Water-soluble chlorophyll protein in Brassicaceae plants is a stress-induced chlorophyll-binding protein

Exciton interactions of chlorophyll tetramer in water-soluble chlorophyll-binding protein BoWSCP

The exciton interaction of four chlorophyll a (Chl a) molecules in a symmetrical tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP was analyzed in the pH range of 3-11. Exciton splitting ΔE = 232 ± 2 cm-1 of the Qy band of Chl a into two subcomponents with relative intensities of 78.1 ± 0.7 % and 21.9 ± 0.7 % was determined by a joint decomposition of the absorption and circular dichroism spectra into Gaussian functions. The exciton coupling parameters were calculated based on the BoWSCP atomic structure in three approximations: the point dipole model, the distributed atomic monopoles, and direct ab initio calculations in the TDDFT/PCM approximation. The Coulomb interactions of monomers were calculated within the continuum model using three values of optical permittivity. The models based on the properties of free Chl a in solution suffer from significant errors both in estimating the absolute value of the exciton interaction and in the relative intensity of exciton transitions. Calculations within the TDDFT/PCM approximation reproduce the experimentally determined parameters of the exciton splitting and the relative intensities of the exciton bands. The following factors of pigment-protein and pigment-pigment interactions were examined: deviation of the macrocycle geometry from the planar conformation of free Chl; the formation of hydrogen bonds between the macrocycle and water molecules; the overlap of wave functions of monomers at close distances. The most significant factor is the geometrical deformation of the porphyrin macrocycle, which leads to an increase in the dipole moment of Chl monomer from 5.5 to 6.9 D and to a rotation of the dipole moment by 15° towards the cyclopentane ring. The contributions of resonant charge-transfer states to the wave functions of the Chl dimer were determined and the transition dipole moments of the symmetric and antisymmetric charge-transfer states were estimated.

Femtosecond Dynamics of Excited States of Chlorophyll Tetramer in Water-Soluble Chlorophyll-Binding Protein BoWSCP

The paper reports on the absorption dynamics of chlorophyll a in a symmetric tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP. It was measured by a broadband femtosecond laser pump-probe spectroscopy within the range from 400 to 750 nm and with a time resolution of 20 fs-200 ps. When BoWSCP was excited in the region of the Soret band at a wavelength of 430 nm, nonradiative intramolecular conversion S3→S1 was observed with a characteristic time of 83 ± 9 fs. When the complex was excited in the region of the Qy band at 670 nm, relaxation transition between two excitonic states of the chlorophyll dimer was observed in the range of 105 ± 10 fs. Absorption spectra of the excited singlet states S1 and S3 of chlorophyll a were obtained. The delocalization of the excited state between exciton-coupled Chl molecules in BoWSCP tetramer changed in time and depended on the excitation energy. When BoWSCP is excited in the Soret band region, an ultrafast photochemical reaction is observed. This could result from the reduction of tryptophan in the vicinity of chlorophyll.

Heterologous expression of the Glycine soja Kunitz-type protease inhibitor GsKTI improves resistance to drought stress and Helicoverpa armigera in transgenic Arabidopsis lines

Kunitz-like protease inhibitors (KTIs) have been identified to play critical roles in insect defense, but evidence for their involvement in drought stress is sparse. The aim of this study was to identify and functionally characterize a Kunitz-like protease inhibitor, GsKTI, from the wild soybean (Glycine soja) variety ED059. Expression patterns suggest that drought stress and insect herbivory may induce GsKTI transcript levels. Transgenic Arabidopsis lines overexpressing GsKTI have been shown to exhibit enhanced drought tolerance by regulating the ABA signaling pathway and increasing xylem cell number. Transgenic Arabidopsis leaves overexpressing GsKTI interfered with insect digestion and thus had a negative effect on the growth of Helicoverpa armigera. It is concluded that GsKTI increases resistance to drought stress and insect attack in transgenic Arabidopsis lines.

Brassica napus Drought-Induced 22-kD Protein (BnD22) Acts Simultaneously as a Cysteine Protease Inhibitor and Chlorophyll-Binding Protein

Class II water-soluble chlorophyll proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins that bind with chlorophyll (Chl) and its derivatives. The physiological function of WSCPs is still unclear, but it is assumed to be involved in stress responses, which is likely related to their Chl-binding and protease inhibition (PI) activities. Yet, the dual function and simultaneous functionality of WSCPs must still be better understood. Here, the biochemical functions of Brassica napus drought-induced 22-kDa protein (BnD22), a major WSCP expressed in B. napus leaves, were investigated using recombinant hexahistidine-tagged protein. We showed that BnD22 inhibited cysteine proteases, such as papain, but not serine proteases. BnD22 was able to bind with Chla or Chlb to form tetrameric complexes. Unexpectedly, BnD22-Chl tetramer displays higher inhibition toward cysteine proteases, indicating (i) simultaneous Chl-binding and PI activities and (ii) Chl-dependent activation of PI activity of BnD22. Moreover, the photostability of BnD22-Chl tetramer was reduced upon binding with the protease. Using three-dimensional structural modeling and molecular docking, we revealed that Chl binding favors interaction between BnD22 and proteases. Despite its Chl-binding ability, the BnD22 was not detected in chloroplasts but rather in the endoplasmic reticulum and vacuole. In addition, the C-terminal extension peptide of BnD22, which cleaved off post-translationally in vivo, was not implicated in subcellular localization. Instead, it drastically promoted the expression, solubility and stability of the recombinant protein.

Chlorophyll a Dimers Bound in the Water-Soluble Protein BoWSCP Photosensitize the Reduction of Cytochrome c

When bound to water-soluble proteins of the WSCP family, chlorophyll molecules form dimers structurally similar to the "special pair" of chlorophylls (bacteriochlorophylls) in photosynthetic reaction centers. Being exposed to red light (λ ≥ 650 nm) in oxygen-free solutions, chlorophyll a dimers harbored by BoWSCP holoproteins (from Brassica oleracea var. botrytis) have sensitized the reduction of cytochrome c. According to absorption and circular dichroism spectroscopy data, the photochemical process did not significantly impair the structure of chlorophyll a molecules as well as their dimers harbored by BoWSCP protein. Adding tris(hydroxymethyl)aminomethane as an electron donor for chlorophyll recovery stimulated the photoreduction of cytochrome c.

Fingerprint and Universal Markovian Closure of Structured Bosonic Environments

We exploit the properties of chain mapping transformations of bosonic environments to identify a finite collection of modes able to capture the characteristic features, or fingerprint, of the environment. Moreover we show that the countable infinity of residual bath modes can be replaced by a universal Markovian closure, namely, a small collection of damped modes undergoing a Lindblad-type dynamics whose parametrization is independent of the spectral density under consideration. We show that the Markovian closure provides a quadratic speedup with respect to standard chain mapping techniques and makes the memory requirement independent of the simulation time, while preserving all the information on the fingerprint modes. We illustrate the application of the Markovian closure to the computation of linear spectra but also to nonlinear spectral response, a relevant experimentally accessible many body coherence witness for which efficient numerically exact calculations in realistic environments are currently lacking.

Characterization, stability, and feasibility of long-term use of light-absorbing components of aqueous spinach extract-based photogalvanic electrolyte

In the present work, the photogalvanic cells have been studied with respect to the photo-stability and the long-term use of the electrolyte based on crude aqueous spinach extract sensitizer for solar energy harvesting. Further, the nature of chemical components present in the old and photo-decayed electrolyte and their current generation capacity has also not been investigated so far otherwise it is of much significance for durable use of the same electrolyte in cells. In earlier studies, the steady-state photo-generation of current for about two hours from crude spinach extract-based cell has been shown during illumination. But, the data for only two hours of the steady-state current generation is not sufficient to show the feasibility of working with photogalvanic cells. Therefore, to fill this research gap of lack of characterization of sensitizers’ molecules of crude spinach extract and lack of study on long-term use of this electrolyte (crude spinach extract-surfactant-reductant-alkali-water), the present extensive study has been done. The observed spectrum of crude spinach extract resembles that of chlorophyll–protein complex showing it is the main chemical component in extract absorbing light. A strong acid adversely affects the extract’s photogalvanics and high pH is friendly to the physiological and photogalvanic activity of the extract. The spectra of illuminated and very old crude spinach extract-NaOH-Sodium lauryl sulfate (NaLS)-Fructose photogalvanic electrolyte solution show negligible absorbance (540–700 nm) and zero absorbance (at 700 nm) suggesting the absence of chlorophyll due to its photo-degradation. When this photo-degraded electrolyte is again illuminated, the power output obtained is nearly equal to that for the first time illuminated fresh electrolyte. The observed current at zero time and after 2641 h from the same electrolyte used in long term is 50 mA cm⁻² and 40 mA cm⁻², respectively. It means that the fresh crude spinach extract, as well as the photo-degraded extract at high pH, are almost equally capable of power generation.

New insights into chlorophyll-WSCP (water-soluble chlorophyll proteins) interactions : The case study of BnD22 (Brassica napus drought-induced 22 kDa)

The water-soluble chlorophyll-proteins (WSCP) of class II from Brassicaceae are non-photosynthetic proteins that bind chlorophylls (Chls) and chlorophyll derivatives. Their physiological roles, biochemical functions and mode of action are still unclear. It is assumed that the WSCPs have a protection function against Chl photodamage during stressful conditions. WSCPs are subdivided into class IIA and class IIB according to their apparent Chla/b binding ratio. Although their Chla/Chlb binding selectivity has been partly characterized, their Chl affinities are not yet precisely defined. For instance, WSCPs IIA do not show any Chl binding preference while WSCPs IIB have greater affinity to Chlb. In this study, we present a novel method for assessment of Chl binding to WSCPs based on the differences of Chl photobleaching rates in a large range of Chl/protein ratios. The protein we have chosen to study WSCP is BnD22, a WSCP IIA induced in the leaves of Brassica napus under water deficit. BnD22 formed oligomeric complexes upon binding to Chla and/or Chlb allowing a protective effect against photodamage. The binding constants indicate that BnD22 binds with high affinity the Chls and with a strong selectivity to Chla. Moreover, dependending of Chl/protein ratio upon reconstitution, two distinct binding events were detected resulting from difference of Chl stoichiometry inside oligomeric complexes.

Magnetophotoselection in the Investigation of Excitonically Coupled Chromophores: The Case of the Water-Soluble Chlorophyll Protein

A magnetophotoselection (MPS) investigation of the photoexcited triplet state of chlorophyll a both in a frozen organic solvent and in a protein environment, provided by the water-soluble chlorophyll protein (WSCP) of Lepidium virginicum, is reported. The MPS experiment combines the photoselection achieved by exciting with linearly polarized light with the magnetic selection of electron paramagnetic resonance (EPR) spectroscopy, allowing the determination of the relative orientation of the optical transition dipole moment and the zero-field splitting tensor axes in both environments. We demonstrate the robustness of the proposed methodology for a quantitative description of the excitonic interactions among pigments. The orientation of the optical transition dipole moments determined by the EPR analysis in WSCP, identified as an appropriate model system, are in excellent agreement with those calculated in the point-dipole approximation. In addition, MPS provides information on the electronic properties of the triplet state, localized on a single chlorophyll a pigment of the protein cluster, in terms of orientation of the zero-field splitting tensor axes in the molecular frame.

High-throughput sequencing data revealed genotype-specific changes evoked by heat stress in crown tissue of barley sdw1 near-isogenic lines

BACKGROUND

High temperature shock is becoming increasingly common in our climate, affecting plant growth and productivity. The ability of a plant to survive stress is a complex phenomenon. One of the essential tissues for plant performance under various environmental stimuli is the crown. However, the molecular characterization of this region remains poorly investigated. Gibberellins play a fundamental role in whole-plant stature formation. This study identified plant stature modifications and crown-specific transcriptome re-modeling in gibberellin-deficient barley sdw1.a (BW827) and sdw1.d (BW828) mutants exposed to increased temperature.

RESULTS

The deletion around the sdw1 gene in BW827 was found to encompass at least 13 genes with primarily regulatory functions. A bigger genetic polymorphism of BW828 than of BW827 in relation to wild type was revealed. Transcriptome-wide sequencing (RNA-seq) revealed several differentially expressed genes involved in gibberellin metabolism and heat response located outside of introgression regions. It was found that HvGA20ox4, a paralogue of the HvGA20ox2 gene, was upregulated in BW828 relative to other genotypes, which manifested as basal internode elongation. The transcriptome response to elevated temperature differed in the crown of sdw1.a and sdw1.d mutants; it was most contrasting for HvHsf genes upregulated under elevated temperature in BW828, whereas those specific to BW827 were downregulated. In-depth examination of sdw1 mutants revealed also some differences in their phenotypes and physiology.

CONCLUSIONS

We concluded that despite the studied sdw1 mutants being genetically related, their heat response seemed to be genotype-specific and observed differences resulted from genetic background diversity rather than single gene mutation, multiple gene deletion, or allele-specific expression of the HvGA20ox2 gene. Differences in the expressional reaction of genes to heat in different sdw1 mutants, found to be independent of the polymorphism, could be further explained by in-depth studies of the regulatory factors acting in the studied system. Our findings are particularly important in genetic research area since molecular response of crown tissue has been marginally investigated, and can be useful for wide genetic research of crops since barley has become a model plant for them.

Towards a quantitative description of excitonic couplings in photosynthetic pigment-protein complexes: quantum chemistry driven multiscale approaches

A structure-based quantitative calculation of excitonic couplings between photosynthetic pigments has to describe the dynamical polarization of the protein/solvent environment of the pigments, giving rise to reaction field and screening effects. Here, this challenging problem is approached by combining the fragment molecular orbital (FMO) method with the polarizable continuum model (PCM). The method is applied to compute excitonic couplings between chlorophyll a (Chl a) pigments of the water-soluble chlorophyll-binding protein (WSCP). By calibrating the vacuum dipole strength of the 0–0 transition of the Chl a chromophores according to experimental data, an excellent agreement between calculated and experimental linear absorption and circular dichroism spectra of WSCP is obtained. The effect of the mutual polarization of the pigment ground states is calculated to be very small. The simple Poisson-Transition-charge-from-Electrostatic-potential (Poisson-TrEsp) method is found to accurately describe the screening part of the excitonic coupling, obtained with FMO/PCM. Taking into account that the reaction field effects of the latter method can be described by a scalar constant leads to an improvement of Poisson-TrEsp that is expected to provide the basis for simple and realistic calculations of optical spectra and energy transfer in photosynthetic light-harvesting complexes. In addition, we present an expression for the estimation of Huang–Rhys factors of high-frequency pigment vibrations from experimental fluorescence line-narrowing spectra that takes into account the redistribution of oscillator strength by the interpigment excitonic coupling. Application to WSCP results in corrected Huang–Rhys factors that are less than one third of the original values obtained by the standard electronic two-state analysis that neglects the above redistribution. These factors are important for the estimation of the dipole strength of the 0–0 transition of the chromophores and for the development of calculation schemes for the spectral density of the exciton-vibrational coupling.

The importance of reaction field and screening effects on the excitonic couplings is demonstrated, and from quantum-chemical calculations a single scaling factor is derived that can be used to improve simple models based on the Poisson equation.

Direct Assembly in Aqueous Solutions of Stable Chlorophyllide Complexes with Type II Water-soluble Chlorophyll Proteins

Water-soluble chlorophyll-binding proteins (WSCPs) from Brassicaceae constitute a small family of non-photosynthetic proteins that may provide a useful benchmark and model system for studying molecular aspects of chlorophyll-protein interactions such as the tuning of absorption and emission spectra, and binding selectivity. WSCP apo-proteins are readily expressed by recombinant DNA techniques and can be assembled in vitro with natural and synthetic chlorophyll derivatives. The complexes with native chlorophylls are exceptionally stable toward thermal dissociation and protein denaturation due to hydrophobic interactions with the chlorophyll's phytyl chains that stabilize the core of the WSCP tetrameric complexes. However, assembly requires the use of detergents or water-in-oil emulsions to introduce the hydrophobic pigments into the water-soluble apo-proteins. Here, we explore the direct assembly of recombinant WSCPs with the water-soluble phytyl-free chlorophyll analogue chlorophyllide a in aqueous solutions. We show that the complexes formed by mixing chlorophyllide and WSCP apo-proteins are exclusively tetrameric, and while they lack the extreme thermostability of the respective chlorophyll complexes, they are still thermostable up to around 60°C. Their absorption and CD spectra are very similar to the chlorophyll complexes albeit slight peak shifts and broadening of the bands indicate variations in pigment and protein conformations, and less rigid structures. Simplifying the assembly process of WSCPs opens new possibilities for their use in modelling natural chlorophyll-protein complexes, and as templates for designing novel artificial protein-pigment complexes.

Proteomic Insight into the Symbiotic Relationship of Pinus massoniana Lamb and Suillus luteus towards Developing Al-Stress Resistance

Global warming significantly impacts forest range areas by increasing soil acidification or aluminum toxicity. Aluminum (Al) toxicity retards plant growth by inhibiting the root development process, hindering water uptake, and limiting the bioavailability of other essential micronutrients. Pinus massoniana (masson pine), globally recognized as a reforestation plant, is resistant to stress conditions including biotic and abiotic stresses. This resistance is linked to the symbiotic relationship with diverse ectomycorrhizal fungal species. In the present study, we investigated the genetic regulators as expressed proteins, conferring a symbiotic relationship between Al-stress resistance and Suillus luteus in masson pine. Multi-treatment trials resulted in the identification of 12 core Al-stress responsive proteins conserved between Al stress conditions with or without S. luteus inoculation. These proteins are involved in chaperonin CPN60-2, protein refolding and ATP-binding, Cu-Zn-superoxide dismutase precursor, oxidation-reduction process, and metal ion binding, phosphoglycerate kinase 1, glycolytic process, and metabolic process. Furthermore, 198 Al responsive proteins were identified specifically under S. luteus-inoculation and are involved in gene regulation, metabolic process, oxidation-reduction process, hydrolase activity, and peptide activity. Chlorophyll a-b binding protein, endoglucanase, putative spermidine synthase, NADH dehydrogenase, and glutathione-S-transferase were found with a significant positive expression under a combined Al and S. luteus treatment, further supported by the up-regulation of their corresponding genes. This study provides a theoretical foundation for exploiting the regulatory role of ectomycorrhizal inoculation and associated genetic changes in resistance against Al stress in masson pine.

How water-soluble chlorophyll protein extracts chlorophyll from membranes

Water-soluble chlorophyll proteins (WSCPs) found in Brassicaceae are non-photosynthetic proteins that bind only a small number of chlorophylls. Their biological function remains unclear, but recent data indicate that WSCPs are involved in stress response and pathogen defense as producers of reactive oxygen species and/or Chl-regulated protease inhibitors. For those functions, WSCP apoprotein supposedly binds Chl to become physiologically active or inactive, respectively. Thus, Chl-binding seems to be a pivotal step for the biological function of WSCP. WSCP can extract Chl from the thylakoid membrane but little is known about the mechanism of how Chl is sequestered from the membrane into the binding sites. Here, we investigate the interaction of WSCP with the thylakoid membrane in detail. The extraction of Chl from the thylakoid by WSCP apoprotein is a slow and inefficient reaction, because WSCP presumably does not directly extract Chl from other Chl-binding proteins embedded in the membrane. WSCP apoprotein interacts with model membranes that contain the thylakoid lipids MGDG, DGDG or PG, and can extract Chl from those. Furthermore, the WSCP-Chl complex, once formed, no longer interacts with membranes. We concluded that the surroundings of the WSCP pigment-binding site are involved in the WSCP-membrane interaction and identified a ring of hydrophobic amino acids with two conserved Trp residues around the Chl-binding site. Indeed, WSCP variants, in which one of the Trp residues was exchanged for Phe, still interact with the membrane but are no longer able to extract Chl.

The pigment binding behaviour of water-soluble chlorophyll protein (WSCP)

Water-soluble chlorophyll proteins (WSCPs) are homotetrameric proteins that bind four chlorophyll (Chl) molecules in identical binding sites, which makes WSCPs a good model to study protein-pigment interactions. In a previous study, we described preferential binding of Chl a or Chl b in various WSCP versions. Chl b binding is preferred when a hydrogen bond can be formed between the C7 formyl of the chlorin macrocycle and the protein, whereas Chl a is preferred when Chl b binding is sterically unfavorable. Here, we determined the binding affinities and kinetics of various WSCP versions not only for Chl a/b, but also for chlorophyllide (Chlide) a/b and pheophytin (Pheo) a/b. Altered KD values are responsible for the Chl a/b selectivity in WSCP whereas differences in the reaction kinetics are neglectable in explaining different Chl a/b preferences. WSCP binds both Chlide and Pheo with a lower affinity than Chl, which indicates the importance of the phytol chain and the central Mg2+ ion as interaction sites between WSCP and pigment. Pheophorbide (Pheoide), lacking both the phytol chain and the central Mg2+ ion, can only be bound as Pheoide b to a WSCP that has a higher affinity for Chl b than Chl a, which underlines the impact of the C7 formyl-protein interaction. Moreover, WSCP was able to bind protochlorophyllide and Mg-protoporphyrin IX, which suggests that neither the size of the π electron system of the macrocycle nor the presence of a fifth ring at the macrocycle notably affect the binding to WSCP. WSCP also binds heme to form a tetrameric complex, suggesting that heme is bound in the Chl-binding site.

How the Protein Environment Can Tune the Energy, the Coupling, and the Ultrafast Dynamics of Interacting Chlorophylls: The Example of the Water-Soluble Chlorophyll Protein

The interplay between active molecules and the protein environment in light-harvesting complexes tunes the photophysics and the dynamical properties of pigment-protein complexes in a subtle way, which is not fully understood. Here we characterized the photophysics and the ultrafast dynamics of four variants of the water-soluble chlorophyll protein (WSCP) as an ideal model system to study the behavior of strongly interacting chlorophylls. We found that when coordinated by the WSCP protein, the presence of the formyl group in chlorophyll b replacing the methyl group in chlorophyll a strongly affects the exciton energy and the dynamics of the system, opening up the possibility of tuning the photophysics and the transport properties of multichromophores by engineering specific interactions with the surroundings.

On the Stability of the Water-Soluble Chlorophyll-Binding Protein (WSCP) Studied by Molecular Dynamics Simulations

The water-soluble chlorophyll-binding protein (WSCP) is assumed to be not a part of the photosynthetic process. Applying molecular dynamics (MD) simulations, we aimed to obtain insight into the exceptional stability of WSCP. We analyzed dynamical features such as the hydrogen bond network, flexibility, and force distributions. The WSCP structure contains two cysteines at the interfaces of every protein chain, which are in close contact with the cysteines of the other dimer. We tested if a connection of these cysteines between different protein chains influences the dynamical behavior to investigate any influences on the thermal stability. We find that the hydrogen bond network is very stable regardless of the presence or absence of the hypothetical disulfide bridges and/or the chlorophyll units. Furthermore, it is found that the phytyl chains of the chlorophyll units are extremely flexible, much more than what is seen in crystal structures. Nonetheless, they seem to protect a photochemically active site of the chlorophylls over the complete simulation time. Finally, we also find that a cavity in the chlorophyll-surrounding sheath exists, which may allow access for individual small molecules to the core of WSCP.

How water-mediated hydrogen bonds affect chlorophyll a/b selectivity in Water-Soluble Chlorophyll Protein

The Water-Soluble Chlorophyll Protein (WSCP) of Brassicaceae is a remarkably stable tetrapyrrole-binding protein that, by virtue of its simple design, is an exceptional model to investigate the interactions taking place between pigments and their protein scaffold and how they affect the photophysical properties and the functionality of the complexes. We investigated variants of WSCP from Lepidium virginicum (Lv) and Brassica oleracea (Bo), reconstituted with Chlorophyll (Chl) b, to determine the mechanisms by which the different Chl binding sites control their Chl a/b specificities. A combined Raman and crystallographic investigation has been employed, aimed to characterize in detail the hydrogen-bond network involving the formyl group of Chl b. The study revealed a variable degree of conformational freedom of the hydrogen bond networks among the WSCP variants, and an unexpected mixed presence of hydrogen-bonded and not hydrogen-bonded Chls b in the case of the L91P mutant of Lv WSCP. These findings helped to refine the description of the mechanisms underlying the different Chl a/b specificities of WSCP versions, highlighting the importance of the structural rigidity of the Chl binding site in the vicinity of the Chl b formyl group in granting a strong selectivity to binding sites.

Beneficial implications of sugar beet proteinase inhibitor BvSTI on plant architecture and salt stress tolerance in Lotus corniculatus L

Food demands of increasing human population dictate intensification of livestock production, however, environmental stresses could jeopardize producers' efforts. Forage legumes suffer from yield losses and poor nutritional status due to salinity increase of agricultural soils. As tools aimed to reduce negative impacts of biotic or abiotic stresses, proteinase inhibitors (PIs) have been promoted for biotechnological improvements. In order to increase tolerance of Lotus corniculatus L. to salt stress, serine PI, BvSTI, was introduced into this legume using Agrobacterium rhizogenes, with final transformation efficiency of 4.57%. PCR, DNA gel-blot, RT-PCR and in-gel protein activity assays confirmed the presence and activity of BvSTI products in transformed lines. Plants from three selected transgenic lines (21, 73 and 109) showed significant alterations in overall phenotypic appearance, corresponding to differences in BvSTI accumulation. Lines 73 and 109 showed up to 7.3-fold higher number of tillers and massive, up to 5.8-fold heavier roots than in nontransformed controls (NTC). Line 21 was phenotypically similar to NTC, accumulated less BvSTI transcripts and did not exhibit an additional band of recombinant trypsin inhibitor as seen in lines 73 and 109. Exposure of the transgenic lines to NaCl revealed different levels of salt stress susceptibility. The NaCl sensitivity index, based on morphological appearance and chlorophyll concentrations showed that lines 73 and 109 were significantly less affected by salinity than NTC or line 21. High level of BvSTI altered morphology and delayed salt stress related senescence, implicating BvSTI gene as a promising tool for salinity tolerance improvement trials in L. corniculatus.

New homologues of Brassicaceae water-soluble chlorophyll proteins shed light on chlorophyll binding, spectral tuning, and molecular evolution

Type-II water-soluble chlorophyll (Chl) proteins (WSCPs) of Brassicaceae are promising models for understanding how protein sequence and structure affect Chl binding and spectral tuning in photosynthetic Chl-protein complexes. However, to date, their use has been limited by the small number of known WSCPs, which also limited understanding their physiological roles. To overcome these limitations, we performed a phylogenetic analysis to compile a more comprehensive and complete set of natural type-II WSCP homologues. The identified homologues were heterologously expressed in Escherichia coli, purified, tested for assembly with chlorophylls, and spectroscopically characterized. The analyses led to the discovery of previously unrecognized type-IIa and IIb subclass WSCPs, as well as of a new subclass that did not bind chlorophylls. Further analysis by ancestral sequence reconstruction yielded sequences of putative ancestors of the three subclasses, which were subsequently recombinantly expressed in E. coli, purified and characterized. Combining the phylogenetic and spectroscopic data with molecular structural information revealed distinct Chl-binding motifs, and identified residues critically impacting spectral tuning. The distinct Chl-binding properties of the WSCP archetypes suggest that the non-Chl-binding subclass evolved from a Chl-binding ancestor that most likely lost its Chl-binding capacity upon localization in the plant tissues with low Chl content. This dual evolutionary trajectory is consistent with WSCPs association with the Kunitz-type protease inhibitors superfamily, and indications of their inhibitory activity in response to various forms of stress in plants. These findings suggest new directions for exploring the physiological roles of WSCPs and the correlation, if any, between Chl-binding and protease inhibition functionality.

Chlorophyll a/b binding-specificity in water-soluble chlorophyll protein

We altered the chlorophyll (Chl) binding sites in various versions of water-soluble chlorophyll protein (WSCP) by amino acid exchanges to alter their preferences for either Chl a or Chl b. WSCP is ideally suited for this mutational analysis since it forms a tetrameric complex with only four identical Chl binding sites. A loop of 4-6 amino acids is responsible for Chl a versus Chl b selectivity. We show that a single amino acid exchange within this loop changes the relative Chl a/b affinities by a factor of 40. We obtained crystal structures of this WSCP variant binding either Chl a or Chl b. The Chl binding sites in these structures were compared with those in the major light-harvesting complex (LHCII) of the photosynthetic apparatus in plants to search for similar structural features involved in Chl a/b binding specificity.

Complete enzyme set for chlorophyll biosynthesis in Escherichia coli

Chlorophylls are essential cofactors for photosynthesis, which sustains global food chains and oxygen production. Billions of tons of chlorophylls are synthesized annually, yet full understanding of chlorophyll biosynthesis has been hindered by the lack of characterization of the Mg-protoporphyrin IX monomethyl ester oxidative cyclase step, which confers the distinctive green color of these pigments. We demonstrate cyclase activity using heterologously expressed enzyme. Next, we assemble a genetic module that encodes the complete chlorophyll biosynthetic pathway and show that it functions in Escherichia coli. Expression of 12 genes converts endogenous protoporphyrin IX into chlorophyll a, turning E. coli cells green. Our results delineate a minimum set of enzymes required to make chlorophyll and establish a platform for engineering photosynthesis in a heterotrophic model organism.

The complex world of plant protease inhibitors: Insights into a Kunitz-type cysteine protease inhibitor of Arabidopsis thaliana

Plants have evolved an intricate regulatory network of proteases and corresponding protease inhibitors (PI), which operate in various biological pathways and serve diverse spatiotemporal functions during the sedentary life of a plant. Intricacy of the regulatory network can be anticipated from the observation that, depending on the developmental stage and environmental cue(s), either a single PI or multiple PIs regulate the activity of a given protease. On the other hand, the same PI often interacts with different targets at different places, necessitating another level of fine control to be added in planta. Here, it is reported on how the activity of a papain-like cysteine protease dubbed RD21 (RESPONSIVE TO DESICCATION 21) is differentially regulated by serpin and Kunitz PIs over plant development and how this mechanism contributes to defenses against herbivorous arthropods and microbial pests.

Water-Soluble Chlorophyll Protein (WSCP) Stably Binds Two or Four Chlorophylls

Water-soluble chlorophyll proteins (WSCPs) of class IIa from Brassicaceae form tetrameric complexes containing one chlorophyll (Chl) per apoprotein but no carotenoids. The complexes are remarkably stable toward dissociation and protein denaturation even at 100 °C and extreme pH values, and the Chls are partially protected against photooxidation. There are several hypotheses that explain the biological role of WSCPs, one of them proposing that they function as a scavenger of Chls set free upon plant senescence or pathogen attack. The biochemical properties of WSCP described in this paper are consistent with the protein acting as an efficient and flexible Chl scavenger. At limiting Chl concentrations, the recombinant WSCP apoprotein binds substoichiometric amounts of Chl (two Chls per tetramer) to form complexes that are as stable toward thermal dissociation, denaturation, and photodamage as the fully pigmented ones. If more Chl is added, these two-Chl complexes can bind another two Chls to reach the fully pigmented state. The protection of WSCP Chls against photodamage has been attributed to the apoprotein serving as a diffusion barrier for oxygen, preventing its access to triplet excited Chls and, thus, the formation of singlet oxygen. By contrast, the sequential binding of Chls by WSCP suggests a partially open or at least flexible structure, raising the question of how WSCP photoprotects its Chls without the help of carotenoids.

Serpin1 and WSCP differentially regulate the activity of the cysteine protease RD21 during plant development in Arabidopsis thaliana

Proteolytic enzymes (proteases) participate in a vast range of physiological processes, ranging from nutrient digestion to blood coagulation, thrombosis, and beyond. In plants, proteases are implicated in host recognition and pathogen infection, induced defense (immunity), and the deterrence of insect pests. Because proteases irreversibly cleave peptide bonds of protein substrates, their activity must be tightly controlled in time and space. Here, we report an example of how nature evolved alternative mechanisms to fine-tune the activity of a cysteine protease dubbed RD21 (RESPONSIVE TO DESICCATION-21). One mechanism in the model plant Arabidopsis thaliana studied here comprises irreversible inhibition of RD21's activity by Serpin1, whereas the other mechanism is a result of the reversible inhibition of RD21 activity by a Kunitz protease inhibitor named water-soluble chlorophyll-binding protein (WSCP). Activity profiling, complex isolation, and homology modeling data revealed unique interactions of RD21 with Serpin1 and WSCP, respectively. Expression studies identified only partial overlaps in Serpin1 and WSCP accumulation that explain how RD21 contributes to the innate immunity of mature plants and arthropod deterrence of seedlings undergoing skotomorphogenesis and greening.

Biosynthesis of Chlorophyll a in a Purple Bacterial Phototroph and Assembly into a Plant Chlorophyll-Protein Complex

Improvements to photosynthetic efficiency could be achieved by manipulating pigment biosynthetic pathways of photosynthetic organisms in order to increase the spectral coverage for light absorption. The development of organisms that can produce both bacteriochlorophylls and chlorophylls is one way to achieve this aim, and accordingly we have engineered the bacteriochlorophyll-utilizing anoxygenic phototroph Rhodobacter sphaeroides to make chlorophyll a. Bacteriochlorophyll and chlorophyll share a common biosynthetic pathway up to the precursor chlorophyllide. Deletion of genes responsible for the bacteriochlorophyll-specific modifications of chlorophyllide and replacement of the native bacteriochlorophyll synthase with a cyanobacterial chlorophyll synthase resulted in the production of chlorophyll a. This pigment could be assembled in vivo into the plant water-soluble chlorophyll protein, heterologously produced in Rhodobacter sphaeroides, which represents a proof-of-principle for the engineering of novel antenna complexes that enhance the spectral range of photosynthesis.

A potent bidirectional promoter from the monocot cereal Eleusine coracana

Ragi bifunctional α-amylase-trypsin inhibitor (RBI) of Eleusine coracana (L.) Gaertn. (finger millet) simultaneously inhibits α-amylase and trypsin. In continuation of previous work on the cloning, expression and characterization of RBI, a bidirectional promoter from finger millet was explored on the basis of experimental observations. Two trypsin inhibitors were identified while purifying RBI from a trypsin-Sepharose column eluent. Using an FPLC gel filtration column, these three inhibitors were purified to homogeneity and subjected to MALDI-TOF-TOF-MS/MS analysis and N-terminal sequencing. Both ragi trypsin inhibitors (RTIs) showed the same N-terminal sequence and considerable sequence similarity to RBI, indicating the presence of a multigene protease inhibitor family in finger millet. To gain insight into the evolution of these genes, the upstream region of RBI was explored by Genome Walking. Interestingly, on sequencing, a genome walking product of ∼1 Kb showed presence of an N-terminal RBI specific primer sequence twice but in opposite directions and leaving an intervening region of ∼0.9 Kb. The intervening region was presumed to represent an E. coracana bidirectional promoter (EcBDP), intuitively having a divergent RBI-RTI gene pair at two sides. For assaying the bidirectionality of promoter activity, a dual reporter GUS-GFP vector construct was made for plant expression containing the reporter genes at two ends of EcBDP, which was used to transform Agrobacterium tumefaciens LBA 4404. Transient plant transformation by recombinant Agrobacterium cells was carried out in onion scale epidermal cells and finger millet seedling leaves. Simultaneous expression of GUS and GFP under EcBDP established it as a potent natural bidirectional promoter from monocot origin, thereby potentially having vast application in cereal gene manipulations. In addition, inducibility of the EcBDP by either abscisic acid or cold treatment, as determined by transient transformation in onion, would substantiate more precise control of gene expression to mitigate the effects of adverse environmental conditions.

Water in Oil Emulsions: A New System for Assembling Water-soluble Chlorophyll-binding Proteins with Hydrophobic Pigments

Chlorophylls (Chls) and bacteriochlorophylls (BChls) are the primary cofactors that carry out photosynthetic light harvesting and electron transport. Their functionality critically depends on their specific organization within large and elaborate multisubunit transmembrane protein complexes. In order to understand at the molecular level how these complexes facilitate solar energy conversion, it is essential to understand protein-pigment, and pigment-pigment interactions, and their effect on excited dynamics. One way of gaining such understanding is by constructing and studying complexes of Chls with simple water-soluble recombinant proteins. However, incorporating the lipophilic Chls and BChls into water-soluble proteins is difficult. Moreover, there is no general method, which could be used for assembly of water-soluble proteins with hydrophobic pigments. Here, we demonstrate a simple and high throughput system based on water-in-oil emulsions, which enables assembly of water-soluble proteins with hydrophobic Chls. The new method was validated by assembling recombinant versions of the water-soluble chlorophyll binding protein of Brassicaceae plants (WSCP) with Chl a. We demonstrate the successful assembly of Chl a using crude lysates of WSCP expressing E. coli cell, which may be used for developing a genetic screen system for novel water-soluble Chl-binding proteins, and for studies of Chl-protein interactions and assembly processes.

Hole-Burning Spectroscopy on Excitonically Coupled Pigments in Proteins: Theory Meets Experiment

A theory for the calculation of resonant and nonresonant hole-burning (HB) spectra of pigment–protein complexes is presented and applied to the water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The theory is based on a non-Markovian line shape theory (Renger and MarcusJ. Chem. Phys.2002, 116, 9997) and includes exciton delocalization, vibrational sidebands, and lifetime broadening. An earlier approach by Reppert (J. Phys. Chem. Lett.2011, 2, 2716) is found to describe nonresonant HB spectra only. Here we present a theory that can be used for a quantitative description of HB data for both nonresonant and resonant burning conditions. We find that it is important to take into account the excess energy of the excitation in the HB process. Whereas excitation of the zero-phonon transition of the lowest exciton state, that is, resonant burning allows the protein to access only its conformational substates in the neighborhood of the preburn state, any higher excitation gives the protein full access to all conformations present in the original inhomogeneous ensemble. Application of the theory to recombinant WSCP from cauliflower, reconstituted with chlorophyll a or chlorophyll b, gives excellent agreement with experimental data by Pieper et al. (J. Phys. Chem. B2011, 115, 405321417356) and allows us to obtain an upper bound of the lifetime of the upper exciton state directly from the HB experiments in agreement with lifetimes measured recently in time domain 2D experiments by Alster et al. (J. Phys. Chem. B2014, 118, 352424627983).

An Ethylene-Protected Achilles' Heel of Etiolated Seedlings for Arthropod Deterrence

A small family of Kunitz protease inhibitors exists in Arabidopsis thaliana, a member of which (encoded by At1g72290) accomplishes highly specific roles during plant development. Arabidopsis Kunitz-protease inhibitor 1 (Kunitz-PI;1), as we dubbed this protein here, is operative as cysteine PI. Activity measurements revealed that despite the presence of the conserved Kunitz-motif the bacterially expressed Kunitz-PI;1 was unable to inhibit serine proteases such as trypsin and chymotrypsin, but very efficiently inhibited the cysteine protease RESPONSIVE TO DESICCATION 21. Western blotting and cytolocalization studies using mono-specific antibodies recalled Kunitz-PI;1 protein expression in flowers, young siliques and etiolated seedlings. In dark-grown seedlings, maximum Kunitz-PI;1 promoter activity was detected in the apical hook region and apical parts of the hypocotyls. Immunolocalization confirmed Kunitz-PI;1 expression in these organs and tissues. No transmitting tract (NTT) and HECATE 1 (HEC1), two transcription factors previously implicated in the formation of the female reproductive tract in flowers of Arabidopsis, were identified to regulate Kunitz-PI;1 expression in the dark and during greening, with NTT acting negatively and HEC1 acting positively. Laboratory feeding experiments with isopod crustaceans such as Porcellio scaber (woodlouse) and Armadillidium vulgare (pillbug) pinpointed the apical hook as ethylene-protected Achilles' heel of etiolated seedlings. Because exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and mechanical stress (wounding) strongly up-regulated HEC1-dependent Kunitz-PI;1 gene expression, our results identify a new circuit controlling herbivore deterrence of etiolated plants in which Kunitz-PI;1 is involved.

DUF538 protein superfamily is predicted to be chlorophyll hydrolyzing enzymes in plants

The possible hydrolytic activity towards chlorophyll molecules was predicted for DUF538 protein superfamily in plants. It was examined by using computational as well as experimental tools including in vitro chlorophyll degradation, antioxidant compounds production and in vivo real-time gene expression tests. Comparison of the computational data with the experimental results indicated that DUF538 proteins might be chlorophyll hydrolyzing enzyme (most probably carboxyesterase) which degrade chlorophyll molecules (66 % per 12 hrs) to produce new compounds (1.8 fold per 12 hrs) with antioxidant properties. The relevance of DUF538 gene expression level with the chlorophyll contents (2.8 fold increase per chlorophyll content of 50 %) of the drought-stressed leaves showed that chlorophyll degradation by DUF538 is most probably induced in response to stress stimuli. Despite membranous chlorophyll catabolic pathways, DUF538-dependent reactions is predicted to be occurred in the cytosol of the under stressed plants. We addressed as to whether chlorophyll breakdown to antioxidant compounds by DUF538 is a defense mechanism of plants against stress stimuli, in vivo? This question is going to be investigated in our next research project.

A Kunitz-type protease inhibitor regulates programmed cell death during flower development in Arabidopsis thaliana

Flower development and fertilization are tightly controlled in Arabidopsis thaliana. In order to permit the fertilization of a maximum amount of ovules as well as proper embryo and seed development, a subtle balance between pollen tube growth inside the transmitting tract and pollen tube exit from the septum is needed. Both processes depend on a type of programmed cell death that is still poorly understood. Here, it is shown that a Kunitz protease inhibitor related to water-soluble chlorophyll proteins of Brassicaceae (AtWSCP, encoded by At1g72290) is involved in controlling cell death during flower development in A. thaliana. Genetic, biochemical, and cell biology approaches revealed that WSCP physically interacts with RD21 (RESPONSIVE TO DESICCATION) and that this interaction in turn inhibits the activity of RD21 as a pro-death protein. The regulatory circuit identified depends on the restricted expression of WSCP in the transmitting tract and the septum epidermis. In a respective Atwscp knock-out mutant, flowers exhibited precocious cell death in the transmitting tract and unnatural death of septum epidermis cells. As a consequence, apical-basal pollen tube growth, fertilization of ovules, as well as embryo development and seed formation were perturbed. Together, the data identify a unique mechanism of cell death regulation that fine-tunes pollen tube growth.

Water-soluble chlorophyll-binding proteins from Arabidopsis thaliana and Raphanus sativus target the endoplasmic reticulum body

Background

Non-photosynthetic chlorophyll (Chl) proteins called water-soluble Chl-binding proteins are distributed in Brassicaceae plants. Brassica oleracea WSCP (BoWSCP) and Lepidium virginicum WSCP (LvWSCP) are highly expressed in leaves and stems, while Arabidopsis thaliana WSCP (AtWSCP) and Raphanus sativus WSCP (RshWSCP) are highly transcribed in floral organs. BoWSCP and LvWSCP exist in the endoplasmic reticulum (ER) body. However, the subcellular localization of AtWSCP and RshWSCP is still unclear. To determine the subcellular localization of these WSCPs, we constructed transgenic plants expressing Venus-fused AtWSCP or RshWSCP.

Results

Open reading frames corresponding to full-length AtWSCP and RshWSCP were cloned and ligated between the cauliflower mosaic virus 35S promoter and Venus, a gene encoding a yellow fluorescent protein. We introduced the constructs into A. thaliana by the floral dip method. We succeeded in constructing a number of transformants expressing Venus-fused chimeric AtWSCP (AtWSCP::Venus) or RshWSCP (RshWSCP::Venus). We detected fluorescence derived from the chimeric proteins using a fluorescence microscope system. In cotyledons, fluorescence derived from AtWSCP::Venus and RshWSCP::Venus was detected in spindle structures. The spindle structures altered their shape to a globular form under blue light excitation. In true leaves, the number of spindle structures was drastically reduced. These observations indicate that the spindle structure was the ER body.

Conclusions

AtWSCP and RshWSCP have the potential for ER body targeting like BoWSCP and LvWSCP.

Constitutive over-expression of rice chymotrypsin protease inhibitor gene OCPI2 results in enhanced growth, salinity and osmotic stress tolerance of the transgenic Arabidopsis plants

Protease inhibitors are involved primarily in defense against pathogens. In recent years, these proteins have also been widely implicated in response of plants to diverse abiotic stresses. Rice chymotrypsin protease inhibitor gene OCPI2 is highly induced under salt and osmotic stresses. The construct containing the complete coding sequence of OCPI2 cloned downstream to CaMV35S promoter was transformed in Arabidopsis and single copy, homozygous transgenic lines were produced. The transgenic plants exhibited significantly enhanced tolerance to NaCl, PEG and mannitol stress as compared to wild type plants. Importantly, the vegetative and reproductive growth of transgenic plants under unstressed, control conditions was also enhanced: transgenic plants were more vigorous than wild type, resulting into higher yield in terms of silique number. The RWC values and membrane stability index of transgenic in comparison to wild type plants was higher. Higher proline content was observed in the AtOCPI2 lines, which was associated with higher transcript expression of pyrroline-5-carboxylate synthase and lowered levels of proline dehydrogenase genes. The chymotrypsin protease activities were lower in the transgenic as against wild type plants, under both unstressed, control as well as stressed conditions. It thus appears that rice chymotrypsin protease inhibitor gene OCPI2 is a useful candidate gene for genetic improvement of plants against salt and osmotic stress.

Water-soluble chlorophyll protein is involved in herbivore resistance activation during greening of Arabidopsis thaliana

Water-soluble chlorophyll proteins (WSCPs) constitute a small family of unusual chlorophyll (Chl)-binding proteins that possess a Kunitz-type protease inhibitor domain. In Arabidopsis thaliana, a WSCP has been identified, named AtWSCP, that forms complexes with Chl and the Chl precursor chlorophyllide (Chlide) in vitro. AtWSCP exhibits a quite unexpected expression pattern for a Chl binding protein and accumulated to high levels in the apical hook of etiolated plants. AtWSCP expression was negatively light-regulated. Transgenic expression of AtWSCP fused to green fluorescent protein (GFP) revealed that AtWSCP is localized to cell walls/apoplastic spaces. Biochemical assays identified AtWSCP as interacting with RD21 (responsive to desiccation 21), a granulin domain-containing cysteine protease implicated in stress responses and defense. Reconstitution experiments showed tight interactions between RD21 and WSCP that were relieved upon Chlide binding. Laboratory feeding experiments with two herbivorous isopod crustaceans, Porcellio scaber (woodlouse) and Armadillidium vulgare (pillbug), identified the apical hook as Achilles' heel of etiolated plants and that this was protected by RD21 during greening. Because Chlide is formed in the apical hook during seedling emergence from the soil, our data suggest an unprecedented mechanism of herbivore resistance activation that is triggered by light and involves AtWSCP.

Are tyrosine residues involved in the photoconversion of the water-soluble chlorophyll-binding protein of Chenopodium album?

Non-photosynthetic and hydrophilic chlorophyll (Chl) proteins, called water-soluble Chl-binding proteins (WSCPs), are distributed in various species of Chenopodiaceae, Amaranthaceae, Polygonaceae and Brassicaceae. Based on their photoconvertibility, WSCPs are categorised into two classes: Class I (photoconvertible) and Class II (non-photoconvertible). Chenopodium album WSCP (CaWSCP; Class I) is able to convert the chlorin skeleton of Chl a into a bacteriochlorin-like skeleton under light in the presence of molecular oxygen. Potassium iodide (KI) is a strong inhibitor of the photoconversion. Because KI attacks tyrosine residues in proteins, tyrosine residues in CaWSCP are considered to be important amino acid residues for the photoconversion. Recently, we identified the gene encoding CaWSCP and found that the mature region of CaWSCP contained four tyrosine residues: Tyr13, Tyr14, Tyr87 and Tyr134. To gain insight into the effect of the tyrosine residues on the photoconversion, we constructed 15 mutant proteins (Y13A, Y14A, Y87A, Y134A, Y13-14A, Y13-87A, Y13-134A, Y14-87A, Y14-134A, Y87-134A, Y13-14-87A, Y13-14-134A, Y13-87-134A, Y14-87-134A and Y13-14-87-134A) using site-directed mutagenesis. Amazingly, all the mutant proteins retained not only chlorophyll-binding activity, but also photoconvertibility. Furthermore, we found that KI strongly inhibited the photoconversion of Y13-14-87-134A. These findings indicated that the four tyrosine residues are not essential for the photoconversion.

Effect of near-infrared irradiation on photoconversion of the water-soluble chlorophyll-binding protein of Chenopodium album

We investigated the effects of near-infrared irradiation on the photoconversion of Chenopodium album water-soluble chlorophyll-binding protein (CaWSCP) in the presence of sodium hydrosulfite and found a further photoconversion from CP742 to CP763, a novel form of CaWSCP. Interestingly, one-third of the absorption peak at 668 nm was recovered in CP763, but re-irradiation under oxidative conditions eliminated the photo convertibility of CaWSCP.

Identification of genes encoding photoconvertible (Class I) water-soluble chlorophyll-binding proteins from Chenopodium ficifolium

Photoconvertible water-soluble chlorophyll-binding proteins, called Class I WSCPs, have been detected in Chenopodiaceae, Amaranthaceae and Polygonaceae plant species. To date, Chenopodium album WSCP (CaWSCP) is the only cloned gene encoding a Class I WSCP. In this study, we identified two cDNAs encoding Chenopodium ficifolium Class I WSCPs, CfWSCP1, and CfWSCP2. Sequence analyses revealed that the open reading frames of CfWSCP1 and CfWSCP2 were 585 and 588 bp, respectively. Furthermore, both CfWSCPs contain cystein2 and cystein30, which are essential for the chlorophyll-binding ability of CaWSCP. Recombinant CfWSCP1 and CfWSCP2, expressed in Escherichia coli as hexa-histidine fusion proteins (CfWSCP1-His and CfWSCP2-His), formed inclusion bodies; however, we were able to solubilize these using a buffer containing 8 M urea and then refold them by dialysis. The refolded CfWSCP1-His and CfWSCP2-His could bind chlorophylls and exhibited photoconvertibility, confirming that the cloned CfWSCPs are further examples of Class I WSCPs.

Cysteine-2 and Cys30 are essential for chlorophyll-binding activity of the water-soluble chlorophyll-binding protein (WSCP) of Chenopodium album

Chenopodium album has a non-photosynthetic chlorophyll protein known as the water-soluble chlorophyll (Chl)-binding protein (WSCP). The C. album WSCP (CaWSCP) is able to photoconvert the chlorin skeleton of Chl a into a bacteriochlorin-like skeleton. Reducing reagents such as β-mercaptoethanol or dithiothreitol inhibit photoconversion, indicating that S-S bridge(s) in CaWSCP are quite important for it. Recently, we found that the mature region of CaWSCP contains five cysteine residues; Cys2, Cys30, Cys48, Cys63, and Cys144. To identify which cysteine residues are involved in the photoconversion, we generated five mutants (C2S, C30S, C48S, C63S, and C144S) by site-directed mutagenesis. Interestingly, C48S, C63S, and C144S mutants showed the same Chl-binding activity and photoconvertibility as those of the recombinant wild-type CaWSCP-His, while the C2S and C30S mutants completely lost Chl-binding activity. Our findings indicated that the S-S bridge between Cys2 and Cys30 in each CaWSCP subunit is essential for Chl-binding activity.

Leaf senescence and nitrogen remobilization efficiency in oilseed rape (Brassica napus L.)

Despite its worldwide economic importance for food (oil, meal) and non-food (green energy and chemistry) uses, oilseed rape has a low nitrogen (N) use efficiency (NUE), mainly due to the low N remobilization efficiency (NRE) observed during the vegetative phase when sequential leaf senescence occurs. Assuming that improvement of NRE is the main lever for NUE optimization, unravelling the cellular mechanisms responsible for the recycling of proteins (the main N source in leaf) during sequential senescence is a prerequisite for identifying the physiological and molecular determinants that are associated with high NRE. The development of a relevant molecular indicator (SAG12/Cab) of leaf senescence progression in combination with a (15)N-labelling method were used to decipher the N remobilization associated with sequential senescence and to determine modulation of this process by abiotic factors especially N deficiency. Interestingly, in young leaves, N starvation delayed senescence and induced BnD22, a water-soluble chlorophyll-binding protein that acts against oxidative alterations of chlorophylls and exhibits a protease inhibitor activity. Through its dual function, BnD22 may help to sustain sink growth of stressed plants and contribute to a better utilization of N recycled from senescent leaves, a physiological trait that could improve NUE. Proteomics approaches have revealed that proteolysis involves chloroplastic FtsH protease in the early stages of senescence, aspartic protease during the course of leaf senescence, and the proteasome β1 subunit, mitochondria processing protease and SAG12 (cysteine protease) during the later senescence phases. Overall, the results constitute interesting pathways for screening genotypes with high NRE and NUE.

Three-step photoconversion of only three subunits of the water-soluble chlorophyll-binding protein tetramer from Chenopodium album

Water-soluble chlorophyll (Chl)-binding proteins (WSCPs) have been found in various plants. WSCPs are categorized into two classes based on their photoconvertibility: Class I (photoconvertible) and Class II (non-photoconvertible). Based on their absorption peaks, which occur in the red wavelengths, the pre- and post-photoconverted forms of Chenopodium album WSCP (CaWSCP) are called CP668 and CP742, respectively. Although various biochemical and biophysical properties of CaWSCP have already been characterized, questions remain regarding the structural dynamics of the photoconversion from CP668 to CP742, and the relationship between the photoconversion activity and incident light wavelength. To address how the wavelength of incident light affects the photoconversion, we performed time-course analyses of CaWSCP photoconversion by using light-emitting diodes that emit either white light, or at the discrete wavelengths 670, 645, 525, 470, or 430 nm. The most efficient photoconversion was observed under irradiation at 430 nm. Less efficient photoconversion was observed under irradiation with 670, 645, 470, or 525 nm light, in that order. The relationship between photoconversion activity and wavelength corresponded with the absorption peak intensities of Chls in the CaWSCP complex. The observed time dependence of the A(742)/A(668) ratio during photoconversion of the CaWSCP complex indicated that the photoconversion from CP668 to CP742 occurs in a three-step reaction, and that only three subunits in the complex could be photoconverted.