BBX32 dampens E3 ligase activity to promote greening in emerging seedlings.

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-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.