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

RsOBP2a, a member of OBF BINDING PROTEIN transcription factors, inhibits two chlorophyll degradation genes in green radish

The green radish (Raphanus sativus L.) contains abundant chlorophyll (Chl). DOF-type transcription factor OBF BINDING PROTEIN (OBP) plays crucial functions in plant growth, development, maturation and responses to various abiotic stresses. However, the metabolism by which OBP transcription factors regulate light-induced Chl metabolism in green radish is not well understood. In this study, six OBP genes were identified from the radish genome, distributed unevenly across five chromosomes. Among these genes, RsOBP2a showed significantly higher expression in the green flesh compared to the white flesh of green radish. Analysis of promoter elements suggested that RsOBPs might be involved in stress responses, particularly in light-related processes. Overexpression of RsOBP2a led to increase Chl levels in cotyledons and adventitious roots of radish, while silencing RsOBP2a expression through TYMV-induced gene silencing accelerated leaf senescence. Further investigations revealed that RsOBP2a was localized in the nucleus and served as a transcriptional repressor. RsOBP2a was induced by light and directly suppressed the expression of STAYGREEN (SGR) and RED CHLOROPHYLL CATABOLITE REDUCTASE (RCCR), thereby delaying senescence in radish. Overall, a novel regulatory model involving RsOBP2a, RsSGR, and RsRCCR was proposed to govern Chl metabolism in response to light, offering insights for the enhancement of green radish germplasm.

Alternaria TeA toxin activates a chloroplast retrograde signaling pathway to facilitate JA-dependent pathogenicity

The chloroplast is a critical battleground in the arms race between plants and pathogens. Among microbe-secreted mycotoxins, tenuazonic acid (TeA), produced by the genus Alternaria and other phytopathogenic fungi, inhibits photosynthesis, leading to a burst of photosynthetic singlet oxygen (1O2) that is implicated in damage and chloroplast-to-nucleus retrograde signaling. Despite the significant crop damage caused by Alternaria pathogens, our understanding of the molecular mechanism by which TeA promotes pathogenicity and cognate plant defense responses remains fragmentary. We now reveal that A. alternata induces necrotrophic foliar lesions by harnessing EXECUTER1 (EX1)/EX2-mediated chloroplast-to-nucleus retrograde signaling activated by TeA toxin-derived photosynthetic 1O2 in Arabidopsis thaliana. Mutation of the 1O2-sensitive EX1-W643 residue or complete deletion of the EX1 singlet oxygen sensor domain compromises expression of 1O2-responsive nuclear genes and foliar lesions. We also found that TeA toxin rapidly induces nuclear genes implicated in jasmonic acid (JA) synthesis and signaling, and EX1-mediated retrograde signaling appears to be critical for establishing a signaling cascade from 1O2 to JA. The present study sheds new light on the foliar pathogenicity of A. alternata, during which EX1-dependent 1O2 signaling induces JA-dependent foliar cell death.

Natural variation in STAYGREEN contributes to low-temperature tolerance in cucumber

Low-temperature (LT) stress threatens cucumber production globally; however, the molecular mechanisms underlying LT tolerance in cucumber remain largely unknown. Here, using a genome-wide association study (GWAS), we found a naturally occurring single nucleotide polymorphism (SNP) in the STAYGREEN (CsSGR) coding region at the gLTT5.1 locus associated with LT tolerance. Knockout mutants of CsSGR generated by clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated nuclease 9 exhibit enhanced LT tolerance, in particularly, increased chlorophyll (Chl) content and reduced reactive oxygen species (ROS) accumulation in response to LT. Moreover, the C-repeat Binding Factor 1 (CsCBF1) transcription factor can directly activate the expression of CsSGR. We demonstrate that the LT-sensitive haplotype CsSGRHapA , but not the LT-tolerant haplotype CsSGRHapG could interact with NON-YELLOW COLORING 1 (CsNYC1) to mediate Chl degradation. Geographic distribution of the CsSGR haplotypes indicated that the CsSGRHapG was selected in cucumber accessions from high latitudes, potentially contributing to LT tolerance during cucumber cold-adaptation in these regions. CsSGR mutants also showed enhanced tolerance to salinity, water deficit, and Pseudoperonospora cubensis, thus CsSGR is an elite target gene for breeding cucumber varieties with broad-spectrum stress tolerance. Collectively, our findings provide new insights into LT tolerance and will ultimately facilitate cucumber molecular breeding.

The transcription factor BnaA9.WRKY47 coordinates leaf senescence and nitrogen remobilization in Brassica napus

Nitrogen (N) is an essential macronutrient for plants, and its remobilization is key for adaptation to deficiency stress. However, there is limited understanding of the regulatory mechanisms of N remobilization in the important crop species Brassica napus (oilseed rape). Here, we report the identification of a transcription factor, BnaA9.WRKY47, that is induced by N starvation in a canola variety. At the seedling stage, BnaA9.WRKY47-overexpressing (OE) lines displayed earlier senescence of older leaves and preferential growth of juvenile leaves compared to the wild type under N starvation. At the field scale, the seed yield was significantly increased in the BnaA9.WRKY47-OE lines compared with the wild type when grown under N deficiency conditions and, conversely, it was reduced in BnaA9.WRKY47-knockout mutants. Biochemical analyses demonstrated that BnaA9.WRKY47 directly activates BnaC7.SGR1 to accelerate senescence of older leaves. In line with leaf senescence, the concentration of amino acids in the older leaves of the OE lines was elevated, and the proportion of plant N that they contained was reduced. This was associated with BnaA9.WRKY47 activating the amino acid permease BnaA9.AAP1 and the nitrate transporter BnaA2.NRT1.7. Thus, the expression of BnaA9.WRKY47 efficiently facilitated N remobilization from older to younger leaves or to seeds. Taken together, our results demonstrate that BnaA9.WRKY47 up-regulates the expression of BnaC7.SGR1, BnaA2.NRT1.7, and BnaA9AAP1, thus promoting the remobilization of N in B. napus under starvation conditions.

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.

Underlying mechanism of accelerated cell death and multiple disease resistance in a maize lethal leaf spot 1 allele

Multiple disease resistance (MDR) in maize has attracted increasing attention. However, the interplay between cell death and metabolite changes and their contributions to MDR remains elusive in maize. In this study, we identified a mutant named as lesion mimic 30 (les30) that showed 'suicidal' lesion formation in the absence of disease and had enhanced resistance to the fungal pathogen Curvularia lunata. Using map-based cloning, we identified the causal gene encoding pheophorbide a oxidase (PAO), which is known to be involved in chlorophyll degradation and MDR, and is encoded by LETHAL LEAF SPOT1 (LLS1). LLS1 was found to be induced by both biotic and abiotic stresses. Transcriptomics analysis showed that genes involved in defense responses and secondary metabolite biosynthesis were mildly activated in leaves of the les30 mutant without lesions, whilst they were strongly activated in leaves with lesions. In addition, in les30 leaves with lesions, there was overaccumulation of defense-associated phytohormones including jasmonic acid and salicylic acid, and of phytoalexins including phenylpropanoids, lignin, and flavonoids, suggesting that their biosynthesis was activated in a lesion-dependent manner. Taken together, our study implies the existence of an interactive amplification loop of interrupted chlorophyll degradation, cell death, expression of defense-related genes, and metabolite changes that results in suicidal lesion formation and MDR, and this has the potential to be exploited by genetic manipulation to improve maize disease resistance.

A Novel Algicidal Bacterium and Its Effects against the Toxic Dinoflagellate Karenia mikimotoi (Dinophyceae)

The toxic dinoflagellate Karenia mikimotoi is a harmful algal bloom-forming species in coastal areas around the world. It produces ichthyotoxins and hemolytic toxins, with deleterious effects on marine ecosystems. In this study, the bacterium Pseudoalteromonas sp. FDHY-MZ2, with high algicidal efficiency against K. mikimotoi, was isolated from a bloom event. Based on the results, it completely lysed K. mikimotoi cells within 24 h 0.5% (vol/vol), with the algicidal activity of the supernatant of the bacterium culture. Algal cell wall fragmentation occurred, leading to cell death. There was a marked decline in various photochemical traits. When treated with the supernatant, cellulase, pheophorbide a oxygenase (PAO) and cyclin B genes were significantly increased, suggesting induced cell wall deterioration, chloroplast degradation and cell cycle regulation of K. mikimotoi cells. In addition, the expression levels of reactive oxygen species (ROS) scavenging gene was significantly inhibited, indicating that the ROS removal system was damaged. The bacterial culture was dried to obtain the spray-dried powder, which showed algicidal activity rates of 92.2 and 100% against a laboratory K. mikimotoi culture and a field microcosm of Karlodinium sp. bloom within 24 h with the addition of 0.04% mass fraction powder. Our results demonstrate that FDHY-MZ2 is a suitable strain for K. mikimotoi and Karlodinium sp. blooms management. In addition, this study provides a new strategy for the anthropogenic control of harmful algal bloom-forming species in situ. IMPORTANCE K. mikimotoi is a noxious algal bloom-forming species that cause damaging of the aquaculture industry and great financial losses. Bacterium with algicidal activity is an ideal agency to inhibit the growth of harmful algae. In this approach application, the bacterium with high algicidal activity is required and the final management material is ideal for easy-to-use. The algicidal characteristics are also needed to understand the effects of the bacterium for managing strategy exploration. In this study, we isolated a novel algicidal bacterium with extremely high lysis efficiency for K. mikimotoi. The algicidal characteristics of the bacterium as well as the chemical and molecular response of K. mikimotoi with the strain challenge were examined. Finally, the algicidal powder was explored for application. The results demonstrate that FDHY-MZ2 is suitable for K. mikimotoi and Karlodinium sp. blooms controlling, and this study provides a new strategy for algicidal bacterium application.

Suppressing chlorophyll degradation by silencing OsNYC3 improves rice resistance to Rhizoctonia solani, the causal agent of sheath blight

Necrotrophic fungus Rhizoctonia solani Kühn (R. solani) causes serious diseases in many crops worldwide, including rice and maize sheath blight (ShB). Crop resistance to the fungus is a quantitative trait and resistance mechanism remains largely unknown, severely hindering the progress on developing resistant varieties. In this study, we found that resistant variety YSBR1 has apparently stronger ability to suppress the expansion of R. solani than susceptible Lemont in both field and growth chamber conditions. Comparison of transcriptomic profiles shows that the photosynthetic system including chlorophyll biosynthesis is highly suppressed by R. solani in Lemont but weakly in YSBR1. YSBR1 shows higher chlorophyll content than that of Lemont, and inducing chlorophyll degradation by dark treatment significantly reduces its resistance. Furthermore, three rice mutants and one maize mutant that carry impaired chlorophyll biosynthesis all display enhanced susceptibility to R. solani. Overexpression of OsNYC3, a chlorophyll degradation gene apparently induced expression by R. solani infection, significantly enhanced ShB susceptibility in a high-yield ShB-susceptible variety '9522'. However, silencing its transcription apparently improves ShB resistance without compromising agronomic traits or yield in field tests. Interestingly, altering chlorophyll content does not affect rice resistance to blight and blast diseases, caused by biotrophic and hemi-biotrophic pathogens, respectively. Our study reveals that chlorophyll plays an important role in ShB resistance and suppressing chlorophyll degradation induced by R. solani infection apparently improves rice ShB resistance. This discovery provides a novel target for developing resistant crop to necrotrophic fungus R. solani.

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 Emerging Role for Chloroplasts in Disease and Defense

Chloroplasts are key players in plant immune signaling, contributing to not only de novo synthesis of defensive phytohormones but also the generation of reactive oxygen and nitrogen species following activation of pattern recognition receptors or resistance (R) proteins. The local hypersensitive response (HR) elicited by R proteins is underpinned by chloroplast-generated reactive oxygen species. HR-induced lipid peroxidation generates important chloroplast-derived signaling lipids essential to the establishment of systemic immunity. As a consequence of this pivotal role in immunity, pathogens deploy effector complements that directly or indirectly target chloroplasts to attenuate chloroplast immunity (CI). Our review summarizes the current knowledge of CI signaling and highlights common pathogen chloroplast targets and virulence strategies. We address emerging insights into chloroplast retrograde signaling in immune responses and gaps in our knowledge, including the importance of understanding chloroplast heterogeneity and chloroplast involvement in intraorganellular interactions in host immunity.

The miR528-AO Module Confers Enhanced Salt Tolerance in Rice by Modulating the Ascorbic Acid and Abscisic Acid Metabolism and ROS Scavenging

The monocot lineage-specific miR528 was previously established as a multistress regulator. However, it remains largely unclear how miR528 participates in response to salinity stress in rice. Here, we show that miR528 positively regulates rice salt tolerance by down-regulating a gene encoding l-ascorbate oxidase (AO), thereby bolstering up the AO-mediated abscisic acid (ABA) synthesis and ROS scavenging. Overexpression of miR528 caused a substantial increase in ascorbic acid (AsA) and ABA contents but a significant reduction in ROS accumulation, resulting in the enhanced salt tolerance of rice plants. Conversely, knockdown of miR528 or overexpression of AO stimulated the expression of the AO gene, hence lowering the level of AsA, a critical antioxidant that promotes the ABA content but reduces the ROS level, and then compromising rice tolerance to salinity. Together, the findings reveal a novel mechanism of the miR528-AO module-mediated salt tolerance by modulating the processes of AsA and ABA metabolism as well as ROS detoxification, which adds a new regulatory role to the miR528-AO stress defense pathway in rice.

Comparative anatomical and transcriptomic analyses of the color variation of leaves in Aquilaria sinensis

Color variation in plant tissues is a common phenomenon accompanied with a series of biological changes. In this study, a special-phenotype Aquilaria sinensis (GS) with color variation of leaf was firstly reported, and DNA barcode sequences showed GS samples could not be discriminated clearly with the normal A. sinensis sample (NS), which suggested that the variety was not the cause of the GS formation. To reveal the characteristics of GS compared to NS, the anatomical and transcriptome sequencing studies were carried out. In microscopic observation, the leaves of golden-vein-leaf sample (LGS) and normal-vein-leaf sample (LNS) showed significant differences including the area of the included phloem in midrib and the thickness parameters of palisade and spongy tissues; the stems of golden-vein-leaf sample (SGS) and normal-vein-leaf sample (SNS) were also different in many aspects such as the area of vessels and included phloem. In addition, the structure of chloroplast was more complete in the midrib of LNS than that of LGS, and some particles suspected as virus were found through transmission electron microscope as well. Genes upregulated in LGS in contrast with LNS were mainly enriched in photosynthesis. As for stems, most of the genes upregulated in SGS compared to SNS were involved in translation and metabolism processes. The pathways about photosynthesis and chlorophyll metabolism as well as some important transcription factors may explain the molecular mechanism of the unique phenotypes of leaves and the genes related to suberin biosynthesis may result in the difference of stems. In addition, the genes about defense response especially biotic stress associated with numerous pathogenesis-related (PR) genes upregulated in LGS compared to LNS indicated that the pathogen may be the internal factor. Taken together, our results reveal the macro- and micro-phenotype variations as well as gene expression profiles between GS and NS, which could provide valuable clues for elucidating the mechanism of the color variation of Aquilaria.

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.

Leaf senescence: progression, regulation, and application

Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants' fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds of senescence-associated genes (SAGs) via reverse genetic research in model plant Arabidopsis thaliana as well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

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.

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

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

Chloroplast immunity illuminated

The chloroplast has recently emerged as pivotal to co-ordinating plant defence responses and as a target of plant pathogens. Beyond its central position in oxygenic photosynthesis and primary metabolism - key targets in the complex virulence strategies of diverse pathogens - the chloroplast integrates, decodes and responds to environmental signals. The capacity of chloroplasts to synthesize phytohormones and a diverse range of secondary metabolites, combined with retrograde and reactive oxygen signalling, provides exquisite flexibility to both perceive and respond to biotic stresses. These processes also represent a plethora of opportunities for pathogens to evolve strategies to directly or indirectly target 'chloroplast immunity'. This review covers the contribution of the chloroplast to pathogen associated molecular pattern and effector triggered immunity as well as systemic acquired immunity. We address phytohormone modulation of immunity and surmise how chloroplast-derived reactive oxygen species underpin chloroplast immunity through indirect evidence inferred from genetic modification of core chloroplast components and direct pathogen targeting of the chloroplast. We assess the impact of transcriptional reprogramming of nuclear-encoded chloroplast genes during disease and defence and look at future research challenges.

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

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

Phyllobilins from Senescence-Associated Chlorophyll Breakdown in the Leaves of Basil (Ocimum basilicum) Show Increased Abundance upon Herbivore Attack

In view of the common use of the herb basil (Ocimum basilicum) in nutrition and in phytomedicine, the contents of its leaves are of obvious interest. In extracts of fresh yellowish-green basil leaves, phyllobilins (PBs), which are bilin-type catabolites of chlorophyll (Chl), were detected using high-performance liquid chromatography (HPLC). Two such PBs, provisionally named Ob-nonfluorescent chlorophyll catabolite (NCC)-40 and Ob-YCC-45, exhibited previously unknown structures that were delineated by a thorough spectroscopic characterization. When basil leaves were infested with aphids or thrips or underwent fungal infections, areas with chlorosis were observed. HPLC analyses of the infested parts of leaves compared to those of the healthy parts showed a significant accumulation of PBs in the infested areas, demonstrating that the senescence-associated pheophorbide a oxygenase/phyllobilin (PAO/PB) pathway is activated by herbivore feeding and fungal infection.

Organized Disassembly of Photosynthesis During Programmed Cell Death Mediated By Long Chain Bases

In plants, pathogen triggered programmed cell death (PCD) is frequently mediated by polar lipid molecules referred as long chain bases (LCBs) or ceramides. PCD interceded by LCBs is a well-organized process where several cell organelles play important roles. In fact, light-dependent reactions in the chloroplast have been proposed as major players during PCD, however, the functional aspects of the chloroplast during PCD are largely unknown. For this reason, we investigated events that lead to disassembly of the chloroplast during PCD mediated by LCBs. To do so, LCB elevation was induced with Pseudomonas syringae pv. tomato (a non-host pathogen) or Fumonisin B1 in Phaseolus vulgaris. Then, we performed biochemical tests to detect PCD triggering events (phytosphingosine rises, MPK activation and H2O2 generation) followed by chloroplast structural and functional tests. Observations of the chloroplast, via optical phenotyping methods combined with microscopy, indicated that the loss of photosynthetic linear electron transport coincides with the organized ultrastructure disassembly. In addition, structural changes occurred in parallel with accumulation of H2O2 inside the chloroplast. These features revealed the collapse of chloroplast integrity and function as a mechanism leading to the irreversible execution of the PCD promoted by LCBs.

Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles

Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.

Oxygen and ROS in Photosynthesis

Oxygen is a natural acceptor of electrons in the respiratory pathway of aerobic organisms and in many other biochemical reactions. Aerobic metabolism is always associated with the formation of reactive oxygen species (ROS). ROS may damage biomolecules but are also involved in regulatory functions of photosynthetic organisms. This review presents the main properties of ROS, the formation of ROS in the photosynthetic electron transport chain and in the stroma of chloroplasts, and ROS scavenging systems of thylakoid membrane and stroma. Effects of ROS on the photosynthetic apparatus and their roles in redox signaling are discussed.

Leaf Senescence: The Chloroplast Connection Comes of Age

Leaf senescence is a developmental process critical for plant fitness, which involves genetically controlled cell death and ordered disassembly of macromolecules for reallocating nutrients to juvenile and reproductive organs. While natural leaf senescence is primarily associated with aging, it can also be induced by environmental and nutritional inputs including biotic and abiotic stresses, darkness, phytohormones and oxidants. Reactive oxygen species (ROS) are a common thread in stress-dependent cell death and also increase during leaf senescence. Involvement of chloroplast redox chemistry (including ROS propagation) in modulating cell death is well supported, with photosynthesis playing a crucial role in providing redox-based signals to this process. While chloroplast contribution to senescence received less attention, recent findings indicate that changes in the redox poise of these organelles strongly affect senescence timing and progress. In this review, the involvement of chloroplasts in leaf senescence execution is critically assessed in relation to available evidence and the role played by environmental and developmental cues such as stress and phytohormones. The collected results indicate that chloroplasts could cooperate with other redox sources (e.g., mitochondria) and signaling molecules to initiate the committed steps of leaf senescence for a best use of the recycled nutrients in plant reproduction.

Functional Dissection of the Chickpea (Cicer arietinum L.) Stay-Green Phenotype Associated with Molecular Variation at an Ortholog of Mendel's I Gene for Cotyledon Color: Implications for Crop Production and Carotenoid Biofortification

"Stay-green" crop phenotypes have been shown to impact drought tolerance and nutritional content of several crops. We aimed to genetically describe and functionally dissect the particular stay-green phenomenon found in chickpeas with a green cotyledon color of mature dry seed and investigate its potential use for improvement of chickpea environmental adaptations and nutritional value. We examined 40 stay-green accessions and a set of 29 BC2F4-5 stay-green introgression lines using a stay-green donor parent ICC 16340 and two Indian elite cultivars (KAK2, JGK1) as recurrent parents. Genetic studies of segregating populations indicated that the green cotyledon trait is controlled by a single recessive gene that is invariantly associated with the delayed degreening (extended chlorophyll retention). We found that the chickpea ortholog of Mendel's I locus of garden pea, encoding a SGR protein as very likely to underlie the persistently green cotyledon color phenotype of chickpea. Further sequence characterization of this chickpea ortholog CaStGR1 (CaStGR1, for carietinum stay-green gene 1) revealed the presence of five different molecular variants (alleles), each of which is likely a loss-of-function of the chickpea protein (CaStGR1) involved in chlorophyll catabolism. We tested the wild type and green cotyledon lines for components of adaptations to dry environments and traits linked to agronomic performance in different experimental systems and different levels of water availability. We found that the plant processes linked to disrupted CaStGR1 gene did not functionality affect transpiration efficiency or water usage. Photosynthetic pigments in grains, including provitaminogenic carotenoids important for human nutrition, were 2-3-fold higher in the stay-green type. Agronomic performance did not appear to be correlated with the presence/absence of the stay-green allele. We conclude that allelic variation in chickpea CaStGR1 does not compromise traits linked to environmental adaptation and agronomic performance, and is a promising genetic technology for biofortification of provitaminogenic carotenoids in chickpea.

The C-terminal cysteine-rich motif of NYE1/SGR1 is indispensable for its function in chlorophyll degradation in Arabidopsis

The C-terminal cysteine-rich motif of NYE1/SGR1 affects chlorophyll degradation likely by mediating its self-interaction and conformational change, and somehow altering its Mg-dechelating activity in response to the changing redox potential. During green organ senescence in plants, the most prominent phenomenon is the degreening caused by net chlorophyll (Chl) loss. NON-YELLOWING1/STAY-GREEN1 (NYE1/SGR1) was recently reported to be able to dechelates magnesium (Mg) from Chl a to initiate its degradation, but little is known about the domain/motif basis of its functionality. In this study, we carried out a protein truncation assay and identified a conserved cysteine-rich motif (CRM, P-X3-C-X3-C-X-C2-F-P-X5-P) at its C terminus, which is essential for its function. Genetic analysis showed that all four cysteines in the CRM were irreplaceable, and enzymatic assays demonstrated that the mutation of each of the four cysteines affected its Mg-dechelating activity. The CRM plays a critical role in the conformational change and self-interaction of NYE1 via the formation of inter- and intra-molecular disulfide bonds. Our results may provide insight into how NYE1 responds to rapid redox changes during leaf senescence and in response to various environmental stresses.

Jasmonate production through chlorophyll a degradation by Stay-Green in Arabidopsis thaliana

Leaf color change through chlorophyll degradation is a characteristic symptom of senescence. Magnesium removal from chlorophyll a is the initial step in chlorophyll a degradation, in a reaction catalyzed by Stay-Green (SGR). Arabidopsis thaliana has three SGR homologs, SGR1, SGR2, and SGR-like. When SGR1 is overexpressed, both chlorophyll a and b are degraded and leaves turn yellow. This process is visually identical to senescence, suggesting that SGR1 overexpression affects various physiological processes in plants. To examine this possibility, gene expression associated with chlorophyll metabolism and senescence was analyzed following dexamethasone-inducible SGR1 introduction into Arabidopsis. When SGR1 was overexpressed following 18 h of dexamethasone treatment, genes involved in chlorophyll degradation were upregulated, as were senescence-associated genes. These observations suggested that chlorophyll a degradation promotes senescence. As jasmonate is the plant hormone responsible for senescence and was expected to be involved in the regulation of gene expression after dexamethasone treatment, the level of jasmonoyl-isoleucine, the active form of jasmonate, was measured. The jasmonoyl-isoleucine level increased slightly after 10 h of SGR1 overexpression, and this increase became significant after 18 h. These observations suggest that jasmonate is produced through chlorophyll a degradation and affects the promotion of senescence.

STAYGREEN, STAY HEALTHY: a loss-of-susceptibility mutation in the STAYGREEN gene provides durable, broad-spectrum disease resistances for over 50 years of US cucumber production

The Gy14 cucumber (Cucumis sativus) is resistant to oomyceteous downy mildew (DM), bacterial angular leaf spot (ALS) and fungal anthracnose (AR) pathogens, but the underlying molecular mechanisms are unknown. Quantitative trait locus (QTL) mapping for the disease resistances in Gy14 and further map-based cloning identified a candidate gene for the resistant loci, which was validated and functionally characterized by spatial-temporal gene expression profiling, allelic diversity and phylogenetic analysis, as well as local association studies. We showed that the triple-disease resistances in Gy14 were controlled by the cucumber STAYGREEN (CsSGR) gene. A single nucleotide polymorphism (SNP) in the coding region resulted in a nonsynonymous amino acid substitution in the CsSGR protein, and thus disease resistance. Genes in the chlorophyll degradation pathway showed differential expression between resistant and susceptible lines in response to pathogen inoculation. The causal SNP was significantly associated with disease resistances in natural and breeding populations. The resistance allele has undergone selection in cucumber breeding. The durable, broad-spectrum disease resistance is caused by a loss-of-susceptibility mutation of CsSGR. Probably, this is achieved through the inhibition of reactive oxygen species over-accumulation and phytotoxic catabolite over-buildup in the chlorophyll degradation pathway. The CsSGR-mediated host resistance represents a novel function of this highly conserved gene in plants.

Dose-dependent effects of 1O2 in chloroplasts are determined by its timing and localization of production

In plants, highly reactive singlet oxygen (1O2) is known to inhibit photosynthesis and to damage the cell as a cytotoxin. However, more recent studies have also proposed 1O2 as a signal. In plants under stress, not only 1O2 but also other reactive oxygen species (ROS) are generated simultaneously, thus making it difficult to link a particular response to the release of 1O2 and establish a signaling role for this ROS. This obstacle has been overcome by the identification of conditional mutants of Arabidopsis thaliana that selectively generate 1O2 and trigger various 1O2-mediated responses. In chloroplasts of these mutants, chlorophyll or its biosynthetic intermediates may act as a photosensitizer and generate 1O2. These 1O2-mediated responses are not only dependent on the dosage of 1O2 but also are determined by the timing and suborganellar localization of its production. This spatial- and temporal-dependent variability of 1O2-mediated responses emphasizes the importance of 1O2 as a highly versatile and short-lived signal that acts throughout the life cycle of a plant.

Genetic architecture of quantitative flower and leaf traits in a pair of sympatric sister species of Primulina

Flowers and leaves each represent suites of functionally interrelated traits that are often involved in species divergence and local adaptation. However, a major unresolved issue is how the individual component traits that make up a complex trait such as a flower evolve in a coordinated fashion to retain a high degree of functionality. We use a quantitative trait loci (QTL) approach to elucidate the genetic architecture of divergence in flower and leaf traits between the sister species Primulina depressa and Primulina danxiaensis, which grow sympatrically but in contrasting microhabitats. We found that flower traits were controlled by multiple QTL of small effect, while leaf physiological and morphological traits tended to be controlled by QTL of larger effect. The observed floral integration, manifested by a high degree overlap in both individual trait QTL and QTL for principal component scores (PCA QTL), may have been critical for evolutionary divergence of floral morphology in relation to their pollinators. This overlap suggests that direct selection on only one or a few of the component traits could have caused substantial divergence in other floral traits due to genetic correlations, while the low QTL overlap between floral and vegetative traits suggests that these trait suites are genetically unlinked and can evolve independently in response to different selective pressures corresponding to their distinct functions.

STAYGREEN (CsSGR) is a candidate for the anthracnose (Colletotrichum orbiculare) resistance locus cla in Gy14 cucumber

Map-based cloning identified a candidate gene for resistance to the anthracnose fungal pathogen Colletotrichum orbiculare in cucumber, which reveals a novel function for the highly conserved STAYGREEN family genes for host disease resistance in plants. Colletotrichum orbiculare is a hemibiotrophic fungal pathogen that causes anthracnose disease in cucumber and other cucurbit crops. No host resistance genes against the anthracnose pathogens have been cloned in crop plants. Here, we reported fine mapping and cloning of a resistance gene to the race 1 anthracnose pathogen in cucumber inbred lines Gy14 and WI 2757. Phenotypic and QTL analysis in multiple populations revealed that a single recessive gene, cla, was underlying anthracnose resistance in both lines, but WI2757 carried an additional minor-effect QTL. Fine mapping using 150 Gy14 × 9930 recombinant inbred lines and 1043 F₂ individuals delimited the cla locus into a 32 kb region in cucumber Chromosome 5 with three predicted genes. Multiple lines of evidence suggested that the cucumber STAYGREEN (CsSGR) gene is a candidate for the anthracnose resistance locus. A single nucleotide mutation in the third exon of CsSGR resulted in the substitution of Glutamine in 9930 to Arginine in Gy14 in CsSGR protein which seems responsible for the differential anthracnose inoculation responses between Gy14 and 9930. Quantitative real-time PCR analysis indicated that CsSGR was significantly upregulated upon anthracnose pathogen inoculation in the susceptible 9930, while its expression was much lower in the resistant Gy14. Investigation of allelic diversities in natural cucumber populations revealed that the resistance allele in almost all improved cultivars or breeding lines of the U.S. origin was derived from PI 197087. This work reveals an unknown function for the highly conserved STAYGREEN (SGR) family genes for host disease resistance in plants.

The major leaf ferredoxin Fd2 regulates plant innate immunity in Arabidopsis

Ferredoxins, the major distributors for electrons to various acceptor systems in plastids, contribute to redox regulation and antioxidant defence in plants. However, their function in plant immunity is not fully understood. In this study, we show that the expression of the major leaf ferredoxin gene Fd2 is suppressed by Pseudomonas syringae pv. tomato (Pst) DC3000 infection, and that knockout of Fd2 (Fd2-KO) in Arabidopsis increases the plant's susceptibility to both Pst DC3000 and Golovinomyces cichoracearum. On Pst DC3000 infection, the Fd2-KO mutant accumulates increased levels of jasmonic acid and displays compromised salicylic acid-related immune responses. Fd2-KO also shows defects in the accumulation of reactive oxygen species induced by pathogen-associated molecular pattern-triggered immunity. However, Fd2-KO shows enhanced R-protein-mediated resistance to Pst DC3000/AvrRpt2 infection, suggesting that Fd2 plays a negative role in effector-triggered immunity. Furthermore, Fd2 interacts with FIBRILLIN4 (FIB4), a harpin-binding protein localized in chloroplasts. Interestingly, Fd2, but not FIB4, localizes to stromules that extend from chloroplasts. Taken together, our results demonstrate that Fd2 plays an important role in plant immunity.

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.

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.

Loss in photosynthesis during senescence is accompanied by an increase in the activity of β-galactosidase in leaves of Arabidopsis thaliana: modulation of the enzyme activity by water stress

The precise nature of the developmental modulation of the activity of cell wall hydrolases that breakdown the wall polysaccharides to maintain cellular sugar homeostasis under sugar starvation environment still remains unclear. In this work, the activity of β-galactosidase (EC 3.2.1.23), a cell-wall-bound enzyme known to degrade the wall polysaccharides, has been demonstrated to remarkably enhance during senescence-induced loss in photosynthesis in Arabidopsis thaliana. The enhancement in the enzyme activity reaches a peak at the terminal phase of senescence when the rate of photosynthesis is at its minimum. Although the precise nature of chemistry of the interface between the decline in photosynthesis and enhancement in the activity of the enzyme could not be fully resolved, the enhancement in its activity in dark and its suppression in light or with exogenous sugars may indicate the involvement of loss of photosynthetic production of sugars as a key factor that initiates and stimulates the activity of the enzyme. The hydrolase possibly participates in the catabolic network of cell wall polysaccharides to produce sugars for execution of energy-dependant senescence program in the background of loss of photosynthesis. Drought stress experienced by the senescing leaves accelerates the decline in photosynthesis with further stimulation in the activity of the enzyme. The stress recovery of photosynthesis and suppression of the enzyme activity on withdrawal of stress support the proposition of photosynthetic modulation of the cell-wall-bound enzyme activity.

Pathogen-Induced Leaf Chlorosis: Products of Chlorophyll Breakdown Found in Degreened Leaves of Phytoplasma-Infected Apple (Malus × domestica Borkh.) and Apricot (Prunus armeniaca L.) Trees Relate to the Pheophorbide a Oxygenase/Phyllobilin Pathway

Phytoplasmoses such as apple proliferation (AP) and European stone fruit yellows (ESFY) cause severe economic losses in fruit production. A common symptom of both phytoplasma diseases is early yellowing or leaf chlorosis. Even though chlorosis is a well-studied symptom of biotic and abiotic stresses, its biochemical pathways are hardly known. In particular, in this context, a potential role of the senescence-related pheophorbide a oxygenase/phyllobilin (PaO/PB) pathway is elusive, which degrades chlorophyll (Chl) to phyllobilins (PBs), most notably to colorless nonfluorescent Chl catabolites (NCCs). In this work, we identified the Chl catabolites in extracts of healthy senescent apple and apricot leaves. In extracts of apple tree leaves, a total of 12 Chl catabolites were detected, and in extracts of leaves of the apricot tree 16 Chl catabolites were found. The seven major NCC fractions in the leaves of both fruit tree species were identical and displayed known structures. All of the major Chl catabolites were also found in leaf extracts from AP- or ESFY-infected trees, providing the first evidence that the PaO/PB pathway is relevant also for pathogen-induced chlorosis. This work supports the hypothesis that Chl breakdown in senescence and phytoplasma infection proceeds via a common pathway in some members of the Rosaceae family.

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.

Light Suppresses Bacterial Population through the Accumulation of Hydrogen Peroxide in Tobacco Leaves Infected with Pseudomonas syringae pv. tabaci

Pseudomonas syringae pv. tabaci (Pst) is a hemibiotrophic bacterial pathogen responsible for tobacco wildfire disease. Although considerable research has been conducted on the tobacco plant's tolerance to Pst, the role of light in the responses of the photosystems to Pst infection is poorly understood. This study aimed to elucidate the underlying mechanisms of the reduced photosystem damage in tobacco leaves due to Pst infection under light conditions. Compared to dark conditions, Pst infection under light conditions resulted in less chlorophyll degradation and a smaller decline in photosynthetic function. Although the maximal quantum yield of photosystem II (PSII) and the activity of the photosystem I (PSI) complex decreased as Pst infection progressed, damage to PSI and PSII after infection was reduced under light conditions compared to dark conditions. Pst was 17-fold more abundant in tobacco leaves under dark compared to light conditions at 3 days post inoculation (dpi). Additionally, H2O2 accumulated to a high level in tobacco leaves after Pst infection under light conditions; although to a lesser extent, H2O2 accumulation was also significant under dark conditions. Pretreatment with H2O2 alleviated chlorotic lesions and decreased Pst abundance in tobacco leaves at 3 dpi under dark conditions. MV pretreatment had the same effects under light conditions, whereas 3-(3,4-dichlorophenyl)-1,1-dimethylurea pretreatment aggravated chlorotic lesions and increased the Pst population. These results indicate that chlorotic symptoms and the size of the bacterial population are each negatively correlated with H2O2 accumulation. In other words, light appears to suppress the Pst population in tobacco leaves through the accumulation of H2O2 during infection.

SGRL can regulate chlorophyll metabolism and contributes to normal plant growth and development in Pisum sativum L

Among a set of genes in pea (Pisum sativum L.) that were induced under drought-stress growth conditions, one encoded a protein with significant similarity to a regulator of chlorophyll catabolism, SGR. This gene, SGRL, is distinct from SGR in genomic location, encoded carboxy-terminal motif, and expression through plant and seed development. Divergence of the two encoded proteins is associated with a loss of similarity in intron/exon gene structure. Transient expression of SGRL in leaves of Nicotiana benthamiana promoted the degradation of chlorophyll, in a manner that was distinct from that shown by SGR. Removal of a predicted transmembrane domain from SGRL reduced its activity in transient expression assays, although variants with and without this domain reduced SGR-induced chlorophyll degradation, indicating that the effects of the two proteins are not additive. The combined data suggest that the function of SGRL during growth and development is in chlorophyll re-cycling, and its mode of action is distinct from that of SGR. Studies of pea sgrL mutants revealed that plants had significantly lower stature and yield, a likely consequence of reduced photosynthetic efficiencies in mutant compared with control plants under conditions of high light intensity.

Licensed to Kill: Mitochondria, Chloroplasts, and Cell Death

Programmed cell death (PCD) is crucial in plant organogenesis and survival. In this review the involvement of mitochondria and chloroplasts in PCD execution is critically assessed. Recent findings support a central role for mitochondria in PCD, with newly identified components of the mitochondrial electron transport chain (mETC), FOF1 ATP synthase, cardiolipins, and ATPase AtOM66. While chloroplasts received less attention, their contribution to PCD is well supported, suggesting that they possibly contribute by producing reactive oxygen species (ROS) in the presence of light or even contribute through cytochrome f release. Finally we discuss two working models where mitochondria and chloroplasts could cooperatively execute PCD: mitochondria initiate the commitment steps and recruit chloroplasts for swift execution or, alternatively, mitochondria and chloroplasts could operate in parallel.

Cloning and characterization of the pepper CaPAO gene for defense responses to salt-induced leaf senescence

BACKGROUND

Pheophorbide a oxygenase (PAO) is an important enzyme in the chlorophyll catabolism pathway and is involved in leaf senescence. It opens the porphyrin macrocycle of pheophorbide a and finally forms the primary fluorescent chlorophyll catabolite. Previous studies have demonstrated the function of PAO during cell death. However, the characterizaton of PAO during leaf senescence induced by environmental factors is not well understood.

METHODS

Homology-based cloning and RACE techniques were used to obtain the full-length cDNA of the CaPAO gene. CaPAO expression was determined by quantitative real-time PCR. Function of CaPAO gene were studied using virus-induced gene silencing and transgenic techniques with tobacco plants (Nicotiana tabacum).

RESULTS

A novel PAO gene CaPAO was isolated from pepper (Capsicum annuum L.). The full-length CaPAO cDNA is comprised of 1838 bp, containing an open reading frame of 1614 bp, and encodes a 537 amino acid protein. This deduced protein belongs to the Rieske-type iron-sulfur superfamily, containing a conserved Rieske cluster. CaPAO expression, as determined by quantitative real-time PCR, was higher in leaves than roots, stems and flowers. It was upregulated by abscisic acid, methyl jasmonate and salicylic acid. Moreover, CaPAO was significantly induced by high salinity and osmotic stress treatments and also was regulated by Phytophthora capsici. The virus-induced gene silencing technique was used to silence the CaPAO gene in pepper plants. After 3 days of high salt treatment, the chlorophyll breakdown of CaPAO-silenced pepper plants was retarded. RD29A promoter-inducible expression vector was constructed and transferred into tobacco plant. After 7 days of salt treatment, the leaves of transgenic plants were severely turned into yellow, the lower leaves showed necrotic symptom and chlorophyll content was significantly lower than that in the control plants.

CONCLUSIONS

The expression of CaPAO gene was induced in natural senescence and various stresses. The CaPAO gene may be related to defense responses to various stresses and play an important role in salt-induced leaf senescence.

Transcriptomic and metabolomic analyses identify a role for chlorophyll catabolism and phytoalexin during Medicago nonhost resistance against Asian soybean rust

Asian soybean rust (ASR) caused by Phakopsora pachyrhizi is a devastating foliar disease affecting soybean production worldwide. Understanding nonhost resistance against ASR may provide an avenue to engineer soybean to confer durable resistance against ASR. We characterized a Medicago truncatula-ASR pathosystem to study molecular mechanisms of nonhost resistance. Although urediniospores formed appressoria and penetrated into epidermal cells of M. truncatula, P. pachyrhizi failed to sporulate. Transcriptomic analysis revealed the induction of phenylpropanoid, flavonoid and isoflavonoid metabolic pathway genes involved in the production of phytoalexin medicarpin in M. truncatula upon infection with P. pachyrhizi. Furthermore, genes involved in chlorophyll catabolism were induced during nonhost resistance. We further characterized one of the chlorophyll catabolism genes, Stay-green (SGR), and demonstrated that the M. truncatula sgr mutant and alfalfa SGR-RNAi lines showed hypersensitive-response-like enhanced cell death upon inoculation with P. pachyrhizi. Consistent with transcriptomic analysis, metabolomic analysis also revealed the accumulation of medicarpin and its intermediate metabolites. In vitro assay showed that medicarpin inhibited urediniospore germination and differentiation. In addition, several triterpenoid saponin glycosides accumulated in M. truncatula upon inoculation with P. pachyrhizi. In summary, using multi-omic approaches, we identified a correlation between phytoalexin production and M. truncatula defense responses against ASR.

Proteomics of effector-triggered immunity (ETI) in plants

Effector-triggered immunity (ETI) was originally termed gene-for-gene resistance and dates back to fundamental observations of flax resistance to rust fungi by Harold Henry Flor in the 1940s. Since then, genetic and biochemical approaches have defined our current understanding of how plant “resistance” proteins recognize microbial effectors. More recently, proteomic approaches have expanded our view of the protein landscape during ETI and contributed significant advances to our mechanistic understanding of ETI signaling. Here we provide an overview of proteomic techniques that have been used to study plant ETI including both global and targeted approaches. We discuss the challenges associated with ETI proteomics and highlight specific examples from the literature, which demonstrate how proteomics is advancing the ETI research field.

Novel insights into the Citrus sinensis nonhost response suggest photosynthesis decline, abiotic stress networks and secondary metabolism modifications

Plants are constantly exposed to stress factors. Biotic stress is produced by living organisms such as pathogens, whereas abiotic stress by unfavourable environmental conditions. In Citrus species, one of the most important fruit crops in the world, these stresses generate serious limitations in productivity. Through biochemical and transcriptomic assays, we had previously characterised the Citrus sinensis (L.) Osbeck nonhost response to Xanthomonas campestris pv. vesicatoria (Doidge), in contrast to Asiatic citrus canker infection caused by Xanthomonas citri subsp. citri (Hasse). A hypersensitive response (HR) including changes in the expression of several transcription factors was reported. Here, a new exhaustive analysis of the Citrus sinensis transcriptomes previously obtained was performed, allowing us to detect the over-representation of photosynthesis, abiotic stress and secondary metabolism processes during the nonhost HR. The broad downregulation of photosynthesis-related genes was correlated with an altered photosynthesis physiology. The high number of heat shock proteins and genes related to abiotic stress, including aquaporins, suggests that stresses crosstalk. Additionally, the secondary metabolism exhibited lignin and carotenoid biosynthesis modifications and expression changes in the cell rescue GSTs. In conclusion, novel features of the Citrus nonhost HR, an important part of the plants' defence against disease that has yet to be fully exploited in plant breeding programs, are presented.

Effects of Heat Shock and 2,4-D Treatment on Morphological and Physiological Characteristics of Microspores and Microspore-Derived Doubled Haploid Plants in Brassica napus L

BACKGROUND

Stresses such as heat shock, starvation, or osmotic is essential to lead isolated microspores towards embryogenesis. Despite the effectiveness of stresses in embryogenesis, they exert adverse effects on metabolism and growth of the regenerated plants.

OBJECTIVES

The effects of heat shock and 2,4-D treatment on total protein content of treated microspores, morphological and physiological characteristics of the doubled haploid (DH) plants were assessed.

MATERIALS AND METHODS

Buds containing mid- to late- uninucleate microspores were used for microspore culture. Microspores were isolated and cultured in NLN-13 medium and incubated at 30ºC for 14 days or treated with 2,4-D (35 mg.L^-1) for 30 min to induce embryogenesis. Microspore-derived embryos were transferred onto B5 medium for plantlet regeneration. Ploidy level of the regenerated plantlets was determined using Partec flow cytometry. Spectrophotometric readings were carried out at 490, 663 and 645 nm to determine Chl a-b and carotenoids contents. TRIzol and cetyl-threeethyl-ammonium bromide (CTAB) were used for protein extraction from microspores and leaves. Length and width of stomata and pollen grains were also photographed using light microscope (Olympus).

RESULTS

Applied stressors significantly reduced total protein content of treated microspores however, protein content and concentration of chlorophyll a and b of the DH plants were only increased by heat shock treatment when compared with the donor plant 'Hyola 420'. In contrast, carotenoids were not affected by applied stressors. Longer and wider stomata were observed by 2,4-D treatment but, the length of pollen grains was significantly decreased following heat shock and 2,4-D treatment.

CONCLUSIONS

Total protein content of cultured microspores, concentration of chlorophyll a and b, length and width of stomata of microspore-derived doubled haploid plants were significantly affected by the type of inductive stresses. However, carotenoids were more stable and not affected by applied stressors.

An Arabidopsis soluble chloroplast proteomic analysis reveals the participation of the Executer pathway in response to increased light conditions

The Executer1 and Executer2 proteins have a fundamental role in the signalling pathway mediated by singlet oxygen in chloroplast; nonetheless, not much is known yet about their specific activity and features. Herein, we have followed a differential-expression proteomics approach to analyse the impact of Executer on the soluble chloroplast protein abundance in Arabidopsis. Because singlet oxygen plays a significant role in signalling the oxidative response of plants to light, our analysis also included the soluble chloroplast proteome of plants exposed to a moderate light intensity in the time frame of hours. A number of light- and genotype-responsive proteins were detected, and mass-spectrometry identification showed changes in abundance of several photosynthesis- and carbon metabolism-related proteins as well as proteins involved in plastid mRNA processing. Our results support the participation of the Executer proteins in signalling and control of chloroplast metabolism, and in the regulation of plant response to environmental changes.

Transcriptome responses to Ralstonia solanacearum infection in the roots of the wild potato Solanum commersonii

BACKGROUND

Solanum commersonii is a wild potato species that exhibits high tolerance to both biotic and abiotic stresses and has been used as a source of genes for introgression into cultivated potato. Among the interesting features of S. commersonii is resistance to the bacterial wilt caused by Ralstonia solanacearum, one of the most devastating bacterial diseases of crops.

RESULTS

In this study, we used deep sequencing of S. commersonii RNA (RNA-seq) to analyze the below-ground plant transcriptional responses to R. solanacearum. While a majority of S. commersonii RNA-seq reads could be aligned to the Solanum tuberosum Group Phureja DM reference genome sequence, we identified 2,978 S. commersonii novel transcripts through assembly of unaligned S. commersonii RNA-seq reads. We also used RNA-seq to study gene expression in pathogen-challenged roots of S. commersonii accessions resistant (F118) and susceptible (F97) to the pathogen. Expression profiles obtained from read mapping to the S. tuberosum reference genome and the S. commersonii novel transcripts revealed a differential response to the pathogen in the two accessions, with 221 (F118) and 644 (F97) differentially expressed genes including S. commersonii novel transcripts in the resistant and susceptible genotypes. Interestingly, 22.6% of the F118 and 12.8% of the F97 differentially expressed genes had been previously identified as responsive to biotic stresses and half of those up-regulated in both accessions had been involved in plant pathogen responses. Finally, we compared two different methods to eliminate ribosomal RNA from the plant RNA samples in order to allow dual mapping of RNAseq reads to the host and pathogen genomes and provide insights on the advantages and limitations of each technique.

CONCLUSIONS

Our work catalogues the S. commersonii transcriptome and strengthens the notion that this species encodes specific genes that are differentially expressed to respond to bacterial wilt. In addition, a high proportion of S. commersonii-specific transcripts were altered by R. solanacearum only in F118 accession, while phythormone-related genes were highly induced in F97, suggesting a markedly different response to the pathogen in the two plant accessions studied.

Key players of singlet oxygen-induced cell death in plants

The production of reactive oxygen species (ROS) is an unavoidable consequence of oxygenic photosynthesis. Singlet oxygen ((1)O2) is a highly reactive species to which has been attributed a major destructive role during the execution of ROS-induced cell death in photosynthetic tissues exposed to excess light. The study of the specific biological activity of (1)O2 in plants has been hindered by its high reactivity and short lifetime, the concurrent production of other ROS under photooxidative stress, and limited in vivo detection methods. However, during the last 15 years, the isolation and characterization of two (1)O2-overproducing mutants in Arabidopsis thaliana, flu and ch1, has allowed the identification of genetically controlled (1)O2 cell death pathways and a (1)O2 acclimation pathway that are triggered at sub-cytotoxic concentrations of (1)O2. The study of flu has revealed the control of cell death by the plastid proteins EXECUTER (EX)1 and EX2. In ch1, oxidized derivatives of β-carotene, such as β-cyclocitral and dihydroactinidiolide, have been identified as important upstream messengers in the (1)O2 signaling pathway that leads to stress acclimation. In both the flu and ch1 mutants, phytohormones act as important promoters or inhibitors of cell death. In particular, jasmonate has emerged as a key player in the decision between acclimation and cell death in response to (1)O2. Although the flu and ch1 mutants show many similarities, especially regarding their gene expression profiles, key differences, such as EXECUTER-independent cell death in ch1, have also been observed and will need further investigation to be fully understood.

Plant innate immunity--sunny side up?

Reactive oxygen species (ROS)- and calcium- dependent signaling pathways play well-established roles during plant innate immunity. Chloroplasts host major biosynthetic pathways and have central roles in energy production, redox homeostasis, and retrograde signaling. However, the organelle's importance in immunity has been somehow overlooked. Recent findings suggest that the chloroplast also has an unanticipated function as a hub for ROS- and calcium-signaling that affects immunity responses at an early stage after pathogen attack. In this opinion article, we discuss a chloroplastic calcium-ROS signaling branch of plant innate immunity. We propose that this chloroplastic branch acts as a light-dependent rheostat that, through the production of ROS, influences the severity of the immune response.

Programmed cell death in plants: A chloroplastic connection

Programmed cell death (PCD) is an integral cellular program by which targeted cells culminate to demise under certain developmental and pathological conditions. It is essential for controlling cell number, removing unwanted diseased or damaged cells and maintaining the cellular homeostasis. The details of PCD process has been very well elucidated and characterized in animals but similar understanding of the process in plants has not been achieved rather the field is still in its infancy that sees some sporadic reports every now and then. The plants have 2 energy generating sub-cellular organelles- mitochondria and chloroplasts unlike animals that just have mitochondria. The presence of chloroplast as an additional energy transducing and ROS generating compartment in a plant cell inclines to advocate the involvement of chloroplasts in PCD execution process. As chloroplasts are supposed to be progenies of unicellular photosynthetic organisms that evolved as a result of endosymbiosis, the possibility of retaining some of the components involved in bacterial PCD by chloroplasts cannot be ruled out. Despite several excellent reviews on PCD in plants, there is a void on an update of information at a place on the regulation of PCD by chloroplast. This review has been written to provide an update on the information supporting the involvement of chloroplast in PCD process and the possible future course of the field.