Abstract
Phytochromes are the principle photoreceptors in light-regulated plant development, primarily acting via translocation of the light-activated photoreceptor into the nucleus and subsequent gene regulation. However, several independent lines of evidence indicate unambiguously that an additional cytoplasmic signaling mechanism must exist. Directional responses in filament tip cells of the moss Physcomitrella patens are steered by phy4 which has been shown to interact physically with the blue light receptor phototropin at the plasma membrane. This complex might perceive and transduce vectorial information leading to cytoskeleton reorganization and finally a directional growth response. We developed yeast two-hybrid procedures using photochemically functional, full-length phy4 as bait in Physcomitrella cDNA library screens and growth assays under different light conditions, revealing Pfr-dependent interactions possibly associated with phytochrome cytoplasmic signaling. Candidate proteins were then expressed in planta with fluorescent protein tags to determine their intracellular localization in darkness and red light. Of 14 candidates, 12 were confirmed to interact with phy4 in planta using bimolecular fluorescence complementation. We also used database information to study their expression patterns relative to those of phy4. We discuss the likely functional characteristics of these holophytochrome-interacting proteins (HIP’s) and their possible roles in signaling.
Highlights
Plant phytochromes absorb predominantly red (R; ∼660 nm) and far-red (FR; ∼710–730 nm) light, thereby steering physiological responses including seed germination, de-etiolation, shade avoidance, and flowering
The directional responses in higher plants mediated by the blue light (B) absorbing photoreceptor phototropin are modulated by phytochromes: this might derive from phytochrome effects on gene expression, several of these responses are still seen in mutants in which nuclear translocation is defective, implying that farred absorbing form of phytochrome (Pfr) is physiologically active in the cytoplasm (Rösler et al, 2007; Kami et al, 2012)
We developed yeast two-hybrid (Y2H) methods to screen for phy4-interacting proteins that might be involved in the phy4 cytoplasmic signaling pathway
Summary
Plant phytochromes absorb predominantly red (R; ∼660 nm) and far-red (FR; ∼710–730 nm) light, thereby steering physiological responses including seed germination, de-etiolation, shade avoidance, and flowering. Phototropic growth of moss filament tip cells is steered by R in a FR-reversible manner, indicating that the photoreceptor is phytochrome This cannot result from transcription/translation regulation because directional information would inevitably be lost in the process. Chloroplast translocation too is regulated vectorially by phy (Kadota et al, 2000; Mittmann et al, 2004) In both the fern Adiantum and the alga Mougeotia chloroplast movements are steered vectorially in a R/FR-reversible manner, indicative of phytochrome action – but in these cases via neochrome, a chimeric photoreceptor comprising a phytochrome sensory module attached to a phototropin. The field of phytochrome cytoplasmic signaling has been reviewed recently (Hughes, 2013)
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