Arabidopsis GLK transcription factors interact with ABA-INSENSITIVE4 to modulate cotyledon greening in light-exposed etiolated seedlings

During seedling etiolation in darkness, the biosynthesis of protochlorophyllide (Pchlide) and the development of etioplasts must be strictly controlled to prevent photooxidative damage upon light exposure. The transcription factors GLK1 and GLK2 are central regulators of chlorophyll biosynthesis and chloroplast biogenesis. Here, we show that GLK1 and GLK2 interact with ABSCISIC ACID INSENSITIVE 4 (ABI4). We reveal that GLKs and ABI4 have antagonistic functions in cotyledon greening of light-exposed etiolated seedlings: compared to the wild type, abi4 mutants, similar to a transgenic line overexpressing GLK2, accumulated more Pchlide in dark-grown seedlings, while glk1 glk2 mutants contained less Pchlide. The high Pchlide levels in etiolated abi4 mutants and GLK2 overexpressors were inefficiently photoreduced upon light exposure, leading to a significant accumulation of 1O2 in the cotyledons after the dark-to-light transition. This corresponded to low cotyledon greening rates and low seedling survival. Additionally, we identified eight PhANGs involved in Pchlide biosynthesis and etioplast development, whose transcript accumulation patterns may contribute to the photobleaching of etiolated abi4 mutants and GLK2 overexpressors. Importantly, the high Pchlide content, low cotyledon greening rate, high 1O2 level and high PhANG induction in abi4 mutant seedlings were fully dependent on GLK1 and GLK2, indicating that ABI4 acts upstream of GLKs. Since ABI4 does not regulate GLK transcript level, and ABI4 physically interacts with GLK proteins, these data suggest that ABI4 inhibits GLK1 and GLK2 activities in etiolated seedlings to prevent high Pchlide accumulation which would lead to high 1O2 levels and seedling death upon exposure to light.