The Role of Phytochromes in Triggering Plant Developmental Transitions


Light is an essential stimulus for energy production, plant growth, development and adaptation to constantly changing environmental conditions. Plants sense different wavelengths of light through the action of specialised families of photoreceptor proteins. The molecular and physiological role of the phytochrome family of red/far‐red light receptors is well characterised. Upon light activation, phytochromes trigger multicomponent signalling cascades to induce fundamental cellular processes ranging from gene expression to protein abundance and nuclear architecture to regulate various aspects of plant development. Phytochrome responses are cell autonomous but in some cases mediate interorgan communication to control major developmental and adaptive responses such as flowering initiation and shade avoidance. Reciprocal interactions and integration between phytochrome and circadian signalling pathways are essential for optimising growth and reproductive success during the life cycle of the model plant Arabidopsis thaliana.

Key Concepts

  • Light is a vital informational signal for plant survival and growth.
  • The phytochrome family of plant photoreceptors acts as molecular photoswitches in response to red and far‐red light.
  • Plant phytochrome signal transduction regulates molecular and cellular processes.
  • Phytochromes induce cell‐autonomous responses and interorgan communication.
  • Phytochromes regulate light‐induced developmental transitions as well as adaptation to growth under dense canopy.
  • Plant phytochromes have antagonistic and synergistic roles in regulating photoperiodic flowering in Arabidopsis.
  • Phytochromes inform the endogenous clock about seasonal and daily changes to optimise plant growth and establish reproductive success.

Keywords: light; photoreceptors; phytochromes; development; photomorphogenesis; shade avoidance; flowering; signal integration

Figure 1. (a) Domain composition of plant phytochromes. The amino‐terminal photosensing region consists of a PAS‐like domain, a GAF domain that covalently binds the chromophore phytochromobilin and a PHY‐to‐chrome‐specific domain. The carboxy‐terminal regulatory region consists of two PAS domains and a Histidine‐related kinase domain. (b) Molecular and physiological functions of plant phytochromes. Phytochromes actively regulate a series of cellular processes in response to red and far‐red light and play major roles in inducing major developmental transitions and adaptive responses to red/far‐red light.
Figure 2. Phytochromes are photoswitchable molecules. (a) Plant phytochromes exist in two photoreversible states: Pr absorbs red light and Pfr absorbs far‐red light. (b) Red light induces rapid nuclear translocation of phytochromes in Arabidopsis. Photoactivated phytochromes concentrate in subnuclear domains also known as photobodies.
Figure 3. Phytochromes regulate major aspects of plant development and physiology: (a) seed germination, (b) photomorphogenesis, (c) stomatal opening and development, (d) entrainment of the clock, (e) photoperiodic flowering and (f) shade avoidance response.
Figure 4. Signal transduction pathway mediating the shade avoidance response. R:FR light is perceived in the cotyledons primarily by phyB. PhyB (Pfr) dissociates from PIFs. Accumulation of PIFs leads to induction of YUCCA expression that triggers auxin biosynthesis. Newly synthesised auxin translocates to other organs to promote their elongation.
Figure 5. Photoperiodic control of flowering initiation. (a) Light and endogenous rhythms regulate CONSTANS protein levels. In the dark, the COP1/SPA complex mediates CO degradation. PhyA protects CO from proteasomal degradation. PhyB targets CO for degradation and also induces CO gene expression. Cryptochromes and FKF1 clock receptor stabilise CO protein abundance. Accumulation of CO induces FT expression in plant leaves. (b) FT travels from leaves to the shoot apical meristem (SAM) to initiate flowering by activating the expression of floral development genes, such as AP1. Collectively, red/far‐red and blue light receptors act cooperatively and antagonistically to induce flowering initiation at the optimal time.


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Further Reading

Shim JS and Imaizumi T (2015) Circadian clock and photoperiodic response in Arabidopsis: From seasonal flowering to redox homeostasis. Biochemistry 54 (2): 157–170. DOI: 10.1021/bi500922q.

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Kaiserli, Eirini, and Chory, Joanne(Mar 2016) The Role of Phytochromes in Triggering Plant Developmental Transitions. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0023714]