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Deconstructing and repurposing the light-regulated interplay between Arabidopsis phytochromes and interacting factors.

ABSTRACT: Phytochrome photoreceptors mediate adaptive responses of plants to red and far-red light. These responses generally entail light-regulated association between phytochromes and other proteins, among them the phytochrome-interacting factors (PIF). The interaction with Arabidopsis thaliana phytochrome B (AtPhyB) localizes to the bipartite APB motif of the A. thaliana PIFs (AtPIF). To address a dearth of quantitative interaction data, we construct and analyze numerous AtPIF3/6 variants. Red-light-activated binding is predominantly mediated by the APB N-terminus, whereas the C-terminus modulates binding and underlies the differential affinity of AtPIF3 and AtPIF6. We identify AtPIF variants of reduced size, monomeric or homodimeric state, and with AtPhyB affinities between 10 and 700 nM. Optogenetically deployed in mammalian cells, the AtPIF variants drive light-regulated gene expression and membrane recruitment, in certain cases reducing basal activity and enhancing regulatory response. Moreover, our results provide hitherto unavailable quantitative insight into the AtPhyB:AtPIF interaction underpinning vital light-dependent responses in plants.

SUBMITTER: Golonka D

PROVIDER: S-EPMC6888877 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Phytochromes are red and far-red light photoreceptors that regulate various aspects of plant development. One of the less-understood roles of phytochromes is the inhibition of hypocotyl negative gravitropism, which refers to the loss of hypocotyl gravitropism and resulting random growth direction in red or far-red light. This light response allows seedlings to curve toward blue light after emergence from the soil and enhances seedling establishment in the presence of mulch. Phytochromes inhibit hypocotyl negative gravitropism by inhibiting four phytochrome-interacting factors (PIF1, PIF3, PIF4, PIF5), as shown by hypocotyl agravitropism of dark-grown pif1 pif3 pif4 pif5 quadruple mutants. We show that phytochromes inhibit negative gravitropism by converting starch-filled gravity-sensing endodermal amyloplasts to other plastids with chloroplastic or etioplastic features in red or far-red light, whereas PIFs promote negative gravitropism by inhibiting the conversion of endodermal amyloplasts to etioplasts in the dark. By analyzing transgenic plants expressing PIF1 with an endodermis-specific SCARECROW promoter, we further show that endodermal PIF1 is sufficient to inhibit the conversion of endodermal amyloplasts to etioplasts and hypocotyl negative gravitropism of the pif quadruple mutant in the dark. Although the functions of phytochromes in gravitropism and chloroplast development are normally considered distinct, our results indicate that these two functions are closely related.

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Project description:Phytochromes (Phy) and phytochrome-interacting factor (PIF) transcription factors constitute a major signaling module that controls plant development in response to red and far-red light. A low red:far-red ratio is interpreted as shading by neighbor plants and induces cell elongation-a phenomenon called shade-avoidance syndrome (SAS). PAR1 and its closest homolog PAR2 are negative regulators of SAS; they belong to the HLH transcription factor family that lacks a typical basic domain required for DNA binding, and they are believed to regulate gene expressions through DNA binding transcription factors that are yet to be identified. Here, we show that light signal stabilizes PAR1 protein and PAR1 interacts with PIF4 and inhibits PIF4-mediated gene activation. DNA pull-down and chromatin immunoprecipitation (ChIP) assays showed that PAR1 inhibits PIF4 DNA binding in vitro and in vivo. Transgenic plants overexpressing PAR1 (PAR1OX) are insensitive to gibberellin (GA) or high temperature in hypocotyl elongation, similarly to the pifq mutant. In addition to PIF4, PAR1 also interacts with PRE1, a HLH transcription factor activated by brassinosteroid (BR) and GA. Overexpression of PRE1 largely suppressed the dwarf phenotype of PAR1OX. These results indicate that PAR1-PRE1 and PAR1-PIF4 heterodimers form a complex HLH/bHLH network regulating cell elongation and plant development in response to light and hormones.

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Arabidopsis HEMERA/pTAC12 initiates photomorphogenesis by phytochromes.

Project description:Light plays a profound role in plant development, yet how photoreceptor excitation directs phenotypic plasticity remains elusive. One of the earliest effects of light is the regulated translocation of the red/far-red photoreceptors, phytochromes, from the cytoplasm to subnuclear foci called phytochrome nuclear bodies. The function of these nuclear bodies is unknown. We report the identification of hemera, a seedling lethal mutant of Arabidopsis with altered phytochrome nuclear body patterns. hemera mutants are impaired in all phytochrome responses examined, including proteolysis of phytochrome A and phytochrome-interacting transcription factors. HEMERA was identified previously as pTAC12, a component of a plastid complex associated with transcription. Here, we show that HEMERA has a function in the nucleus, where it acts specifically in phytochrome signaling, is predicted to be structurally similar to the multiubiquitin-binding protein, RAD23, and can partially rescue yeast rad23mutants. Together, these results implicate phytochrome nuclear bodies as sites of proteolysis.

| S-EPMC2935685 | biostudies-literature

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