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The perception of light signals by the phytochrome family of photoreceptors has a crucial influence on almost all aspects of growth and development throughout a plant's life cycle. The holistic regulatory networks orchestrated by phytochromes, including conformational switching, subcellular localization, direct protein-protein interactions, transcriptional and posttranscriptional regulations, and translational and posttranslational controls to promote photomorphogenesis, are highly coordinated and regulated at multiple levels. During the past decade, advances using innovative approaches have substantially broadened our understanding of the sophisticated mechanisms underlying the phytochrome-mediated light signaling pathways. This review discusses and summarizes these discoveries of the role of the modular structure of phytochromes, phytochrome-interacting proteins, and their functions; the reciprocal modulation of both positive and negative regulators in phytochrome signaling; the regulatory roles of phytochromes in transcriptional activities, alternative splicing, and translational regulation; and the kinases and E3 ligases that modulate PHYTOCHROME INTERACTING FACTORs to optimize photomorphogenesis. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

High ambient temperature due to global warming has a profound influence on plant growth and development at all stages of life cycle. Plant response to high ambient temperature termed thermomorphogenesis is characterized by hypocotyl and petiole elongation, and hyponastic growth at seedling stage. However, the molecular mechanism of thermomorphogenesis is still rudimentary. Here, we show that a set of four SUPPRESSOR OF PHYA-105 (SPA) genes is required for thermomorphogenesis. Consistently, SPAs are necessary for global gene expression changes in response to high ambient temperature. SPA1 level is unaffected, while the thermosensor phyB is stabilized in the spaQ mutant at high ambient temperature. Furthermore, in the absence of four SPA genes, the pivotal transcription factor PIF4 fails to accumulate, indicating a role of SPAs in regulating the phyB-PIF4 module at high ambient temperature. SPA1 directly phosphorylates PIF4 in vitro, and a mutant SPA1 affecting the kinase activity fails to rescue the PIF4 level as well as the thermo-insensitive phenotype of spaQ, suggesting that the SPA1 kinase activity is necessary for thermomorphogenesis. Taken together, these data suggest that SPAs are new components that integrate light and temperature signaling via fine tuning the phyB-PIF4 module.