BBX32 dampens E3 ligase activity to promote greening in emerging seedlings

ABSTRACT Germinating seeds undergo elaborate de‐etiolation developmental transitions upon initial soil emergence. As central transcription factors promoting cotyledon greening, the abundance of ETHYLENE‐INSENSITIVE 3 (EIN3) and PHYTOCHROME‐INTERACTING FACTOR 3 (PIF3) are strictly controlled by physically associating themselves with the EIN3‐BINDING F BOX PROTEINS 1 and 2 (EBF1/2) for ubiquitination. Here, we report that the B‐box zinc‐finger protein BBX32, as a positive regulator during seedling de‐etiolation. BBX32 is robustly elevated during the dark‐to‐light transitions. Constitutively expressing BBX32 ultimately protects against severe photobleaching damage by synchronizing the accumulation of protochlorophyllide (Pchlide) and the differentiation of etioplast–chloroplast apparatus in buried seedlings. Specifically, BBX32 directly interacts with EIN3, PIF3 and EBF1/2. These associations disrupt the assembly of the SCF EBF1/2 ‐EIN3/PIF3 E3 ligation protein complexes, thus dampening E3 ligase activity and robustly controlling EIN3/PIF3 stability. Under soil conditions, BBX32‐ox largely rescues the greening deficiency of EBF1ox , and all EIN3ox/bbx32 seedlings override the bbx32 mutant defect and successfully turn green. Both biochemical findings and genetic evidence reveal a novel regulatory paradigm by which the B‐box protein dampens the E3 ligase binding activity to achieve green seedlings upon changes in light or soil environmental conditions.