Distinct phytochrome actions in nonvascular plants revealed by targeted inactivation of phytobilin biosynthesis.

The red/far-red light photoreceptor phytochrome mediates photomorphological responses in plants. For light sensing and signaling, phytochromes need to associate with open-chain tetrapyrrole molecules as the chromophore. Biosynthesis of tetrapyrrole chromophores requires members of ferredoxin-dependent bilin reductases (FDBRs). It was shown that LONG HYPOCOTYL 2 (HY2) is the only FDBR in flowering plants producing the phytochromobilin (PΦB) for phytochromes. However, in the moss Physcomitrella patens, we found a second FDBR that catalyzes the formation of phycourobilin (PUB), a tetrapyrrole pigment usually found as the protein-bound form in cyanobacteria and red algae. Thus, we named the enzyme PUB synthase (PUBS). Severe photomorphogenic phenotypes, including the defect of phytochrome-mediated phototropism, were observed in Physcomitrella patens when both HY2 and PUBS were disrupted by gene targeting. This indicates HY2 and PUBS function redundantly in phytochrome-mediated responses of nonvascular plants. Our studies also show that functional PUBS orthologs are found in selected lycopod and chlorophyte genomes. Using mRNA sequencing for transcriptome profiling, we demonstrate that expression of the majority of red-light-responsive genes are misregulated in the pubs hy2 double mutant. These studies showed that moss phytochromes rapidly repress expression of genes involved in cell wall organization, transcription, hormone responses, and protein phosphorylation but activate genes involved in photosynthesis and stress signaling during deetiolation. We propose that, in nonvascular plants, HY2 and PUBS produce structurally different but functionally similar chromophore precursors for phytochromes. Holophytochromes regulate biological processes through light signaling to efficiently reprogram gene expression for vegetative growth in the light.

Phytochrome structure and signaling mechanisms.

Phytochromes are a widespread family of red/far-red responsive photoreceptors first discovered in plants, where they constitute one of the three main classes of photomorphogenesis regulators. All phytochromes utilize covalently attached bilin chromophores that enable photoconversion between red-absorbing (P(r)) and far-red-absorbing (P(fr)) forms. Phytochromes are thus photoswitchable photosensors; canonical phytochromes have a conserved N-terminal photosensory core and a C-terminal regulatory region, which typically includes a histidine-kinase-related domain. The discovery of new bacterial and cyanobacterial members of the phytochrome family within the last decade has greatly aided biochemical and structural characterization of this family, with the first crystal structure of a bacteriophytochrome photosensory core appearing in 2005. This structure and other recent biochemical studies have provided exciting new insights into the structure of phytochrome, the photoconversion process that is central to light sensing, and the mechanism of signal transfer by this important family of photoreceptors.

2.2 microm axial resolution optical coherence tomography based on a 400 nm-bandwidth superluminescent diode.

We demonstrate 2.2 microm axial resolution optical coherence tomography (OCT) in 1.1-1.7 microm wavelength regime by using a nonidentical multiple-quantum-well (MQW) superluminescent diode (SLD) with record-bandwidth emission. The compact, low-cost, and reliable light source with extreme broadband emission demonstrates significant potentials for spectroscopic and commercial OCT applications requiring ultrahigh spatial resolution.

Genome-wide analysis of light-regulated alternative splicing mediated by photoreceptors in Physcomitrella patens.

BACKGROUND: Light is one of the most important factors regulating plant growth and development. Light-sensing photoreceptors tightly regulate gene expression to control photomorphogenic responses. Although many levels of gene expression are modulated by photoreceptors, regulation at the mRNA splicing step remains unclear. RESULTS: We performed high-throughput mRNA sequencing to analyze light-responsive changes in alternative splicing in the moss Physcomitrella patens, and found that a large number of alternative splicing events were induced by light in the moss protonema. Light-responsive intron retention preferentially occurred in transcripts involved in photosynthesis and translation. Many of the alternatively spliced transcripts were expressed from genes with a function relating to splicing or light signaling, suggesting a potential impact on pre-mRNA splicing and photomorphogenic gene regulation in response to light. Moreover, most light-regulated intron retention was induced immediately upon light exposure, while motif analysis identified a repetitive GAA motif that may function as an exonic regulatory cis element in light-mediated alternative splicing. Further analysis in gene-disrupted mutants was consistent with a function for multiple red-light photoreceptors in the upstream regulation of light-responsive alternative splicing. CONCLUSIONS: Our results indicate that intensive alternative splicing occurs in non-vascular plants and that, during photomorphogenesis, light regulates alternative splicing with transcript selectivity. We further suggest that alternative splicing is rapidly fine-tuned by light to modulate gene expression and reorganize metabolic processes, and that pre-mRNA cis elements are involved in photoreceptor-mediated splicing regulation.

A light-independent allele of phytochrome B faithfully recapitulates photomorphogenic transcriptional networks.

Dominant gain-of-function alleles of Arabidopsis phytochrome B were recently shown to confer light-independent, constitutive photomorphogenic (cop) phenotypes to transgenic plants (Su and Lagarias, 2007). In the present study, comparative transcription profiling experiments were performed to assess whether the pattern of gene expression regulated by these alleles accurately reflects the process of photomorphogenesis in wild-type Arabidopsis. Whole-genome transcription profiles of dark-grown phyAphyB seedlings expressing the Y276H mutant of phyB (YHB) revealed that YHB reprograms about 13% of the Arabidopsis transcriptome in a light-independent manner. The YHB-regulated transcriptome proved qualitatively similar to but quantitatively greater than those of wild-type seedlings grown under 15 or 50 micromol m(-2) m(-1) continuous red light (Rc). Among the 2977 genes statistically significant two-fold (SSTF) regulated by YHB in the absence of light include those encoding components of the photosynthetic apparatus, tetrapyrrole/pigment biosynthetic pathways, and early light-responsive signaling factors. Approximately 80% of genes SSTF regulated by Rc were also YHB-regulated. Expression of a notable subset of 346 YHB-regulated genes proved to be strongly attenuated by Rc, indicating compensating regulation by phyC-E and/or other Rc-dependent processes. Since the majority of these 346 genes are regulated by the circadian clock, these results suggest that phyA- and phyB-independent light signaling pathway(s) strongly influence clock output. Together with the unique plastid morphology of dark-grown YHB seedlings, these analyses indicate that the YHB mutant induces constitutive photomorphogenesis via faithful reconstruction of phyB signaling pathways in a light-independent fashion.

Light-independent phytochrome signaling mediated by dominant GAF domain tyrosine mutants of Arabidopsis phytochromes in transgenic plants.

The photoreversibility of plant phytochromes enables continuous surveillance of the ambient light environment. Through expression of profluorescent, photoinsensitive Tyr-to-His mutant alleles of Arabidopsis thaliana phytochrome B (PHYB(Y276H)) and Arabidopsis phytochrome A (PHYA(Y242H)) in transgenic Arabidopsis plants, we demonstrate that photoconversion is not a prerequisite for phytochrome signaling. PHYB(Y276H)-expressing plants exhibit chromophore-dependent constitutive photomorphogenesis, light-independent phyB(Y276H) nuclear localization, constitutive activation of genes normally repressed in darkness, and light-insensitive seed germination. Fluence rate analyses of transgenic plants expressing PHYB(Y276H), PHYA(Y242H), and other Y(GAF) mutant alleles of PHYB demonstrate that a range of altered light-signaling activities are associated with mutation of this residue. We conclude that the universally conserved GAF domain Tyr residue, with which the bilin chromophore is intimately associated, performs a critical role in coupling light perception to signal transduction by plant phytochromes.

Tic21 is an essential translocon component for protein translocation across the chloroplast inner envelope membrane.

An Arabidopsis thaliana mutant defective in chloroplast protein import was isolated and the mutant locus, cia5, identified by map-based cloning. CIA5 is a 21-kD integral membrane protein in the chloroplast inner envelope membrane with four predicted transmembrane domains, similar to another potential chloroplast inner membrane protein-conducting channel, At Tic20, and the mitochondrial inner membrane counterparts Tim17, Tim22, and Tim23. cia5 null mutants were albino and accumulated unprocessed precursor proteins. cia5 mutant chloroplasts were normal in targeting and binding of precursors to the chloroplast surface but were defective in protein translocation across the inner envelope membrane. Expression levels of CIA5 were comparable to those of major translocon components, such as At Tic110 and At Toc75, except during germination, at which stage At Tic20 was expressed at its highest level. A double mutant of cia5 At tic20-I had the same phenotype as the At tic20-I single mutant, suggesting that CIA5 and At Tic20 function similarly in chloroplast biogenesis, with At Tic20 functioning earlier in development. We renamed CIA5 as Arabidopsis Tic21 (At Tic21) and propose that it functions as part of the inner membrane protein-conducting channel and may be more important for later stages of leaf development.

Import pathways of chloroplast interior proteins and the outer-membrane protein OEP14 converge at Toc75.

Most chloroplast outer-membrane proteins are synthesized at their mature size without cleavable targeting signals. Their insertion into the outer membrane is insensitive to thermolysin pretreatment of chloroplasts and does not require ATP. It has therefore been assumed that insertion of outer-membrane proteins proceeds through a different pathway from import into the interior of chloroplasts, which requires a thermolysin-sensitive translocon complex and ATP. Here, we show that a model outer-membrane protein, OEP14, competed with the import of a chloroplast interior protein, indicating that the two import pathways partially overlapped. Cross-linking studies showed that, during insertion, OEP14 was associated with Toc75, a thermolysin-resistant component of the outer-membrane protein-conducting channel that mediates the import of interior-targeted precursor proteins. Whereas almost no OEP14 inserted into protein-free liposomes, OEP14 inserted into proteoliposomes containing reconstituted Toc75 with a high efficiency. Taken together, our data indicate that Toc75 mediates OEP14 insertion, and therefore plays a dual role in the targeting of proteins to the outer envelope membrane and interior of chloroplasts.