Chl Degradation Research Articles

The H3K27me3 demethylase REF6 promotes leaf senescence through directly activating major senescence regulatory and functional genes in Arabidopsis.

The roles of histone demethylation in the regulation of plant flowering, disease resistance, rhythmical response, and seed germination have been elucidated recently; however, how histone demethylation affects leaf senescence remains largely unclear. In this study, we exploited yeast one-hybrid (Y1H) to screen for the upstream regulators of NONYELLOWING1 (NYE1), and identified RELATIVE OF EARLY FLOWERING6 (REF6), a histone H3 lysine 27 tri-methylation (H3K27me3) demethylase, as a putative binding protein of NYE1 promoter. By in vivo and in vitro analyses, we demonstrated that REF6 directly binds to the motif CTCGYTY in NYE1/2 promoters through its zinc finger domain and positively regulates their expression. Loss-of-function of REF6 delayed chlorophyll (Chl) degradation, whereas overexpression of REF6 accelerated Chl degradation. Subsequently, we revealed that REF6 positively regulates the general senescence process by directly up-regulating ETHYLENE INSENSITIVE 2 (EIN2), ORESARA1 (ORE1), NAC-LIKE, ACTIVATED BY AP3/PI (NAP), PYRUVATE ORTHOPHOSPHATE DIKINASE (PPDK), PHYTOALEXIN DEFICIENT 4 (PAD4), LIPOXYGENASE 1 (LOX1), NAC DOMAIN CONTAINING PROTEIN 3 (AtNAC3), and NAC TRANSCRIPTION FACTOR-LIKE 9 (NTL9), the key regulatory and functional genes predominantly involved in the regulation of developmental leaf senescence. Importantly, loss-of-function of REF6 increased H3K27me3 levels at all the target Senescence associated genes (SAGs). We therefore conclusively demonstrate that H3K27me3 methylation represents an epigenetic mechanism prohibiting the premature transcriptional activation of key developmentally up-regulated senescence regulatory as well as functional genes in Arabidopsis.

Application of electrostatic atomized water particle suppresses calyx discoloration in relation to postharvest quality of mangosteen (Garcinia mangostana L.)

Commercial exporting of mangosteen (Garcinia mangostana L.) is faced with rapid discoloration by degreening of calyx in relation to the loss of postharvest quality. Utilization of electrostatic atomized water particles (EAWPs) has been shown to reduce the chlorophyll (Chl) degradation of green calyxes and postharvest quality during storage. Fruit were treated with EAWPs for 0, 10, 20, 30, 40, 50 and 60 min, and then kept in the dark at 20 °C and 90 ± 5% RH. Treatments with EAWPs for 40 min were found to be appropriate treatments to maintain green calyx freshness by suppressing color change in the calyx and peel of mangosteen fruits, and produced hue angle and a* value higher than that of the control. This treatment efficiently delayed the reduction of Chl a and b contents, and Chl degrading enzyme activities in the calyx. Production of Chl derivatives including chlorophyllide a, pheophorbide a, 132-hydroxychlorophyll a and pheophytin a were detected during storage in the calyx of mangosteen treated with EAWPs for 40 min and were higher than that of the control. Hydrogen peroxide content in EAWPs-treated calyxes was detected at a high level at day 0 of storage and was lower than that of the control. Total anthocyanin content, cyanidin-3-sophoroside and cyanidin-3-glucoside were lower in the treated peel than in the control. SEM observation revealed that treatment of mangosteen with EAWPs for 40 min may induce closing stomata, and reduced guard cells shriveling in the mangosteen calyx. The results obtained in this study suggested that treatment with EAWPs for 40 min could delay degradation of Chl by controlling its catabolites, inhibited Chl degrading enzyme activities, and positively affected the calyx and peel quality of mangosteen in storage.

Chlorophyll Degradation and Light-harvesting Complex II Aggregate Formation During Dark-induced Leaf Senescence in Arabidopsis Pheophytinase Mutants

The stay-green mutant of Arabidopsis thaliana, ore10 forms stable light-harvesting complex II (LHCII) aggregates during dark-induced senescence, which showed a single base deletion (G1351) in the coding region of the pheophytinase (PPH) gene. PPH specifically dephytylates the Mg-free chlorophyll (Chl) pigment pheophytin, yielding pheophorbide. In both ore10 and pph-1 mutants, pheophytin a accumulated due to the deficiency of PPH gene, but the amount was relatively smaller than that of degraded Chl, and most of the pheophytin a was bound to the stable LHCII forming aggregates with some other Chl-protein (CP) complexes. Comparison of Chl a/b ratios in thylakoids, aggregates, and LHCII indicated that the suppression of Chl b to Chl a conversion was stronger when Chl b reductase was missing and weak when PPH is missing in the large Chl catabolic complex, which allowed the partial degradation of Chl b. These results suggest that the PPH-dependent pathway is not specific for LHCII, but common for all CP complexes, including LHCII. In PPH-deficient mutants, the degradation of LHCII was suppressed by the formation of aggregates, and some of the remaining CP complexes and pheophytin a were included in the aggregates. Non-included CP complexes were degraded via an unknown mechanism.

Exogenous Melatonin Counteracts NaCl-Induced Damage by Regulating the Antioxidant System, Proline and Carbohydrates Metabolism in Tomato Seedlings.

Melatonin, a natural agent, has multiple functions in animals as well as in plants. However, its possible roles in plants under abiotic stress are not clear. Nowadays, soil salinity is a major threat to global agriculture because a high soil salt content causes multiple stresses (hyperosmotic, ionic, and oxidative). Therefore, the aim of the present study was to explore: (1) the involvement of melatonin in biosynthesis of photosynthetic pigments and in regulation of photosynthetic enzymes, such as carbonic anhydrase (CA) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco); (2) the role of melatonin in osmoregulation by proline and carbohydrate metabolism; and (3) the function of melatonin in the antioxidant defense system under salinity. Outcomes of the study reveal that under non-saline conditions, application of melatonin (20 and 50 µM) improved plant growth, viz. shoot length, root length, shoot fresh weight (FW), root FW, shoot dry weight (DW), root DW and leaf area and physio-biochemical parameters [chlorophyll (Chl) a and b, proline (Pro) and total soluble carbohydrates (TSC) content, and increased the activity of CA and Rubisco]. However, tomato seedlings treated with NaCl exhibited enhanced Chl degradation, electrolyte leakage (EL), malondialdehyde (MDA) and reactive oxygen species (ROS; superoxide and hydrogen peroxide). ROS were detected in leaf and root. Interestingly, application of melatonin improved plant growth and reduced EL, MDA and ROS levels through upregulation of photosynthesis enzymes (CA, Rubisco), antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase and ascorbate reductase) and levels of non-enzymatic antioxidants [ascorbate (ASC) and reduced glutathione (GSH)], as well as by affecting the ASC—GSH cycle. Additionally, exogenous melatonin also improved osmoregulation by increasing the content of TSC, Pro and Δ1-pyrroline-5-carboxylate synthetase activity. These results suggest that melatonin has beneficial effects on tomato seedlings growth under both stress and non-stress conditions. Melatonin’s role in tolerance to salt stress may be associated with the regulation of enzymes involved in photosynthesis, the antioxidant system, metabolism of proline and carbohydrate, and the ASC—GSH cycle. Also, melatonin could be responsible for maintaining the high ratios of GSH/GSSG and ASC/DHA.

Mg-dechelatase is involved in the formation of photosystem II but not in chlorophyll degradation in Chlamydomonas reinhardtii.

The STAY-GREEN (SGR) gene encodes Mg-dechelatase which catalyzes the conversion of chlorophyll (Chl) a to pheophytin (Pheo) a. This reaction is the first and most important regulatory step in the Chl degradation pathway. Conversely, Pheo a is an indispensable molecule in photosystem (PS) II, suggesting the involvement of SGR in the formation of PSII. To investigate the physiological functions of SGR, we isolated Chlamydomonas sgr mutants by screening an insertion-mutant library. The sgr mutants had reduced maximum quantum efficiency of PSII (Fv /Fm ) and reduced Pheo a levels. These phenotypes were complemented by the introduction of the Chlamydomonas SGR gene. Blue Native polyacrylamide gel electrophoresis and immunoblotting analysis showed that although PSII levels were reduced in the sgr mutants, PSI and light-harvesting Chl a/b complex levels were unaffected. Under nitrogen starvation conditions, Chl degradation proceeded in the sgr mutants as in the wild type, indicating that ChlamydomonasSGR is not required for Chl degradation and primarily contributes to the formation of PSII. In contrast, in the Arabidopsis sgr triple mutant (sgr1 sgr2 sgrL), which completely lacks SGR activity, PSII was synthesized normally. These results suggest that the Arabidopsis SGR participates in Chl degradation while the ChlamydomonasSGR participates in PSII formation despite having the same catalytic property.

Three LcABFs are Involved in the Regulation of Chlorophyll Degradation and Anthocyanin Biosynthesis During Fruit Ripening in Litchi chinensis.

During litchi (Litchi chinensis Sonn.) fruit ripening, two major physiological changes, degreening (Chl degradation) and pigmentation (anthocyanin biosynthesis), are visually apparent. However, the specific factor triggering this important transition is still unclear. In the present study, we found that endogenous ABA content increased sharply when Chl breakdown was initiated and the ABA level peaked just before the onset of anthocyanin accumulation, suggesting that ABA plays an important role during litchi fruit pigmentation. We characterized three ABSCISIC ACID RESPONSE ELEMENT-BINDING FACTORs (LcABF1/2/3) belonging to group A of the basic leucine zipper (bZIP) transcription factors previously shown to be involved in ABA signaling under abiotic stress. LcABF1 transcripts increased at the onset of Chl degradation, and the expression of LcABF3 accumulated in parallel with anthocyanin biosynthesis. In addition, dual luciferase and yeast one-hybrid assays indicated that LcABF1/2 recognized ABA-responsive elements in the promoter region of Chl degradation-related genes (PAO and SGR), while LcABF2/3 bound the promoter region of LcMYB1 and anthocyanin biosynthesis-related structural genes. Indeed, Nicotiana benthamiana leaves transiently expressing LcABF1/2 showed a senescence phenomenon with Chl degradation, and LcABF3 overexpression increased the accumulation of anthocyanin via activation of LcMYB1, which is the key determinant of anthocyanin biosynthesis. These data indicate that LcABF1/2/3 are important transcriptional regulators of ABA-dependent litchi fruit ripening involved in both Chl degradation and anthocyanin biosynthesis.

Knockdown of STAYGREEN in Perennial Ryegrass (Lolium perenne L.) Leads to Transcriptomic Alterations Related to Suppressed Leaf Senescence and Improved Forage Quality.

Chl breakdown is a hallmark of leaf senescence. Protein degradation is tightly associated with accelerated Chl catabolism during leaf senescence. Therefore, blocking or reducing Chl breakdown and thereby improving Chl and leaf protein contents is desirable for agronomic improvement in perennial forage grasses. Perennial ryegrass (Lolium perenne L.) is one principle cool-season forage grass in temperate areas throughout the world. In this study, the perennial ryegrass STAY-GREEN gene (LpSGR) was cloned and characterized. LpSGR was highly expressed in developmentally or dark-induced senescent leaves. LpSGR was subcellularly localized in chloroplast and interacted with the other Chl catabolic enzymes. RNA interference (RNAi) of LpSGR in perennial ryegrass blocked the degradation of Chl, resulting in increased Chl content and photochemical efficiency in senescent leaves. The RNAi transgenic plants had significantly improved forage quality, with up to 46.1% increased protein content in the harvested biomass. Transcriptome comparison revealed that suppression of LpSGR led to multiple alterations in metabolic pathways in locations inside the chloroplast. Most transcription factors of senescence-associated hormonal signaling pathways (e.g. ABA, ethylene and jasmonic acid) had decreased expression levels in the RNAi plants. These results provided a foundation for the further study on the regulatory mechanism of LpSGR in perennial ryegrass for the purpose of forage improvement with delayed leaf senescence and higher forage quality.

Ascorbic acid improves the tolerance of wheat plants to lead toxicity

ABSTRACTAmong the heavy metals (HMs), lead (Pb) is considered as a toxic HM which adversely affects growth and development of crop plants. The present experiment was aimed to investigate the potential role of ascorbic acid (ASC) in the reversal of Pb-inhibited nitrogen and sulfur assimilation enzymes activity and activity of photosynthesis enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and growth response in wheat plants. Wheat seedlings were subjected to 0 mM (control) and 0.2 mM and 0.6 mM of ASC with and without 2 mM of Pb. Plants treated with Pb exhibited the following reduced growth characteristics (root length, shoot length, root fresh weight (FW), shoot FW, root dry weight (DW) and shoot DW). A decrease was also observed in the activity of Rubisco and ATP sulfurylase (ATP-S), relative water content (RWC), accumulation of total chlorophyll (Total Chl) and content of nutrients [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg)] in Pb-treated plants. However, an increase in Chl degradation and in the activity of O-acetylserine(thiol)lyase (OAS-TL) and accumulation of cysteine (Cys), malondialdehyde (MDA) and hydrogen peroxide (H2O2) was observed in plants under Pb stress. On the contrary, exogenous application of ASC mitigated the Pb-toxicity-induced oxidative damage by enhancing the activities of antioxidant enzymes, such as superoxide dismutase, catalase and glutathione reductase. Improved activity of antioxidant enzymes suppressed the formation of MDA and H2O2, which was reflected in the form of improved growth characteristics. Moreover, ASC induced improvement in plants defense systems by reduced Chl degradation and improved the content of essential nutrients (N, P, K, Ca and Mg) and Cys, RWC and the activity of Rubisco, ATP-S, NR and OAS-TL.

Exogenous glutathione attenuates lead-induced oxidative stress in wheat by improving antioxidant defense and physiological mechanisms

ABSTRACTThis study aims at investigate how exogenous glutathione (GSH, 1.0 mM) affects the oxidative stress and antioxidant defense in wheat seedlings under lead (Pb) stress [0.5 and 1.0 mM Pb(NO3)2]. Lead treatment decreased growth, leaf relative water content, and chlorophyll (chl) content whereas raised proline (Pro) level. Lead stress increased H2O2 content, generation rate, and membrane lipid peroxidation. Addition of Pb also disrupted antioxidant enzyme activities and status of endogenous ascorbate and GSH pool. The increase of methylglyoxal was evident under Pb stress. Glutathione supplementation under Pb stress increased antioxidant redox pool and augmented the activities of antioxidant enzymes, and decreased ROS production. Exogenous supplementation of GSH reverted the increase in the methylglyoxal level due to Pb stress due to increased activities of glyoxalase enzymes. Exogenous GSH also regulated Pro, well-maintained tissue water status and prevented chl degradation and increased plant growth and biomass. Abbreviations: AO- ascorbate oxidase; APX- ascorbate peroxidase; AsA- ascorbic acid (ascorbate); BSA- bovine serum albumin; CAT- catalase; CDNB-1-chloro-2, 4-dinitrobenzene; chl- chlorophyll; DHA- dehydroascorbate; DHAR- dehydroascorbate reductase; DTNB- 5,5′-dithio-bis (2-nitrobenzoic acid); EDTA- ethylenediaminetetraacetic acid; Gly I- glyoxalase I; Gly II- glyoxalase II; GR- glutathione reductase; GSH- reduced glutathione; GSSG- oxidized glutathione; GPX- glutathione peroxidase; GST- glutathione S-transferase; MDA- malondialdehyde; MDHA- monodehydroascorbate; MDHAR- monodehydroascorbate reductase; MG- methylglyoxal; NADPH- nicotinamide adenine dinucleotide phosphate; NBT- nitroblue tetrazolium chloride; Pb- lead; PEG- polyethylene glycol; Pro- proline, ROS- reactive oxygen species; RWC- relative water content; SLG- S-d-lactoylglutathione; SOD- superoxide dismutase; TBA- thiobarbituric acid; TCA- trichloroacetic acid.

Transcriptome analysis in petals and leaves of chrysanthemums with different chlorophyll levels

BackgroundChlorophylls (Chls) are magnesium-containing tetrapyrrole macromolecules responsible for the green color in plants. The Chl metabolic pathway has been intensively studied and nearly all the enzymes involved in the pathway have been identified and characterized. Synthesis and activity of these enzymes are tightly regulated in tissue- and developmental stage–specific manners. Leaves contain substantial amounts of Chls because Chls are indispensable for photosynthesis. In contrast, petals generally contain only trace amounts of Chls, which if present would mask the bright petal color. Limited information is available about the mechanisms that control such tissue-specific accumulation of Chls.ResultsTo identify the regulatory steps that control Chl accumulation, we compared gene expression in petals and leaves of chrysanthemum cultivars with different Chl levels. Microarray and quantitative real-time PCR analyses showed that the expression levels of Chl biosynthesis genes encoding glutamyl-tRNA reductase, Mg-protoporphyrin IX chelatase, Mg-protoporphyrin IX monomethylester cyclase, and protochlorophyllide oxidoreductase were well associated with Chl content: their expression levels were lower in white petals than in green petals, and were highest in leaves. Among Chl catabolic genes, expression of STAY-GREEN, encoding Mg-dechelatase, which is a key enzyme controlling Chl degradation, was considerably higher in white and green petals than in leaves. We searched for transcription factor genes whose expression was well related to Chl level in petals and leaves and found three such genes encoding MYB113, CONSTANS-like 16, and DREB and EAR motif protein.ConclusionsFrom our transcriptome analysis, we assume that a low rate of Chl biosynthesis and a high rate of Chl degradation lead to the absence of Chls in white chrysanthemum petals. We identified several candidate transcription factors that might affect Chl accumulation in chrysanthemum petals. Functional analysis of these transcription factors will provide a basis for future molecular studies of tissue-specific Chl accumulation.

Phytohormone and Light Regulation of Chlorophyll Degradation.

Degreening, due to the net loss of chlorophyll (Chl), is the most prominent symptom during the processes of leaf senescence, fruit ripening, and seed maturation. Over the last decade or so, extensive identifications of Chl catabolic genes (CCGs) have led to the revelation of the biochemical pathway of Chl degradation. As such, exploration of the regulatory mechanism of the degreening process is greatly facilitated. During the past few years, substantial progress has been made in elucidating the regulation of Chl degradation, particularly via the mediation of major phytohormones' signaling. Intriguingly, ethylene and abscisic acid's signaling have been demonstrated to interweave with light signaling in mediating the regulation of Chl degradation. In this review, we briefly summarize this progress, with an effort on providing a framework for further investigation of multifaceted and hierarchical regulations of Chl degradation.

NYEs/SGRs-mediated chlorophyll degradation is critical for detoxification during seed maturation in Arabidopsis.

In the seed industry, chlorophyll (Chl) fluorescence is often used as a major non-invasive reporter of seed maturation and quality. Breakdown of Chl is a proactive process during the late stage of seed maturation, as well as during leaf senescence and fruit ripening. However, the biological significance of this process is still unclear. NYE1 and NYE2 are Mg-dechelatases, catalyzing the first rate-limiting step of Chl a degradation. Loss-of-function of both NYE1 and NYE2 not only results in a nearly complete retention of Chl during leaf senescence, but also produces green seeds in Arabidopsis. In this study, we showed that Chl retention in the nye1 nye2 double-mutant caused severe photo-damage to maturing seeds. Upon prolonged light exposure, green seeds of nye1 nye2 gradually bleached out and eventually lost their germination capacity. This organ-specific photosensitive phenotype is likely due to an over-accumulation of free Chl, which possesses photosensitizing properties and causes a burst of reactive oxygen species upon light exposure. As expected, a similar, albeit much milder, photosensitive phenotype was observed in the seeds of d1 d2, a green-seed mutant defective in NYE/SGR orthologous genes in soybean. Taken together, our data suggest that efficient NYEs-mediated Chl degradation is critical for detoxification during seed maturation.

A R2R3-MYB Gene LfMYB113 is Responsible for Autumn Leaf Coloration in Formosan sweet gum (Liquidambar formosana Hance).

The regulation of autumn leaf coloration in deciduous trees has long been an enigma. Due to the fact that different coloration phenotypes may be considered when planting, more understanding of the regulation mechanism is needed. In this study, a R2R3-MYB transcription factor gene LfMYB113 was identified from a subtropical deciduous tree species Formosan sweet gum (Liquidambar formosana Hance). The expression patterns of LfMYB113 in four selected phenotypes were different and were positively correlated with leaf anthocyanin content. In a 35S::LfMYB113 transgenic Nicotiana tabacum plant, both the early and late genes in the anthocyanin biosynthetic pathway were shown to be up-regulated. It was also shown that LfMYB113 can activate the promoter sequence of LfDFR1 and LfDFR2. Transient overexpression of LfMYB113 in Nicotiana benthamiana showed strong anthocyanin accumulation and pre-senescence; the latter was confirmed by up-regulation of senescence-associated genes. In addition, the activation of proLfSGR::YFP by LfMYB113 in transient experiments indicated that LfMYB113 may have a role in regulation of Chl degradation. To our knowledge, this is the first time a R2R3-MYB transcription factor has been functionally identified as one of the key regulators of autumn leaf coloration and autumn leaf senescence.

Suppressor of Overexpression of CO 1 Negatively Regulates Dark-Induced Leaf Degreening and Senescence by Directly Repressing Pheophytinase and Other Senescence-Associated Genes in Arabidopsis.

Although the biochemical pathway of chlorophyll (Chl) degradation has been largely elucidated, how Chl is rapidly yet coordinately degraded during leaf senescence remains elusive. Pheophytinase (PPH) is the enzyme for catalyzing the removal of the phytol group from pheophytin a, and PPH expression is significantly induced during leaf senescence. To elucidate the transcriptional regulation of PPH, we used a yeast (Saccharomyces cerevisiae) one-hybrid system to screen for its trans-regulators. SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), a key flowering pathway integrator, was initially identified as one of the putative trans-regulators of PPH After dark treatment, leaves of an SOC1 knockdown mutant (soc1-6) showed an accelerated yellowing phenotype, whereas those of SOC1-overexpressing lines exhibited a partial stay-green phenotype. SOC1 and PPH expression showed a negative correlation during leaf senescence. Substantially, SOC1 protein could bind specifically to the CArG box of the PPH promoter in vitro and in vivo, and overexpression of SOC1 significantly inhibited the transcriptional activity of the PPH promoter in Arabidopsis (Arabidopsis thaliana) protoplasts. Importantly, soc1-6 pph-1 (a PPH knockout mutant) double mutant displayed a stay-green phenotype similar to that of pph-1 during dark treatment. These results demonstrated that SOC1 inhibits Chl degradation via negatively regulating PPH expression. In addition, measurement of the Chl content and the maximum photochemical efficiency of photosystem II of soc1-6 and SOC1-OE leaves after dark treatment suggested that SOC1 also negatively regulates the general senescence process. Seven SENESCENCE-ASSOCIATED GENES (SAGs) were thereafter identified as its potential target genes, and NONYELLOWING1 and SAG113 were experimentally confirmed. Together, we reveal that SOC1 represses dark-induced leaf Chl degradation and senescence in general in Arabidopsis.

Rice 7-Hydroxymethyl Chlorophyll a Reductase Is Involved in the Promotion of Chlorophyll Degradation and Modulates Cell Death Signaling.

The loss of green coloration via chlorophyll (Chl) degradation typically occurs during leaf senescence. To date, many Chl catabolic enzymes have been identified and shown to interact with light harvesting complex II to form a Chl degradation complex in senescing chloroplasts; this complex might metabolically channel phototoxic Chl catabolic intermediates to prevent oxidative damage to cells. The Chl catabolic enzyme 7-hydroxymethyl Chl a reductase (HCAR) converts 7-hydroxymethyl Chl a (7-HMC a) to Chl a. The rice (Oryza sativa) genome contains a single HCAR homolog (OsHCAR), but its exact role remains unknown. Here, we show that an oshcar knockout mutant exhibits persistent green leaves during both dark-induced and natural senescence, and accumulates 7-HMC a and pheophorbide a (Pheo a) in green leaf blades. Interestingly, both rice and Arabidopsis hcar mutants exhibit severe cell death at the vegetative stage; this cell death largely occurs in a light intensity-dependent manner. In addition, 7-HMC a treatment led to the generation of singlet oxygen (1O2) in Arabidopsis and rice protoplasts in the light. Under herbicide-induced oxidative stress conditions, leaf necrosis was more severe in hcar plants than in wild type, and HCAR-overexpressing plants were more tolerant to reactive oxygen species than wild type. Therefore, in addition to functioning in the conversion of 7-HMC a to Chl a in senescent leaves, HCAR may play a critical role in protecting plants from high light-induced damage by preventing the accumulation of 7-HMC a and Pheo a in developing and mature leaves at the vegetative stage.

The Mystery of Mendel's Stay-Green: Magnesium Stays Chelated in Chlorophylls

Large-scale chlorophyll (Chl) degradation, along with concomitant anthocyanin biosynthesis, creates the magnificent canopy scenery in the autumn. Dynamic Chl degradation greatly facilitates plant growth and development by remobilizing nutrients from senescing organs to rapidly developing ones, seeds and storage organs in particular. In the last decade or so, the major Chl degradation pathway, termed the pheophorbide a oxygenase (PAO) pathway, has been extensively characterized by identifying Chl catabolic genes (CCGs) or enzymes (CCEs).

Variability of bioturbation in various sediment types and on different spatial scales in the southwestern Baltic Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 557:31-49 (2016) - DOI: https://doi.org/10.3354/meps11837 Variability of bioturbation in various sediment types and on different spatial scales in the southwestern Baltic Sea Claudia Morys*, Stefan Forster, Gerhard Graf Department Marine Biology, University of Rostock, Albert-Einstein-Str. 3, 18059 Rostock, Germany *Corresponding author: claudia.morys@uni-rostock.de ABSTRACT: Variability of bioturbation on different spatial scales was revealed through a survey at 6 stations in the southwestern Baltic Sea with different sediment types, salinities and macrozoobenthic communities. At each station, 6 sampling locations were investigated with 4 cores each (24 cores per station). The cores were analyzed for vertical chlorophyll (chl) profiles, which were modeled with both a local (tracer distribution decreasing exponentially with depth indicative of diffusive transport, DB) and a non-local (presence of subsurface maximum of the tracer, injection flux J and ingestion rate r) mixing model developed by Soetaert et al. (1996; J Mar Res 54:1207–1227). Degradation of chl was determined experimentally by an incubation of fresh sediment under anoxic, dark conditions and provided decay constants kD of 0.01 d-1 for mud and 0.02 d-1 for sand. Mixing depths reach 7.1 ± 1.6 cm at stations in the west (except Lübeck Bay, LB), 2 cm deeper than at stations in the east, which reach 5.2 ± 1.7 cm (including LB), mainly depending on the macrozoobenthic community present. Bioturbation intensities indicate high variability between closely located sampling sites as well as across the southern Baltic Sea, and depend on the food supply from the water column. Stations indicate a difference in local mixing (DB) of a factor of 20 and in non-local processes (J) of 6. Non-local transports account for 33 to 50% of the investigated area in the west and for 70 to 100% in the east. The statistical description of the results indicates the necessity of high sampling effort when using chl as a particle tracer. KEY WORDS: Bioturbation · Local and non-local transport · Chlorophyll degradation · Mixing depth Full text in pdf format PreviousNextCite this article as: Morys C, Forster S, Graf G (2016) Variability of bioturbation in various sediment types and on different spatial scales in the southwestern Baltic Sea. Mar Ecol Prog Ser 557:31-49. https://doi.org/10.3354/meps11837 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 557. Online publication date: September 28, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

ANAC032 Positively Regulates Age-Dependent and Stress-Induced Senescence in Arabidopsis thaliana.

Members of the NAC transcription factor family have been implicated in the regulation of different processes of plant development including senescence. In this study, the role of ANAC032 is analyzed in Arabidopsis thaliana (Col-0). ANAC032 is shown to act as a transcriptional activator and its expression is induced in senescing leaves as well as in dark-treated detached leaves. Analysis of transgenic overexpressors (OXs) and chimeric repressors (SRDXs) of ANAC032 indicates that ANAC032 positively regulates age-dependent and dark-induced leaf senescence. Quantitative real-time PCR analysis showed that ANAC032 regulates leaf senescence mainly through the modulation of expression of the senescence-associated genes AtNYE1, SAG113 and SAUR36/SAG201, which are involved in Chl degradation, and ABA and auxin promotion of senescence, respectively. In addition, ANAC032 expression is induced by a range of oxidative and abiotic stresses. As a result, ANAC032 overexpression lines exhibited enhanced leaf senescence when challenged with different oxidative (3-aminotriazole, fumonisin B1 and high light) and abiotic stress (osmotic and salinity) conditions compared with the wild type. In contrast, ANAC032 SRDX lines displayed the opposite phenotype. ANAC032 transgenic lines showed altered 2,4-D-mediated root tip swelling and root inhibition responses when compared with the wild type. The altered response to auxin, oxidative and abiotic stress treatments in ANAC032 transgenic lines involves differential accumulation of H2O2 compared with the wild type. Taken together, these results indicate that ANAC032 is an important transcription factor that positively regulates age-dependent and stress-induced senescence in A. thaliana by modulating reactive oxygen species production.

ABF2, ABF3, and ABF4 Promote ABA-Mediated Chlorophyll Degradation and Leaf Senescence by Transcriptional Activation of Chlorophyll Catabolic Genes and Senescence-Associated Genes in Arabidopsis

Chlorophyll (Chl) degradation is an integral process of leaf senescence, and NYE1/SGR1 has been demonstrated as a key regulator of Chl catabolism in diverse plant species. In this study, using yeast one-hybrid screening, we identified three abscisic acid (ABA)-responsive element (ABRE)-binding transcription factors, ABF2 (AREB1), ABF3, and ABF4 (AREB2), as the putative binding proteins of the NYE1 promoter. Through the transactivation analysis, electrophoretic mobility shift assay, and chromatin immunoprecipitation, we demonstrated that ABF2, ABF3, and ABF4 directly bound to and activated the NYE1 promoter in vitro and in vivo. ABA is a positive regulator of leaf senescence, and exogenously applied ABA can accelerate Chl degradation. The triple mutant of the ABFs, abf2abf3abf4, as well as two ABA-insensitive mutants, abi1-1 and snrk2.2/2.3/2.6, exhibited stay-green phenotypes after ABA treatment, along with decreased induction of NYE1 and NYE2 expression. In contrast, overexpression of ABF4 accelerated Chl degradation upon ABA treatment. Interestingly, ABF2/3/4 could also activate the expression of two Chl catabolic enzyme genes, PAO and NYC1, by directly binding to their promoters. In addition, abf2abf3abf4 exhibited a functional stay-green phenotype, and senescence-associated genes (SAGs), such as SAG29 (SWEET15), might be directly regulated by the ABFs. Taken together, our results suggest that ABF2, ABF3, and ABF4 likely act as key regulators in mediating ABA-triggered Chl degradation and leaf senescence in general in Arabidopsis.

Protection of Chloroplast Membranes by VIPP1 Rescues Aberrant Seedling Development in Arabidopsis nyc1 Mutant.

Chlorophylls (Chl) in photosynthetic apparatuses, along with other macromolecules in chloroplasts, are known to undergo degradation during leaf senescence. Several enzymes involved in Chl degradation, by which detoxification of Chl is safely implemented, have been identified. Chl degradation also occurs during embryogenesis and seedling development. Some genes encoding Chl degradation enzymes such as Chl b reductase (CBR) function during these developmental stages. Arabidopsis mutants lacking CBR (NYC1 and NOL) have been reported to exhibit reduced seed storability, compromised germination, and cotyledon development. In this study, we examined aberrant cotyledon development and found that NYC1 is solely responsible for this phenotype. We inferred that oxidative damage of chloroplast membranes caused the aberrant cotyledon. To test the inference, we attempted to trans-complement nyc1 mutant with overexpressing VIPP1 protein that is unrelated to Chl degradation but which supports chloroplast membrane integrity. VIPP1 expression actually complemented the aberrant cotyledon of nyc1, whereas stay-green phenotype during leaf senescence remained. The swollen chloroplasts observed in unfixed cotyledons of nyc1, which are characteristics of chloroplasts receiving envelope membrane damage, were recovered by overexpressing VIPP1. These results suggest that chloroplast membranes are a target for oxidative damage caused by the impairment in Chl degradation. Trans-complementation of nyc1 with VIPP1 also suggests that VIPP1 is useful for protecting chloroplasts against oxidative stress.