Characterization of the requirements for localization of phytochrome B to nuclear bodies

Abstract

Phytochromes are red-and far-red-sensing photoreceptors that detect the quantity, quality, and duration of light throughout the entire life cycle of plants. Phytochromes accumulate in the cytoplasm in the dark. As one of the earliest responses after light illumination, phytochromes localize to the nucleus where they become associated with discrete nuclear bodies (NBs). Here, we describe the steady-state dynamics of Arabidopsis phytochrome B (phyB) localization in response to different light conditions and define four phyB subnuclear localization patterns: diffuse nuclear localization, small and numerous NBs only, both small and large NBs, and large NBs only. We show that phyB nuclear import is not sufficient for phyB NB formation. Rather, phyB accumulation in NBs is mainly determined by the percentage of the total amount of phyB protein that is in the active phyB conformer, with large NBs always correlating with strong phyB responses. A genetic screen to identify determinants required for subnuclear localization of phyB resulted in several phyB mutants, mutants deficient in phytochrome chromophore biosynthesis, and mutations in at least one previously uninvestigated locus. This study lays the groundwork for future investigations to identify the molecular mechanisms of light-regulated partitioning of plant photoreceptors to discrete subnuclear domains.

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

“…By contrast, in dim R or in light with a low R-to-FR ratio, PBG is localized to many small photobodies or is evenly dispersed in the nucleoplasm, and seedlings are taller (Chen et al, 2003). Together, these results support the idea that the localization of phyB to photobodies correlates tightly with the degree of hypocotyl growth inhibition (Chen et al, 2003).…”

“…Consistent with this notion, in high-intensity R, PBG localizes exclusively to a few large photobodies with diameters between 1 and 2 mm, and seedlings are correspondingly short (Chen et al, 2003(Chen et al, , 2010. By contrast, in dim R or in light with a low R-to-FR ratio, PBG is localized to many small photobodies or is evenly dispersed in the nucleoplasm, and seedlings are taller (Chen et al, 2003). Together, these results support the idea that the localization of phyB to photobodies correlates tightly with the degree of hypocotyl growth inhibition (Chen et al, 2003).…”

“…Light conditions that shift the Pfr/Pr equilibrium in favor of Pfr promote the localization of phyB to large photobodies. Consistent with this notion, in high-intensity R, PBG localizes exclusively to a few large photobodies with diameters between 1 and 2 mm, and seedlings are correspondingly short (Chen et al, 2003(Chen et al, , 2010. By contrast, in dim R or in light with a low R-to-FR ratio, PBG is localized to many small photobodies or is evenly dispersed in the nucleoplasm, and seedlings are taller (Chen et al, 2003).…”

“…Accumulating evidence supports the notion that photobody localization of phyB is required for the phytochrome-mediated inhibition of hypocotyl growth . For example, in continuous R, the steady-state photobody localization pattern (size and number) of phyB fused to GFP (PBG) is determined by the percentage of phyB in Pfr (Yamaguchi et al, 1999;Chen et al, 2003). Light conditions that shift the Pfr/Pr equilibrium in favor of Pfr promote the localization of phyB to large photobodies.…”

“…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).…”