UC Riverside

Plant growth and development are governed by intricate signaling networks, including the strigolactone (SL) and karrikin (KAR) pathways. Despite sharing similar perception and signaling mechanisms, they generate distinct developmental responses. This dissertation delves into the molecular basis of the interactions between the SL receptor DWARF14 (D14) and the transcriptional repressor SMAX1-LIKE7 (SMXL7), alongside the KAR-responsive transcriptional regulation executed by the KAR receptor KARRIKIN INSENSITIVE 2 (KAI2) and SUPPRESSOR OF MAX2 1 (SMAX1) in Arabidopsis thaliana.Employing site-directed and random mutagenesis on SMXL7 revealed crucial residues affecting its specificity with D14. Interestingly, many identified residues are neither D14-interfacing nor surface-exposed, indicating a sophisticated mechanism that modulates this interaction. This finding challenges the prevailing view that interaction specificity mainly hinges on interface mutations and offers insights into the co-evolution of signaling molecules. Structure-Function analyses of SMAX1 and SMXL7 demonstrated the pivotal role of the N domain in mediating downstream KAR and SL responses. Genetic tests with domain swapping and deletion analyses on SMAX1 and SMXL7 confirmed the essential function of the N domain in controlling downstream transcription. A yeast two-hybrid assay with an Arabidopsis transcription factor library identified 32 transcription factors interacting with SMAX1, whose interactions are mediated via the N domain. To uncover SMAX1's target proteins and genes, we employed proximity labeling and estradiol-inducible systems centered on the SMAX1 N domain as a crucial regulation domain. Ongoing efforts aim to investigate SMAX1-interacting proteins to highlight its regulatory mechanism. Furthermore, an estradiol-inducible system was utilized to explore early transcriptional responses to SMAX1 variants despite challenges, discussing potential optimizations for the system. This research advances our understanding of the molecular mechanisms underlying the specificity and evolutionary trajectory of SL signaling. It also elucidates the regulatory functions of SMAX1 and SMXL7 in plant development, setting the stage for future explorations into the dynamic interactions between hormone signaling pathways and their transcriptional outcomes.