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

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 ( At PhyB) localizes to the bipartite APB motif of the A. thaliana PIFs ( At PIF). To address a dearth of quantitative interaction data, we construct and analyze numerous At PIF3/6 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 At PIF3 and At PIF6. We identify At PIF variants of reduced size, monomeric or homodimeric state, and with At PhyB affinities between 10 and 700 nM. Optogenetically deployed in mammalian cells, the At PIF 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 At PhyB: At PIF interaction underpinning vital light-dependent responses in plants. David Golonka et al. report the epitopes in Arabidopsis phytochrome-interacting factors (PIF) that underlie light-dependent interactions with phytochrome B. They identify compact PIF variants that enable light-activated gene expression and membrane recruitment with reduced basal activity and enhanced regulatory response.