Biology subjects

Hoecker, U.

Publications and source records attributed to Hoecker, U..

4 recordsLinked to original sources

bioRxiv2026

Functional divergence of the repressors of photomorphogenesis SPA2 and SPA3 during Arabidopsis seedling deetiolation

The COP1/SPA ubiquitin ligase is a key repressor of photomorphogenesis that is inactivated by photoreceptors to initiate light signalling. The four SPA proteins (SPA1-SPA4) confer functional specificity to COP1 during plant growth, yet the underlying molecular mechanisms remain unclear. Here, we used a domain-swap approach in transgenic seedlings to address the functional divergence of SPA2 and SPA3. We show that the respective N-terminal kinase domain determines the contrasting protein stabilities of SPA2 and SPA3 in light-grown seedlings. The instability of SPA2 correlates with a specific ability of the SPA2 N-terminal domain to bind phytochrome A in the light, suggesting that phytochrome A promotes the CUL4DET1/COP1-dependent degradation of SPA2 but not of SPA3. We uncover that the coiled-coiled and WD-repeat domains of SPA2 and SPA3 substantially differ in their activity in repression of photomorphogenesis, with those of SPA2 being more active repressors than those of SPA3. Thus, SPA2 combines a potent repressor activity with light-induced instability. We conclude that the evolution of SPA2 instability in response to light counterbalances its inherent strong repressor activity, thereby allowing seedling etiolation in darkness followed by rapid reduction in COP1 activity through SPA2 degradation upon light-exposure as seedlings emerge from soil to initiate photosynthetic growth. HighlightThe repressor of light signaling SPA2 combines a phytochrome A-interacting instability domain with a potent repressor domain to allow greatly contrasting activities of COP1 in skoto- and photomorphogenesis.

plant biology↗

bioRxiv2026

Arabidopsis GLK transcription factors interact with ABI4 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. Significance statementThe transition from darkness to light is a critical moment that determines seedling establishment and survival. We report here that the transcription factor ABI4 inhibits GLK1 and GLK2 activities during seedling etiolation to repress Pchlide biosynthesis in darkness, thereby limiting singlet oxygen accumulation and seedling death upon exposure to light.

plant biology↗

bioRxiv2022

Environmental gradients reveal stress hubs predating plant terrestrialization

Plant terrestrialization brought forth the land plants (embryophytes). Embryophytes account for most of the biomass on land and evolved from streptophyte algae in a singular event. Recent advances have unraveled the first full genomes of the closest algal relatives of land plants; among the first such species was Mesotaenium endlicherianum. Here, we used fine-combed RNAseq in tandem with photophysiological assessment on Mesotaenium exposed to a continuous range of temperature and light cues. Our data establish a grid of 42 different conditions, resulting in 128 transcriptomes and ~1.5 Tbp (~9.9 billion reads) of data to study combinatory effects of stress response using clustering along gradients. We describe major hubs in genetic networks underpinning stress response and acclimation in the molecular physiology of Mesotaenium. Our data suggest that lipid droplet formation, plastid and cell wall-derived signals denominate molecular programs since more than 600 million years of streptophyte evolution--before plants made their first steps on land.

evolutionary biology↗

bioRxiv2020

Genomic evidence reveals SPA-regulated developmental and metabolic pathways in dark-grown Arabidopsis seedlings

Photomorphogenesis is repressed in the dark mainly by an E3 ubiquitin ligase complex comprising CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and four homologous proteins called SUPPRESSOR OF PHYA-105 (SPA1-SPA4) in Arabidopsis. This complex induces the ubiquitination and subsequent degradation of positively acting transcription factors (e.g., HY5, HFR1, PAP1 and others) in the dark to repress photomorphogenesis. Genomic evidence showed a large number of genes regulated by COP1 in the dark, of which many are direct targets of HY5. However, the genomic basis for the constitute photomorphogenic phenotype of spaQ remains unknown. Here, we show that >7200 genes are differentially expressed in the spaQ background compared to wild-type in the dark. Comparison of the RNA Sequencing (RNA-Seq) data between cop1 and spaQ revealed a large overlapping set of genes regulated by the COP1-SPA complex. In addition, many of the genes coordinately regulated by the COP1-SPA complex are also regulated by HY5 directly and indirectly. Taken together, our data reveal that SPA proteins repress photomorphogenesis by controlling gene expression in concert with COP1, likely through regulating the abundance of downstream transcription factors in light signaling pathways. Moreover, SPA proteins may function both in a COP1-dependent and -independent manner in regulating many biological processes and developmental pathways in Arabidopsis. Summary statementComparison of transcriptome analyses between cop1 and spaQ mutants reveal overlapping pathways regulated by COP1 and SPAs.

plant biology↗