Chlorophyll limitation in plants remodels and balances the photosynthetic apparatus by changing the accumulation of photosystems I and II through two different approaches

Proteome and Metabolome Analyses of Albino Bracts in Davidia involucrata

Although the mechanisms underlying albino phenotypes have been examined in model plants and major crops, our knowledge of bract albinism is still in its infancy. Davidia involucrata, a relic plant called dove tree, is best known for the intriguing trait with a pair of white bracts covering the capitula. Here, comparative physiological, cytological, proteomic, and metabolomic analyses were performed to dissect the albinism mechanism of D. involucrata bracts. The bracts exhibited low chlorophyll and carotenoid contents, reduced photosynthetic efficiency, and impaired chloroplast structure. The severe deficiency of photosynthetic pigments and the substantial decrease in cuticle thickness made the bracts light-sensitive. In total, 1134 differentially expressed proteins (DEPs) were obtained between bracts and leaves. Pathway enrichment analysis of DEPs revealed that photosynthetic pigment biosynthesis and photosynthesis were suppressed, whereas protein processing in endoplasmic reticulum, flavonoid biosynthesis, and the ubiquitin-proteasome system (UPS) were activated in bracts. Strikingly, DEPs implicated in chloroplast development, including PPR and AARS proteins, were mainly down-regulated in bracts. We further investigated albinism-induced metabolic changes and detected 412 differentially abundant metabolites (DAMs). Among them, enhanced flavonoids accumulation can plausibly explain the role of bracts in pollinator attraction. Amino acids and their derivatives in bracts showed remarkably increased abundance, which might be causally linked to enhanced UPS function. Our work could lay foundations for understanding albinism mechanisms and adaptive significance of plant bracts and facilitate future utilization of D. involucrata resources.

Agrobacterium-mediated transformation of B. juncea reveals that BjuLKP2 functions in plant yellowing

KEY MESSAGE

A stable Agrobacterium-mediated transformation system was constructed for B. juncea, and BjuLKP2 was overexpressed, leading to plant yellowing. A stable and efficient transformation system is necessary to verify gene functions in plants. To establish an Agrobacterium-mediated transformation system for B. juncea, various factors, including the explant types, hormone combination and concentration, infection time and concentration, were optimized. Eventually, a reliable system was established, and two BjuLKP2 overexpression (OE) lines, which displayed yellowing of cotyledons, shoot tips, leaves and flower buds, as well as a decrease in total chlorophyll content, were generated. qRT-PCR assays revealed significant upregulation of five chlorophyll synthesis genes and downregulation of one gene in the BjuLKP2 OE line. Furthermore, antioxidant capacity assays revealed reduced activities of APX, CAT and SOD, while POD activity increased in the BjuLKP2 OE26. Additionally, the kinetic determination of chlorophyll fluorescence induction suggested a decrease in the photosynthetic ability of BjuLKP2 OE26. GUS assays revealed the expression of BjuLKP2 in various tissues, including the roots, hypocotyls, cotyledons, leaf vasculature, trichomes, sepals, petals, filaments, styles and stigma bases, but not in seeds. Scanning electron revealed alterations in chloroplast ultrastructure in both the sponge and palisade tissue. Collectively, these findings indicate that BjuLKP2 plays a role in plant yellowing through a reduction in chlorophyll content and changes in chloroplasts structure.

J, Hase T, Jensen P, Hanke GT. Ferredoxin C2 is required for chlorophyll biosynthesis and accumulation of photosynthetic antennae in Arabidopsis

Ferredoxins (Fd) are small iron-sulphur proteins, with sub-types that have evolved for specific redox functions. Ferredoxin C2 (FdC2) proteins are essential Fd homologues conserved in all photosynthetic organisms and a number of different FdC2 functions have been proposed in angiosperms. Here we use RNAi silencing in Arabidopsis thaliana to generate a viable fdC2 mutant line with near-depleted FdC2 protein levels. Mutant leaves have ~50% less chlorophyll a and b, and chloroplasts have poorly developed thylakoid membrane structure. Transcriptomics indicates upregulation of genes involved in stress responses. Although fdC2 antisense plants show increased damage at photosystem II (PSII) when exposed to high light, PSII recovers at the same rate as wild type in the dark. This contradicts literature proposing that FdC2 regulates translation of the D1 subunit of PSII, by binding to psbA transcript. Measurement of chlorophyll biosynthesis intermediates revealed a build-up of Mg-protoporphyrin IX, the substrate of the aerobic cyclase. We localise FdC2 to the inner chloroplast envelope and show that the FdC2 RNAi line has a disproportionately lower protein abundance of antennae proteins, which are nuclear-encoded and must be refolded at the envelope after import.

Physiological and proteome studies of maize (Zea mays L.) in response to leaf removal under high plant density

BACKGROUND

Under high plant density, intensifying competition among individual plants led to overconsumption of energy and nutrients and resulted in an almost dark condition in the lower strata of the canopy, which suppressed the photosynthetic potential of the shaded leaves. Leaf removal could help to ameliorate this problem and increase crop yields. To reveal the mechanism of leaf removal in maize, tandem mass tags label-based quantitative analysis coupled with liquid chromatography-tandem mass spectrometry were used to capture the differential protein expression profiles of maize subjected to the removal of the two uppermost leaves (S₂), the four uppermost leaves (S₄), and with no leaf removal as control (S₀).

RESULTS

Excising leaves strengthened the light transmission rate of the canopy and increased the content of malondialdehyde, whereas decreased the activities of superoxide dismutase and peroxidase. Two leaves removal increased the photosynthetic capacity of ear leaves and the grain yield significantly, whereas S₄ decreased the yield markedly. Besides, 239 up-accumulated proteins and 99 down-accumulated proteins were identified between S₂ and S₀, which were strongly enriched into 30 and 23 functional groups; 71 increased proteins and 42 decreased proteins were identified between S₄ and S₀, which were strongly enriched into 22 and 23 functional groups, for increased and decreased proteins, respectively.

CONCLUSIONS

Different defoliation levels had contrastive effects on maize. The canopy light transmission rate was strengthened and proteins related to photosynthetic electron-transfer reaction were up-regulated significantly for treatment S₂, which improved the leaf photosynthetic capacity, and obtained a higher grain yield consequently. In contrast, S₄ decreased the grain yield and increased the expressions of proteins and genes associated with fatty acid metabolism. Besides, both S₂ and S₄ exaggerated the defensive response of maize in physiological and proteomic level. Although further studies are required, the results in our study provide new insights to the further improvement in maize grain yield by leaf removal.

iTRAQ-Based Quantitative Proteomic Analysis of Cotton Roots and Leaves Reveals Pathways Associated with Salt Stress

Salinity is a major abiotic stress that affects plant growth and development. In this study, we performed a proteomic analysis of cotton roots and leaf tissue following exposure to saline stress. 611 and 1477 proteins were differentially expressed in the roots and leaves, respectively. In the roots, 259 (42%) proteins were up-regulated and 352 (58%) were down-regulated. In the leaves, 748 (51%) proteins were up-regulated and 729 (49%) were down-regulated. On the basis of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, we concluded that the phenylalanine metabolism and starch and sucrose metabolism were active for energy homeostasis to cope with salt stress in cotton roots. Moreover, photosynthesis, pyruvate metabolism, glycolysis / gluconeogenesis, carbon fixation in photosynthetic organisms and phenylalanine metabolism were inhabited to reduce energy consumption. Characterization of the signaling pathways will help elucidate the mechanism activated by cotton in response to salt stress.

Linking chlorophyll biosynthesis to a dynamic plastoquinone pool

Chlorophylls are essential cofactors in photosynthesis. All steps in the chlorophyll pathway are well characterized except for the cyclase reaction in which the fifth ring of the chlorophyll molecule is formed during conversion of Mg-protoporphyrin IX monomethyl ester into Protochlorophyllide. The only subunit of the cyclase identified so far, is AcsF (Xantha-l in barley and Chl27 in Arabidopsis). This subunit contains a typical consensus di-iron-binding sequence and belongs to a subgroup of di-iron proteins, such as the plastid terminal oxidase (PTOX) in the chloroplast and the alternative oxidase (AOX) found in mitochondria. In order to complete the catalytic cycle, the irons of these proteins need to be reduced from Fe(3+) to Fe(2+) and either a reductase or another form of reductant is required. It has been reported that the alternative oxidase (AOX) and the plastid terminal oxidase (PTOX) utilize the di-iron center to oxidise ubiquinol and plastoquinol, respectively. In this paper, we have used a specific inhibitor of di-iron proteins as well as Arabidopsis and barley mutants affected in regulation of photosynthetic electron flow, to show that the cyclase step indeed is directly coupled to the plastoquinone pool. Thus, plastoquinol might act as an electron donor for the cyclase reaction and thereby fulfil the role of a cyclase reductase. That would provide a functional connection between the redox status of the thylakoids and the biosynthesis of chlorophyll.

Phytochrome B Mediates the Regulation of Chlorophyll Biosynthesis through Transcriptional Regulation of ChlH and GUN4 in Rice Seedlings

Accurate regulation of chlorophyll synthesis is crucial for chloroplast formation during the greening process in angiosperms. In this study, we examined the role of phytochrome B (phyB) in the regulation of chlorophyll synthesis in rice seedlings (Oryza sativa L.) through the characterization of a pale-green phenotype observed in the phyB mutant grown under continuous red light (Rc) irradiation. Our results show that the Rc-induced chlorophyll accumulation can be divided into two components—a phyB-dependent and a phyB-independent component, and that the pale-green phenotype is caused by the absence of the phyB-dependent component. To elucidate the role of the missing component we established an Rc-induced greening experiment, the results of which revealed that several genes encoding proteins on the chlorophyll branch were repressed in the phyB mutant. Notable among them were ChlH and GUN4 genes, which encode subunit H and an activating factor of magnesium chelatase (Mg-chelatase), respectively, that were largely repressed in the mutant. Moreover, the kinetic profiles of chlorophyll precursors suggested that Mg-chelatase activity simultaneously decreased with the reduction in the transcript levels of ChlH and GUN4. These results suggest that phyB mediates the regulation of chlorophyll synthesis through transcriptional regulation of these two genes, whose products exert their action at the branching point of the chlorophyll biosynthesis pathway. Reduction of 5-aminolevulinic acid (5-ALA) synthesis could be detected in the mutant, but the kinetic profiles of chlorophyll precursors indicated that it was an event posterior to the reduction of the Mg-chelatase activity. It means that the repression of 5-ALA synthesis should not be a triggering event for the appearance of the pale-green phenotype. Instead, the repression of 5-ALA synthesis might be important for the subsequent stabilization of the pale-green phenotype for preventing excessive accumulation of hazardous chlorophyll precursors, which is an inevitable consequence of the reduction of Mg-chelatase activity.

iTRAQ-based quantitative proteomics analysis of Brassica napus leaves reveals pathways associated with chlorophyll deficiency

Photosynthesis, the primary source of plant biomass, is important for plant growth and crop yield. Chlorophyll is highly abundant in plant leaves and plays essential roles in photosynthesis. We recently isolated a chlorophyll-deficient mutant (cde1) from ethyl methanesulfonate (EMS) mutagenized Brassica napus. Herein, quantitative proteomics analysis using the iTRAQ approach was conducted to investigate cde1-induced changes in the proteome. We identified 5069 proteins from B. napus leaves, of which 443 showed differential accumulations between the cde1 mutant and its corresponding wild-type. The differentially accumulated proteins were found to be involved in photosynthesis, porphyrin and chlorophyll metabolism, biosynthesis of secondary metabolites, carbon fixation, spliceosome, mRNA surveillance and RNA degradation. Our results suggest that decreased abundance of chlorophyll biosynthetic enzymes and photosynthetic proteins, impaired carbon fixation efficiency and disturbed redox homeostasis might account for the reduced chlorophyll contents, impaired photosynthetic capacity and increased lipid peroxidation in this mutant. Epigenetics was implicated in the regulation of gene expression in cde1, as proteins involved in DNA/RNA/histone methylation and methylation-dependent chromatin silencing were up-accumulated in the mutant. Biological significance Photosynthesis produces more than 90% of plant biomass and is an important factor influencing potential crop yield. The pigment chlorophyll plays essential roles in light harvesting and energy transfer during photosynthesis. Mutants deficient in chlorophyll synthesis have been used extensively to investigate the chlorophyll metabolism, development and photosynthesis. However, limited information is available with regard to the changes of protein profiles upon chlorophyll deficiency. Here, a combined physiological, histological, proteomics and molecular analysis revealed several important pathways associated with chlorophyll deficiency. This work provides new insights into the regulation of chlorophyll biosynthesis and photosynthesis in higher plants and these findings may be applied to genetic engineering for high photosynthetic efficiency in crops.

LCAA, a novel factor required for magnesium protoporphyrin monomethylester cyclase accumulation and feedback control of aminolevulinic acid biosynthesis in tobacco

Low Chlorophyll Accumulation A (LCAA) antisense plants were obtained from a screen for genes whose partial down-regulation results in a strong chlorophyll deficiency in tobacco (Nicotiana tabacum). The LCAA mutants are affected in a plastid-localized protein of unknown function, which is conserved in cyanobacteria and all photosynthetic eukaryotes. They suffer from drastically reduced light-harvesting complex (LHC) contents, while the accumulation of all other photosynthetic complexes per leaf area is less affected. As the disturbed accumulation of LHC proteins could be either attributable to a defect in LHC biogenesis itself or to a bottleneck in chlorophyll biosynthesis, chlorophyll synthesis rates and chlorophyll synthesis intermediates were measured. LCAA antisense plants accumulate magnesium (Mg) protoporphyrin monomethylester and contain reduced protochlorophyllide levels and a reduced content of CHL27, a subunit of the Mg protoporphyrin monomethylester cyclase. Bimolecular fluorescence complementation assays confirm a direct interaction between LCAA and CHL27. 5-Aminolevulinic acid synthesis rates are increased and correlate with an increased content of glutamyl-transfer RNA reductase. We suggest that LCAA encodes an additional subunit of the Mg protoporphyrin monomethylester cyclase, is required for the stability of CHL27, and contributes to feedback-control of 5-aminolevulinic acid biosynthesis, the rate-limiting step of chlorophyll biosynthesis.

Reorganization of photosystem II is involved in the rapid photosynthetic recovery of desert moss Syntrichia caninervis upon rehydration

The moss Syntrichia caninervis (S. caninervis) is one of the dominant species in biological soil crusts of deserts. It has long been the focus of scientific research because of its ecological value. Moreover, S. caninervis has a special significance in biogenesis research because it is characterized by its fast restoration of photosynthesis upon onset of rehydration of the desiccated organism. In order to study the mechanisms of rapid photosynthetic recovery in mosses upon rewatering, we investigated the kinetics of the recovery process of photosynthetic activity in photosystem (PS) II, with an indirect assessment of the photochemical processes based on chlorophyll (Chl) fluorescence measurements. Our results showed that recovery can be divided into two phases. The fast initial phase, completed within 3 min, was characterized by a quick increase in maximal quantum efficiency of PSII (F(v)/F(m)). Over 50% of the PSII activities, including excitation energy transfer, oxygen evolution, charge separation, and electron transport, recovered within 0.5 min after rehydration. The second, slow phase was dominated by the increase of plastoquinone (PQ) reduction and the equilibrium of the energy transport from the inner antenna to the reaction center (RC) of PSII. Analysis of the recovery process in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) revealed that blocking the electron transport from Q(A) to Q(B) did not hamper Chl synthesis or Chl organization in thylakoid membranes under light conditions. A de novo chloroplast protein synthesis was not necessary for the initial recovery of photochemical activity in PSII. In conclusion, the moss's ability for rapid recovery upon rehydration is related to Chl synthesis, quick structural reorganization of PSII, and fast restoration of PSII activity without de novo chloroplast protein synthesis.