Phytochrome Signaling: Time to Tighten up the Loose Ends Hai Wang, Haiyang Wang  Molecular Plant  Volume 8, Issue 4, Pages 540-551 (April 2015) DOI: 10.1016/j.molp.2014.11.021 Copyright © 2015 The Author Terms and Conditions

Figure 1 Localization of Phytochromes in Two Distinct Forms of Nuclear Bodies. (A and B) Transient phyA NBs are formed upon pulses (5 min) of R (A) or FR (B) light treatment. (C) Stable phyA NBs after being exposed to prolonged (5 h) FR light. (D and E) Transient phyB NBs are formed after pulses (5 min) of R (D), but not FR (E) treatment. (F) Prolonged R light exposure (4 days) induces the formation of stable phyB NBs. Note that transient NBs are smaller and more numerous than stable NBs. cys, phyA-GFP in the cytosol speckles; pl, plastids; el, etioplasts; nu, nucleus; The dashed lines encircle the nuclei in (C) and (E); nus, spotted nuclear areas of phyB-GFP in (B). (A), (B), (D), and (E) were modified from Kircher et al. (1999) with permission from Copyright Clearance Center. (C) was from Lin et al. (2007) with permission from Copyright Clearance Center. (F) was adapted from Kevei et al. (2007) with permission from Copyright Clearance Center. Molecular Plant 2015 8, 540-551DOI: (10.1016/j.molp.2014.11.021) Copyright © 2015 The Author Terms and Conditions

Figure 2 The Escort Model Showing Phytochrome Action in Transcriptional Control. In the escort model, phytochromes directly associate with downstream gene promoters by interaction with various sets of transcription factors (TF) and coregulators (C), thus facilitating differential regulation of many aspects of plant growth, development, and stress responses, which in sum constitute an integrated response to light (Modified from Figure 9, Chen et al., 2014). Molecular Plant 2015 8, 540-551DOI: (10.1016/j.molp.2014.11.021) Copyright © 2015 The Author Terms and Conditions

Figure 3 Multiple Layers of Antagonistic Interplay between phys and PIFs. Light-activated phyB inhibits PIF (except PIF7) activity by direct association with and phosphorylation of PIFs, which leads to the recruitment of LRB E3 ligases and subsequent mutually assured destruction of both PIFs and phyB. In addition, some members of PIFs were found to enhance the activity of COP1, which in turn facilitates the degradation of phys. Molecular Plant 2015 8, 540-551DOI: (10.1016/j.molp.2014.11.021) Copyright © 2015 The Author Terms and Conditions

Figure 4 Regulation of Stem Elongation, Lateral Bud Outgrowth, and Flowering by phyB Involves Intercellular and Interorgan Signaling. When plants are shaded (under low R:FR), inactivation of phyB in leaves leads to de-repression of PIFs, which in turn promotes auxin biosynthesis and elongation of stem and petiole. In addition to auxin, other unknown mobile signals may also be involved in this process. Promotion of flowering by shade is controlled by up-regulation of CO, which results from inactivation of phyB. In addition, CO-independent regulation of FT by phyB may also exist. However, it is still unknown how the signal from phyB in mesophyll cells affects the expression of CO and FT in the vascular bundles. phyB also negatively regulates BRC1 and BRC2, two repressors of axillary bud outgrowth. Presumably this regulation requires an as yet unidentified long-distance mobile signal connecting phyB in leaves and BRC1/BRC2 in axillary buds. Branching is also negatively regulated by shoot apex-derived auxin and root-derived strigolactone, and positively regulated by cytokinin synthesized in the root and stem. Molecular Plant 2015 8, 540-551DOI: (10.1016/j.molp.2014.11.021) Copyright © 2015 The Author Terms and Conditions