Nucleocytoplasmic Partitioning of the Plant Photoreceptors Phytochrome A, B, C, D, and E Is Regulated Differentially by Light and Exhibits a Diurnal Rhythm

Abstract

The phytochrome family of plant photoreceptors has a central role in the adaptation of plant development to changes in ambient light conditions. The individual phytochrome species regulate different or partly overlapping physiological responses. We generated transgenic Arabidopsis plants expressing phytochrome A to E:green fluorescent protein (GFP) fusion proteins to assess the biological role of intracellular compartmentation of these photoreceptors in lightregulated signaling. We show that all phytochrome:GFP fusion proteins were imported into the nuclei. Translocation of these photoreceptors into the nuclei was regulated differentially by light. Light-induced accumulation of phytochrome species in the nuclei resulted in the formation of speckles. The appearance of these nuclear structures exhibited distinctly different kinetics, wavelengths, and fluence dependence and was regulated by a diurnal rhythm. Furthermore, we demonstrate that the import of mutant phytochrome B:GFP and phytochrome A:GFP fusion proteins, shown to be defective in signaling in vivo, is regulated by light but is not accompanied by the formation of speckles. These results suggest that (1) the differential regulation of the translocation of phytochrome A to E into nuclei plays a role in the specification of functions, and (2) the appearance of speckles is a functional feature of phytochrome-regulated signaling.

Plant organellar protein targeting: a traffic plan still under construction

“…For example, the nucleocytoplasmic partitioning of photoreceptors is influenced by light. Subcellular localization of phytochromes A, B, C, D and E, and the ability to form speckle structures following nuclear import, demonstrates a direct response to light and a diurnal rhythm [37].…”

Photobody Localization of Phytochrome B Is Tightly Correlated with Prolonged and Light-Dependent Inhibition of Hypocotyl Elongation in the Dark

“…At the cellular level, one of the earliest responses to light is the translocation of photoactivated phytochromes from the cytoplasm to subnuclear foci called phytochrome speckles or photobodies (Yamaguchi et al, 1999;Kircher et al, 2002;Chen and Chory, 2011;Van Buskirk et al, 2012). Since the initial observation of photobodies 15 years ago (Yamaguchi et al, 1999), the necessity of photobodies in phytochrome signaling has been debated.…”

“…Most loss-of-function phyB alleles are defective in phyB localization to large photobodies and are taller than the wild type (Kircher et al, 2002;Chen et al, 2003;Matsushita et al, 2003;Nito et al, 2013), whereas in gain-of-function phyB mutants, phyB localizes to large photobodies under dim light and hypocotyl elongation is more restricted than in the wild type (Ádám et al, 2011;Medzihradszky et al, 2013;Zhang et al, 2013). In the most extreme case, phyB Y276H (YHB), a constitutively active phyB mutant, localizes to large photobodies regardless of light conditions and can inhibit hypocotyl growth even in the dark (Su and Lagarias, 2007).…”

Insight into nuclear body formation of phytochromes through stochastic modelling and experiment

“…Such detailed information about NBs is presently unavailable and hence we shall leave α and β general. The parameters a and b are the association and dissociation rates having the standard units Since the late phyB NBs which we are modelling, are stable under constant experimental conditions [13,25], we will be deriving expressions for the number and size distributions of the NBs in steady-state conditions.…”

“…For estimation of the error bounds we used standard uncertainty propagation [34]. The average number of fundamental building blocks (phyB dimers) per NB denoted as m can then be computed from equation (13). The resulting estimated parameter values are given in table 1.…”