Arabidopsis GOLDEN2-LIKE transcription factors interact with ABA-INSENSITIVE 4 to modulate cotyledon greening in light-exposed etiolated seedlings.

The transcription factors GOLDEN2-LIKE1 (GLK1) and GLK2 are central regulators of chlorophyll biosynthesis and chloroplast biogenesis. 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 with the wild type, abi4 mutants, similar to seedlings overexpressing GLK2, accumulated more protochlorophyllide in dark-grown seedlings, while glk1 glk2 mutants contained less protochlorophyllide. The protochlorophyllide in etiolated abi4 mutants and GLK2 overexpressors was inefficiently photoreduced upon light exposure, leading to accumulation of 1O2. This corresponded to low cotyledon greening rates and low seedling survival. We also identified photosynthesis-associated nuclear genes (PhANGs) whose transcript accumulation patterns may contribute to the seedling photobleaching pattern. Importantly, the high protochlorophyllide 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 GLK genes. Since ABI4 did not regulate GLK transcript levels while it physically interacted with GLK proteins, these data suggest that ABI4 inhibits GLK1 and GLK2 activities in etiolated seedlings to prevent high protochlorophyllide accumulation which would lead to high 1O2 levels and seedling death upon light exposure.