The photosynthetic apparatus of plants responds to changing light quantity and quality with coordinated changes in both the light-harvesting antennae of the photosystems and the amounts of electron transport components and ATP synthase. These compositional modulations are accompanied by changes in thylakoid membrane organisation and photosynthetic capacity. It is now clear that there is a dynamic continuum of organisation and function of the photosynthetic apparatus from the appressed granal and non-appressed stroma thylakoids within a chloroplast, to different chloroplasts within a leaf, to leaves within and between species. While it is very unlikely that there is a unique solution to photosynthesis in the sun or shade, substantial changes in composition, and hence thylakoid membrane organisation and function, are elicited as part of sun/shade responses.

(u.s.A. SUMMARYClass II spinach chloroplasts were fragmented by passage through the French pressure cell (French press), and the fragments were separated. by fractional centrifugation. Fragments sedimenting between lOQO X g and 10,000 X g (10K) .have a lower chl a/chl b ratio and lower P-700 content than whole chloroplasts.· Fragments -sedimenting between 4o.·ooo x g and 160,000 x g (160K) have a much higher chl a/chl b ratio (6.0) and amuch higher P-700 content (1 P-700/105 chlorophylls) than whole chloroplasts. The chlorophyll and cytochrome contents of the Frenc::hJ>ress fractions are similar to those found in fractions iso-,.lated after digitonin disruption.The 160K fraction performs photosystem 1 but not photosystem 2 reactions. The lOK fraction contains both photosystems. Electrophoresis of sodium dodecylsulfate solubilized lOK and 160K fractions gives further evidence for this distribution of photosystems.

1. Mesophyll and parenchyma-sheath chloroplasts of maize leaves were separated by density fractionation in non-aqueous media. 2. An investigation of the distribution of photosynthetic enzymes indicated that the mesophyll chloroplasts probably contain the entire leaf complement of pyruvate,P(i) dikinase, NADP-specific malate dehydrogenase, glycerate kinase and nitrite reductase and most of the adenylate kinase and pyrophosphatase. The fractionation pattern of phosphopyruvate carboxylase suggested that this enzyme may be associated with the bounding membrane of mesophyll chloroplasts. 3. Ribulose diphosphate carboxylase, ribose phosphate isomerase, phosphoribulokinase, fructose diphosphate aldolase, alkaline fructose diphosphatase and NADP-specific ;malic' enzyme appear to be wholly localized in the parenchyma-sheath chloroplasts. Phosphoglycerate kinase and NADP-specific glyceraldehyde phosphate dehydrogenase, on the other hand, are distributed approximately equally between the two types of chloroplast. 4. After exposure of illuminated leaves to (14)CO(2) for 25sec., labelled malate, aspartate and 3-phosphoglycerate had similar fractionation patterns, and a large proportion of each was isolated with mesophyll chloroplasts. Labelled fructose phosphates and ribulose phosphates were mainly isolated in fractions containing parenchyma-sheath chloroplasts, and dihydroxyacetone phosphate had a fractionation pattern intermediate between those of C(4) dicarboxylic acids and sugar phosphates. 6. These results indicate that the mesophyll and parenchyma-sheath chloroplasts have a co-operative function in the operation of the C(4)-dicarboxylic acid pathway. Possible routes for the transfer of carbon from C(4) dicarboxylic acids to sugars are discussed.

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Critical illness in COVID-19 is an extreme and clinically homogeneous disease phenotype that we have previously shown1 to be highly efficient for discovery of genetic associations2. Despite the advanced stage of illness at presentation, we have shown that host genetics in patients who are critically ill with COVID-19 can identify immunomodulatory therapies with strong beneficial effects in this group3. Here we analyse 24,202 cases of COVID-19 with critical illness comprising a combination of microarray genotype and whole-genome sequencing data from cases of critical illness in the international GenOMICC (11,440 cases) study, combined with other studies recruiting hospitalized patients with a strong focus on severe and critical disease: ISARIC4C (676 cases) and the SCOURGE consortium (5,934 cases). To put these results in the context of existing work, we conduct a meta-analysis of the new GenOMICC genome-wide association study (GWAS) results with previously published data. We find 49 genome-wide significant associations, of which 16 have not been reported previously. To investigate the therapeutic implications of these findings, we infer the structural consequences of protein-coding variants, and combine our GWAS results with gene expression data using a monocyte transcriptome-wide association study (TWAS) model, as well as gene and protein expression using Mendelian randomization. We identify potentially druggable targets in multiple systems, including inflammatory signalling (JAK1), monocyte–macrophage activation and endothelial permeability (PDE4A), immunometabolism (SLC2A5 and AK5), and host factors required for viral entry and replication (TMPRSS2 and RAB2A).

Abstract. A chlorophyll-deficient mutant of pea (Pisum sativum) was found as a spontaneous mutation of the variety Greenfeast. Total chlorophyll of the mutant leaves was about one-half that of normal pea leaves per mg dry weight, and the ratio of chl a:chl b ranged from 10 to 18, compared with 3 for normal pea. In each generation the mutant plants gave rise to normal and mutant plants and lethal plants with yellow leaves.For a normal pea plant, CO2 uptake was saturated at about 60,000 lux, whereas with mutant leaves, the rate of CO2 uptake was still increasing at 113,000 lux. At 113,000 lux the mutant and normal leaves showed similar rates of CO2 fixation per unit area of leaf surface, but on a chlorophyll basis the mutant leaves were twice as active. Hill reaction measurements on isolated chloroplasts also showed that the mutant chloroplasts werc saturated at higher intensities than the normal, and that the activity of the mutant was at least double that of the normal on a chlorophyll basis.It is suggested that the photosynthetic units of the mutant chloroplasts contain about half the number of chlorophyll molecules as compared to the normal photosynthetic units.Electron microscopy of leaf sections of normal and mutant leaves showed that the mutant chloropla,sts oontain fewer lamellae per chloroplast and fewer lamellae per granum. The lethal chloroplasts, which were virtually devoid of chlorophyll, were characterized by an absence of grana.

Abstract. Peas were grown in controlled environments (12h white fluorescent light. ∼47 μmol photons m‐2 s 1/12 dark, 25 °C), using (1) 15‐min far‐red illumination at the end of each photoperiod (brief FR) to simulate the increase in the far‐red/red ratio near the end of the day, and (2) high levels of supplementary far‐red light (red:far‐red ratio=0.04) during the entire photoperiod (long‐term FR) to simulate extreme shade conditions under a plant canopy. Brief FR illumination led to marked morphological effects attributable to phytochrome regulation, namely, an increase in internodal length, but a decrease in leaflet area, chloroplast size and chlorophyll content per chloroplast compared with the control. Significantly, brief FR illumination had little or no effect on the amounts of the major chloroplast components (ribulose 1.5‐biphosphate carboxylase, adenosine triphosphate synthase, cytochrome b/f complex and Photosystem II) relative to chlorophyll or Photosystem I, and the leaf photosynthetic capacities per unit chlorophyll were similar. In contrast, supplementing high levels of far‐red light during the entire photoperiod not only led to the phytochrome effects above, but there was also a marked increase in leaf photosynthetic capacity per unit chlorophyll. due to increased amounts of the major chloroplast components relative to chlorophyll or Photosystem I. We hypothesize that supplementary far‐red light, absorbed by Photosystem I, induced an increase in the major chloroplast components by a photosynthetic feedback mechanism. In fully greened leaves, we propose that the two photosystems themselves, rather than phytochrome, may be the predominent sensors of light quantity in triggering modulations of the stoichiometries of chloroplast components, which in turn lead to varying photosynthetic capacities.

The effect of light quality on the composition, function and structure of the thylakoid membranes, as well as on the photosynthetic rates of intact fronds from Asplenium australasicum, a shade plant, grown in blue, white, or red light of equal intensity (50 microeinsteins per square meter per second) was investigated. When compared with those isolated from plants grown in white and blue light, thylakoids from plants grown in red light have higher chlorophyll a/chlorophyll b ratios and lower amounts of light-harvesting chlorophyll a/b-protein complexes than those grown in blue light. On a chlorophyll basis, there were higher levels of PSII reaction centers, cytochrome f and coupling factor activity in thylakoids from red light-rown ferns, but lower levels of PSI reaction centers and plastoquinone. The red light-grown ferns had a higher PSII/ PSI reaction center ratio of 4.1 compared to 2.1 in blue light-grown ferns, and a larger apparent PSI unit size and a lower PSI unit size. The CO2 assimilation rates in fronds from red light-grown ferns were lower on a unit area or fresh weight basis, but higher on a chlorophyll basis, reflecting the higher levels of electron carriers and electron transport in the thylakoids.The structure of thylakoids isolated from plants grown under the three light treatments was similar, with no significant differences in the number of thylakoids per granal stack or the ratio of appressed membrane length/ nonippressed membrane length. The large freeze-fracture particles had the same size in the red-, blue-, and white-grown ferns, but there were some differences in their density. Light quality is an important factor in the regulation of the composition and function of thylakoid membranes, but the effects depend upon the plant species.The development of etiolated seedlings, in terms of plant growth, chloroplast structure, and function, is greatly influenced by the prevailing light quality (4,8,9,11,17,18,28,29). It has been reported that both developing and mature seedlings grown under blue light, have higher Chl a/Chl b ratios (4,11,17), higher prenylquinone content and lower xanthophyll to carotene ratios (11), and higher photosynthetic rates (17), than those grown under red light. These variations were also accompanied by differences in the ultrastructure of the chloroplasts with chloroplasts from plants grown under red light exhibiting higher grana content than those from plants grown under blue light (4, 1 1, 18). It was proposed (1 1, 18) that the differences in thylakoid composition, photosynthetic activity, and chloroplast structure of plants adapted to low intensity blue light are similar to 'sun ' (Fig. 1); note the instrument is not corrected beyond 700 nm.Chloroplasts were isolated from fully mature fronds as previously described (14) except that 0.1% BSA was present in the homogenization buffer. Thylakoids were washed in glass-distilled H20 followed by two washes and resuspension in 50 mM Tricine (pH 8.0). Aliquots of thylakoid membranes were stored in liquid N2. Total ...