Differential phosphorylation of the N‐terminal extension regulates phytochrome B signaling

Abstract

Summary Phytochrome B (phyB) is an excellent light quality and quantity sensor that can detect subtle changes in the light environment. The relative amounts of the biologically active photoreceptor (phyB Pfr) are determined by the light conditions and light independent thermal relaxation of Pfr into the inactive phyB Pr, termed thermal reversion. Little is known about the regulation of thermal reversion and how it affects plants’ light sensitivity. In this study we identified several serine/threonine residues on the N‐terminal extension (NTE) of Arabidopsis thaliana phyB that are differentially phosphorylated in response to light and temperature, and examined transgenic plants expressing nonphosphorylatable and phosphomimic phyB mutants. The NTE of phyB is essential for thermal stability of the Pfr form, and phosphorylation of S86 particularly enhances the thermal reversion rate of the phyB Pfr–Pr heterodimer in vivo. We demonstrate that S86 phosphorylation is especially critical for phyB signaling compared with phosphorylation of the more N‐terminal residues. Interestingly, S86 phosphorylation is reduced in light, paralleled by a progressive Pfr stabilization under prolonged irradiation. By investigating other phytochromes (phyD and phyE) we provide evidence that acceleration of thermal reversion by phosphorylation represents a general mechanism for attenuating phytochrome signaling.

Increasing ambient temperature progressively disassembles Arabidopsis phytochrome B from individual photobodies with distinct thermostabilities

“…The variation in thermostability or the Pfr:Pfr conformation among PBs could be due to differences in modifications of phyB itself and/or interactions with other PB constituents. For example, phosphorylation of phyB's N-terminal module can largely influence the stability and activity of the Pfr [70][71][72] . In particular, phosphorylation of Ser86 destabilizes the Pfr:Pfr form and attenuates PB localization 70,72 .…”

“…For example, phosphorylation of phyB's N-terminal module can largely influence the stability and activity of the Pfr [70][71][72] . In particular, phosphorylation of Ser86 destabilizes the Pfr:Pfr form and attenuates PB localization 70,72 . Three phyB-interacting proteins have been reported to stabilize the Pfr form by suppressing its thermal reversion, they are ARABIDOPSIS RESPONSE REGULATOR 4 (ARR4) 73 , PHOTOPERIODIC CONTROL OF HYPOCOTYL 1 (PCH1) [50][51][52] , and PCH1-LIKE (PCHL) 52 .…”

Increases in ambient temperature progressively disassembleArabidopsisphytochrome B from individual photobodies with distinct thermostabilities

“…The fact that PBs exhibit a range of thermostabilities in ambient temperature implies that the stability of the Pfr:Pfr of phyB is different among PBs. This could be due to different phyB binding partners or slightly different active phyB forms among 70,72 . Three phyB-interacting proteins have been reported to stabilize the Pfr form by suppressing its dark reversion, they are ARABIDOPSIS RESPONSE REGULATOR 4 (ARR4) 73 , PHOTOPERIODIC CONTROL OF HYPOCOTYL 1 (PCH1) [50][51][52] , and PCH1-LIKE (PCHL) 52 .…”

Thermal Reversion of Plant Phytochromes

“…Novel data reveal that phyD and phyE are again very different from phyA and phyB regarding their thermal reversion properties in Arabidopsis. PhyE Pfr is very stable without showing detectable Pfr reversion at 22 C during 4 h of darkness and consequently accumulates high relative Pfr/Ptot levels already at very low light intensities (Vicziá n et al, 2019). The data further indicate that phyD could be thermally extremely unstable, even preventing detection by in vivo spectroscopy that relies on a certain stability of the two photointerconvertible conformations (Vicziá n et al, 2019).…”