Disease relevance of PHYA

• This change stabilizes the light-labile PHYA protein in light and causes a 100-fold shift in the threshold for far-red light sensitivity [1].
• We show here that the dwarf response is related to a reduction in active gibberellins (GAs) in tobacco (Nicotiana tabacum) overexpressing oat phytochrome A under the control of the cauliflower mosaic virus (CaMV) 35S promoter and can be suppressed by foliar applications of gibberellic acid [2].

High impact information on PHYA

• In contrast, the exaggerated temperature response of cryptochrome-2 mutants is caused by temperature-dependent redundancy with the phytochrome A photoreceptor [3].
• SPA1 can localize to the nucleus, suggesting a possible function in phytochrome A-specific regulation of gene expression [4].
• Phytochrome B is the primary high-intensity red light photoreceptor for circadian control, and phytochrome A acts under low-intensity red light [5].
• Cryptochrome 1 and phytochrome A both act to transmit low-fluence blue light to the clock [5].
• Both the Pr and the Pfr forms of phyA, as well as the PHYA apoprotein, are ubiquitinated by COP1 in vitro [6].

Biological context of PHYA

• This mutation carries a single amino acid substitution at residue 631, from valine to methionine (V631M), in the core region within the C-terminal half of PHYA [7].
• The triple transcription start site arrangement is similar to that of pea PHYA but different from the single start site of oat, rice and maize PHYA genes, indicating a possible monocot-dicot difference [8].
• It was concluded that the pef1 mutant is defective in both PHYA- and PHYB-mediated signaling pathways, and may represent a lesion in an early step of the phytochrome signal transduction pathway [9].
• One such mutant, pef1, was selectively insensitive to both red and far-red light in the inhibition of hypocotyl elongation response; a classic phytochrome phenotype mediated by both PHYA and PHYB [9].
• Thus, PHYA and PHYB play a key role in mediating red-light-dependent positive phototropism in roots [10].

Anatomical context of PHYA

• Difference spectra and Western blot analysis showed normal concentrations of PHYA photoreceptor apoprotein, which appeared photochemically active [9].
• Far-red light blocks greening of Arabidopsis seedlings via a phytochrome A-mediated change in plastid development [11].
• The gradual swelling of protoplasts that prevails under background red light is shown to be a phytochrome-mediated response in which phytochrome A contributes more than phytochrome B [12].
• Upon light exposure, COP1 migrates to the cytosol allowing photomorphogenesis to proceed but the residual nuclear pool down-regulates light signaling mediated by phytochrome A. Here we show that weak alleles of cop1 exhibit reverse photomorphogenic responses i.e. reduced rather than enhanced cotyledon unfolding under red light compared to darkness [13].

Associations of PHYA with chemical compounds

• Our data suggest that FIN219 may define a critical link for phytochrome A-mediated far-red inactivation of COP1 and a possible cross-talk juncture between auxin regulation and phytochrome signaling [14].
• The hy8-3 mutant that expresses wild-type levels of photochemically active phytochrome A has a glycine-to-glutamate missense mutation at residue 727 in the C-terminal domain of the phyA sequence [15].
• Opposing roles of phytochrome A and phytochrome B in early cryptochrome-mediated growth inhibition [16].
• Overexpression of Arabidopsis phytochrome B inhibits phytochrome A function in the presence of sucrose [17].
• Recombinant FyPP efficiently dephosphorylated oat phytochrome A in the presence of Fe(2+) or Zn(2+) in a spectral form-dependent manner [18].
• De-methylation of phyA' in the DNA methyl transferase I (met1) mutant background increased PHYA expression and restored the wild-type phenotype, confirming the pivotal role of exonic CG methylation in maintaining the altered epigenetic state [19].

Physical interactions of PHYA

• PIL5 preferentially interacts with the Pfr forms of Phytochrome A (PhyA) and Phytochrome B (PhyB) [20].
• Arabidopsis FHY3 defines a key phytochrome A signaling component directly interacting with its homologous partner FAR1 [21].

Regulatory relationships of PHYA

• Arabidopsis FHY1 protein stability is regulated by light via phytochrome A and 26S proteasome [22].
• Phytochrome A negatively or positively regulates phytochrome B signaling, depending on light conditions [23].
• Effects of synergistic signaling by phytochrome A and cryptochrome1 on circadian clock-regulated catalase expression [24].
• To examine the efficacy of this cellular site, oat phytochrome A was also expressed using Arabidopsis chlorophyll a/b-binding protein (CAB) and the Arabidopsis ubiquitin (UBQ1) promoters [2].
• Therefore, neither phytochrome A nor hytochrome B appears to regulate GA biosynthesis from GA12 to GA4 during seed germination, since the conversion of GA12 to GA9 is regulated by one enzyme (GA 20-oxidase) [25].

Other interactions of PHYA

• The induction of randomization was confirmed using lines that express to different levels PHYA and PHYB cDNAs [26].
• Expression analysis revealed that PIL5 repressed the expression of GA biosynthetic genes (GA3ox1 and GA3ox2), and activated the expression of a GA catabolic gene (GA2ox) in both PHYA- and PHYB-dependent germination assays [27].
• FHY1: a phytochrome A-specific signal transducer [28].
• A null mutation in PHYA impaired the membrane depolarization and prevented the early cry1-dependent phase of growth inhibition as effectively and with the same time course as mutations in CRY1 [16].
• The PHYC gene of Arabidopsis. Absence of the third intron found in PHYA and PHYB [29].

Analytical, diagnostic and therapeutic context of PHYA

• Using oligonucleotide microarrays to measure expression profiles in wild-type and phytochrome A (phyA) null-mutant Arabidopsis seedlings, we have shown that 10% of the genes represented on the array are regulated by phyA in response to a continuous far-red light signal [30].
• The dynamic behavior of phytochrome A (phyA) in seedlings of the model plant Arabidopsis was examined by in vivo spectroscopy and by western and northern blotting [31].
• Immunoblotting revealed that the need of phytochrome E for germination in FR was not caused by altered phytochrome A levels [32].

References