Light signaling in plants-a selective history

Abstract

In addition to providing the radiant energy that drives photosynthesis, sunlight carries signals that enable plants to grow, develop and adapt optimally to the prevailing environment. Here we trace the path of research that has led to our current understanding of the cellular and molecular mechanisms underlying the plant's capacity to perceive and transduce these signals into appropriate growth and developmental responses. Because a fully comprehensive review was not possible, we have restricted our coverage to the phytochrome and cryptochrome classes of photosensory receptors, while recognizing that the phototropin and UV classes also contribute importantly to the full scope of light-signal monitoring by the plant.

Figure 1.

Scheme depicting Hendrick's proposed photoreversible switch-like behavior of the photoreceptor responsible for plant responses to the red/far-red region of the light spectrum. Pr, red-light absorbing form. Pfr, far-red light absorbing form.

Figure 2.

Photoactivated phys directly regulates target-gene expression via molecular interaction with PIF transcription factors. In the dark (left), phytochromes are synthesized in their biologically inactive Pr form, which is localized to the cytosol. PIFs are constitutively nuclear-localized transcription factors that bind to G-box (CACGTG) sequence elements in the promoters of many light-regulated genes. Arabidopsis seedlings undergo etiolated development (skotomorphogenesis). Upon red light illumination (right), phytochromes are converted to the biologically active Pfr form. The Pfr form translocates into the nucleus, binds to PIFs, and induces direct-target gene expression. The phy-PIF interaction within the nucleus results in rapid phosphorylation, ubiquitination, and degradation of PIFs. This light-induced degradation of PIFs results in activation of PIF-repressed genes and suppression of PIF-activated genes. The transcriptional network elicited by light exposure drives photomorphogenic development.

Figure 3.

Light-triggered photobody formation by LLPS facilitates convergence of cryptochrome and phytochrome signaling pathways directly at the genome interface. The diagram depicts the nuclear importation of photoactivated phy and the condensation of both the photoactivated phytochromes and CRY by LLPS. This light-induced LLPS of the photoreceptors facilitates protein-protein interactions both between the phy and CRY molecules and with the numerous photoreceptor-bound signaling proteins, denoted W, X, Y, and Z. These multilateral interactions regulate processes that include chromatin remodeling, transcription, RNA metabolism (splicing, modification, degradation, etc), protein metabolism (translation, modification, degradation, etc), and photomorphogenesis.