24^ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1 that is independent of the EAR motif had a global effect on gene expression. Taken together, these results reveal that the non-transcriptional regulatory activities of SMAX1 and SMXL2 mediates the karrikin-regulated seedling response to red light.