Molecular cloning, functional expression and characterisation of RCC reductase involved in chlorophyll catabolism

Identification of Photosynthesis Characteristics and Chlorophyll Metabolism in Leaves of Citrus Cultivar (Harumi) with Varying Degrees of Chlorosis

In autumn and spring, citrus leaves with a Ponkan (Citrus reticulata Blanco cv. Ponkan) genetic background (Harumi, Daya, etc.) are prone to abnormal physiological chlorosis. The effects of different degrees of chlorosis (normal, mild, moderate and severe) on photosynthesis and the chlorophyll metabolism of leaves of Citrus cultivar (Harumi) were studied via field experiment. Compared with severe chlorotic leaves, the results showed that chlorosis could break leaf metabolism balance, including reduced chlorophyll content, photosynthetic parameters, antioxidant enzyme activity and enzyme activity related to chlorophyll synthesis, increased catalase and decreased enzyme activity. In addition, the content of chlorophyll synthesis precursors showed an overall downward trend expected for uroporphyrinogen III. Furthermore, the relative expression of genes for chlorophyll synthesis (HEMA1, HEME2, HEMG1 and CHLH) was down-regulated to some extent and chlorophyll degradation (CAO, CLH, PPH, PAO and SGR) showed the opposite trend with increased chlorosis. Changes in degradation were more significant. In general, the chlorosis of Harumi leaves might be related to the blocked transformation of uroporphyrinogen III (Urogen III) to coproporphyrinogen III (Coprogen III), the weakening of antioxidant enzyme system activity, the weakening of chlorophyll synthesis and the enhancement in degradation.

Physiological and comparative transcriptome analysis of the response and adaptation mechanism of the photosynthetic function of mulberry (Morus alba L.) leaves to flooding stress

Flooding has become one of the major abiotic stresses that seriously affects plant growth and development owing to changes in the global precipitation pattern. Mulberry (Morus alba L.) is a desirable tree spePhysocarpus amurensis Maxim andcies with high ecological and economic benefits. To reveal the response and adaptive mechanisms of the photosynthetic functions of mulberry leaves to flooding stress, chlorophyll synthesis, photosynthetic electron transfer and the Calvin cycle were investigated by physiological studies combined with an analysis of the transcriptome. Flooding stress inhibited the synthesis of chlorophyll (Chl) and decreased its content in mulberry leaves. The sensitivity of Chl a to flooding stress was higher than that of Chl b owing to the changes of CHLG (LOC21385082) and CAO (LOC21408165) that encode genes during chlorophyll synthesis. The levels of expression of Chl b reductase NYC (LOC112094996) and NYC (LOC21385774), which are involved in Chl b degradation, were upregulated on the fifteenth day of flooding, which accelerated the transformation of Chl b to Chl a, and upregulated the expression of PPH (LOC21385040) and PAO (LOC21395013). This accelerated the degradation of chlorophyll. Flooding stress significantly inhibited the photosynthetic function of mulberry leaves. A Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of differentially expressed genes under different days of flooding stress indicated significant enrichment in Photosynthesis-antenna proteins (map00196), Photosynthesis (map00195) and Carbon fixation in photosynthetic organisms (map00710). On the fifth day of flooding, 7 and 5 genes that encode antenna proteins were identified on LHCII and LHCI, respectively. They were significantly downregulated, and the degree of downregulation increased as the trees were flooded longer. Therefore, the power of the leaves to capture solar energy and transfer this energy to the reaction center was reduced. The chlorophyll fluorescence parameters and related changes in the expression of genes in the transcriptome indicated that the PSII and PSI of mulberry leaves were damaged, and their activities decreased under flooding stress. On the fifth day of flooding, electron transfer on the PSII acceptor side of mulberry leaves was blocked, and the oxygen-evolving complex (OEC) on the donor side was damaged. On the tenth day of flooding, the thylakoid membranes of mulberry leaves were damaged. Five of the six coding genes that mapped to the OEC were significantly downregulated. Simultaneously, other coding genes located at the PSII reaction center and those located at the PSI reaction center, including Cytb6/f, PC, Fd, FNR and ATP, were also significantly downregulated. In addition, the gas exchange parameters (P_n, G_s, T_r, and C_i) of the leaves decreased after 10 days of flooding stress primarily owing to the stomatal factor. However, on the fifteenth day of flooding, the value for the intracellular concentration of CO₂ was significantly higher than that on the tenth day of flooding. In addition, the differentially expressed genes identified in the Calvin cycle were significantly downregulated, suggesting that in addition to stomatal factors, non-stomatal factors were also important factors that mediated the decrease in the photosynthetic capacity of mulberry leaves. In conclusion, the inhibition of growth of mulberry plants caused by flooding stress was primarily related to the inhibition of chlorophyll synthesis, antenna proteins, photosynthetic electron transfer and the Calvin cycle. The results of this study provide a theoretical basis for the response and mechanism of adaptation of the photosynthetic function of mulberry to flooding stress.

Low and high storage temperature inhibited the coloration of mandarin fruit (Citrus unshiu Marc.) with different mechanism

BACKGROUND

Peel color regulated by pigment metabolism is one of the most crucial indicators affecting the commodity values of citrus fruit. Storage temperature is a vital environmental factor that regulates the fruit pigmentation.

RESULTS

Results showed that the peel coloring process was significantly inhibited when mandarin fruit were stored at 5 and 32 °C with normal coloring at 25 °C as the control. However, the inhibitive mechanisms of 5 and 32 °C storage were different. At 5 °C, higher levels of CcNYC and CcCHL2 were detected, which indicated that 5 °C induces the circulation of chlorophyll rather than inhibits chlorophyll degradation. CcPSY2, CcCHYB, and CcZEP exhibited higher expression levels in fruit stored at 5 °C, which accelerated the accumulation of carotenoids. In fruit stored at 32 °C, CcNYC, CcPAO, and CcCHL2 exhibited lower expression levels than those fruit stored at 5 °C, and the expressions of CcPSY2, CcCHYB, and CcZEP were down regulated, implying the carotenoid synthesis was suppressed.

CONCLUSION

Storage at 5 °C inhibited the postharvest coloring of mandarin fruit mainly by activating the cycle of chlorophyll, although it promotes the accumulation of carotenoids at the same time, but chlorophyll covers the color of carotenoids. Storage at 32 °C inhibited mandarin fruit coloring mainly by inhibiting the degradation of chlorophyll. Compared with the change of individual chlorophyll or carotenoid content, the change of the ratio of chlorophyll and carotenoid had a more important role in the coloration of mandarin fruit. This research offers valuable details for understanding the effect of temperature on the coloring process of postharvest citrus fruit. © 2022 Society of Chemical Industry.

Grapevine (Vitis vinifera) responses to salt stress and alkali stress: transcriptional and metabolic profiling

BACKGROUND

Soil salinization and alkalization are widespread environmental problems that limit grapevine (Vitis vinifera L.) growth and yield. However, little is known about the response of grapevine to alkali stress. This study investigated the differences in physiological characteristics, chloroplast structure, transcriptome, and metabolome in grapevine plants under salt stress and alkali stress.

RESULTS

We found that grapevine plants under salt stress and alkali stress showed leaf chlorosis, a decline in photosynthetic capacity, a decrease in chlorophyll content and Rubisco activity, an imbalance of Na⁺ and K⁺, and damaged chloroplast ultrastructure. Fv/Fm decreased under salt stress and alkali stress. NPQ increased under salt stress whereas decreased under alkali stress. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed the differentially expressed genes (DEGs) induced by salt stress and alkali stress were involved in different biological processes and have varied molecular functions. The expression of stress genes involved in the ABA and MAPK signaling pathways was markedly altered by salt stress and alkali stress. The genes encoding ion transporter (AKT1, HKT1, NHX1, NHX2, TPC1A, TPC1B) were up-regulated under salt stress and alkali stress. Down-regulation in the expression of numerous genes in the 'Porphyrin and chlorophyll metabolism', 'Photosynthesis-antenna proteins', and 'Photosynthesis' pathways were observed under alkali stress. Many genes in the 'Carbon fixation in photosynthetic organisms' pathway in salt stress and alkali stress were down-regulated. Metabolome showed that 431 and 378 differentially accumulated metabolites (DAMs) were identified in salt stress and alkali stress, respectively. L-Glutamic acid and 5-Aminolevulinate involved in chlorophyll synthesis decreased under salt stress and alkali stress. The abundance of 19 DAMs under salt stress related to photosynthesis decreased. The abundance of 16 organic acids in salt stress and 22 in alkali stress increased respectively.

CONCLUSIONS

Our findings suggested that alkali stress had more adverse effects on grapevine leaves, chloroplast structure, ion balance, and photosynthesis than salt stress. Transcriptional and metabolic profiling showed that there were significant differences in the effects of salt stress and alkali stress on the expression of key genes and the abundance of pivotal metabolites in grapevine plants.

Transcriptome Analysis and Intraspecific Variation in Spanish Fir (Abies pinsapo Boiss.)

Spanish fir (Abies pinsapo Boiss.) is an endemic, endangered tree that has been scarcely investigated at the molecular level. In this work, the transcriptome of Spanish fir was assembled, providing a large catalog of expressed genes (22,769), within which a high proportion were full-length transcripts (12,545). This resource is valuable for functional genomics studies and genome annotation in this relict conifer species. Two intraspecific variations of A. pinsapo can be found within its largest population at the Sierra de las Nieves National Park: one with standard green needles and another with bluish-green needles. To elucidate the causes of both phenotypes, we studied different physiological and molecular markers and transcriptome profiles in the needles. "Green" trees showed higher electron transport efficiency and enhanced levels of chlorophyll, protein, and total nitrogen in the needles. In contrast, needles from "bluish" trees exhibited higher contents of carotenoids and cellulose. These results agreed with the differential transcriptomic profiles, suggesting an imbalance in the nitrogen status of "bluish" trees. Additionally, gene expression analyses suggested that these differences could be associated with different epigenomic profiles. Taken together, the reported data provide new transcriptome resources and a better understanding of the natural variation in this tree species, which can help improve guidelines for its conservation and the implementation of adaptive management strategies under climatic change.

Evolution of chlorophyll degradation is associated with plant transition to land

Chlorophyll, the central pigment of photosynthesis, is highly photo‐active and degraded enzymatically during leaf senescence. Merging comparative genomics and metabolomics, we evaluate the extent to which the chlorophyll detoxification pathway has evolved in Viridiplantae. We argue that cytosolic detoxification of phyllobilins in particular was a critical process to the green lineage’s transition to land.

Effects of different processing methods on the chlorophyll structure in kiwifruit

Kiwifruit puree was treated with high and normal temperature withal pressure as independent variables to determinate the structural changes of chlorophyll derivatives. Two groups of colored elution samples were identified...

Identification of Key Genes in 'Luang Pratahn', Thai Salt-Tolerant Rice, Based on Time-Course Data and Weighted Co-expression Networks

Salinity is an important environmental factor causing a negative effect on rice production. To prevent salinity effects on rice yields, genetic diversity concerning salt tolerance must be evaluated. In this study, we investigated the salinity responses of rice (Oryza sativa) to determine the critical genes. The transcriptomes of 'Luang Pratahn' rice, a local Thai rice variety with high salt tolerance, were used as a model for analyzing and identifying the key genes responsible for salt-stress tolerance. Based on 3' Tag-Seq data from the time course of salt-stress treatment, weighted gene co-expression network analysis was used to identify key genes in gene modules. We obtained 1,386 significantly differentially expressed genes in eight modules. Among them, six modules indicated a significant correlation within 6, 12, or 48h after salt stress. Functional and pathway enrichment analysis was performed on the co-expressed genes of interesting modules to reveal which genes were mainly enriched within important functions for salt-stress responses. To identify the key genes in salt-stress responses, we considered the two-state co-expression networks, normal growth conditions, and salt stress to investigate which genes were less important in a normal situation but gained more impact under stress. We identified key genes for the response to biotic and abiotic stimuli and tolerance to salt stress. Thus, these novel genes may play important roles in salinity tolerance and serve as potential biomarkers to improve salt tolerance cultivars.

Research Progress in the Interconversion, Turnover and Degradation of Chlorophyll

Chlorophylls (Chls, Chl a and Chl b) are tetrapyrrole molecules essential for photosynthetic light harvesting and energy transduction in plants. Once formed, Chls are noncovalently bound to photosynthetic proteins on the thylakoid membrane. In contrast, they are dismantled from photosystems in response to environmental changes or developmental processes; thus, they undergo interconversion, turnover, and degradation. In the last twenty years, fruitful research progress has been achieved on these Chl metabolic processes. The discovery of new metabolic pathways has been accompanied by the identification of enzymes associated with biochemical steps. This article reviews recent progress in the analysis of the Chl cycle, turnover and degradation pathways and the involved enzymes. In addition, open questions regarding these pathways that require further investigation are also suggested.

An evergreen mind and a heart for the colors of fall

With the finest biochemical and molecular approaches, convincing explorative strategies, and long-term vision, Stefan Hörtensteiner succeeded in elucidating the biochemical pathway responsible for chlorophyll degradation. After having contributed to the identification of key chlorophyll degradation products in the course of the past 25 years, he gradually identified and characterized most of the crucial players in the PAO/phyllobilin degradation pathway of chlorophyll. He was one of the brightest plant biochemists of his generation, and his work opened doors to a better understanding of plant senescence, tetrapyrrole homeostasis, and their complex regulation. He sadly passed away on 5 December 2020, aged 57.

NOL-mediated functional stay-green traits in perennial ryegrass (Lolium perenne L.) involving multifaceted molecular factors and metabolic pathways regulating leaf senescence

Loss of chlorophyll (Chl) is a hallmark of leaf senescence, which may be regulated by Chl catabolic genes, including NON-YELLOW COLORING 1 (NYC1)-like (NOL). The objective of this study was to determine molecular factors and metabolic pathways underlying NOL regulation of leaf senescence in perennial grass species. LpNOL was cloned from perennial ryegrass (Lolium perenne L.) and found to be highly expressed in senescent leaves. Transient overexpression of LpNOL accelerated leaf senescence and Chl b degradation in Nicotiana benthamiana. LpNOL RNA interference (NOLi) in perennial ryegrass not only significantly blocked Chl degradation in senescent leaves, but also delayed initiation and progression of leaf senescence. This study found that NOL, in addition to functioning as a Chl b reductase, could enact the functional stay-green phenotype in perennial grass species, as manifested by increased photosynthetic activities in NOLi plants. Comparative transcriptomic analysis revealed that NOL-mediated functional stay-green in perennial ryegrass was mainly achieved through the modulation of Chl catabolism, light harvesting for photosynthesis, photorespiration, cytochrome respiration, carbohydrate catabolism, oxidative detoxification, and abscisic acid biosynthesis and signaling pathways.

Involvement of CsERF2 in leaf variegation of Cymbidium sinense 'Dharma'

MAIN CONCLUSION

CsERF2, an ethylene response factor, plays a role in leaf variegation. Leaf variegation is a main ornamental characteristic in Cymbidium sinense (C. sinense). However, the mechanisms of leaf color variegation remain largely unclear. In the present study, we analyzed the cytological and physiological features, as well as molecular analyses of leaves from wild-type (WT) and leaf variegation mutants of Cymbidium sinense 'Dharma'. Chloroplasts with typical and functional structures were discovered in WT and green sectors of the mutants leaves (MG), but not in yellow sectors of the mutant leaves (MY). The activities of key enzymes involved in chlorophyll (Chl) degradation and their substrate contents were significantly increased in MY. Genes related to Chl degradation also showed a significant up-regulation in MY. Transcriptomic analysis showed that the expression of all identified ethylene response factors (ERFs) was significantly up-regulated, and the 1-aminocyclopropane-1-carboxylic acid (ACC) content in MY was significantly higher compared with MG. QRT-PCR analysis validated that the expression levels of genes related to Chl degradation could be positively affected by ethylene (ETH) treatment. Stable overexpression of CsERF2 in Nicotiana tabacum (N. tabacum) led to a decrease in Chl content and abnormal chloroplast. Transcriptomic analysis and qRT-PCR results showed that the KEGG pathway related to chloroplast development and function showed significant change in transgenic N. tabacum. Therefore, the leaf color formation of C. sinense was greatly affected by chloroplast development and Chl metabolism. CsERF2 played an important role in leaf variegation of C. sinense.

HEBE, a novel positive regulator of senescence in Solanum lycopersicum

Leaf senescence and plant aging are traits of great interest for breeders. Senescing cells undergo important physiological and biochemical changes, while cellular structures such as chloroplasts are degraded with dramatic metabolic consequences for the whole plant. The possibility of prolonging the photosynthetic ability of leaves could positively impact the plant's life span with benefits for biomass production and metabolite accumulation; plants with these characteristics display a stay-green phenotype. A group of plant transcription factors known as NAC play a pivotal role in controlling senescence: here we describe the involvement of the tomato NAC transcription factor Solyc12g036480, which transcript is present in leaves and floral buds. Since its silencing delays leaf senescence and prevents plants from ageing, we renamed Solyc12g0364 HḖBĒ, for the Greek goddess of youth. In this manuscript we describe how HEB downregulation negatively affects the progression of senescence, resulting in changes in transcription of senescence-promoting genes, as well as the activity of enzymes involved in chlorophyll degradation, thereby explaining the stay-green phenotype.

Dark/Light Treatments Followed by γ-Irradiation Increase the Frequency of Leaf-Color Mutants in Cymbidium

Radiation randomly induces chromosomal mutations in plants. However, it was recently found that the frequency of flower-color mutants could be specifically increased by upregulating anthocyanin pathway gene expression before radiation treatments. The mechanisms of chlorophyll biosynthesis and degradation are active areas of plant study because chlorophyll metabolism is closely connected to photosynthesis. In this study, we determined the dark/light treatment conditions that resulted in upregulation of the expression levels of six chlorophyll pathway genes, uroporphyrinogen III synthase (HEMD), uroporphyrinogen III decarboxylase (HEME2), NADPH-protochlorophyllide oxidoreductase (POR) A (PORA), chlorophyll synthase (CHLG), chlorophyllase (CLH2), and red chlorophyll catabolite reductase (RCCR), and measured their effects on the γ-irradiation-induced frequencies of leaf-color mutants in two Cymbidium cultivars. To degrade chlorophyll in rhizomes, 60–75 days of dark treatment were required. To upregulate the expressions of chlorophyll pathway genes, 10 days of light treatment appeared to be optimal. Dark/light treatments followed by γ-irradiation increased chlorophyll-related leaf mutants by 1.4- to 2.0-fold compared with γ-ray treatment alone. Dark/light treatments combined with γ-irradiation increased the frequency of leaf-color mutants in Cymbidium, which supports the wider implementation of a plant breeding methodology that increases the mutation frequency of a target trait by controlling the expression of target trait-related genes.

iTRAQ-Based Proteomic Analysis of Watermelon Fruits in Response to Cucumber green mottle mosaic virus Infection

Cucumber green mottle mosaic virus (CGMMV) is an important viral pathogen on cucurbit plants worldwide, which can cause severe fruit decay symptoms on infected watermelon (usually called “watermelon blood flesh”). However, the molecular mechanism of this disease has not been well understood. In this study, we employed the isobaric tags for relative and absolute quantitation (iTRAQ) technique to analyze the proteomic profiles of watermelon fruits in response to CGMMV infection. A total of 595 differentially accumulated proteins (DAPs) were identified, of which 404 were upregulated and 191 were downregulated. Functional annotation analysis showed that these DAPs were mainly involved in photosynthesis, carbohydrate metabolism, secondary metabolite biosynthesis, plant–pathogen interaction, and protein synthesis and turnover. The accumulation levels of several proteins related to chlorophyll metabolism, pyruvate metabolism, TCA cycle, heat shock proteins, thioredoxins, ribosomal proteins, translation initiation factors, and elongation factors were strongly affected by CGMMV infection. Furthermore, a correlation analysis was performed between CGMMV-responsive proteome and transcriptome data of watermelon fruits obtained in our previous study, which could contribute to comprehensively elucidating the molecular mechanism of “watermelon blood flesh”. To confirm the iTRAQ-based proteome data, the corresponding transcripts of ten DAPs were validated by determining their abundance via quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). These results could provide a scientific basis for in-depth understanding of the pathogenic mechanisms underlying CGMMV-induced “watermelon blood flesh”, and lay the foundation for further functional exploration and verification of related genes and proteins.

Cryptic chlorophyll breakdown in non-senescent green Arabidopsis thaliana leaves

Chlorophyll (Chl) breakdown is a diagnostic visual process of leaf senescence, which furnishes phyllobilins (PBs) by the PAO/phyllobilin pathway. As Chl breakdown disables photosynthesis, it appears to have no role in photoactive green leaves. Here, colorless PBs were detected in green, non-senescent leaves of Arabidopsis thaliana. The PBs from the green leaves had structures entirely consistent with the PAO/phyllobilin pathway and the mutation of a single Chl catabolic enzyme completely abolished PBs with the particular modification. Hence, the PAO/phyllobilin pathway was active in the absence of visible senescence and expression of genes encoding Chl catabolic enzymes was observed in green Arabidopsis leaves. PBs accumulated to only sub-% amounts compared to the Chls present in the green leaves, excluding a substantial contribution of Chl breakdown from rapid Chl turnover associated with photosystem II repair. Indeed, Chl turnover was shown to involve a Chl a dephytylation and Chl a reconstitution cycle. However, non-recyclable pheophytin a is also liberated in the course of photosystem II repair, and is proposed here to be scavenged and degraded to the observed PBs. Hence, a cryptic form of the established pathway of Chl breakdown is indicated to play a constitutive role in photoactive leaves.

In Search of Bioactivity - Phyllobilins, an Unexplored Class of Abundant Heterocyclic Plant Metabolites from Breakdown of Chlorophyll

The fate of the green plant pigment chlorophyll (Chl) in de-greening leaves has long been a fascinating biological puzzle. In the course of the last three decades, various bilin-type products of Chl breakdown have been identified, named phyllobilins (PBs). Considered 'mere' leftovers of a controlled biological Chl detoxification originally, the quest for finding relevant bioactivities of the PBs has become a new paradigm. Indeed, the PBs are abundant in senescent leaves, in ripe fruit and in some vegetables, and they display an exciting array of diverse heterocyclic structures. This review outlines briefly which types of Chl breakdown products occur in higher plants, describes basics of their bio-relevant structural and chemical properties and gives suggestions as to 'why' the plants produce vast amounts of uniquely 'decorated' heterocyclic compounds. Clearly, it is worthwhile to consider crucial metabolic roles of PBs in plants, which may have practical consequences in agriculture and horticulture. However, PBs are also part of our plant-based nutrition and their physiological and pharmacological effects in humans are of interest, as well.

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.

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.

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.

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.

A liquid chromatography-mass spectrometry platform for the analysis of phyllobilins, the major degradation products of chlorophyll in Arabidopsis thaliana

During senescence, chlorophyll is broken down to a set of structurally similar, but distinct linear tetrapyrrolic compounds termed phyllobilins. Structure identification of phyllobilins from over a dozen plant species revealed that modifications at different peripheral positions may cause complex phyllobilin composition in a given species. For example, in Arabidopsis thaliana wild-type, eight different phyllobilins have structurally been characterized to date. Accurate phyllobilin identification and quantification, which classically have been performed by high performance liquid chromatography (HPLC) and UV/vis detection, are, however, hampered because of their similar physiochemical properties and vastly differing abundances in plant extracts. Here we established a rapid method for phyllobilin identification and quantification that couples ultra-HPLC with high-resolution/high-precision tandem mass spectrometry. Using Arabidopsis wild-type and mutant lines that are deficient in specific phyllobilin-modifying reactions, we identified a total of 16 phyllobilins, among them two that have not been described before in Arabidopsis. The single and collision-induced dissociation tandem mass spectrometry data of all 16 Arabidopsis phyllobilins were collected in a mass spectrometry library, which is available to the scientific community. The library allows rapid detection and quantification of phyllobilins within and across Arabidopsis genotypes and we demonstrate its potential use for high-throughput approaches and genome-wide association studies in chlorophyll breakdown. By extending the library with phyllobilin data from other plant species in the future, we aim providing a tool for chlorophyll metabolite analysis as a measure of senescence for practical applications, such as post-harvest quality control.

The role of ANAC072 in the regulation of chlorophyll degradation during age- and dark-induced leaf senescence

ANAC072 positively regulates both age- and dark-induced leaf senescence through activating the transcription of NYE1. Leaf senescence is integral to plant development, which is age-dependent and strictly regulated by internal and environmental signals. Although a number of senescence-related mutants and senescence-associated genes (SAGs) have been identified and characterized in the past decades, the general regulatory network of leaf senescence is still far from being elucidated. Here, we report the role of ANAC072, an SAG identified through bioinformatics analysis, in the regulation of chlorophyll degradation during natural and dark-induced leaf senescence. The expression of ANAC072 was increased with advancing leaf senescence in Arabidopsis. Leaf degreening was significantly delayed under normal or dark-induced conditions in anac072-1, a knockout mutant of ANAC072, with a higher chlorophyll level detected. In contrast, an overexpression mutant, anac072-2, with ANAC072 transcription markedly upregulated, showed an early leaf-yellowing phenotype. Consistently, senescent leaves of the loss-of-function mutant anac072-1 exhibited delays in the decrease of photosynthesis efficiency of photosystem II (F v/F m ratio) and the increase of plasma membrane ion leakage rate as compared with corresponding leaves of wild-type Col-0 plants, whereas the overexpression mutant anac072-2 showed opposite changes. Our data suggest that ANAC072 plays a positive role during natural and dark-induced leaf senescence. In addition, the transcript level of NYE1, a key regulatory gene in chlorophyll degradation, relied on the function of ANAC072. Combining these analyses with electrophoretic mobility shift assay and chromatin immunoprecipitation, we demonstrated that ANAC072 directly bound to the NYE1 promoter in vitro and in vivo, so ANAC072 may promote chlorophyll degradation by directly upregulating the expression of NYE1.

Chlorophyll Catabolites in Fall Leaves of the Wych Elm Tree Present a Novel Glycosylation Motif

Fall leaves of the common wych elm tree (Ulmus glabra) were studied with respect to chlorophyll catabolites. Over a dozen colorless, non‐fluorescent chlorophyll catabolites (NCCs) and several yellow chlorophyll catabolites (YCCs) were identified tentatively. Three NCC fractions were isolated and their structures were characterized by spectroscopic means. Two of these, Ug‐NCC‐27 and Ug‐NCC‐43, carried a glucopyranosyl appendage. Ug‐NCC‐53, the least polar of these NCCs, was identified as the formal product of an intramolecular esterification of the propionate and primary glucopyranosyl hydroxyl groups of Ug‐NCC‐43. Thus, the glucopyranose moiety and three of the pyrrole units of Ug‐NCC‐53 span a 20‐membered ring, installing a bicyclo[17.3.1]glycoside moiety. This structural motif is unprecedented in heterocyclic natural products, according to a thorough literature search. The remarkable, three‐dimensional bicyclo[17.3.1]glycoside architecture reduces the flexibility of the linear tetrapyrrole. This feature of Ug‐NCC‐53 is intriguing, considering the diverse biological effects of known bicyclo[n.3.1]glycosidic natural products.

Transcriptome analysis of sweet orange trees infected with 'Candidatus Liberibacter asiaticus' and two strains of Citrus Tristeza Virus

BACKGROUND

Huanglongbing (HLB) and tristeza, are diseases of citrus caused by a member of the α-proteobacteria, 'Candidatus Liberibacter asiaticus' (CaLas), and Citrus tristeza virus (CTV) respectively. HLB is a devastating disease, but CTV strains vary from very severe to very mild. Both CaLas and CTV are phloem-restricted. The CaLas-B232 strain and CTV-B6 cause a wide range of severe and similar symptoms. The mild strain CTV-B2 doesn't induce significant symptoms or damage to plants.

RESULTS

Transcriptome profiles obtained through RNA-seq revealed 611, 404 and 285 differentially expressed transcripts (DETs) after infection with CaLas-B232, CTV-B6 and CTV-B2. These DETs were components of a wide range of pathways involved in circadian rhythm, cell wall modification and cell organization, as well as transcription factors, transport, hormone response and secondary metabolism, signaling and stress response. The number of transcripts that responded to both CTV-B6 and CaLas-B232 was much larger than the number of transcripts that responded to both strains of CTV or to both CTV-B2 and CaLas-B232. A total of 38 genes were assayed by RT-qPCR and the correlation coefficients between Gfold and RT-qPCR were 0.82, 0.69, 0.81 for sweet orange plants infected with CTV-B2, CTV-B6 and CaLas-B232, respectively.

CONCLUSIONS

The number and composition of DETs reflected the complexity of symptoms caused by the pathogens in established infections, although the leaf tissues sampled were asymptomatic. There were greater similarities between the sweet orange in response to CTV-B6 and CaLas-B232 than between the two CTV strains, reflecting the similar physiological changes caused by both CTV-B6 and CaLas-B232. The circadian rhythm system of plants was perturbed by all three pathogens, especially by CTV-B6, and the ion balance was also disrupted by all three pathogens, especially by CaLas-B232. Defense responses related to cell wall modification, transcriptional regulation, hormones, secondary metabolites, kinases and stress were activated by all three pathogens but with different patterns. The transcriptome profiles of Citrus sinensis identified host genes whose expression is affected by the presence of a pathogen in the phloem without producing symptoms (CTV-B2), and host genes whose expression leads to induction of symptoms in the plant (CTV-B6, CaLas-B232).

Breakdown of Chlorophyll in Higher Plants--Phyllobilins as Abundant, Yet Hardly Visible Signs of Ripening, Senescence, and Cell Death

Fall colors have always been fascinating and are still a remarkably puzzling phenomenon associated with the breakdown of chlorophyll (Chl) in leaves. As discovered in recent years, nongreen bilin-type Chl catabolites are generated, which are known as the phyllobilins. Collaborative chemical-biological efforts have led to the elucidation of the key Chl-breakdown processes in senescent leaves and in ripening fruit. Colorless and largely photoinactive phyllobilins are rapidly produced from Chl, apparently primarily as part of a detoxification program. However, fluorescent Chl catabolites accumulate in some senescent leaves and in peels of ripe bananas and induce a striking blue glow. The structural features, chemical properties, and abundance of the phyllobilins in the biosphere suggest biological roles, which still remain to be elucidated.

The NAC transcription factor ANAC046 is a positive regulator of chlorophyll degradation and senescence in Arabidopsis leaves

Chlorophyll (Chl) degradation occurs during leaf senescence, embryo degreening, bud breaking, and fruit ripening. The Chl catabolic pathway has been intensively studied and nearly all the enzymes involved are identified and characterized; however, the molecular regulatory mechanisms of this pathway are largely unknown. In this study, we performed yeast one-hybrid screening using a transcription factor cDNA library to search for factors controlling the expression of Chl catabolic genes. We identified ANAC046 as a common regulator that directly binds to the promoter regions of NON-YELLOW COLORING1, STAY-GREEN1 (SGR1), SGR2, and PHEOPHORBIDE a OXYGENASE. Transgenic plants overexpressing ANAC046 exhibited an early-senescence phenotype and a lower Chl content in comparison with the wild-type plants, whereas loss-of-function mutants exhibited a delayed-senescence phenotype and a higher Chl content. Microarray analysis of ANAC046 transgenic plants showed that not only Chl catabolic genes but also senescence-associated genes were positively regulated by ANAC046. We conclude that ANAC046 is a positive regulator of Arabidopsis leaf senescence and exerts its effect by controlling the expression of Chl catabolic genes and senescence-associated genes.

Leaf Senescence by Magnesium Deficiency

Magnesium ions (Mg(2+)) are the second most abundant cations in living plant cells, and they are involved in various functions, including photosynthesis, enzyme catalysis, and nucleic acid synthesis. Low availability of Mg(2+) in an agricultural field leads to a decrease in yield, which follows the appearance of Mg-deficient symptoms such as chlorosis, necrotic spots on the leaves, and droop. During the last decade, a variety of physiological and molecular responses to Mg(2+) deficiency that potentially link to leaf senescence have been recognized, allowing us to reconsider the mechanisms of Mg(2+) deficiency. This review focuses on the current knowledge about the physiological responses to Mg(2+) deficiency including a decline in transpiration, accumulation of sugars and starch in source leaves, change in redox states, increased oxidative stress, metabolite alterations, and a decline in photosynthetic activity. In addition, we refer to the molecular responses that are thought to be related to leaf senescence. With these current data, we give an overview of leaf senescence induced by Mg deficiency.

Jasmonic acid promotes degreening via MYC2/3/4- and ANAC019/055/072-mediated regulation of major chlorophyll catabolic genes

Degreening caused by rapid chlorophyll (Chl) degradation is a characteristic event during green organ senescence or maturation. Pheophorbide a oxygenase gene (PAO) encodes a key enzyme of Chl degradation, yet its transcriptional regulation remains largely unknown. Using yeast one-hybrid screening, coupled with in vitro and in vivo assays, we revealed that Arabidopsis MYC2/3/4 basic helix-loop-helix proteins directly bind to PAO promoter. Overexpression of the MYCs significantly enhanced the transcriptional activity of PAO promoter in Arabidopsis protoplasts, and methyl jasmonate (MeJA) treatment greatly induced PAO expression in wild-type Arabidopsis plants, but the induction was abolished in myc2 myc3 myc4. In addition, MYC2/3/4 proteins could promote the expression of another Chl catabolic enzyme gene, NYC1, as well as a key regulatory gene of Chl degradation, NYE1/SGR1, by directly binding to their promoters. More importantly, the myc2 myc3 myc4 triple mutant showed a severe stay-green phenotype, whereas the lines overexpressing the MYCs showed accelerated leaf yellowing upon MeJA treatment. These results suggest that MYC2/3/4 proteins may mediate jasmonic acid (JA)-induced Chl degradation by directly activating these Chl catabolic genes (CCGs). Three NAC family proteins, ANAC019/055/072, downstream from MYC2/3/4 proteins, could also directly promote the expression of a similar set of CCGs (NYE1/SGR1, NYE2/SGR2 and NYC1) during Chl degradation. In particular, anac019 anac055 anac072 triple mutant displayed a severe stay-green phenotype after MeJA treatment. Finally, we revealed that MYC2 and ANAC019 may interact with each other and synergistically enhance NYE1 expression. Together, our study reveals a hierarchical and coordinated regulatory network of JA-induced Chl degradation.

Genes and quantitative genetic variation involved with senescence in cells, organs, and the whole plant

Senescence, the deterioration of morphological, physiological, and reproductive functions with age that ends with the death of the organism, was widely studied in plants. Genes were identified that are linked to the deterioration of cells, organs and the whole plant. It is, however, unclear whether those genes are the source of age dependent deterioration or get activated to regulate such deterioration. Furthermore, it is also unclear whether such genes are active as a direct consequence of age or because they are specifically involved in some developmental stages. At the individual level, it is the relationship between quantitative genetic variation, and age that can be used to detect the genetic signature of senescence. Surprisingly, the latter approach was only scarcely applied to plants. This may be the consequence of the demanding requirements for such approaches and/or the fact that most research interest was directed toward plants that avoid senescence. Here, I review those aspects in turn and call for an integrative genetic theory of senescence in plants. Such conceptual development would have implications for the management of plant genetic resources and generate progress on fundamental questions raised by aging research.

Arabidopsis genes, AtNPR1, AtTGA2 and AtPR-5, confer partial resistance to soybean cyst nematode (Heterodera glycines) when overexpressed in transgenic soybean roots

BACKGROUND

Extensive studies using the model system Arabidopsis thaliana to elucidate plant defense signaling and pathway networks indicate that salicylic acid (SA) is the key hormone triggering the plant defense response against biotrophic and hemi-biotrophic pathogens, while jasmonic acid (JA) and derivatives are critical to the defense response against necrotrophic pathogens. Several reports demonstrate that SA limits nematode reproduction.

RESULTS

Here we translate knowledge gained from studies using Arabidopsis to soybean. The ability of thirty-one Arabidopsis genes encoding important components of SA and JA synthesis and signaling in conferring resistance to soybean cyst nematode (SCN: Heterodera glycines) are investigated. We demonstrate that overexpression of three of thirty-one Arabidoposis genes in transgenic soybean roots of composite plants decreased the number of cysts formed by SCN to less than 50% of those found on control roots, namely AtNPR1(33%), AtTGA2 (38%), and AtPR-5 (38%). Three additional Arabidopsis genes decreased the number of SCN cysts by 40% or more: AtACBP3 (53% of the control value), AtACD2 (55%), and AtCM-3 (57%). Other genes having less or no effect included AtEDS5 (77%), AtNDR1 (82%), AtEDS1 (107%), and AtPR-1 (80%), as compared to control. Overexpression of AtDND1 greatly increased susceptibility as indicated by a large increase in the number of SCN cysts (175% of control).

CONCLUSIONS

Knowledge of the pathogen defense system gained from studies of the model system, Arabidopsis, can be directly translated to soybean through direct overexpression of Arabidopsis genes. When the genes, AtNPR1, AtGA2, and AtPR-5, encoding specific components involved in SA regulation, synthesis, and signaling, are overexpressed in soybean roots, resistance to SCN is enhanced. This demonstrates functional compatibility of some Arabidopsis genes with soybean and identifies genes that may be used to engineer resistance to nematodes.

Update on the biochemistry of chlorophyll breakdown

In land plants, chlorophyll is broken down to colorless linear tetrapyrroles in a highly conserved multi-step pathway. The pathway is termed the 'PAO pathway', because the opening of the chlorine macrocycle present in chlorophyll catalyzed by pheophorbide a oxygenase (PAO), the key enzyme of the pathway, provides the characteristic structural basis found in all further downstream chlorophyll breakdown products. To date, most of the biochemical steps of the PAO pathway have been elucidated and genes encoding many of the chlorophyll catabolic enzymes been identified. This review summarizes the current knowledge on the biochemistry of the PAO pathway and provides insight into recent progress made in the field that indicates that the pathway is more complex than thought in the past.

Plant type ferredoxins and ferredoxin-dependent metabolism

Ferredoxin (Fd) is a small [2Fe-2S] cluster-containing protein found in all organisms performing oxygenic photosynthesis. Fd is the first soluble acceptor of electrons on the stromal side of the chloroplast electron transport chain, and as such is pivotal to determining the distribution of these electrons to different metabolic reactions. In chloroplasts, the principle sink for electrons is in the production of NADPH, which is mostly consumed during the assimilation of CO2 . In addition to this primary function in photosynthesis, Fds are also involved in a number of other essential metabolic reactions, including biosynthesis of chlorophyll, phytochrome and fatty acids, several steps in the assimilation of sulphur and nitrogen, as well as redox signalling and maintenance of redox balance via the thioredoxin system and Halliwell-Asada cycle. This makes Fds crucial determinants of the electron transfer between the thylakoid membrane and a variety of soluble enzymes dependent on these electrons. In this article, we will first describe the current knowledge on the structure and function of the various Fd isoforms present in chloroplasts of higher plants and then discuss the processes involved in oxidation of Fd, introducing the corresponding enzymes and discussing what is known about their relative interaction with Fd.

Nitric oxide deficiency accelerates chlorophyll breakdown and stability loss of thylakoid membranes during dark-induced leaf senescence in Arabidopsis

Nitric oxide (NO) has been known to preserve the level of chlorophyll (Chl) during leaf senescence. However, the mechanism by which NO regulates Chl breakdown remains unknown. Here we report that NO negatively regulates the activities of Chl catabolic enzymes during dark-induced leaf senescence. The transcriptional levels of the major enzyme genes involving Chl breakdown pathway except for RED CHL CATABOLITE REDUCTASE (RCCR) were dramatically up-regulated during dark-induced Chl degradation in the leaves of Arabidopsis NO-deficient mutant nos1/noa1 that exhibited an early-senescence phenotype. The activity of pheide a oxygenase (PAO) was higher in the dark-induced senescent leaves of nos1/noa1 compared with wild type. Furthermore, the knockout of PAO in nos1/noa1 background led to pheide a accumulation in the double mutant pao1 nos1/noa1, which retained the level of Chl during dark-induced leaf senescence. The accumulated pheide a in darkened leaves of pao1 nos1/noa1 was likely to inhibit the senescence-activated transcriptional levels of Chl catabolic genes as a feed-back inhibitory effect. We also found that NO deficiency led to decrease in the stability of photosynthetic complexes in thylakoid membranes. Importantly, the accumulation of pheide a caused by PAO mutations in combination with NO deficiency had a synergistic effect on the stability loss of thylakoid membrane complexes in the double mutant pao1 nos1/noa1 during dark-induced leaf senescence. Taken together, our findings have demonstrated that NO is a novel negative regulator of Chl catabolic pathway and positively functions in maintaining the stability of thylakoid membranes during leaf senescence.

Chlorophyll catabolites in senescent leaves of the lime tree (Tilia cordata)

In cold extracts of senescent leaves of the Lime tree (Tilia cordata), two colorless nonfluorescent chlorophyll catabolites (NCCs) were identified, named Tc-NCC-1 and Tc-NCC-2, as well as a polar yellow chlorophyll catabolite (YCC), named Tc-YCC. The constitution of the two NCCs was determined by spectroscopic means. In addition, a tentative structure was derived for Tc-YCC. The three chlorophyll degradation products exhibited tetrapyrrolic structures, as are typical of NCCs or YCCs, and turned out to be rather polar, due to a glucopyranosyl group at their 8(2)-position. At their 3-positions, the more polar Tc-NCC-1 carried a 1,2-dihydroxyethyl group and the less polar Tc-NCC-2 a vinyl group. Tc-YCC was identified as the product of an oxidation of Tc-NCC-1.

Accelerated cell death 2 suppresses mitochondrial oxidative bursts and modulates cell death in Arabidopsis

The Arabidopsis ACCELERATED CELL DEATH 2 (ACD2) protein protects cells from programmed cell death (PCD) caused by endogenous porphyrin-related molecules like red chlorophyll catabolite or exogenous protoporphyrin IX. We previously found that during bacterial infection, ACD2, a chlorophyll breakdown enzyme, localizes to both chloroplasts and mitochondria in leaves. Additionally, acd2 cells show mitochondrial dysfunction. In plants with acd2 and ACD2 (+) sectors, ACD2 functions cell autonomously, implicating a pro-death ACD2 substrate as being cell non-autonomous in promoting the spread of PCD. ACD2 targeted solely to mitochondria can reduce the accumulation of an ACD2 substrate that originates in chloroplasts, indicating that ACD2 substrate molecules are likely to be mobile within cells. Two different light-dependent reactive oxygen bursts in mitochondria play prominent and causal roles in the acd2 PCD phenotype. Finally, ACD2 can complement acd2 when targeted to mitochondria or chloroplasts, respectively, as long as it is catalytically active: the ability to bind substrate is not sufficient for ACD2 to function in vitro or in vivo. Together, the data suggest that ACD2 localizes dynamically during infection to protect cells from pro-death mobile substrate molecules, some of which may originate in chloroplasts, but have major effects on mitochondria.

Correlation of leaf senescence and gene expression/activities of chlorophyll degradation enzymes in harvested Chinese flowering cabbage (Brassica rapa var. parachinensis)

Chinese flowering cabbage is one of the main leafy vegetables produced in China. They have a rapid leaf yellowing due to chlorophyll degradation after harvest that limits their marketing. In the present study, leaf senescence of the cabbages was manipulated by ethylene and 6-benzyl aminopurine (6-BA) treatment to investigate the correlation of leaf senescence and chlorophyll degradation related to gene expression/activities in the darkness. The patterns of several senescence associated markers, including a typical marker, the expression of senescence-associated gene SAG(12), demonstrated that ethylene accelerated leaf senescence of the cabbages, while 6-BA retarded this progress. Similar to the trends of BrSAG(12) gene expression, strong activation in the expression of three chlorophyll degradation related genes, pheophytinase (BrPPH), pheophorbide a oxygenase (BrPAO) and red chlorophyll catabolite reductase (BrRCCR), was detected in ethylene treated and control leaves during the incubation, while no evident increase was recorded in 6-BA treated leaves. The overall dynamics of Mg-dechelatase activities in all treatments displayed increasing trends during the senescence process, and a delayed increase in the activities was observed for 6-BA treated leaves. However, chlorophyllase activity as well as the expression of BrChlase1 and BrChlase2 decreased with the incubation in all treatments. Taken together, the expression of BrPPH, BrPAO and BrRCCR, and the activity of Mg-dechelatase was closely associated with the chlorophyll degradation during the leaf senescence process in harvested Chinese flowering cabbages under dark conditions.

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

Arabidopsis cell death in compatible and incompatible interactions with Alternaria brassicicola

Two strains of necrotrophic Alternaria brassicicola, Ab40857 and Ab42464, are virulent on Korean cabbage and several wild types of Arabidopsis thaliana. Interaction between Ab42464 and Col-0 was compatible, whereas interaction between Ab40857 and Col-0 was incompatible. The loss of defense, no death (dnd) 1 function abrogated the compatibility between Ab42464 and Col-0, and the accelerated cell death (acd) 2 mutation attenuated the Col-0's resistance against Ab40857. These two fungal strains induced PR1 transcription in Col-0. Ab40857 accelerated transcription of PDF1.2, THI2.1, CAT, and POX by 12 h compared to those challenged with Ab42464. More abundant cell death was observed in Col-0 infected with Ab42464, however, callose deposition was evident in the incompatible interaction. Remarkably, Ab40857-infected areas of acd2-2 underwent rampant cell death and Ab42464 triggered callose production in dnd1-1. Furthermore, the incompatibility between Ab40857 and Col-0 was nullified by the coronatine-insensitive 1 (coi1) and phytoalexin-deficient 3 (pad3) mutations but not by nonexpresser of PR genes (npr1) and pad4. Ab40857 induced abundant cell death in pad3. Taken together, cell death during the early infection stage is a key determinant that discriminates between a compatible interaction and an incompatible one, and the resistance within Col-0 against Ab40857 is dependent on a defense-signaling pathway mediated by jasmonic acid and PAD3.

Chlorophyllase versus pheophytinase as candidates for chlorophyll dephytilation during senescence of broccoli

Degradation of chlorophylls during senescence is a highly regulated process which requires the concerted action of several enzymes. Traditionally, it has been stated that the dismantling process of the chlorophyll molecule begins with a dephytilation step, followed by Mg(2+) removal and other breakdown reactions. Recently, new evidence suggests the possibility of a rearrangement in the first two steps of this process, occurring Mg(2+) removal prior to the loss of the phytol side chain. With the purpose of approximating to the real sequential order of these reactions and to assess if dephytilation occurs on intact (catalyzed by chlorophyllase) or Mg-free (catalyzed by pheophytinase) chlorophyll, expression of both genes was analyzed in broccoli tissue during senescence. Samples of broccoli florets treated with plant hormones, such as cytokinin and ethylene were utilized, as to assess the effect of such compounds on the expression of these genes. Results showed that chlorophyllase expression did not correlate to typical expression patterns for genes related to senescence, since a decrease in expression during senescence was found for one of the two chlorophyllase genes analyzed, and the hormonal-treatment effects on gene expression did not match those observed on chlorophyll content for both chlorophyllase genes. Pheophytinase expression patterns, on the other hand, displayed an increase in the first 3 days of induced senescence, followed by lower expression values towards the end of the experiment. Samples subjected to postharvest treatments mostly showed an inhibition of pheophytinase expression, especially in samples in which degradation of chlorophylls had been delayed. These results suggest that pheophytinase expression correlates to the visual manifestation of postharvest treatments, supporting the possibility that this enzyme is responsible for the dephytilation step in chlorophyll breakdown.

Proteome analysis of the Albugo candida-Brassica juncea pathosystem reveals that the timing of the expression of defence-related genes is a crucial determinant of pathogenesis

White rust, caused by Albugo candida, is a serious pathogen of Brassica juncea (Indian mustard) and poses a potential hazard to the presently developing canola-quality B. juncea industry worldwide. A comparative proteomic study was undertaken to explore the molecular mechanisms that underlie the defence responses of Brassica juncea to white rust disease caused by the biotrophic oomycete Albugo candida. Nineteen proteins showed reproducible differences in abundance between a susceptible (RH 819) and a resistant variety (CBJ 001) of B. juncea following inoculation with A. candida. The identities of all 19 proteins were successfully established through Q-TOF MS/MS. Five of these proteins were only detected in the resistant variety and showed significant differences in their abundance at various times following pathogen inoculation in comparison to mock-inoculated plants. Among these was a thaumatin-like protein (PR-5), a protein not previously associated with the resistance of B. juncea towards A. candida. One protein, peptidyl-prolyl cis/trans isomerase (PPIase) isoform CYP20-3, was only detected in the susceptible variety and increased in abundance in response to the pathogen. PPIases have recently been discovered to play an important role in pathogenesis by suppressing the host cell's immune response. For a subset of seven proteins examined in more detail, an increase in transcript abundance always preceded their induction at the proteome level. These findings are discussed within the context of the A. candida-Brassica juncea pathosystem, especially in relation to host resistance to this pathogen.

Chlorophyll breakdown in higher plants

Chlorophyll breakdown is an important catabolic process of leaf senescence and fruit ripening. Structure elucidation of colorless linear tetrapyrroles as (final) breakdown products of chlorophyll was crucial for the recent delineation of a chlorophyll breakdown pathway which is highly conserved in land plants. Pheophorbide a oxygenase is the key enzyme responsible for opening of the chlorin macrocycle of pheophorbide a characteristic to all further breakdown products. Degradation of chlorophyll was rationalized by the need of a senescing cell to detoxify the potentially phototoxic pigment, yet recent investigations in leaves and fruits indicate that chlorophyll catabolites could have physiological roles. This review updates structural information of chlorophyll catabolites and the biochemical reactions involved in their formation, and discusses the significance of chlorophyll breakdown. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Accumulation of chlorophyll catabolites photosensitizes the hypersensitive response elicited by Pseudomonas syringae in Arabidopsis

• The staygreen (SGR) gene encodes a chloroplast-targeted protein which promotes chlorophyll degradation via disruption of light-harvesting complexes (LHCs). • Over-expression of SGR in Arabidopsis (SGR-OX) in a Columbia-0 (Col-0) background caused spontaneous necrotic flecking. To relate this to the hypersensitive response (HR), Col-0, SGR-OX and RNAi SGR (SGRi) lines were challenged with Pseudomonas syringae pv tomato (Pst) encoding the avirulence gene avrRpm1. Increased and decreased SGR expression, respectively, accelerated and suppressed the kinetics of HR-cell death. In Col-0, SGR transcript increased at 6  h after inoculation (hai) when tissue electrolyte leakage indicated the initiation of cell death. • Excitation of the chlorophyll catabolite pheophorbide (Pheide) leads to the formation of toxic singlet oxygen ((1)O(2)). Pheide was first detected at 6  hai with Pst avrRpm1 and was linked to (1)O(2) generation and correlated with reduced Pheide a oxygenase (PaO) protein concentrations. The maximum quantum efficiency of photosystem II (F(v)/F(m)), quantum yield of electron transfer at photosystem II (φPSII), and photochemical quenching (qP) decreased at 6  hai in Col-0 but not in SGRi. Disruption of photosynthetic electron flow will cause light-dependent H(2)O(2) generation at 6  hai. • We conclude that disruption of LHCs, possibly influenced by SGR, and absence of PaO produce phototoxic chlorophyll catabolites and oxidative stress leading to the HR.