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The aureochrome photoreceptor PtAUREO1a is a highly effective blue light switch in diatoms

ABSTRACT: Aureochromes represent a unique type of blue-light photoreceptors that possess a blue-light sensing flavin-binding LOV-domain and a DNA-binding bZIP domain. Therefore, in contrast to other photoreceptors, aureochromes are considered as light-driven transcription factors. As a member of the essential group of marine primary producers, the pennate diatom Phaeodactylum tricornutum possesses four aureochromes (PtAUREO1a, 1b, 1c, 2). We here show a dramatic change in the global gene expression pattern of P. tricornutum cells after a shift from red to blue light. About 75% of the genes show significantly changed transcript levels already after 10 and 60 min of blue light exposure, which includes genes of major transcription factors as well as other photoreceptors. Very surprisingly, in two independent PtAureo1a knockout lines, this light induced regulation of gene expression is almost completely abolished. Such a massive and fast transcriptional change depending on only one single photoreceptor is so far unprecedented. We hence conclude that PtAUREO1a plays a key role in light regulation.

ORGANISM(S): Phaeodactylum tricornutum

PROVIDER: GSE158698 | GEO | 2020/10/12

REPOSITORIES: GEO

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Project description:Diatoms, albeit being only distantly related with higher plants, harbour a plant-like cryptochrome (CryP) that was proposed to act as a photoreceptor required for the regulation of photosynthetic proteins. Importantly, plant cryptochromes are assumed to be involved in the regulation of developmental processes that are only needed in multicellular organisms; their role in the unicellular diatoms to date is mostly enigmatic. To elucidate the function of this plant-like cryptochrome in a unicellular species, we examined the role of CryP in the regulation of transcription in the diatom Phaeodactylum tricornutum by comparative RNA-seq of wild type and CryP knock-down mutants, under prolonged darkness and one hour after onset of blue light. In total, mRNAs of 12,298 genes were identified and more than 70% of the genes could be sorted into functional bins. We here show that CryP influences gene expression in three different ways: some genes are regulated by CryP under blue light only, others independent of the light condition and surprisingly some are regulated by CryP only in darkness. Changes under blue light included only around 4% of total genes, and genes regulated in any condition were distributed over almost all functional categories. Expression of photosynthesis genes was lowered by CryP under BL, in contrast to the overall enhancement in wild-type cells under the same conditions. Interestingly, CryP also exerted an influence on two other photoreceptors: the genes for phytochrome (Pt-DPH) and CPF1, another cryptochrome of Phaeodactylum tricornutum, were down-regulated by CryP independent of the light condition. However, the regulatory effects of the different photoreceptors on transcriptional output were independent. The influence of CryP on the expression of other photoreceptors hints to the existence of a regulatory signalling network in diatoms that includes several cryptochromes and phytochrome, whereby CryP acts as a regulator of transcript abundance under light as well as in darkness.

Project description:We have studied the transcriptional, metabolic and photo-physiological responses to light of different spectral quality in the marine diatom Phaeodactylum tricornutum through time-series studies of cultures exposed to equal doses of photosynthetically usable radiation of blue, green and red light. The experiments showed that short-term differences in gene expression and profiles are mainly light quality-dependent. Transcription of photosynthesis-associated nuclear genes was activated mainly through a light quality-independent mechanism likely to rely on chloroplast-to-nucleus signaling. In contrast, genes encoding proteins important for photoprotection and PSII repair were highly dependent on a blue light receptor-mediated signal. Changes in energy transfer efficiency by light-harvesting pigments were spectrally dependent; furthermore, a declining trend in photosynthetic efficiency was observed in red light. The combined results suggest that diatoms possess a light quality-dependent ability to activate photoprotection and efficient repair of photodamaged PSII. In spite of approximately equal numbers of PSII-absorbed quanta in blue, green and red light, the spectral quality of light is important for diatom responses to ambient light conditions. Continuous, axenic culturing of P. tricornutum was done as described in Nymark et al. (2009). The cultures were incubated at 15M-BM-0C under cool white fluorescent light (Philips TLD 36W/96) providing a scalar irradiance (EPAR) of 100 M-NM-<mol m-2 s-1 under continuous white light (CWL) conditions. Upon the onset of the experiment the cultures were synchronized by 48 h dark-treatment (D48). Thereafter the algae were exposed to blue light (BL), green light (GL) or red light (RL) provided by a waveband specific LED panel (SL3500, Photon Systems Instruments). The algae were exposed to 0.5 h, 6 h or 24 h of: 1) 230 M-NM-<mol m-2 s-1 of RL, 2) 100 M-NM-<mol m-2 s-1 of GL, 3) 50 M-NM-<mol m-2 s-1 of BL or 4) 100 M-NM-<mol m-2 s-1 of WL respectively. Three biological replicas for each of the treatments were harvested. Data for the white light exposure and 48h dark treatment is described in GSE42039, PMID: 23520530.

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The aureochrome photoreceptor PtAUREO1a is a highly effective blue light switch in diatoms

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