Chlorophyll Catabolism Research Articles

Transcriptional regulation of chlorophyll-catabolic genes associated with exogenous chemical effects and genotypic variations in heat-induced leaf senescence for perennial ryegrass

Heat-induced leaf senescence is hallmarked with accelerated chlorophyll (Chl) catabolism. The involvement of Chl catabolic genes (CCGs) in heat-induced leaf senescence is not well characterized. The objectives of this study were to investigate transcriptional changes in CCGs associated with exogenous chemical effects and genotypic variations during heat-induced leaf senescence in perennial ryegrass (Lolium perenne L.), and to characterize its underlying mechanisms. Perennial ryegrass plants of ‘Buena Vista’ and ‘Pinnacle’ differing in heat tolerance were exposed to heat stress at 35/30 °C (day/night) and control condition at 25/20 °C (day/night) in growth chambers and foliar sprayed with the following chemicals: salicylic acid (SA), 6-benzylaminopurine (6-BA; a cytokinin), calcium dichloride (CaCl2), aminoethoxyvinylglycine (AVG; an ethylene biosynthesis inhibitor), sodium nitroprusside (SNP, a nitric oxide donor), and urea (N, nitrogen source). Higher Chl content, photochemical efficiency and membrane stability were found in ‘Buena Vista’ than ‘Pinnacle’ whereas lower expression levels of three CCGs (LpNYC1, LpNOL, and LpPPH) were detected in the former genotype under heat stress. Exogenous application of SA, 6-BA, CaCl2, AVG, SNP, and N suppressed heat-induced Chl decline and expression of CCGs in heat sensitive genotype ‘Pinnacle’ under heat stress. Heat-induced oxidative damage was alleviated by these compounds, as indicated by significantly lower reactive oxygen species (ROS) content and higher activities of ROS scavenging enzymes. The expression levels of CCGs were negatively correlated to ROS level, including O2− and H2O2, while positively correlated to activities of ROS scavenging enzymes, such as CAT and SOD. The current results suggest that the transcript levels of LpNYC1, LpNOL, and LpPPH were negatively correlated with heat-induced leaf senescence, and down-regulation of CCGs genes could suppress heat-induced leaf senescence and oxidative damages, conferring heat tolerance in perennial ryegrass.

Expression of IbVPE1 from sweet potato in Arabidopsis affects leaf development, flowering time and chlorophyll catabolism

BackgroundSince their discovery, vacuolar processing enzymes (VPEs) have consistently been investigated as programmed cell death (PCD) initiators and participants in plant development and responses to biotic or abiotic stresses, in part due to similarities with the apoptosis regulator caspase-1. However, recent studies show additional functions of VPE in tomatoes, specifically in sucrose accumulation and fruit ripening.ResultsHerein, we evaluated the functions of VPE from sweetpotato, initially in expression pattern analyses of IbVPE1 during development and senescence. Subsequently, we identified physiological functions by overexpressing IbVPE1 in Arabidopsis thaliana, and showed reduced leaf sizes and numbers and early flowering, and elucidated the underlying molecular mechanisms.ConclusionsThe present data demonstrate functions of the VPE gene family in development and senescence and in regulation of flowering times, leaf sizes and numbers, and senescence phenotypes in Arabidopsis thaliana.

Catabolism and bioactive properties of chlorophylls

Chlorophylls are fundamental molecules for life and probably the most important of all natural pigments. Although chlorophyll breakdown is inevitably associated with the loss of green color during ripening or senescence, the truth is that chlorophyll molecules continuously degrade through so-called chlorophyll turnover (lifetime of hours). In addition to their primary function, chlorophylls are in demand due to their bioactive properties. Recently, several chlorophyll derivatives have shown health promoting activities. This review also provides an updated outline of the most recent advances that have occurred in the deciphering mechanism of chlorophyll catabolism; it will also describe the novel biological properties associated with chlorophyll derivatives.

Chlorophyll catabolism precedes changes in chloroplast structure and proteome during leaf senescence.

The earliest visual changes of leaf senescence occur in the chloroplast as chlorophyll is degraded and photosynthesis declines. Yet, a comprehensive understanding of the sequence of catabolic events occurring in chloroplasts during natural leaf senescence is still missing. Here, we combined confocal and electron microscopy together with proteomics and biochemistry to follow structural and molecular changes during Arabidopsis leaf senescence. We observed that initiation of chlorophyll catabolism precedes other breakdown processes. Chloroplast size, stacking of thylakoids, and efficiency of PSII remain stable until late stages of senescence, whereas the number and size of plastoglobules increase. Unlike catabolic enzymes, whose level increase, the level of most proteins decreases during senescence, and chloroplast proteins are overrepresented among these. However, the rate of their disappearance is variable, mostly uncoordinated and independent of their inherent stability during earlier developmental stages. Unexpectedly, degradation of chlorophyll‐binding proteins lags behind chlorophyll catabolism. Autophagy and vacuole proteins are retained at relatively high levels, highlighting the role of extra‐plastidic degradation processes especially in late stages of senescence. The observation that chlorophyll catabolism precedes all other catabolic events may suggest that this process enables or signals further catabolic processes in chloroplasts.

Taming chlorophylls by early eukaryotes underpinned algal interactions and the diversification of the eukaryotes on the oxygenated Earth

Extant eukaryote ecology is primarily sustained by oxygenic photosynthesis, in which chlorophylls play essential roles. The exceptional photosensitivity of chlorophylls allows them to harvest solar energy for photosynthesis, but on the other hand, they also generate cytotoxic reactive oxygen species. A risk of such phototoxicity of the chlorophyll must become particularly prominent upon dynamic cellular interactions that potentially disrupt the mechanisms that are designed to quench photoexcited chlorophylls in the phototrophic cells. Extensive examination of a wide variety of phagotrophic, parasitic, and phototrophic microeukaryotes demonstrates that a catabolic process that converts chlorophylls into nonphotosensitive 132,173-cyclopheophorbide enols (CPEs) is phylogenetically ubiquitous among extant eukaryotes. The accumulation of CPEs is identified in phagotrophic algivores belonging to virtually all major eukaryotic assemblages with the exception of Archaeplastida, in which no algivorous species have been reported. In addition, accumulation of CPEs is revealed to be common among phototrophic microeukaryotes (i.e., microalgae) along with dismantling of their secondary chloroplasts. Thus, we infer that CPE-accumulating chlorophyll catabolism (CACC) primarily evolved among algivorous microeukaryotes to detoxify chlorophylls in an early stage of their evolution. Subsequently, it also underpinned photosynthetic endosymbiosis by securing close interactions with photosynthetic machinery containing abundant chlorophylls, which led to the acquisition of secondary chloroplasts. Our results strongly suggest that CACC, which allowed the consumption of oxygenic primary producers, ultimately permitted the successful radiation of the eukaryotes throughout and after the late Proterozoic global oxygenation.

Elevated CO2 delayed the chlorophyll degradation and anthocyanin accumulation in postharvest strawberry fruit

Colour is an important quality attribute for the consumer’s acceptability of fruit. Elevated CO2 was applied to strawberry fruit to explore its influence on chlorophyll catabolism and anthocyanin synthesis. The results showed that 20% CO2 delayed the changes of a* and b* values in strawberry fruit. The degradation of chlorophyll was delayed in CO2 treated fruit by inhibiting the activities of chlorophyllase and down-regulating the expression of FaChl b reductase, FaPAO and FaRCCR. In addition, lower concentration of anthocyanins and lower activity of PAL, C4H, 4CL and CHS were recorded under the effect of 20% CO2. Meanwhile, qRT-PCR analysis showed that 13 genes involved in the phenylpropanoid pathway and the flavonoid biosynthesis pathway were also down-regulated under CO2 stress. However, no residual effect on pigment metabolism was observed when elevated CO2 was removed. Our study provided new insights into the regulation of elevated CO2 in the role of pigment metabolism in postharvest.

Complementation studies of the Arabidopsis fc1 mutant substantiate essential functions of ferrochelatase 1 during embryogenesis and salt stress.

Ferrochelatase (FC) is the final enzyme for haem formation in the tetrapyrrole biosynthesis pathway and encoded by two genes in higher plants. FC2 exists predominantly in green tissue, whereas FC1 is constitutively expressed. We intended to substantiate the specific roles of FC1. The embryo-lethal fc1-2 mutant was used to express the two genomic FC-encoding sequences under the FC1 and FC2 promoter and explore the complementation of the FC1 deficiency. Apart from the successful complementation with FC1, expression of FC2 under control of the FC1 promoter (pFC1::FC2) compensates for missing FC1 but not by FC2 promoter expression. The complementing lines pFC1FC2(fc1/fc1) succeeded under standard growth condition but failed under salt stress. The pFC1FC2(fc1/fc1) line exhibited symptoms of leaf senescence, including accelerated loss of haem and chlorophyll and elevated gene expression for chlorophyll catabolism. In contrast, ectopic FC1 expression (p35S::FC1) resulted in increased chlorophyll accumulation. The limited ability of FC2 to complement fc1 is explained by a faster turnover of FC2 mRNA during stress. It is suggested that FC1-produced haem is essential for embryogenesis and stress response. The pFC1::FC2 expression readily complements the fc1-2 embryo lethality, whereas higher FC1 transcript content contributes essentially to stress tolerance.

Characterization of the Leaf Color Mutant hy and Identification of the Mutated Gene in Chinese Cabbage

Leaf color mutants play an important role in our understanding of chlorophyll biosynthesis and catabolism. In this study, we obtained a yellow-green leaf mutant hy in an ethyl methanesulfonate mutagenized population of chinese cabbage (Brassica rapa ssp. pekinensis). The hy phenotype was controlled by a recessive allele at a single locus. The intrinsic photochemical activity of photosystem II (PSII) is impaired in hy, suggesting that absorbed light energy is not efficiently transferred from the light-harvesting complexes antenna to the PSII reaction centers and dissipated as heat or fluorescence. We measured chlorophyll content and chlorophyll precursors and analyzed the expression of key genes in the chlorophyll synthetic pathway in hy and wild type. The mutation phenotype was consistent with inhibited expression of chlorophyll a oxygenase (CAO) gene in the chlorophyll synthetic pathway. In mutant hy, CAO cDNA was cloned so that a C to T mutation at 1099 bp caused a conserved proline (Pro) to serine (Ser) mutation at the 367th amino acid in C-domain, which changed the secondary structure of CAO protein. We speculate that the mutation amino acid changed in the C-domain may affect the catalytic function in mutant CAO.

The Direct Involvement of Dark-Induced Tic55 Protein in Chlorophyll Catabolism and Its Indirect Role in the MYB108-NAC Signaling Pathway during Leaf Senescence in Arabidopsis thaliana.

The chloroplast relies on proteins encoded in the nucleus, synthesized in the cytosol and subsequently transported into chloroplast through the protein complexes Toc and Tic (Translocon at the outer/inner membrane of chloroplasts). A Tic complex member, Tic55, contains a redox-related motif essential for protein import into chloroplasts in peas. However, Tic55 is not crucial for protein import in Arabidopsis. Here, a tic55-II-knockout mutant of Arabidopsis thaliana was characterized for Tic55 localization, its relationship with other translocon proteins, and its association with plant leaf senescence when compared to the wild type. Individually darkened leaves (IDLs) obtained through dark-induced leaf senescence were used to demonstrate chlorophyll breakdown and its relationship with plant senescence in the tic55-II-knockout mutant. The IDLs of the tic55-II-knockout mutant contained higher chlorophyll concentrations than those of the wild type. Our microarray analysis of IDLs during leaf senescence identified seven senescence-associated genes (SAGs) that were downregulated in the tic55-II-knockout mutant: ASP3, APG7, DIN2, DIN11, SAG12, SAG13, and YLS9. Real-time quantitative PCR confirmed the reliability of microarray analysis by showing the same expression patterns with those of the microarray data. Thus, Tic55 functions in dark-induced aging in A. thaliana by indirectly regulating downstream SAGs expression. In addition, the expression of four NAC genes, including ANAC003, ANAC010, ANAC042, and ANAC075 of IDL treated tic55-II-knockout mutant appeared to be downregulated. Yeast one hybrid assay revealed that only ANAC003 promoter region can be bound by MYB108, suggesting that a MYB-NAC regulatory network is involved in dark-stressed senescence.

Molecular Mechanisms Preventing Senescence in Response to Prolonged Darkness in a Desiccation-Tolerant Plant.

The desiccation-tolerant plant Haberlea rhodopensis can withstand months of darkness without any visible senescence. Here, we investigated the molecular mechanisms of this adaptation to prolonged (30 d) darkness and subsequent return to light. H. rhodopensis plants remained green and viable throughout the dark treatment. Transcriptomic analysis revealed that darkness regulated several transcription factor (TF) genes. Stress- and autophagy-related TFs such as ERF8, HSFA2b, RD26, TGA1, and WRKY33 were up-regulated, while chloroplast- and flowering-related TFs such as ATH1, COL2, COL4, RL1, and PTAC7 were repressed. PHYTOCHROME INTERACTING FACTOR4, a negative regulator of photomorphogenesis and promoter of senescence, also was down-regulated. In response to darkness, most of the photosynthesis- and photorespiratory-related genes were strongly down-regulated, while genes related to autophagy were up-regulated. This occurred concomitant with the induction of SUCROSE NON-FERMENTING1-RELATED PROTEIN KINASES (SnRK1) signaling pathway genes, which regulate responses to stress-induced starvation and autophagy. Most of the genes associated with chlorophyll catabolism, which are induced by darkness in dark-senescing species, were either unregulated (PHEOPHORBIDE A OXYGENASE, PAO; RED CHLOROPHYLL CATABOLITE REDUCTASE, RCCR) or repressed (STAY GREEN-LIKE, PHEOPHYTINASE, and NON-YELLOW COLORING1). Metabolite profiling revealed increases in the levels of many amino acids in darkness, suggesting increased protein degradation. In darkness, levels of the chloroplastic lipids digalactosyldiacylglycerol, monogalactosyldiacylglycerol, phosphatidylglycerol, and sulfoquinovosyldiacylglycerol decreased, while those of storage triacylglycerols increased, suggesting degradation of chloroplast membrane lipids and their conversion to triacylglycerols for use as energy and carbon sources. Collectively, these data show a coordinated response to darkness, including repression of photosynthetic, photorespiratory, flowering, and chlorophyll catabolic genes, induction of autophagy and SnRK1 pathways, and metabolic reconfigurations that enable survival under prolonged darkness.

Overexpression of the protein disulfide isomerase AtCYO1 in chloroplasts slows dark-induced senescence in Arabidopsis

BackgroundChlorophyll breakdown is the most obvious sign of leaf senescence. The chlorophyll catabolism pathway and the associated proteins/genes have been identified in considerable detail by genetic approaches combined with stay-green phenotyping. Arabidopsis CYO1 (AtCYO1), a protein disulfide reductase/isomerase localized in the thylakoid membrane, is hypothesized to assemble the photosystem by interacting with cysteine residues of the subunits.ResultsIn this study, we report that ectopic overexpression of AtCYO1 in leaves induces a stay-green phenotype during darkness, where oxidative conditions favor catabolism. In AtCYO1ox leaves, Fv/Fm and both chlorophyll a and chlorophyll b content remained high during dark-induced senescence. The thylakoid ultrastructure was preserved for a longer time in AtCYO1ox leaves than in wild type leaves. AtCYO1ox leaves maintained thylakoid chlorophyll-binding proteins associated with both PSII (D1, D2, CP43, CP47, LHCB2, and Cyt f) and PSI (PSA-A/B), as well as stromal proteins (Rubisco and ferredoxin-NADP+ reductase). AtCYO1ox did not affect senescence-inducible gene expression for chlorophyll catabolism or accumulation of chlorophyll catabolites.ConclusionsOur results suggest that ectopic overexpression of AtCYO1 had a negative impact on the initiation of chlorophyll degradation and proteolysis within chloroplasts. Our findings cast new light on the redox regulation of protein disulfide bonds for the maintenance of functional chloroplasts.

Transcriptome profiling of Litchi chinensis pericarp in response to exogenous cytokinins and abscisic acid

Anthocyanin biosynthesis in Litchi chinensis is promoted by exogenous abscisic acid (ABA) treatment and inhibited by exogenous N-(2-chloro-pyridin-4-yl)-N′-phenylurea (CPPU) application. However, the mechanisms by which ABA or CPPU regulates anthocyanin biosynthesis are still unclear. To understand the global molecular events of these physiological changes, transcriptome profiling was analyzed in L. chinensis cv. Feizixiao pericarps after 0, 10, and 20 days of exogenous ABA (25 mg/L ABA) and CPPU (4 mg/L) treatment using RNA-seq. Compared with the control, a total of 579 and 827 genes were differently expressed [|log2 fold change| ≥ 1 and P value ≤ 0.005] in ABA- and CPPU-treated pericarp, respectively. Exogenous ABA up-regulated the expressions of genes involved in flavonoid and anthocyanin biosynthesis, including PAL, C4H, CHS, CHI, DFR, LDOX, and GTs. In contrast, exogenous CPPU induced genes related to carbon metabolism, amino acids biosynthesis, and photosynthesis, and down-regulated genes related to anthocyanin biosynthesis. Comparison of transcriptomes in responses to individual treatments with ABA or CPPU revealed that there were cooperative and antagonistic interplay between ABA and cytokinins in litchi fruit ripening. ABA treatment had no significant effect on the genes related to chlorophyll catabolism. On the other hand, CPPU treatment significantly increased the expression of chlorophyll synthesis genes and inhibited the expression of chlorophyll degradation gene (SGR). In addition, ABA and CPPU treatment also affected gene expression in other plant hormone signaling pathways, such as auxin, GA, and ethylene, forming a complex network to regulate anthocyanin biosynthesis. This study provides a valuable overview of global molecular events for studying the mechanisms by which ABA and cytokinins influence anthocyanin biosynthesis in litchi and other fruit trees enriched with anthocyanins.

Mass spectrometry: the indispensable tool for plant metabolomics of colourless chlorophyll catabolites

Senescence and ripening of plant tissues engage the pheophorbide a oxygenase pathway, reducing the chlorophyll content to inactive chlorophyll catabolite products, termed phyllobilins. These products are open-macrocycle derivatives, but present different structural features related to species-dependent enzyme activity. This review encompasses a brief outline of the chlorophyll catabolism pathway, a detailed description of the structural motifs of known phyllobilins, giving details of how mass spectrometry provides hints for the characterization of phyllobilins. The structural approach for the identification of phyllobilins requires several spectroscopic methodologies to reach a complete structural identification, including UV–visible spectroscopy, circular dichroism, nuclear magnetic resonance and mass spectrometry. Among these techniques, mass spectrometry presents several advantages for showing the structural features of phyllobilins, through acquisition of accurate mass, elemental composition, and detection of product ions, which provide valuable structural information. The combination of mass spectra with data-managing and in silico prediction tools greatly enhances the comprehensive building of the phyllobilin structure, and the resolving of the intriguing puzzle of enzymatic and chemical reactions involved in chlorophyll catabolism. Indeed, some strategies based on structural constraints that phyllobilins present, with recent developments in software prediction tools are proposed to foster the unravelling of phyllobilin structures.

Interactive effects of melatonin and cytokinin on alleviating drought-induced leaf senescence in creeping bentgrass (Agrostis stolonifera)

Drought stress induces leaf senescence while cytokinins (CK) and melatonin (ME) play roles in regulating leaf senescence. The objectives of this study were to determine whether ME alone or interacting with CK could suppress drought-induced leaf senescence through mediating CK pathways and chlorophyll catabolism in creeping bentgrass (Agrostis stolonifera). Plants of wild type (‘WT’) (cv. ‘Penncross’) or a transgenic line (‘S41′) harbouring isopentenyltransferase (ipt) gene were foliar sprayed with either 20μM ME or water and exposed to drought stress for 14 d in growth chambers. Exogenous applied ME increased visual quality, photochemical efficiency, chlorophyll content and relative water content, while it suppressed leaf electrolyte leakage, lipid peroxidation and hydrogen peroxide production in both WT and ipt-transgenic plants. Cytokinin-signaling and synthesis genes were also significantly up-regulated in ME-treated ipt-transgenic plants. Dehydration responsive genes (JUB1 and DREB2A) were up-regulated and chlorophyll-degradation genes (Chlase, PPH and Chl-PRX) and enzyme activities were significantly down-regulated by ME application. Endogenous melatonin content and ME-biosynthesis genes (TDC1, SNAT1, and COMT) expression increased in ipt-transgenic plants with ME treatment. ME-alleviation of drought-induced leaf senescence in creeping bentgrass was associated with the down-regulation of chlorophyll catabolism and the synergistically interaction with CK-synthesis gene and signaling pathways.

Effects of blue LED light irradiation on pigment metabolism of ethephon-degreened mandarin fruit

To improve peel color of ethephon-degreened citrus fruit, in this study, the effects of blue light emiting diode (LED) light (450nm) irradiation on chlorophyll and carotenoid metabolism of ethephon-degreened mandarin fruit were evaluated. Deeper and faster coloration were observed in blue LED light-irradiated fruit, accompanied by the accumulation of carotenoid and the degradation of chlorophyll. Blue LED irradiation could shorten the degreening time of ethephon-degreened mandarin fruit. The accumulation of total carotenoid, lutein, violaxanthin, β-cryptoxanthin and zeaxanthin could well explain the faster coloration of the blue LED-irradiated mandarin fruit, while the accumulation of β-cryptoxanthin could well explain the deeper color of the blue LED-irradiated mandarin fruit. For chlorophyll metabolism, the increased expression of chlorophyll catabolism related genes (CitChlase, CitPao and CitRCCR), especially the increased expression of CitChlase might partly explain the sharply decrease of chlorophyll a and b in blue LED-irradiated fruit. However, it seems that both the expression levels of genes in metabolism of carotenoid and chlorophyll could only partly explain the changes in carotenoid and chlorophyll contents, more complex mechanisms might be interacting. Blue LED light may be a potential technique for improving the external color and commercial value of ethephon-degreened citrus fruit.

Efficacy of hot water immersion on lime (Citrus auranifolia, Swingle cv. Paan) fruit packed with ethanol vapor in delaying chlorophyll catabolism

Postharvest quality deterioration in lime (C. aurantifolia Swingle cv. Paan) fruit is indicated by loss of green color as a result of chlorophyll degradation. The effect of hot water immersion on lime fruit packed with ethanol vapor pad (EP) in delaying chlorophyll degradation under storage at 10°C was investigated. Mature green lime fruit weighing on average 250g were immersed in hot water at 50°C for 5min, thereafter, packaged with 0.6g EP or without in polypropylene trays sealed with perforated (6-mm hole) polypropylene film (25μm thick). Postharvest quality, physiological changes and activities of chlorophyll degrading enzymes were determined. The results revealed that hot water immersion and packing fruit with EP effectively delayed chlorophyll degradation through suppressing the activity of chlorophyll degrading enzymes; chlorophyllase, chlorophyll degrading peroxidase, Mg-dechelatase, and pheophytinase in lime fruit. Furthermore, ethylene production as well as respiration was inhibited during the early stages of storage. These results suggest that hot water immersion with EP packaging inhibits chlorophyll degradation in lime fruit through suppression of chlorophyll degrading enzymes as well as ethylene production.

Inhibition of Ethylene Biosynthesis and Perception by 1-Methylcyclopropene and Its Consequences on Chlorophyll Catabolism and Storage Quality of ‘Bosc’ Pears

Bosc is a winter cultivar of european pear (Pyrus communis) that has a relatively short storage life partially due to a high ethylene production rate (EPR) during cold storage. ‘Bosc’ pears were harvested at commercial maturity and treated with gas 1-methylcyclopropene (1-MCP) at 0, 0.15, and 0.3 µL·L−1 and stored at −1.1 °C for 8 months. Results indicated that all 1-MCP treatments inhibited EPR and respiration rate (RR), retarded the degradation of chlorophyll and titratable acidity (TA), and extended storage quality; but inhibited ripening capacity. 1-MCP at 0.15 and 0.3 µL·L−1 had the same efficacy on keeping fruit quality although its higher rate was more efficient on inhibiting EPR and RR. The expression of ethylene synthesis genes (PcACS1, PcACS2, PcACS4, PcACS5, and PcACO1) and receptor genes (PcETR1, PcETR2, and PcERS1) was upregulated in control fruit during storage and they were downregulated significantly by 1-MCP treatments. In contrast, the ethylene receptor genes of PcETR5 and PcCTR1 were downregulated in control fruit during storage and were unaffected by 1-MCP treatments. Although the transcription levels of chlorophyll degradation genes PcPPH, PcNOL, PcSGR, PcRCCR, PcNYC, and PcPAO were all upregulated in control fruit during storage and downregulated by 1-MCP; only PcCHL was downregulated in the control and 1-MCP had no consistent effect on it. The relationship of ethylene biosynthesis/perception with chlorophyll degradation and storage quality in european pears was discussed.

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.ResultsFruit 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.ConclusionsThis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3019-1) contains supplementary material, which is available to authorized users.

Exogenous spermidine delays chlorophyll metabolism in cucumber leaves (Cucumis sativus L.) under high temperature stress

This study evaluated the effects of exogenous spermidine (Spd) on chlorophyll (Chl) biosynthesis and catabolism in the leaves of cucumber (Cucumis sativus L.) seedlings under high temperature stress. Substrate culture experiments were performed using the high temperature-sensitive variety ‘Jinchun No. 2’ in an artificial climate chamber at 42/32 °C with foliar applications of 1.0 mmol L−1 Spd. The results suggested that high temperature stress markedly reduced the leaf Chl concentrations and inhibited plant growth; the harmful effect of high temperature on the cucumber seedlings was mitigated by exogenous Spd, which increased the leaf Chl concentration and promoted plant growth. Under high temperature stress, the conversion of porphobilinogen (PBG) into uroporphyrinogen III (UroIII) in the Chl biosynthetic pathway and the catabolic process of Chl were accelerated. Following the application of exogenous Spd, the conversion of PBG into UroIII was suppressed, and the accumulation of certain intermediates, e.g., protoporphyrin IX (ProtoIX) and Mg-protoporphyrin IX (Mg-ProtoIX), was decreased in the Chl biosynthetic pathway. Additionally, exogenous Spd reduced chlorophyllase (Chlase) and Mg-dechelatase (MDCase) activity and transcript levels and markedly downregulated pheophorbide A oxygenase (PaO), red Chl catabolite reductase (RCCR), Chl b reductase 1 (CBR1) and stay-green reductase 1 (SGR1) transcript levels. These results indicate that although high temperature stress accelerated Chl biosynthesis, it concurrently facilitated Chl catabolism in cucumber leaves. Exogenous Spd delayed the conversion of PBG into UroIII in the Chl biosynthetic pathway, effectively preventing oxidative bleaching of Chl in cucumber leaves. Meanwhile, Spd clearly decreased PaO pathway-related enzyme activity and transcript levels, thereby slowing Chl catabolism and increasing Chl concentrations.

Chlorophyll catabolism in olive fruits (var. Arbequina and Hojiblanca) during maturation

The central reaction of chlorophyll (chl) breakdown pathway occurring during olive fruits maturation is the cleavage of the macrocycle pheophorbide a to a primary fluorescent chl catabolite (pFCC) and it is catalyzed by two enzymes: pheophorbide a oxygenase (PaO) and red chl catabolite reductase (RCCR). In subsequent steps, pFCC is converted to different fluorescent chlorophyll catabolites (FCCs) and nonfluorescent chlorophyll catabolites (NCCs). This work demonstrated that RCCR activity of olive fruits is type II. During the study of evolution of PaO and RCCR activities through the olive fruits maturation in two varieties: Hojiblanca and Arbequina, a significant increase in PaO and RCCR activity was found in ripening stage. In addition, the profile and structure of NCCs present in epicarp of this fruit was studied using HPLC/ESI-TOF-MS. Five different NCCs were defined and for the first time the enzymatic reactions implied in chlorophyll degradations in olive fruits elucidated.