Interaction with plant transcription factors can mediate nuclear import of phytochrome B

Abstract

Phytochromes (phy) are red/far-red-absorbing photoreceptors that regulate the adaption of plant growth and development to changes in ambient light conditions. The nuclear transport of the phytochromes upon light activation is regarded as a key step in phytochrome signaling. Although nuclear import of phyA is regulated by the transport facilitators far red elongated hypocotyl 1 (FHY1) and fhy1-like, an intrinsic nuclear localization signal was proposed to be involved in the nuclear accumulation of phyB. We recently showed that nuclear import of phytochromes can be analyzed in a cell-free system consisting of isolated nuclei of the unicellular green algae Acetabularia acetabulum. We now show that this system is also versatile to elucidate the mechanism of the nuclear transport of phyB. We tested the nuclear transport characteristics of full-length phyB as well as N-and C-terminal phyB fragments in vitro and showed that the nuclear import of phyB can be facilitated by phytochrome-interacting factor 3 (PIF3). In vivo measurements of phyB nuclear accumulation in the absence of PIF1, -3, -4, and -5 indicate that these PIFs are the major transport facilitators during the first hours of deetiolation. Under prolonged irradiations additional factors might be responsible for phyB nuclear transport in the plant.Arabidopsis | basic helix-loop-helix | signal transduction P lant development is strictly regulated by the environmental light conditions. Even though light is ubiquitously present, spectral composition, intensity, and light direction varies depending on the local environment. To adapt on local light conditions plants are equipped with a highly developed repertory of photoreceptors (1, 2).Plant phytochromes are mainly involved in detection of red and far-red light wavelengths of the spectrum. A family of five members in Arabidopsis thaliana regulates the major developmental steps in the life of the plant: germination, photomorphogenesis, and flowering (3, 4). Despite the sequence similarities and identical spectroscopic features, the two major phytochromes, phyA and phyB, show overlapping as well as unique photosensory and functional characteristics (5-7). The light-stable phyB mediates red/far-red reversible low fluence response, whereas the light-labile phyA, is involved in far-red light sensing (high irradiance response) and in responses to very weak light (very low fluence response, VLFR) (8).Phytochromes are cytosolic proteins of 125 kDa that are translocated into the nucleus upon light activation (6, 9-11). Nuclear transport is an essential step in phytochrome signaling and a prerequisite for the interaction with transcription factors in the nucleus (12, 13). Concerning the kinetics of nuclear import, severe differences can be found between the two major plant phytochromes, phyA and phyB (6). PhyA cannot be detected in nuclei of dark-grown plants but is rapidly transported into the nucleus upon light irradiation. In contrast, low levels of phyB are present in the nucleus even in dark-grown seedlings. The nuclea...

PIFs: Systems Integrators in Plant Development

“…Interestingly, PIF-induced phyB degradation does not operate under prolonged shade environments (Leivar et al, 2012a), consistent with the absence of interaction between PIFs and the Pr-inactive form of phy or under SD photoperiods (Soy et al, 2012), where the short light period is insufficient to promote significant slow PIF3-induced phyB degradation. Adding another layer of complexity, it has been reported that the interaction with PIFs mediates the nuclear import of phyB into isolated nuclei of the algae Acetabularia acetabulum (Pfeiffer et al, 2012). Consistent with this observation, the authors showed that phyB is not translocated into the nucleus of Arabidopsis pifq mutants during early irradiation of dark-grown seedlings.…”

A Constitutively Active Allele of Phytochrome B Maintains Circadian Robustness in the Absence of Light

“…Phys containing the G767R mutation are retained in the cytoplasm (Wagner and Quail, 1995;Ni et al, 1999;Matsushita et al, 2003). This has been attributed to the inability of the G767R mutant to interact with PIF3 and then be imported into the nucleus (Pfeiffer et al, 2012). Surprisingly, the double phy mutant partially complemented the phyABCDE null mutant: light-grown YHB-G767R(phyABCDE) lines exhibited expanded cotyledons and shorter hypocotyls compared with the parental phyABCDE seedlings (Fig.…”

ArabidopsisPhytochrome B Promotes SPA1 Nuclear Accumulation to Repress Photomorphogenesis under Far-Red Light

“…Recently, the pif4 pif5 double loss-of-function mutant has been shown to be hypersensitive to low FR fluence rates, without altering phyA concentration (Lorrain et al, 2009). PIF3 facilitates the nuclear import of phyB under R and FR light irradiation (Pfeiffer et al, 2012). Thus, PIFs may be involved in phyB-mediated repression of seedling deetiolation under FR light (Figure 10).…”