Universität zu Köln

Pengxin, Yu ORCID: 0009-0004-8402-3150 (2026). Arabidopsis GLK transcription factors interact with ABI4 to modulate cotyledon greening in light-exposed etiolated seedlings. PhD thesis, Universität zu Köln.

PDF PhD thesis PY_publication_format_corrected.pdf Download (2MB)

Abstract

The sessile lifestyle of plants necessitates complex molecular signalling networks that tightly adjust growth, development and metabolism to the ambient environment. 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. In this process, the transcription factors GLK1 and GLK2 are central regulators of Pchlide 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 etiolated seedlings: abi4 mutants, similar to a transgenic line overexpressing GLK2, accumulated more Pchlide than the wild type in dark-grown seedlings, while glk1 glk2 mutants exhibited lower Pchlide levels than the wild type. These 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. We found that GLK1 and GLK2 transcript levels are not changed in etiolated abi4 mutant seedlings; therefore, ABI4 likely inhibits the activities of the GLK1 and GLK2 proteins through direct protein protein interactions. Although transactivation assays in tobacco did not show this inhibitory effect of ABI4, a similar assay in yeast demonstrated the possibility that ABI4 reduces the transactivational capacity of GLK1. Together, 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.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Pengxin, Yu pyu2@uni-koeln.de https://orcid.org/0009-0004-8402-3150 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-797193
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	CEPLAS - Cluster of Excellence on Plant Sciences
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Plant biology UNSPECIFIED Molecular biology UNSPECIFIED
Date of oral exam:	23 January 2026
Referee:	Name Academic Title Hoecker, Ute Prof. Kopriva, Stanislav Prof.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/79719