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The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5'-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction

The long-day plant Arabidopsis thaliana (L.) Heynh. flowers early in response to brief end-of-day (EOD) exposures to far-red light (FR) following a fluorescent short day of 8 h. FR promotion of flowering was nullified by subsequent brief red light (R) EOD exposure, indicating phytochrome involvement. The EOD response to R or FR is a robust measure of phytochrome action. Along with their wild-type (WT) parents, mutants deficient in either phytochrome A or B responded similarly to the EOD treatments. Thus, neither phytochrome A nor B exclusively regulated flowering, although phytochrome B controlled hypocotyl elongation. Perhaps a third phytochrome species is important for the EOD responses of the mutants and/or their flowering is regulated by the amount of the FR-absorbing form of phytochrome, irrespective of the phytochrome species. Overexpression of phytochrome A or phytochrome B resulted in differing photoperiod and EOD responses among the genotypes. The day-neutral overexpressor of phytochrome A had an EOD response similar to all of the mutants and WTs, whereas R EOD exposure promoted flowering in the overexpressor of phytochrome B and FR EOD exposure inhibited this promotion. The comparisons between relative flowering times and leaf numbers at flowering of the overexpressors and their WTs were not consistent across photoperiods and light treatments, although both phytochromes A and B contributed to regulating flowering of the transgenic plants

Phaseolin, the major seed storage protein of Phaseolus vulgaris L., is degraded in the cotyledons in the first 7—10 d following seed germination. We assayed cotyledon extracts for protease activity by using [3H]phaseolin as a substrate and then fractionated the digestion mixtures by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in order to identify the cleavage products. The cotyledons of 4-d-old seedlings contain an endopeptidase which cleaves the polypeptides of [3H]phaseolin (apparent molecular weights = 51000, 48000, 46000 and 43000) into three discrete clusters of proteolytic fragments (Mrs = 27000, 25000 and 23000). Endopeptidase activity is not detected in the cotyledons until the protein content of these organs starts to decline, shortly after the first day of seedling growth. Endopeptidase activity increases to a maximum level in the cotyledons of 5-d-old seedlings and then declines to a minimum value by day 10. The enzyme was purified 335-fold by ammonium-sulfate precipitation, organomercurial-agarose chromatography, gel filtration and ion-exchange chromatography. The endopeptidase constitutes 0.3% of the protein content in the cotyledons of 4-d-old seedlings. It is a cysteine protease with a single polypeptide chain (Mr = 30000). Optimum hydrolysis of [3H]phaseolin occurs at pH 5. The enzyme is irreversibly inactivated at pH values above 7 and at temperatures above 45° C. The endopeptidase attacks only a limited number of peptide bonds in [3H]phaseolin, without causing any appreciable change in the native molecular weight of the storage protein. The endopeptidase is also able to hydrolyze the bean-seed lectin, phytohemagglutinin. Thus, this enzyme may play a general role in degrading cotyledon proteins of P. vulgaris following seed germination.

To develop a model plant system for efficient functional analysis of mutagenized phytochrome polypeptides, we have overexpressed oat phytochrome A in Arabidopsis thaliana. R1 seedlings from selfed primary transformants segregated for hypocotyl length, when grown in the light, with a ratio of 3 short to 1 of normal length. When homozygous lines were established from these two size classes, accumulation of immunologically detectable oat phytochrome cosegregated with the short-hypocotyl trait. The short-hypocotyl seedlings contained substantially more spectrally active phytochrome than their normal-sized siblings, indicating that the introduced oat protein was photoreversible. The short-hypocotyl phenotype was strictly light-dependent, since no morphological effects or phytochrome overexpression could be seen in etiolated seedlings. Overexpression of only the chromophore-bearing, N-terminal domain of phytochrome A did not induce short hypocotyls in light-grown seedlings, indicating that additional sequence is essential for photoreceptor function. Similarly, overexpression of a full-length sequence mutated at the chromophore attachment site had no effect on phenotype, indicating the absence of any detectable dominant negative effect of the chromophoreless polypeptide on the activity of endogenous Arabidopsis phytochrome. Thus, the readily scorable short-hypocotyl phenotype of Arabidopsis seedlings overexpressing phytochrome A provides a simple visual assay for rapidly monitoring the biological activity of mutagenized phytochrome A polypeptides

A detailed sensitivity analysis was conducted to help to quantify the impacts of various emission control options in terms of potential visibility improvements at class I national parks and wilderness areas in the southeastern United States. Particulate matter (PM) levels were estimated using the Community Multiscale Air Quality (CMAQ) model, and light extinctions were calculated using the modeled PM concentrations. First, likely changes in visibility at class I areas were estimated for 2018. Then, using emission projections for 2018 as a starting point, the sensitivity of light extinction was evaluated by reducing emissions from various source categories by 30%. Source categories to be analyzed were determined using a tiered approach: any category that showed significant impact in one tier was broken into subcategories for further analysis in the next tier. In the first tier, sulfur dioxide (SO₂), nitrogen oxides, ammonia, volatile organic compound, and primary carbon emissions were reduced uniformly over the entire domain. During summer, when most class I areas experience their worst visibility, reduction of SO₂ emissions was the most effective control strategy. In the second tier, SO₂ sources were separated as ground level and elevated. The elevated sources in 10 southeastern states were differentiated from those in the rest of the domain and broken into three subcategories: coal-fired power plant (CPP), other power plant, and other than power plant [i.e., non–electric generating unit (non EGU)]. The SO₂ emissions from the CPP subcategory had the largest impact on visibility at class I areas, followed by the non-EGU subcategory. In the third tier, emissions from these two subcategories were further broken down by state. Most class I areas were affected by the emissions from several states, indicating the regional nature of the haze problem. Here, the visibility responses to all of the aforementioned emission reductions are quantified and deviations from general trends are identified.