Degradation pathway(s) of chlorophyll: what has gene cloning revealed?

TMT-Based Proteomic Analysis Reveals the Molecular Mechanisms of Sodium Pheophorbide A against Black Spot Needle Blight Caused by Pestalotiopsis neglecta in Pinus sylvestris var. mongolica

Black spot needle blight is a minor disease in Mongolian Scots pine (Pinus sylvestris var. mongolica) caused by Pestalotiopsis neglecta, but it can cause economic losses in severe cases. Sodium pheophorbide a (SPA), an intermediate product of the chlorophyll metabolism pathway, is a compound with photoactivated antifungal activity, which has been previously shown to inhibit the growth of P. neglecta. In this study, SPA significantly reduced the incidence and disease index and enhanced the chlorophyll content and antioxidant enzyme activities of P. sylvestris var. mongolica. To further study the molecular mechanism of the inhibition, we conducted a comparative proteomic analysis of P. neglecta mycelia with and without SPA treatment. The cellular proteins were obtained from P. neglecta mycelial samples and subjected to a tandem mass tag (TMT)-labelling LC-MS/MS analysis. Based on the results of de novo transcriptome assembly, 613 differentially expressed proteins (DEPs) (p < 0.05) were identified, of which 360 were upregulated and 253 downregulated. The 527 annotated DEPs were classified into 50 functional groups according to Gene Ontology and linked to 256 different pathways using the Kyoto Encyclopedia of Genes and Genomes database as a reference. A joint analysis of the transcriptome and proteomics results showed that the top three pathways were Amino acid metabolism, Carbohydrate metabolism, and Lipid metabolism. These results provide new viewpoints into the molecular mechanism of the inhibition of P. neglecta by SPA at the protein level and a theoretical basis for evaluating SPA as an antifungal agent to protect forests.

Transcriptome Characterization of Pigment-Related Genes in Jujube (Ziziphus Jujuba Mill.) Peel at Different Growth Stages

One of the most important qualities of jujube fruit is its color. Chlorophyll, carotenoid, and anthocyanin all play important roles in the coloring of jujube fruit. However, few studies have focused on the pigment molecular mechanism. In the present study, jujube peels of 'Sanbianhong' in three growth stages were evaluated for their gene expression characteristics and gene regulation related to pigment formation using the transcriptome sequencing analysis. A total of 84.86 Gb of clean data were obtained in the analysis. In the FS1 vs. FS3, FS1 vs. FS5, and FS3 vs. FS5, 4,530, 11,012, and 9,072 differentially expressed genes (DEGs) were identified, respectively. The inter-group screening among the three comparisons yielded 1430 common DEGs. Among these DEGs, 27, 16, and 28 genes were enriched in chlorophyll, carotenoid, and anthocyanin metabolic pathways, respectively. Twelve genes were chosen at random, and the accuracy of the transcriptome data were confirmed using qRT-PCR. The molecular mechanism underlying the pigmentation of jujube fruit was elucidated at the transcriptome level, which would provide a scientific basis for the subsequent functional studies on the color-regulating genes of jujube fruits.

Effects of Salinity Stress on Growth and Physiological Parameters and Related Gene Expression in Different Ecotypes of Sesuvium portulacastrum on Hainan Island

We conducted a study to examine the growth and physiological changes in 12 different ecotypes of Sesuvium portulacastrum collected from Hainan Island in China. These ecotypes were subjected to different concentrations (0, 200, 400, and 600 mmol/L) of sodium chloride (NaCl) salt stress for 14 days. We also analyzed the expression of metabolic genes related to stress response. Under low salt stress, indicators such as plant height in region K (0 mmol/L: 45% and highest at 200 mmol/L: 80%), internode length (0 mmol/L: 0.38, 200 mmol/L: 0.87, 400 mmol/L: 0.25, and 600 mmol/L: 1.35), as well as leaf area, relative water content, fresh weight, and dry weight exhibited an overall increasing trend with the increase in salt concentration. However, as the salt concentration increased, these indicators showed a decreasing trend. Proline and malondialdehyde contents increased with higher salt concentrations. When the NaCl concentration was 400 mmol/L, MDA content in the leaves was highest in the regions E (196.23%), F (94.28%), J (170.10%), and K (136.08%) as compared to the control group, respectively. Most materials demonstrated a significant decrease in chlorophyll a, chlorophyll b, and total chlorophyll content compared to the control group. Furthermore, the ratio of chlorophyll a to chlorophyll b (Rab) varied among different materials. Using principal component analysis, we identified three ecotypes (L from Xinglong Village, Danzhou City; B from Shuigoupo Village, Lingshui County; and J from Haidongfang Park, Dongfang City) that represented high, medium, and low salt tolerance levels, respectively, based on the above growth and physiological indexes. To further investigate the expression changes of related genes at the transcriptional level, we employed qRT-PCR. The results showed that the relative expression of SpP5CS1, SpLOX1, and SpLOX1 genes increased with higher salt concentrations, which corresponded to the accumulation of proline and malondialdehyde content, respectively. However, the relative expression of SpCHL1a and SpCHL1b did not exhibit a consistent pattern. This study contributes to our understanding of the salt tolerance mechanism in the true halophyte S. portulacastrum, providing a solid theoretical foundation for further research in this field.

SmCIP7, a COP1 interactive protein, positively regulates anthocyanin accumulation and fruit size in eggplant

In higher plants, COP1 (Constitutively Photomorphogenic 1) acts as a central regulator of light-signaling networks and globally conditions the target proteins via the ubiquitin-proteasome pathway. However, the function of COP1-interacting proteins in light-regulated fruit coloration and development remains unknown in Solanaceous plants. Here, a COP1-interacting protein-encoding gene, SmCIP7, expressed specifically in the eggplant (Solanum melongena L.) fruit, was isolated. Gene-specific silencing of SmCIP7 using RNA interference (RNAi) significantly altered fruit coloration, fruit size, flesh browning, and seed yield. SmCIP7-RNAi fruits showed evident repression of the accumulation of anthocyanins and chlorophyll, indicating functional similarities between SmCIP7 and AtCIP7. However, the reduced fruit size and seed yield indicated SmCIP7 had evolved a distinctly new function. With the comprehensive application of HPLC-MS, RNA-seq, qRT-PCR, Y2H, BiFC, LCI, and dual-luciferase reporter system (DLR™), it was found that SmCIP7, a COP1 interactive protein in light signaling promoted anthocyanin accumulation, probably by regulating the transcription of SmTT8. Additionally, the drastic up-regulation of SmYABBY1, a homologous gene of SlFAS, might account for the strongly retarded fruit growth in SmCIP7-RNAi eggplant. Altogether, this study proved that SmCIP7 is an essential regulatory gene to modulate fruit coloration and development, serving as a key gene locus in eggplant molecular breeding.

Interactive effect of 24-epibrassinolide and plant growth promoting rhizobacteria inoculation restores photosynthetic attributes in Brassica juncea L. under chlorpyrifos toxicity

Chlorpyrifos (CP) is a commonly used organophosphorous pesticide that is frequently utilised in the agricultural industry because of its great efficiency and inexpensive cost. The focus of the present study was to assess the impact of CP toxicity on Brassica juncea L. and to unravel the ameliorative potential of phytohormone, 24-epibrassinolide (EBL) mediated plant-microbe (Pseudomonas aeruginosa (B1), Burkholderia gladioli (B2)) interaction in B. juncea L. The maximum significant increment in the total chlorophyll, carotenoids, xanthophyll, anthocyanin and flavonoid content with EBL and B2 treatment in CP stressed B. juncea seedlings on spectrophotometric analysis were observed. Autofluorescence imaging of photosynthetic pigments i.e. chlorophyll, carotenoids, and total phenols with confocal microscopy showed maximum fluorescence with EBL and B2. Furthermore, when compared to CP stressed seedlings, scanning electron microscopy (SEM) study of the abaxial surface of leaves revealed a recovery in stomatal opening. The supplementation of EBL and PGPR (plant growth promoting rhizobacteria) improved the level of psb A (D1 subunit PSII) and psb B (CP 47 subunit of PSII) genes expression. The expression analysis of chalcone synthase (CHS), Phenylalanine ammonialyase (PAL), Phyotene synthase (PSY) with RT-PCR system showed up-regulation in the expression when supplemented with EBL and PGPR. As a result, the current study suggests that EBL and PGPR together, can reduce CP-induced toxicity in B. juncea seedlings and recovering the seedling biomass.

PeCLH2 Gene Positively Regulate Salt Tolerance in Transgenic Populus alba × Populus glandulosa

Salt is an important environmental stress factor, which seriously affects the growth, development and distribution of plants. Chlorophyllase plays an important role in stress response. Nevertheless, little is known about the physiological and molecular mechanism of chlorophyll (Chlase, CLH) genes in plants. We cloned PeCLH2 from Populus euphratica and found that PeCLH2 was differentially expressed in different tissues, especially in the leaves of P. euphratica. To further study the role of PeCLH2 in salt tolerance, PeCLH2 overexpression and RNA interference transgenic lines were established in Populus alba × Populus glandulosa and used for salt stress treatment and physiologic indexes studies. Overexpressing lines significantly improved tolerance to salt treatment and reduced reactive oxygen species production. RNA interference lines showed the opposite. Transcriptome analysis was performed on leaves of control and transgenic lines under normal growth conditions and salt stress to predict genes regulated during salt stress. This provides a basis for elucidating the molecular regulation mechanism of PeCLH2 in response to salt stress and improving the tolerance of poplar under salt stress.

Salinity stress improves antioxidant potential by modulating physio-biochemical responses in Moringa oleifera Lam

Moringa oleifera Lam. is a common edible plant, famous for several nutritional and therapeutic benefits. This study investigates the salt -induced modulations in plant growth, physio-biochemical responses, and antioxidant performance of M. oleifera grown under 0, 50, and 100 mM NaCl concentrations. Results showed that the plant effectively managed moderate salinity (50 mM NaCl) by maintaining succulence, weight ratios, and biomass allocation patterns of both shoot and root with minimal reduction in dry biomass. However, high salinity (100 mM NaCl) remarkably declined all growth parameters. The plant accumulated more Na⁺ and Cl^-, while less K⁺ under salinity as compared to the control. Consequently, osmotic potentials of both root and leaf decreased under salinity, which was corroborated by the high amount of proline and soluble sugars. Increased level of H₂O₂ with significantly unchanged membrane fluidity indicating its role in perceiving and managing stress at moderate salinity. In addition, increased activities of superoxide dismutase, and catalase, with increased glutathione and flavonoid contents suggest an integrated participation of both enzymatic and non-enzymatic antioxidant components in regulating ROS. On the other hand, high salinity caused an outburst of ROS indicated by high H₂O₂, MDA, and electrolyte leakage. As a response, moringa drastically increased the activities of all antioxidant enzymes and contents of antioxidant molecules including ascorbic acid, glutathione, total phenols, and flavonoids with high radical scavenging and reducing power capacities. However, a considerable amount of energy was used in such management resulting in a significant growth reduction at 100 mM NaCl. This study suggests that moringa effectively resisted moderate salinity by modulating physio-biochemical attributes and effectively managing ion toxicity and oxidative stress. Salt stress also enhanced the medicinal potentials of moringa by increasing the contents of antioxidant compounds including ascorbic acid, glutathione, total phenols, and flavonoids and their resulting activities. It can be grown on degraded/ saline lands and biomass of this plant can be used for edible and medicinal purposes, besides providing other benefits in a global climate change scenario.

Dissecting the genetic control of natural variation in sorghum photosynthetic response to drought stress

Drought stress causes crop yield losses worldwide. Sorghum is a C4 species tolerant to moderate drought stress, and its extensive natural variation for photosynthetic traits under water-limiting conditions can be exploited for developing cultivars with enhanced stress tolerance. The objective of this study was to discover genes/genomic regions that control the sorghum photosynthetic capacity under pre-anthesis water-limiting conditions. We performed a genome-wide association study for seven photosynthetic gas exchange and chlorophyll fluorescence traits during three periods of contrasting soil volumetric water content (VWC): control (30% VWC), drought (15% VWC), and recovery (30% VWC). Water stress was imposed with an automated irrigation system that generated a controlled dry-down period for all plants, to perform an unbiased genotypic comparison. A total of 60 genomic regions were associated with natural variation in one or more photosynthetic traits in a particular treatment or with derived variables. We identified 33 promising candidate genes with predicted functions related to stress signaling, oxidative stress protection, hormonal response to stress, and dehydration protection. Our results provide new knowledge about the natural variation and genetic control of sorghum photosynthetic response to drought with the ultimate goal of improving its adaptation and productivity under water stress scenarios.

Melatonin Positively Regulates Both Dark- and Age-Induced Leaf Senescence by Reducing ROS Accumulation and Modulating Abscisic Acid and Auxin Biosynthesis in Cucumber Plants

Melatonin (MT), as a signaling molecule, plays a vital role in regulating leaf senescence in plants. This study aimed to verify the antioxidant roles of MT in delaying dark- or age-induced leaf senescence of cucumber plants. The results showed that endogenous MT responds to darkness and overexpression of CsASMT, the key gene of MT synthesis, and delays leaf senescence stimulated by darkness, as manifested by significantly lower malonaldehyde (MDA) and reactive oxygen species (ROS) contents as well as higher activities and gene expression of antioxidant enzymes compared to the control. Moreover, MT suppressed both age- or dark-induced leaf senescence of cucumber, as evidenced by a decrease in senescence-related gene SAG20 and cell-death-related gene PDCD expression and ROS content and an increase in antioxidant capacity and chlorophyll biosynthesis compared with the H₂O-treated seedlings. Meanwhile, the suppression of age-induced leaf senescence by melatonin was also reflected by the reduction in abscisic acid (ABA) biosynthesis and signaling pathways as well as the promotion of auxin (IAA) biosynthesis and signaling pathways in cucumber plants in the solar greenhouse. Combining the results of the two separate experiments, we demonstrated that MT acts as a powerful antioxidant to alleviate leaf senescence by activating the antioxidant system and IAA synthesis and signaling while inhibiting ABA synthesis and signaling in cucumber plants.

Integrated analysis of the transcriptome, sRNAome, and degradome reveals the network regulating fruit skin coloration in sponge gourd (Luffa cylindrica)

Sponge gourd fruit skin color is an important quality-related trait because it substantially influences consumer preferences. However, little is known about the miRNAs and genes regulating sponge gourd fruit skin coloration. This study involved an integrated analysis of the transcriptome, sRNAome, and degradome of sponge gourd fruit skins with green skin (GS) and white skin (WS). A total of 4,331 genes were differentially expressed between the GS and WS, with 2,442 down-regulated and 1,889 up-regulated genes in WS. The crucial genes involved in chlorophyll metabolism, chloroplast development, and chloroplast protection were identified (e.g., HEMA, CHLM, CRD1, POR, CAO, CLH, SGR, CAB, BEL1-like, KNAT, ARF, and peroxidase genes). Additionally, 167 differentially expressed miRNAs were identified, with 70 up-regulated and 97 down-regulated miRNAs in WS. Degradome sequencing identified 125 differentially expressed miRNAs and their 521 differentially expressed target genes. The miR156, miR159, miR166, miR167, miR172, and miR393 targeted the genes involved in chlorophyll metabolism, chloroplast development, and chloroplast protection. Moreover, a flavonoid biosynthesis regulatory network was established involving miR159, miR166, miR169, miR319, miR390, miR396, and their targets CHS, 4CL, bHLH, and MYB. The qRT-PCR data for the differentially expressed genes were generally consistent with the transcriptome results. Subcellular localization analysis of selected proteins revealed their locations in different cellular compartments, including nucleus, cytoplasm and endoplasmic reticulum. The study findings revealed the important miRNAs, their target genes, and the regulatory network controlling fruit skin coloration in sponge gourd.

Chlorophyllase, a Common Plant Hydrolase Enzyme with a Long History, Is Still a Puzzle

Chlorophyllase (Chlase, CLH) is one of the earliest discovered enzymes present in plants and green algae. It was long considered to be the first enzyme involved in chlorophyll (Chl) degradation, while strong evidence showed that it is not involved in Chl breakdown during leaf senescence. On the other hand, it is possible that CLH is involved in Chl breakdown during fruit ripening. Recently, it was discovered that Arabidopsis CLH1 is located in developing chloroplasts but not in mature chloroplasts, and it plays a role in protecting young leaves from long-term photodamage by catalysing Chl turnover in the photosystem II (PSII) repair cycle. However, there remain other important questions related to CLH. In this article, we briefly reviewed the research progress on CLH and listed the main unanswered questions related to CLH for further study.

Composition and Antioxidant Properties of Pigments of Mediterranean Herbs and Spices as Affected by Different Extraction Methods

This study examined the composition and properties of chlorophyll and carotenoid extracted from the leaves of several Mediterranean evergreen shrubs and subshrubs (Myrtus communis L., Pistacia lentiscus L., Thymus vulgaris L., Salvia officinalis L. and Laurus nobilis L.) commonly used as herbs and spices. In order to fully assess their composition over a wide polarity range, pigments were extracted by successive solvent extraction with hexane, 80% acetone and 96% ethanol. Agitation-assisted extraction (AAE), ultrasound-assisted extraction (UAE) and pressurized liquid extraction (PLE) were employed and compared regarding their effect on the pigments’ yield and composition. Individual chlorophylls and carotenoids were analyzed by HPLC-DAD, while the content of total pigments and the extracts’ antioxidant capacity were determined spectrophotometrically. Throughout the experiments, pheophytin a, b and b’ were dominant chlorophyll molecules, while lutein and β-carotene were dominant carotenoids. Overall, the extracted pigments were determined as being in the range of 73.84–127.60 mg 100 g⁻¹ and were the lowest in T. vulgaris, with no significant differences between other species. M. communis and P. lentiscus had the highest antioxidant capacities, showing a moderate positive correlation with carotenoid and chlorophyll levels. Significant differences were found in the levels of individual pigments with most of them showing a medium level of polarity due to the dissolution in acetone as a medium polar solvent. AAE and PLE demonstrated similar efficacy in the extraction of both carotenoids and chlorophylls; however, preference can be given to PLE, being a novel method with numerous advantages, e.g., shorter extraction time and lower solvent consumption. The examined plant species certainly expressed great diversity and showed the potential for application in the production of various functional products.

Ethylene-activated MdPUB24 mediates ubiquitination of MdBEL7 to promote chlorophyll degradation in apple fruit

Chlorophyll (Chl) degradation is a natural phenomenon that occurs during ripening in many fleshy fruit species, and also during fruit storage. The plant hormone ethylene is a key factor in promoting Chl degradation during fruit storage, but the mechanisms involved in this induction are largely unknown. In this study, an apple (Malus domestica) BEL1-LIKE HOMEODOMAIN transcription factor 7 (MdBEL7), potentially functioning as a transcriptional repressor of the Chl catabolic genes (CCGs), including MdCLH, MdPPH2 and MdRCCR2, was identified as a partner of the ethylene-activated U-box type E3 ubiquitin ligase MdPUB24 in a yeast library screen. Yeast-two-hybrid, co-immunoprecipitation and luciferase complementation imaging assays were then used to verify the interaction between MdBEL7 and MdPUB24. In vitro and in vivo ubiquitination experiments revealed that MdPUB24 functions as an E3 ubiquitin ligase to ubiquitinate MdBEL7, thereby causing its degradation through the 26S proteasome pathway. Transient overexpression of MdPUB24 in apple fruit led to a decrease in MdBEL7 abundance and increased expression of CCG genes, including MdCLH, MdPPH2 and MdRCCR2, as well as greater Chl degradation. Taken together, the data indicated that an ethylene-activated U-box type E3 ubiquitin ligase MdPUB24 directly interacts with and ubiquitinates MdBEL7. Consequent degradation of MdBEL7 results in enhanced expression of MdCLH, MdPPH2 and MdRCCR2, and thus Chl degradation during apple fruit storage. Our results reveal that an ethylene-MdPUB24-MdBEL7 module regulates Chl degradation by post-translational modification during apple fruit storage.

High oxygen atmospheric packaging treatment regulates the postharvest changes of Chinese kale (Brassica oleracea var. alboglabra) during storage

Chinese kale is one of the most popular vegetables in southern China and Asia, but it has a short shelf-life. The effect of high oxygen atmospheric packaging (HOAP) treatment on the respiration rate as well as chlorophyll content and the expression of their metabolism-related genes of the soluble proteins in Chinese kale during storage were assessed. The results showed that Chinese kale subjected to HOAP treatment showed stimulated respiration rate and regulated expression of chlorophyll metabolism-related genes, such as BrChlases, BrPPH (pheophytin pheophorbide hydrolase), BrPAO (pheidea oxygenase gene), BrRCCR (red chlorophyll catabolite reductase), and BrSAG12 (senescence-associated gene), compared to the Chinese kale in the control. The activities of chlorophyll enzymes, that is, Chlase and Mg-dechelatase, were also influenced by HOAP treatment during storage. Furthermore, the total content of soluble proteins was stimulated to accumulate, and the intensity of protein bands, detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiling, increased in HOAP-treated samples. Based on the current results, as well as the results of our previous study regarding HOAP treatment of other vegetables, we speculate that HOAP may function by regulating the respiration rate and the accumulation of functional proteins, especially chlorophyll catabolic and antioxidant enzymes, to maintain the freshness of Chinese kale during storage. PRACTICAL APPLICATION: HOAP treatment could be a potential method for delaying quality changes and extending the shelf-life of Chinese kale after harvest.

Arabidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystem II repair

The proteolytic degradation of the photodamaged D1 core subunit during the photosystem II (PSII) repair cycle is well understood, but chlorophyll turnover during D1 degradation remains unclear. Here, we report that Arabidopsis thaliana CHLOROPHYLLASE 1 (CLH1) plays important roles in the PSII repair process. The abundance of CLH1 and CLH2 peaks in young leaves and is induced by high-light exposure. Seedlings of clh1 single and clh1-1/2-2 double mutants display increased photoinhibition after long-term high-light exposure, whereas seedlings overexpressing CLH1 have enhanced light tolerance compared with the wild type. CLH1 is localized in the developing chloroplasts of young leaves and associates with the PSII-dismantling complexes RCC1 and RC47, with a preference for the latter upon exposure to high light. Furthermore, degradation of damaged D1 protein is retarded in young clh1-1/2-2 leaves after 18-h high-light exposure but is rescued by the addition of recombinant CLH1 in vitro. Moreover, overexpression of CLH1 in a variegated mutant (var2-2) that lacks thylakoid protease FtsH2, with which CLH1 interacts, suppresses the variegation and restores D1 degradation. A var2-2 clh1-1/2-2 triple mutant shows more severe variegation and seedling death. Taken together, these results establish CLH1 as a long-sought chlorophyll dephytylation enzyme that is involved in PSII repair and functions in long-term adaptation of young leaves to high-light exposure by facilitating FtsH-mediated D1 degradation.

Transcriptome Analysis Identified Gene Regulation Networks in Soybean Leaves Perturbed by the Coronatine Toxin

The non-host specific Pseudomonas syringae phytotoxin Coronatine (COR) causes chlorosis and promotes toxicity by inducing physiological changes in plants. We performed transcriptome analysis to better understand plants' transcriptional and metabolic response to COR. Toward this end, mock-treated and COR-treated soybean plants were analyzed by RNA-Seq. A total of 4,545 genes were differentially expressed between the two treatments, of which 2,170 were up-regulated whereas 2,375 were down-regulated in COR treated samples. Gene annotation and pathway analysis conducted using the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) databases revealed that the differential genes were involved in photosynthesis, jasmonic acid (JA) synthesis, signal transduction, and phenylpropane metabolism. This study will provide new insights into COR mediated responses and extend our understanding of COR function in plants.

Characterization of a novel allele encoding pheophorbide a oxygenase in rice

We identified a rapid cell death 2 (rcd2) mutant from an indica cultivar Zhongjian100 mutant bank. The red-brown lesions appeared firstly on young seedling leaves, then gradually merged and the leaves completely withered at the late tillering stage. rcd2 displayed apparent cell death at/around the lesions, accumulation of superoxide anion (O₂^-) and disturbed ROS scavenging system, impaired photosynthetic capacity with significantly reduced chlorophyll content. The lesion formation was controlled by a single recessive nuclear gene and induced by natural light as well as mechanical wounding. A single base mutation (A1726T) at the 6th exon of OsMH_03G0040800 resulted in I576F substitution in the encoding protein, pheophorbide a oxygenase (PAO). Functional complementation could rescue the mutant phenotype and PAO-knockout lines exhibited the similar phenotype to rcd2. The activity of PAO decreased significantly while the content of PAO substrate, pheophorbide a, increased apparently in rcd2. The expression of chlorophyll synthesis/degradation-related genes and the contents of metabolic intermediates were largely changed. Furthermore, the level of chlorophyllide a, the product of chlorophyllase, increased significantly, indicating chlorophyllase might play a role in chlorophyll degradation in rice. Our results suggested that the I576F substitution disrupted PAO function, leading to O₂^- accumulation and chlorophyll degradation breakdown in rice.

Impact of electron beam irradiation on the chlorophyll degradation and antioxidant capacity of mango fruit

At the present, the mechanism of chlorophyll degradation in response to ionizing irradiation in harvested fruits have not been examined. To understand the effect of electron beam (E-beam) irradiation on the chlorophyll degrading pathway in relation to chlorophyll degrading enzymes activity, reactive oxygen species (ROS) and antioxidant capacities of harvested mangoes stored at 13 °C for 16 days were studied. E-beam-treated fruit significantly suppressed the activities of chlorophyll degrading enzymes especially pheophytinase (PPH) and chlorophyll degrading peroxidase (Chl-POX) in the late stage of storage. This resulted in the chlorophyll content being maintained. However, E-beam irradiation did not affect the activities of chlorophyllase (Chlase) and magnesium de-chelatase (MD). The respiration rate, ethylene production, ROS accumulation (hydrogen peroxide [H₂O₂] and superoxide radical [O^-. ₂]) immediately increased after E-beam treatment, following which they significantly decreased in comparison to the control. E-beam treatment enhanced the fruit's antioxidant capacity by activating the activities of catalase (CAT) and ascorbate peroxidase (APX) and glutathione (GSH) content, and inactivated the activity of superoxide dismutase (SOD). Further, it did not affect the activity of glutathione reductase (GR) and glutathione disulfide (GSSG), vitamin C content, or total phenolic content. These results imply that E-beam treatment has the potential to delay chlorophyll degradation by suppressing the Chl-POX and PPH activities as well as reduce ROS production via CAT, APX, and SOD activities and GSH content.

Molecular, structural and biochemical characterization of a novel recombinant chlorophyllase from cyanobacterium Oscillatoria acuminata PCC 6304

Background

Chlorophyllase catalyzes the hydrolysis of chlorophyll and produces chlorophyllide and phytol. Cyanobacterial chlorophyllases are likely to be more highly heterologously expressed than plant chlorophyllases. A novel recombinant chlorophyllase from the cyanobacterium Oscillatoria acuminata PCC 6304 was successfully expressed in Escherichia coli BL21(DE3).

Results

The putative N-terminal 28-amino-acid signal peptide sequence of O. acuminata chlorophyllase (OaCLH) is essential for its activity, but may confer poor solubility on OaCLH. The C-terminal fusion of a 6 × His tag caused a partial loss of activity in recombinant OaCLH, but an N-terminal 6 × His tag did not destroy its activity. The optimal pH and temperature for recombinant OaCLH activity are 7.0 and 40 °C, respectively. Recombinant OaCLH has hydrolysis activities against chlorophyll a, chlorophyll b, bacteriochlorophyll a, and pheophytin a, but prefers chlorophyll b and chlorophyll a as substrates. The results of site-directed mutagenesis experiments indicated that the catalytic triad of OaCLH consists of Ser159, Asp226, and His258.

Conclusions

The high-level expression and broad substrate specificity of recombinant OaCLH make it suitable for genetically engineering and a promising biocatalyst for industrial production, with applications in vegetable oil refining and laundry detergents.

Chlorophyll dephytylation in chlorophyll metabolism: a simple reaction catalyzed by various enzymes

Chlorophyll (Chl) is composed of a tetrapyrrole ring and a phytol tail, which facilitate light energy absorbance and assembly with photosynthetic protein complexes, respectively. Chl dephytylation, the hydrolytic removal of the phytol tail, is considered a pivotal step in diverse physiological processes, such as Chl salvage during repair of the photosystem, the Chl cycle in the adjustment of antenna size, and Chl breakdown in leaf senescence and fruit maturation. Moreover, phytol is a component of the tocopherols, a major form of vitamin E that is essential in the human diet. This phytol mostly comes from Chl hydrolysis. However, the authentic enzyme responsible for Chl dephytylation has proved elusive. CHLOROPHYLLASE (CLH) which was discovered over a century ago, was the first enzyme found to have dephytylation activity in vitro, but its role in Chl metabolism has been questioned and remains under debate. Recently, novel dephytylases, i.e., PHEOPHYTINASE (PPH) and CHLOROPHYLL DEPHYTYLASE1 (CLD1) have emerged from genetic studies, indicating that dephytylation in Chl catabolism involves different players and is more complicated than previously thought. Based on sequence homology, substrate specificity, and subcellular localization, CLH, PPH, and CLD1 belong to different types of dephytylase, which prompted us to re-examine the dilemmas and missing links that still exist in Chl metabolism. This review thus focuses on the hitherto unanswered questions involving the Chl dephytylation reaction by highlighting relevant literature, updating recent progress, and synthesizing ideas.

Screening of key genes responsible for Pennisetum setaceum 'Rubrum' leaf color using transcriptome sequencing

Pennisetum setaceum 'Rubrum' is an ornamental grass plant that produces purple leaves in high-light environments and light purple or green leaves in low-light environments, the latter of which greatly reduces its aesthetic appeal. Therefore, we aimed to identify the key genes associated with leaf coloration and elucidate the molecular mechanisms involved in the color changes in P. setaceum 'Rubrum' leaves. We performed transcriptome sequencing of P. setaceum 'Rubrum' leaves before and after shading. A total of 19,043 differentially expressed genes were identified, and the numbers of upregulated and downregulated genes at T1 stage, when compared with their expression at the T0 stage, were 10,761 and 8,642, respectively. The possible pathways that determine P. setaceum 'Rubrum' leaf color included flavonoid biosynthesis, flavone and flavonol biosynthesis, and carotenoid biosynthesis. There were 31 differentially expressed genes related to chlorophyll metabolism, of which 21 were related to chlorophyll biosynthesis and 10 to chlorophyll degradation, as well as three transcription factors that may be involved in the regulation of chlorophyll degradation. There were 31 key enzyme genes involved in anthocyanin synthesis and accumulation in P. setaceum 'Rubrum' leaves, with four transcription factors that may be involved in the regulation of anthocyanin metabolism. The transcriptome data were verified and confirmed reliable by real-time fluorescence quantitative PCR analysis. These findings provide a genetic basis for improving leaf color in P. setaceum 'Rubrum.'

iTRAQ-based comparative proteomic analysis of differences in the protein profiles of stems and leaves from two alfalfa genotypes

BACKGROUND

To explore the molecular regulatory mechanisms of early stem and leaf development, proteomic analysis was performed on leaves and stems of F genotype alfalfa, with thin stems and small leaves, and M genotype alfalfa, with thick stems and large leaves.

RESULTS

Based on fold-change thresholds of > 1.20 or < 0.83 (p < 0.05), a large number of proteins were identified as being differentially enriched between the M and F genotypes: 249 downregulated and 139 upregulated in stems and 164 downregulated and 134 upregulated in leaves. The differentially enriched proteins in stems were mainly involved in amino acid biosynthesis, phenylpropanoid biosynthesis, carbon fixation, and phenylalanine metabolism. The differentially enriched proteins in leaves were mainly involved in porphyrin and chlorophyll metabolism, phenylpropanoid biosynthesis, starch and sucrose metabolism, and carbon fixation in photosynthetic organisms. Six differentially enriched proteins were mapped onto the porphyrin and chlorophyll metabolism pathway in leaves of the M genotype, including five upregulated proteins involved in chlorophyll biosynthesis and one downregulated protein involved in chlorophyll degradation. Eleven differentially enriched proteins were mapped onto the phenylpropanoid pathway in stems of the M genotype, including two upregulated proteins and nine downregulated proteins.

CONCLUSION

Enhanced chlorophyll synthesis and decreased lignin synthesis provided a reasonable explanation for the larger leaves and lower levels of stem lignification in M genotype alfalfa. This proteomic study aimed to classify the functions of differentially enriched proteins and to provide information on the molecular regulatory networks involved in stem and leaf development.

Comparative Transcriptome Analysis Provides Insights Into Yellow Rind Formation and Preliminary Mapping of the Clyr (Yellow Rind) Gene in Watermelon

As an important appearance trait, the rind color of watermelon fruit affects the commodity value and further determines consumption choices. In this study, a comparative transcriptome analysis was conducted to elucidate the genes and pathways involved in the formation of yellow rind fruit in watermelon using a yellow rind inbred line WT4 and a green rind inbred line WM102. A total of 2,362 differentially expressed genes (DEGs) between WT4 and WM102 at three different stages (0, 7, and 14 DAP) were identified and 9,770 DEGs were obtained by comparing the expression level at 7 DAP and 14 DAP with the former stages of WT4. The function enrichment of DEGs revealed a number of pathways and terms in biological processes, cellular components, and molecular functions that were related to plant pigment metabolism, suggesting that there may be a group of common core genes regulating rind color formation. In addition, next-generation sequencing aided bulked-segregant analysis (BSA-seq) of the yellow rind pool and green rind pool selected from an F₂ population revealed that the yellow rind gene (Clyr) was mapped on the top end of chromosome 4. Based on the BSA-seq analysis result, Clyr was further confined to a region of 91.42 kb by linkage analysis using 1,106 F₂ plants. These results will aid in identifying the key genes and pathways associated with yellow rind formation and elucidating the molecular mechanism of rind color formation in watermelon.

Subcellular localization of chlorophyllase2 reveals it is not involved in chlorophyll degradation during senescence in Arabidopsis thaliana

Chlorophyllase (CLH), which catalyzes the release of the phytol chain from chlorophyll (Chl), has been long considered to catalyze the first step of Chl degradation. Arabidopsis contains two isoforms of CLH (CLH1 and CLH2), and CLH1 was previously demonstrated to be localized in tonoplast and endoplasmic reticulum, and not be involved in Chl degradation. In contrast, CLH2 possesses a predicted signal-peptide for chloroplast localization, and phylogenetic analysis of CLHs in Arabidopsis and other species also indicate that CLH2 forms a different clade than CLH1. Therefore, the possibility remains that CLH2 is involved in the breakdown of Chl. In the current study, clh mutants lacking CLH2 or both CLH isoforms were analyzed after the induction of senescence. Results indicated that the clh knockout lines were still able to degrade Chl at the same rate as wild-type plants. Transgenic Arabidopsis plants were generated that constitutively expressed either CLH2 or CLH2 fused to a yellow fluorescent protein (YFP). Observations made using confocal microscopy indicated that CLH2-YFP was located external to chloroplasts. Additionally, in overexpression plants, CLH2 was enriched in tonoplast and endoplasmic reticulum fractions following membrane fractionation. Based on the collective data, we conclude that CLH2 is not involved in Chl breakdown during senescence in Arabidopsis.

Potential roles of melatonin and sulfur in alleviation of lanthanum toxicity in tomato seedlings

Owing to the active use of rare-earth elements in many areas, it is necessary to study their behavior in the environment and their biological impact on plants. Despite the role of melatonin and sulfur in plant growth, development and abiotic stress tolerance; it is still not clear how they have a strong regulatory influence and synergistic effect on growth, physiological and biochemical characteristics of plants under different environmental stresses. Therefore, this study highlights how melatonin and sulfur together potentially involved in a reversal of lanthanum-inhibited photosynthetic and growth responses in tomato seedlings. Here, we reported that seedlings grown in a medium containing 150 μM lanthanum exhibited increased overproduction of reactive oxygen species (ROS) and lipid peroxidation together with increased Chlorophyll degradation, and activity of chlorophyllase, proline dehydrogenase and glycolate oxidase (GOx), and decreased photosynthesis and growth. However, the application of melatonin and sulfur showed significant responses on tomato seedlings, although the response of their combined treatment was more effective by further increasing photosynthesis and growth under lanthanum toxicity. Melatonin supplied with sulfur suppressed ROS formation, lipid peroxidation and activity of GOx, and increased photosynthesis by upregulating activities of carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase. Also, sulfur supplementation with melatonin to seedlings resulted in an elevation in the accumulation of Chl and proline by increasing δ-aminolevulinic acid and activity of δ-aminolevulinic acid dehydratase and Δ¹-pyrroline-5-carboxylate synthetase activity. The administration of melatonin with sulfur substantially induced upregulation of enzymes (superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) activities involved in the antioxidant system, thereby mitigating ROS-induced oxidative damage. Thus, this study provides strong evidence that melatonin and sulfur have strong regulatory influence and synergistic role in alleviating the adverse effect of lanthanum-toxicity by increasing photosynthesis and growth.

Transcriptome analysis based on a combination of sequencing platforms provides insights into leaf pigmentation in Acer rubrum

BACKGROUND

Red maple (Acer rubrum L.) is one of the most common and widespread trees with colorful leaves. We found a mutant with red, yellow, and green leaf phenotypes in different branches, which provided ideal materials with the same genetic relationship, and little interference from the environment, for the study of complex metabolic networks that underly variations in the coloration of leaves. We applied a combination of NGS and SMRT sequencing to various red maple tissues.

RESULTS

A total of 125,448 unigenes were obtained, of which 46 and 69 were thought to be related to the synthesis of anthocyanins and carotenoids, respectively. In addition, 88 unigenes were presumed to be involved in the chlorophyll metabolic pathway. Based on a comprehensive analysis of the pigment gene expression network, the mechanisms of leaf color were investigated. The massive accumulation of Cy led to its higher content and proportion than other pigments, which caused the redness of leaves. Yellow coloration was the result of the complete decomposition of chlorophyll pigments, the unmasking of carotenoid pigments, and a slight accumulation of Cy.

CONCLUSIONS

This study provides a systematic analysis of color variations in the red maple. Moreover, mass sequence data obtained by deep sequencing will provide references for the controlled breeding of red maple.

Stress-induced changes in the ultrastructure of the photosynthetic apparatus of green microalgae

In photosynthetic organisms including unicellular algae, acclimation to and damage by environmental stresses are readily apparent at the level of the photosynthetic apparatus. Phenotypic manifestations of the stress responses include rapid and dramatic reduction of photosynthetic activity and pigment content aimed at mitigating the risk of photooxidative damage. Although the physiological and molecular mechanisms of these events are well known, the ultrastructural picture of the stress responses is often elusive and frequently controversial. We analyzed an extensive set of transmission electron microscopy images of the microalgal cells obtained across species of Chlorophyta and in a wide range of growth conditions. The results of the analysis allowed us to pinpoint distinct ultrastructural changes typical of normal functioning and emergency reduction of the chloroplast membrane system under high light exposure and/or mineral nutrient starvation. We demonstrate the patterns of the stress-related ultrastructural changes including peculiar thylakoid rearrangements and autophagy-like processes and provide an outlook on their significance for implementation of the stress responses.

Sequencing of Euscaphis konishii Endocarp Transcriptome Points to Molecular Mechanisms of Endocarp Coloration

Flower and fruit colors are of vital importance to the ecology and economic market value of plants. The mechanisms of flower and fruit coloration have been well studied, especially among ornamental flower plants and cultivated fruits. As people pay more attention to exocarp coloration, the endocarp coloration in some species has often been ignored. Here, we report on the molecular mechanism of endocarp coloration in three development stages of Euscaphis konishii. The results show that endocarp reddening is closely related to anthocyanin accumulation, and a total of 86,120 unigenes were assembled, with a mean length of 893 bp (N50 length of 1642 bp). We identified a large number of differentially expressed genes associated with endocarp coloration, including anthocyanin biosynthesis, carotenoid biosynthesis, and chlorophyll breakdown. The genes participating in each step of the anthocyanin biosynthesis were found in the transcriptome dataset, but a few genes were found in the carotenoid biosynthesis and chlorophyll breakdown. In addition, the candidate R2R3-MYB transcription factors and candidate glutathione S-transferase transport genes, which likely regulate the anthocyanin biosynthesis, were identified. This study offers a platform for E. konishii functional genomic research and provides a reference for revealing the regulatory mechanisms of endocarp reddening.

Candidate Genes for Yellow Leaf Color in Common Wheat (Triticum aestivum L.) and Major Related Metabolic Pathways according to Transcriptome Profiling

The photosynthetic capacity and efficiency of a crop depends on the biosynthesis of photosynthetic pigments and chloroplast development. However, little is known about the molecular mechanisms of chloroplast development and chlorophyll (Chl) biosynthesis in common wheat because of its huge and complex genome. Ygm, a spontaneous yellow-green leaf color mutant of winter wheat, exhibits reduced Chl contents and abnormal chloroplast development. Thus, we searched for candidate genes associated with this phenotype. Comparative transcriptome profiling was performed using leaves from the yellow leaf color type (Y) and normal green color type (G) of the Ygm mutant progeny. We identified 1227 differentially expressed genes (DEGs) in Y compared with G (i.e., 689 upregulated genes and 538 downregulated genes). Gene ontology and pathway enrichment analyses indicated that the DEGs were involved in Chl biosynthesis (i.e., magnesium chelatase subunit H (CHLH) and protochlorophyllide oxidoreductase (POR) genes), carotenoid biosynthesis (i.e., β-carotene hydroxylase (BCH) genes), photosynthesis, and carbon fixation in photosynthetic organisms. We also identified heat shock protein (HSP) genes (sHSP, HSP70, HSP90, and DnaJ) and heat shock transcription factor genes that might have vital roles in chloroplast development. Quantitative RT-PCR analysis of the relevant DEGs confirmed the RNA-Seq results. Moreover, measurements of seven intermediate products involved in Chl biosynthesis and five carotenoid compounds involved in carotenoid-xanthophyll biosynthesis confirmed that CHLH and BCH are vital enzymes for the unusual leaf color phenotype in Y type. These results provide insights into leaf color variation in wheat at the transcriptional level.

Photosensitizers in prostate cancer therapy

The search for new therapeutics for the treatment of prostate cancer is ongoing with a focus on the balance between the harms and benefits of treatment. New therapies are being constantly developed to offer treatments similar to radical therapies, with limited side effects. Photodynamic therapy (PDT) is a promising strategy in delivering focal treatment in primary as well as post radiotherapy prostate cancer. PDT involves activation of a photosensitizer (PS) by appropriate wavelength of light, generating transient levels of reactive oxygen species (ROS). Several photosensitizers have been developed with a focus on treating prostate cancer like mTHPC, motexafin lutetium, padoporfin and so on. This article will review newly developed photosensitizers under clinical trials for the treatment of prostate cancer, along with the potential advantages and disadvantages in delivering focal therapy.

Jasmonic acid-induced tolerance to root-knot nematodes in tomato plants through altered photosynthetic and antioxidative defense mechanisms

Plant parasitic nematodes cause severe damage to cultivated crops globally. Management of nematode population is a major concern as chemicals used as nematicides have negative impact on the environment. Natural plant products can be safely used for the control of nematodes. Among various plant metabolites, plant hormones play an essential role in developmental and physiological processes and also assist the plants to encounter stressful conditions. Keeping this in mind, the present study was designed to evaluate the effect of jasmonic acid (JA) on the growth, pigments, polyphenols, antioxidants, osmolytes, and organic acids under nematode infection in tomato seedlings. It was observed that nematode inoculation reduced the growth of seedlings. Treatment with JA improved root growth (32.79%), total chlorophylls (71.51%), xanthophylls (94.63%), anthocyanins (37.5%), and flavonoids content (21.11%) when compared to inoculated seedlings alone. The JA application enhanced the total antioxidant capacity (lipid- and water-soluble antioxidants) by 38.23 and 34.37%, respectively, in comparison to infected seedlings. Confocal studies revealed that there was higher accumulation of glutathione in hormone-treated seedlings under nematode infection. Treatment with JA increased total polyphenols content (74.56%) in comparison to nematode-infested seedlings. JA-treated seedlings also enhanced osmolyte and organic acid contents under nematode stress. Overall, treatment with JA improved growth, enhanced pigment levels, modulated antioxidant content, and enhanced osmolyte and organic acid content in nematode-infected seedlings.

Catalytic and structural properties of pheophytinase, the phytol esterase involved in chlorophyll breakdown

During leaf senescence and fruit ripening, chlorophyll is degraded in a multistep pathway into linear tetrapyrroles called phyllobilins. A key feature of chlorophyll breakdown is the removal of the hydrophobic phytol chain that renders phyllobilins water soluble, an important prerequisite for their ultimate storage in the vacuole of senescent cells. Chlorophyllases had been considered for more than a century to catalyze dephytylation in vivo; however, this was recently refuted. Instead, pheophytinase was discovered as a genuine in vivo phytol hydrolase. While chlorophyllase acts rather unspecifically towards different porphyrin substrates, pheophytinase was shown to specifically dephytylate pheophytin, namely Mg-free chlorophyll. The aim of this work was to elucidate in detail the biochemical and structural properties of pheophytinase. By testing different porphyrin substrates with recombinant pheophytinase from Arabidopsis thaliana we show that pheophytinase has high specificity for the acid moiety of the ester bond, namely the porphyrin ring, while the nature of the alcohol, namely the phytol chain in pheophytin, is irrelevant. In silico modelling of the 3-dimensional structure of pheophytinase and subsequent analysis of site-directed pheophytinase mutant forms allowed the identification of the serine, histidine, and aspartic acid residues that compose the catalytic triad, a classical feature of serine-type hydrolases to which both pheophytinase and chlorophyllase belong. Based on substantial structural differences in the models of Arabidopsis pheophytinase and chlorophyllase 1, we discuss potential differences in the catalytic properties of these two phytol hydrolases.

The biochemistry and molecular biology of chlorophyll breakdown

Chlorophyll breakdown is one of the most obvious signs of leaf senescence and fruit ripening. The resulting yellowing of leaves can be observed every autumn, and the color change of fruits indicates their ripening state. During these processes, chlorophyll is broken down in a multistep pathway, now termed the 'PAO/phyllobilin' pathway, acknowledging the core enzymatic breakdown step catalysed by pheophorbide a oxygenase, which determines the basic linear tetrapyrrole structure of the products of breakdown that are now called 'phyllobilins'. This review provides an update on the PAO/phyllobilin pathway, and focuses on recent biochemical and molecular progress in understanding phyllobilin-modifying reactions as the basis for phyllobilin diversity, on the evolutionary diversity of the pathway, and on the transcriptional regulation of the pathway genes.

The biocontrol agent Pseudomonas chlororaphis PA23 primes Brassica napus defenses through distinct gene networks

Background

The biological control agent Pseudomonas chlororaphis PA23 is capable of protecting Brassica napus (canola) from the necrotrophic fungus Sclerotinia sclerotiorum via direct antagonism. While we have elucidated bacterial genes and gene products responsible biocontrol, little is known about how the host plant responds to bacterial priming on the leaf surface, including global changes in gene activity in the presence and absence of S. sclerotiorum.

Results

Application of PA23 to the aerial surfaces of canola plants reduced the number of S. sclerotiorum lesion-forming petals by 91.1%. RNA sequencing of the host pathogen interface showed that pretreatment with PA23 reduced the number of genes upregulated in response to S. sclerotiorum by 16-fold. By itself, PA23 activated unique defense networks indicative of defense priming. Genes encoding MAMP-triggered immunity receptors detecting flagellin and peptidoglycan were downregulated in PA23 only-treated plants, consistent with post-stimulus desensitization. Downstream, we observed reactive oxygen species (ROS) production involving low levels of H₂O₂ and overexpression of genes associated with glycerol-3-phosphate (G3P)-mediated systemic acquired resistance (SAR). Leaf chloroplasts exhibited increased thylakoid membrane structures and chlorophyll content, while lipid metabolic processes were upregulated.

Conclusion

In addition to directly antagonizing S. sclerotiorum, PA23 primes the plant defense response through induction of unique local and systemic defense networks. This study provides novel insight into the effects of biocontrol agents applied to the plant phyllosphere. Understanding these interactions will aid in the development of biocontrol systems as an alternative to chemical pesticides for protection of important crop systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3848-6) contains supplementary material, which is available to authorized users.

Overexpression of MpCYS4, a phytocystatin gene from Malus prunifolia (Willd.) Borkh., delays natural and stress-induced leaf senescence in apple

Phytocystatins are a well-characterized class of naturally occurring protease inhibitors that prevent the catalysis of papain-like cysteine proteases. The action of cystatins in stress tolerance has been studied intensively, but relatively little is known about their functions in plants during leaf senescence. Here, we examined the potential roles of the apple cystatin, MpCYS4, in leaf photosynthesis as well as the concentrations and composition of leaf proteins when plants encounter natural or stress-induced senescence. Overexpression of this gene in apple rootstock M26 effectively slowed the senescence-related declines in photosynthetic activity and chlorophyll concentrations and prevented the action of cysteine proteinases during the process of degrading proteins (e.g., Rubisco) in senescing leaves. Moreover, MpCYS4 alleviated the associated oxidative damage and enhanced the capacity of plants to eliminate reactive oxygen species by activating antioxidant enzymes such as ascorbate peroxidase, peroxidase, and catalase. Consequently, plant cells were protected against damage from free radicals during leaf senescence. Based on these results, we conclude that MpCYS4 functions in delaying natural and stress-induced senescence of apple leaves.

Melatonin Improves the Photosynthetic Apparatus in Pea Leaves Stressed by Paraquat via Chlorophyll Breakdown Regulation and Its Accelerated de novo Synthesis

The positive effect of melatonin on the function of the photosynthetic apparatus is known, but little is known about the specific mechanisms of melatonin's action in plants. The influence of melatonin on chlorophyll metabolism of 24-day-old Pisum sativum L. seedlings during paraquat (PQ)-induced oxidative stress was investigated in this study. Seeds were hydro-primed with water (H), 50 and 200 μM melatonin/water solutions (H-MEL50, H-MEL200), while non-primed seeds were used as controls (C). Increases in chlorophyllase activity (key enzyme in chlorophyll degradation) and 5-aminolevulinic acid contents (the first compound in the porphyrin synthesis pathway) were observed in H-MEL50 and H-MEL200 leaf disks. This suggests that melatonin may accelerate damaged chlorophyll breakdown and its de novo synthesis during the first hours of PQ treatment. Elevated level of pheophytin in control leaf disks following 24 h of PQ incubation probably was associated with an enhanced rate of chlorophyll degradation through formation of pheophytin as a chlorophyll derivative. This validates the hypothesis that chlorophyllide, considered for many years, as a first intermediate of chlorophyll breakdown is not. This is indicated by the almost unchanged chlorophyll to chlorophyllide ratio after 24 h of PQ treatment. However, prolonged effects of PQ-induced stress (48 h) revealed extensive discolouration of control and water-treated leaf disks, while melatonin treatment alleviated PQ-induced photobleaching. Also the ratio of chlorophyll to chlorophyllide and porphyrin contents were significantly higher in plants treated with melatonin, which may indicate that this indoleamine both retards chlorophyll breakdown and stimulates its de novo synthesis during extended stress. We concluded that melatonin added into the seeds enhances the ability of pea seedlings to accelerate chlorophyll breakdown and its de novo synthesis before stress appeared and for several hours after, while during prolonged PQ incubation melatonin delays chlorophyll degradation.

Gene expression and metabolism preceding soft scald, a chilling injury of 'Honeycrisp' apple fruit

BACKGROUND

'Honeycrisp' is an apple cultivar that is susceptible to soft scald, a chilling injury expressed as necrotic patches on the peel. Improved understanding of metabolism associated with the disorder would improve our understanding of soft scald and contribute to developing more effective management strategies for apple storage. It was expected that specific gene expression and specific metabolite levels in the peel would be linked with soft scald risk at harvest and/or specific time points during cold storage.

RESULTS

Fruit from nine 'Honeycrisp' apple orchards that would eventually develop different incidences of soft scald between 4 and 8 weeks of cold air storage were used to contrast and determine differential transcriptomic and metabolomic changes during storage. Untargeted metabolic profiling revealed changes in a number of distinct pathways preceding and concurrent with soft scald symptom development, including elevated γ-aminobutryic acid (GABA), 1-hexanol, acylated steryl glycosides, and free p-coumaryl acyl esters. At harvest, levels of sesquiterpenoid and triterpenoid acyl esters were relatively higher in peel of fruit that did not later develop the disorder. RNA-seq driven gene expression profiling highlighted possible involvement of genes and associated metabolic processes with soft scald development. These included elevated expression of genes involved in lipid peroxidation and phenolic metabolism in fruit with soft scald, and isoprenoid/brassinosteroid metabolism in fruit that did not develop soft scald. Expression of other stress-related genes in fruit that developed soft scald included chlorophyll catabolism, cell wall loosening, and lipid transport while superoxide dismutases were up-regulated in fruit that did not develop the disorder.

CONCLUSIONS

This study delineates the sequential transcriptomic and metabolomic changes preceding soft scald symptom development. Changes were differential depending on susceptibility of fruit to the disorder and could be attributed to key stress related and mediating pathways.

Immobilization of Chlamydomonas reinhardtii CLH1 on APTES-Coated Magnetic Iron Oxide Nanoparticles and Its Potential in the Production of Chlorophyll Derivatives

Recombinant Chlamydomonas reinhardtii chlorophyllase 1 (CrCLH1) that could catalyze chlorophyll hydrolysis to chlorophyllide and phytol in vitro was successfully expressed in Escherichia coli. The recombinant CrCLH1 was immobilized through covalent binding with a cubic (3-aminopropyl) triethoxysilane (APTES) coating on magnetic iron oxide nanoparticles (MIONPs), which led to markedly improved enzyme performance and decreased biocatalyst costs for potential industrial application. The immobilized enzyme exhibited a high immobilization yield (98.99 ± 0.91 mg/g of gel) and a chlorophyllase assay confirmed that the immobilized recombinant CrCLH1 retained enzymatic activity (722.3 ± 50.3 U/g of gel). Biochemical analysis of the immobilized enzyme, compared with the free enzyme, showed higher optimal pH and pH stability for chlorophyll-a hydrolysis in an acidic environment (pH 3-5). In addition, compared with the free enzyme, the immobilized enzyme showed higher activity in chlorophyll-a hydrolysis in a high temperature environment (50-60 °C). Moreover, the immobilized enzyme retained a residual activity of more than 64% of its initial enzyme activity after 14 cycles in a repeated-batch operation. Therefore, APTES-coated MIONP-immobilized recombinant CrCLH1 can be repeatedly used to lower costs and is potentially useful for the industrial production of chlorophyll derivatives.

Gene expression profiling of the green seed problem in Soybean

BACKGROUND

Due to the climate change of the past few decades, some agricultural areas in the world are now experiencing new climatic extremes. For soybean, high temperatures and drought stress can potentially lead to the "green seed problem", which is characterized by chlorophyll retention in mature seeds and is associated with lower oil and seed quality, thus negatively impacting the production of soybean seeds.

RESULTS

Here we show that heat and drought stress result in a "mild" stay-green phenotype and impaired expression of the STAY-GREEN 1 and STAY-GREEN 2 (D1, D2), PHEOPHORBIDASE 2 (PPH2) and NON-YELLOW COLORING 1 (NYC1_1) genes in soybean seeds of a susceptible soybean cultivar. We suggest that the higher expression of these genes in fully mature seeds of a tolerant cultivar allows these seeds to cope with stressful conditions and complete chlorophyll degradation.

CONCLUSIONS

The gene expression results obtained in this study represent a significant advance in understanding chlorophyll retention in mature soybean seeds produced under stressful conditions. This will open new research possibilities towards finding molecular markers for breeding programs to produce cultivars which are less susceptible to chlorophyll retention under the hot and dry climate conditions which are increasingly common in the largest soybean production areas of the world.

A Novel Recombinant Chlorophyllase1 from Chlamydomonas reinhardtii for the Production of Chlorophyllide Derivatives

Natural chlorophyll metabolites have exhibited physiological activity in vitro. In this study, a recombinant chlorophyllase1 gene from Chlamydomonas reinhardtii (CrCLH1) was isolated and characterized. Recombinant CrCLH1 can perform chlorophyll dephytylation and produce chlorophyllide and phytol. In a transient assay, the subcellular localization of CrCLH1-green fluorescent protein was determined to be outside the chloroplast. Biochemical analyses of the activity of recombinant CrCLH1 indicated that its optimal pH value and temperature are 6.0 and 40 °C, respectively. Enzyme kinetic data revealed that the recombinant CrCLH1 had a higher catalytic efficiency for chlorophyll a than for chlorophyll b and bacteriochlorophyll a. According to high-performance liquid chromatography analysis of chlorophyll hydrolysis, recombinant CrCLH1 catalyzed the conversion of chlorophyll a to pheophorbide a at pH 5. Therefore, recombinant CrCLH1 can be used as a biocatalyst to produce chlorophyllide derivatives.

A wheat ABC transporter contributes to both grain formation and mycotoxin tolerance

The mycotoxin deoxynivalenol (DON) acts as a disease virulence factor for Fusarium fungi, and tolerance of DON enhances wheat resistance to Fusarium head blight (FHB) disease. Two variants of an ATP-binding cassette (ABC) family C transporter gene were cloned from DON-treated wheat mRNA, namely TaABCC3.1 and TaABCC3.2. These represent two of three putative genes identified on chromosomes 3A, 3B, and 3D of the wheat genome sequence. Variant TaABCC3.1 represents the DON-responsive transcript previously associated with DON resistance in wheat. PCR-based mapping and in silico sequence analyses located TaABCC3.1 to the short arm of wheat chromosome 3B (not within the FHB resistance quantitative trait locus Fhb1). In silico analyses of microarray data indicated that TaABCC3 genes are expressed in reproductive tissue and roots, and in response to the DON producer Fusarium graminearum. Gene expression studies showed that TaABCC3.1 is activated as part of the early host response to DON and in response to the FHB defence hormone jasmonic acid. Virus-induced gene silencing (VIGS) confirmed that TaABCC3 genes contributed to DON tolerance. VIGS was performed using two independent viral construct applications: one specifically targeted TaABCC3.1 for silencing, while the other targeted this gene and the chromosome 3A homeologue. In both instances, VIGS resulted in more toxin-induced discoloration of spikelets, compared with the DON effects in non-silenced spikelets at 14 d after toxin treatment (≥2.2-fold increase, P<0.05). Silencing by both VIGS constructs enhanced head ripening, and especially so in DON-treated heads. VIGS of TaABCC3 genes also reduced the grain number by more than 28% (P<0.05), both with and without DON treatment, and the effects were greater for the construct that targeted the two homeologues. Hence, DON-responsive TaABCC3 genes warrant further study to determine their potential as disease resistance breeding targets and their function in grain formation and ripening.

Chlorophyll degradation in the gut of generalist and specialist Lepidopteran caterpillars

Plant feeding herbivores excrete most of the ingested chlorophyll (Chl) as partly degraded derivatives lacking the phytol side chain and the central magnesium ion. An ecological role of digested and degraded Chls in the interactions between insects, their food plant and other insects has been described recently. To gain more information on common degradation patterns in plant-feeding insects, the orals secretions and frass of five Lepidopteran caterpillars covering generalists and specialists, namely Spodoptera littoralis, Spodoptera eridania, Heliothis virescens, Helicoverpa armigera, Manduca sexta, and, for comparison, of the leaf beetle larva Chrysomela lapponica were analyzed for chlorophyll catabolites. The major degradation products were determined as pheohorbide a/b and pyropheophorbide a/b by using LC-MS, LC-NMR, UV, and fluorescence spectrometry. The compounds were not present in fresh leaves of the food plants (Phaseolus lunatus, Nicotiana tabacum). The catabolite spectrum in generalists and specialists was qualitatively similar and could be attributed to the action of gut proteins and the strongly alkaline milieu in the digestive tract. Due to the anaerobic environment of the larval gut, the tetrapyrrole core of the Chl catabolites was not cleaved. Substantial amounts of Chl a/b metabolites were strongly complexed by a protein in the mid-gut.

Analysis of an Arabidopsis heat-sensitive mutant reveals that chlorophyll synthase is involved in reutilization of chlorophyllide during chlorophyll turnover

Chlorophylls, the most abundant pigments in the photosynthetic apparatus, are constantly turned over as a result of the degradation and replacement of the damage-prone reaction center D1 protein of photosystem II. Results from isotope labeling experiments suggest that chlorophylls are recycled by reutilization of chlorophyllide and phytol, but the underlying mechanism is unclear. In this study, by characterization of a heat-sensitive Arabidopsis mutant we provide evidence of a salvage pathway for chlorophyllide a. A missense mutation in CHLOROPHYLL SYNTHASE (CHLG) was identified and confirmed to be responsible for a light-dependent, heat-induced cotyledon bleaching phenotype. Following heat treatment, mutant (chlg-1) but not wild-type seedlings accumulated a substantial level of chlorophyllide a, which resulted in a surge of phototoxic singlet oxygen. Immunoblot analysis suggested that the mutation destabilized the chlorophyll synthase proteins and caused a conditional blockage of esterification of chlorophyllide a after heat stress. Accumulation of chlorophyllide a after heat treatment occurred during recovery in the dark in the light-grown but not the etiolated seedlings, suggesting that the accumulated chlorophyllides were not derived from de novo biosynthesis but from de-esterification of the existing chlorophylls. Further analysis of the triple mutant harboring the CHLG mutant allele and null mutations of CHLOROPHYLLASE1 (CLH1) and CLH2 indicated that the known chlorophyllases are not responsible for the accumulation of chlorophyllide a in chlg-1. Taken together, our results show that chlorophyll synthase acts in a salvage pathway for chlorophyll biosynthesis by re-esterifying the chlorophyllide a produced during chlorophyll turnover.

Chlorophyll degradation: the tocopherol biosynthesis-related phytol hydrolase in Arabidopsis seeds is still missing

Phytyl diphosphate (PDP) is the prenyl precursor for tocopherol biosynthesis. Based on recent genetic evidence, PDP is supplied to the tocopherol biosynthetic pathway primarily by chlorophyll degradation and sequential phytol phosphorylation. Three enzymes of Arabidopsis (Arabidopsis thaliana) are known to be capable of removing the phytol chain from chlorophyll in vitro: chlorophyllase1 (CLH1), CLH2, and pheophytin pheophorbide hydrolase (PPH), which specifically hydrolyzes pheophytin. While PPH, but not chlorophyllases, is required for in vivo chlorophyll breakdown during Arabidopsis leaf senescence, little is known about the involvement of these phytol-releasing enzymes in tocopherol biosynthesis. To explore the origin of PDP for tocopherol synthesis, seed tocopherol concentrations were determined in Arabidopsis lines engineered for seed-specific overexpression of PPH and in single and multiple mutants in the three genes encoding known dephytylating enzymes. Except for modestly increasing tocopherol content observed in the PPH overexpressor, none of the remaining lines exhibited significantly reduced tocopherol concentrations, suggesting that the known chlorophyll-derived phytol-releasing enzymes do not play major roles in tocopherol biosynthesis. Tocopherol content of seeds from double mutants in NONYELLOWING1 (NYE1) and NYE2, regulators of chlorophyll degradation, had modest reduction compared with wild-type seeds, although mature seeds of the double mutant retained significantly higher chlorophyll levels. These findings suggest that NYEs may play limited roles in regulating an unknown tocopherol biosynthesis-related phytol hydrolase. Meanwhile, seeds of wild-type over-expressing NYE1 had lower tocopherol levels, suggesting that phytol derived from NYE1-dependent chlorophyll degradation probably doesn't enter tocopherol biosynthesis. Potential routes of chlorophyll degradation are discussed in relation to tocopherol biosynthesis.