BTB/POZ-MATH proteins regulate Arabidopsis seedling development by promoting auxin-independent degradation of the Aux/IAA protein IAA10

After germination, seedlings undergo etiolated development (skotomorphogenesis), enabling them to grow toward the soil surface. In Arabidopsis (Arabidopsis thaliana), etiolated seedlings exhibit rapid hypocotyl elongation, apical hook formation, and closed cotyledons to protect the meristem. In this study, we found that high-order mutants in the BPM (BTB/POZ-MATH) gene family displayed defects in seedling development, characterized by a shorter hypocotyl, early apical hook opening, and opened cotyledons in the dark. BPM1, BPM2, BPM4, and BPM5 exhibited distinct expression patterns and subcellular localization in etiolated seedlings. In a hypocotyl segment assay, the bpm mutants showed defects in auxin response, indicating impaired auxin signaling in the hypocotyl. Expression of the auxin reporter DR5:GFP was also altered in the bpm1,4,5 mutant in various tissues compared to the wild type. Furthermore, yeast two-hybrid, bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation (Co-IP) assay analyses showed that BPM1 interacts with IAA10. Experiments in protoplasts indicated that BPM1 promotes IAA10 ubiquitylation and degradation, which was supported by greater IAA10 protein accumulation in the bpm1,4,5 mutant background. In addition, IAA10 over-expression resulted in phenotypes similar to those of the bpm mutants, indicating that the BPMs may target the Aux/IAA proteins for ubiquitylation and degradation. Overall, our findings shed light on the key roles of the BPMs in auxin signaling during seedling development.