Biochemistry of Indian summer: physiology of autumnal leaf coloration

Novel Types of Phyllobilins in a Fern - Molecular Reporters of the Evolution of Chlorophyll Breakdown in the Paleozoic Era

Breakdown of chlorophyll (Chl), as studied in angiosperms, follows the pheophorbide a oxygenase/phyllobilin (PaO/PB) pathway, furnishing linear tetrapyrroles, named phyllobilins (PBs). In an investigation with fern leaves we have discovered iso-phyllobilanones (iPBs) with an intriguingly rearranged and oxidized carbon skeleton. We report here a key second group of iPBs from the fern and on their structure analysis. Previously, these additional Chl-catabolites escaped their characterization, since they exist in aqueous media as mixtures of equilibrating isomers. However, their chemical dehydration furnished stable iPB-derivatives that allowed the delineation of the enigmatic structures and chemistry of the original natural catabolites. The structures of all fern-iPBs reflect the early core steps of a PaO/PB-type pathway and the PB-to-iPB carbon skeleton rearrangement. A striking further degradative chemical ring-cleavage was observed, proposed to consume singlet molecular oxygen (1O2). Hence, Chl-catabolites may play a novel active role in detoxifying cellular 1O2. The critical deviations from the PaO/PB pathway, found in the fern, reflect evolutionary developments of Chl-breakdown in the green plants in the Paleozoic era.

Stable water isotopes reveal the onset of bud dormancy in temperate trees, whereas water content is a better proxy for dormancy release

Earlier spring growth onset in temperate forests is a visible effect of global warming that alters global water and carbon cycling. Consequently, it becomes crucial to accurately predict the future spring phenological shifts in vegetation under different climate warming scenarios. However, current phenological models suffer from a lack of physiological insights of tree dormancy and are rarely experimentally validated. Here, we sampled twig cuttings of five deciduous tree species at two climatically different locations (270 and 750 m a.s.l., ~ 2.3 °C difference) throughout the winter of 2019-20. Twig budburst success, thermal time to budburst, bud water content and short-term 2H-labelled water uptake into buds were quantified to link bud dormancy status with vascular water transport efficacy, with the objective of establishing connections between the dormancy status of buds and their effectiveness in vascular water transport. We found large differences in the dormancy status between species throughout the entire investigation period, likely reflecting species-specific environmental requirements to initiate and release dormancy, whereas only small differences in the dormancy status were found between the two studied sites. We found strong 2H-labelled water uptake into buds during leaf senescence, followed by a sharp decrease, which we ascribed to the initiation of endodormancy. However, surprisingly, we did not find a progressive increase in 2H-labelled water uptake into buds as winter advanced. Nonetheless, all examined tree species exhibited a consistent relationship between bud water content and dormancy status. Our results suggest that short-term 2H-labelled water uptake may not be a robust indicator of dormancy release, yet it holds promise as a method for tracking the induction of dormancy in deciduous trees. By contrast, bud water content emerges as a cost-effective and more reliable indicator of dormancy release.

Wheat cis-zeatin-O-glucosyltransferase cZOGT1 interacts with the Ca2+-dependent lipid binding protein TaZIP to regulate senescence

Premature senescence is an important factor affecting wheat yield and quality. Wheat yield can be increased by delaying senescence and prolonging the effective photosynthetic time. Previously, we found that the cis-zeatin-O-glucosyltransferase (cZOGT1) gene plays an important role in the stay-green wheat phenotype. In this study, cZOGT1-overexpressing lines exhibited a delayed senescence phenotype, despite a significant reduction in the total cytokinin content. Further, we found that cZOGT1 interacted with the Ca2+-dependent lipid binding protein TaZIP (cZOGT1-interacting protein), and that a high level of cZOGT1 expression led to the suppression of TaZIP expression, which in turn, reduced abscisic acid (ABA) content. The synergistic reduction in cytokinins and ABA levels eventually caused the stay-green phenotype in cZOGT1-overexpressing lines. This study provides a new theoretical basis to explain the mechanism underlying the wheat stay-green phenotype and provides a genetic resource for wheat molecular-design breeding.

The phenomenon of red and yellow autumn leaves: Hypotheses, agreements and disagreements

Yellow and red autumn leaves are typical of many temperate/boreal woody plants. Since the 19th century, it has been either considered the non-functional outcome of chlorophyll degradation that unmasks the pre-existing yellow and red pigments or that the de novo synthesis of red anthocyanins in autumn leaves indicated that it should have a physiological function, although it was commonly ignored. Defending free amino acids and various other resources released especially following the breakdown of the photosynthetic system, and mobilizing them for storage in other organs before leaf fall, is the cornerstone of both the physiological and anti-herbivory hypotheses about the functions of yellow and red autumn leaf colouration. The complicated phenomenon of conspicuous autumn leaf colouration has received significant attention since the year 2000, especially because ecologists started paying attention to its anti-herbivory potential. The obvious imperfection of the hypotheses put forth in several papers stimulated many other scientists. Hot debates among physiologists, among ecologists, and between physiologists and ecologists have been common since the year 2000, first because the various functions of yellow and red autumn leaf colouration are non-exclusive, and second because many scientists were trained to focus on a single subject. Here, I will review the debates, especially between the photoprotective and the anti-herbivory hypotheses, and describe both the progress in their understanding and the required progress.

Wall-associated kinase-like gene RL1 contributes to red leaves in sorghum

Red leaves are common in trees but rare in cereal crops. Red leaves can be used as raw materials for anthocyanin extraction and may have some adaptive significance for plants. In this study, we discovered a red leaf phenotype in the F₁ hybrids derived from a cross between two sorghum accessions with green leaf. Histological analysis of red leaves and green leaves showed that red compounds accumulate in mesophyll cells and gradually spreads to the entire leaf blade. In addition, we found chloroplasts degraded more quickly in red leaves than in green leaves based on transmission electron microscopy. Metabolic analysis revealed that flavonoids including six anthocyanins are more abundant in red leaves. Moreover, transcriptome analysis revealed that expression of flavonoid biosynthesis genes was upregulated in red leaves. These observations indicate that flavonoids and anthocyanins in particular, are ideal candidates for the red compounds accumulating in red leaves. Segregation analysis of the red leaf phenotype suggested a genetic architecture consisting of three dominant genes, one (RL1 for RED LEAF1) of which we mapped to a 55-kb region on chromosome 7 containing seven genes. Sequencing, reverse transcription-polymerase chain reaction, and transcriptome analysis suggested Sobic.007G214300, encoding a wall-associated kinase, as the most likely candidate for RL1. Fine mapping the red leaf gene and identifying the metabolites that cause red leaf in sorghum provide us with a better understanding of the red leaf phenotype in the natural population of sorghum.

Genetic mechanisms of aging in plants: What can we learn from them?

Plants hold all records in longevity. Their aging is a complex process. In the presented review, we analyzed published data on various aspects of plant aging with focus on any inferences that could shed a light on aging in animals and help to fight it in human. Plant aging can be caused by many factors, such as telomere depletion, genomic instability, loss of proteostasis, changes in intercellular interaction, desynchronosis, autophagy misregulation, epigenetic changes and others. Plants have developed a number of mechanisms to increase lifespan. Among these mechanisms are gene duplication ("genetic backup"), the active work of telomerases, abundance of meristematic cells, capacity of maintaining the meristems permanently active and continuous activity of phytohormones. Plant aging usually occurs throughout the whole perennial life, but could be also seasonal senescence. Study of causes for seasonal aging can also help to uncover the mechanisms of plant longevity. The influence of different factors such as microbiome communities, glycation, alternative oxidase activity, mitochondrial dysfunction on plant longevity was also reviewed. Adaptive mechanisms of long-lived plants are considered. Further comparative study of the mechanisms underlying longevity of plants is necessary. This will allow us to reach a potentially new level of understanding of the aging process of plants.

Linking the Spectra of Decomposing Litter to Ecosystem Processes: Tandem Close-Range Hyperspectral Imagery and Decomposition Metrics

Efforts to monitor terrestrial decomposition dynamics at broad spatial scales are hampered by the lack of a cost-effective and scalable means to track the decomposition process. Recent advances in remote sensing have enabled the simulation of litter spectra throughout decomposition for grasses in general, yet unique decomposition pathways are hypothesized to create subtly different litter spectral signatures with unique ecosystem functional significance. The objectives of this study were to improve spectra–decomposition linkages and thereby enable the more comprehensive monitoring of ecosystem processes such as nutrient and carbon cycles. Using close-range hyperspectral imaging, litter spectra and multiple decomposition metrics were concurrently monitored in four classes of naturally decayed litter under four decomposition treatments. The first principal component accounted for approximately 94% of spectral variation in the close-range imagery and was attributed to the progression of decomposition. Decomposition-induced spectral changes were moderately correlated with the leaf carbon to nitrogen ratio (R2 = 0.52) and sodium hydroxide extractables (R2 = 0.45) but had no correlation with carbon dioxide flux. Temperature and humidity strongly influenced the decomposition process but did not influence spectral variability or the patterns of surface decomposition. The outcome of the study is that litter spectra are linked to important metrics of decomposition and thus remote sensing could be utilized to assess decomposition dynamics and the implications for nutrient recycling at broad spatial scales. A secondary study outcome is the need to resolve methodological challenges related to inducing unique decomposition pathways in a lab environment. Improving decomposition treatments that mimic real-world conditions of temperature, humidity, insolation, and the decomposer community will enable an improved understanding of the impacts of climatic change, which are expected to strongly affect microbially mediated decomposition.

Phyllobilins as a challenging diverse natural product class: Exploration of pharmacological activities

Phyllobilins are a group of chlorophyll-derived bilin-type linear tetrapyrroles, generated in the process of chlorophyll breakdown. Since the first phyllobilin was isolated and characterized in 1991, more and more structures of these chlorophyll catabolites were identified alongside the biochemical players involved in chlorophyll breakdown. In the meantime, phyllobilins are known to occur in a large natural structural variety, and new modifications are still being discovered. Phyllobilins have been regarded as products of chlorophyll detoxification for a very long time, hence they have been completely overlooked as a natural product class in terms of their biological role or pharmacological activity. A change of this paradigm, however, is long overdue. Here, we review the current knowledge of the pharmacological activities of phyllobilins and give an overview of the diverse structural modifications, laying the groundwork for analyzing their role(s) as active components in medicinal plants.

Cold acclimation can specifically inhibit chlorophyll biosynthesis in young leaves of Pakchoi

BACKGROUND

Leaf color is an important trait in breeding of leafy vegetables. Y-05, a pakchoi (Brassica rapa ssp. chinensis) cultivar, displays yellow inner (YIN) and green outer leaves (GOU) after cold acclimation. However, the mechanism of this special phenotype remains elusive.

RESULTS

We assumed that the yellow leaf phenotype of Y-05 maybe caused by low chlorophyll content. Pigments measurements and transmission electron microscopy (TEM) analysis showed that the yellow phenotype is closely related with decreased chlorophyll content and undeveloped thylakoids in chloroplast. Transcriptomes and metabolomes sequencing were next performed on YIN and GOU. The transcriptomes data showed that 4887 differentially expressed genes (DEGs) between the YIN and GOU leaves were mostly enriched in the chloroplast- and chlorophyll-related categories, indicating that the chlorophyll biosynthesis is mainly affected during cold acclimation. Together with metabolomes data, the inhibition of chlorophyll biosynthesis is contributed by blocked 5-aminolevulinic acid (ALA) synthesis in yellow inner leaves, which is further verified by complementary and inhibitory experiments of ALA. Furthermore, we found that the blocked ALA is closely associated with increased BrFLU expression, which is indirectly altered by cold acclimation. In BrFLU-silenced pakchoi Y-05, cold-acclimated leaves still showed green phenotype and higher chlorophyll content compared with control, meaning silencing of BrFLU can rescue the leaf yellowing induced by cold acclimation.

CONCLUSIONS

Our findings suggested that cold acclimation can indirectly promote the expression of BrFLU in inner leaves of Y-05 to block ALA synthesis, resulting in decreased chlorophyll content and leaf yellowing. This study revealed the underlying mechanisms of leaves color change in cold-acclimated Y-05.

Effect of exogenous catechin and salicylic acid on rice productivity under ozone stress: the role of chlorophyll contents, lipid peroxidation, and antioxidant enzymes

Increasing ozone concentration is one of the oxidative stresses that affects rice yield loss in many countries. Catechin and salicylic acid were proposed as tools for alleviating oxidative stress in plants, but their roles in protecting rice productivity under ozone stress still remained unknown. We investigated the mechanism of catechin and salicylic acid on rice under ozone stress at the vegetative stage and at the reproductive stage. Rice was sprayed with catechin and salicylic acid before exposure to ozone in the range of 100-150 ppb (8 h day^-1). Ozone and salicylic acid led to a decrease in chlorophyll contents, magnesium contents, and stomatal conductance. This evidence led to a decrease in rice productivity and quality. In contrast, under rice + catechin, both ambient air and elevated ozone conditions had to higher rice productivity and quality than under rice alone and rice + salicylic acid conditions. Catechin could mitigate ozone stress in rice plants through maintaining chlorophyll contents, magnesium contents, and stomatal conductance. Moreover, catechin could induce an unregulation of ascorbate peroxidase, and catalase genes led to increasing their antioxidant enzyme activity. Increasing of antioxidant enzyme activity under rice + ozone + catechin conditions attributed to lower lipid peroxidation than under rice + ozone especially at vegetative stage. This study confirmed that catechin, which is naturally found in tea leaves, could be used as an ozone protectant. The protective role of catechin on chlorophyll contents and antioxidant systems at the vegetative stage attributed to maintaining rice yield under ozone stress. Graphical abstract.

The red chlorophyll catabolite (RCC) is an inefficient sensitizer of singlet oxygen - photochemical studies of the methyl ester of RCC

The red chlorophyll catabolite (RCC) is a proposed cryptic intermediate of chlorophyll (Chl) breakdown in higher plants. Its accumulation in higher plants is believed to be metabolically suppressed, as RCC is commonly suspected to efficiently sensitize for the formation of the cell poison singlet oxygen (¹O₂). We report here a study on luminescence of the methyl ester of RCC (Me-RCC) and of its capacity to generate ¹O₂ in ethanolic solution. A solution of Me-RCC fluoresces at room temperature with a maximum near 670 nm and features a fluorescence spectrum with pronounced vibrational spacing at 77 K. As shown here, sensitization of the generation of ¹O₂ by Me-RCC in an oxygen-saturated solution in hexadeutero-ethanol occurs with a maximal quantum yield of only about 0.015. This low quantum yield suggests that the specific catabolic suppression of the accumulation of RCC during Chl breakdown is not primarily a countermeasure against the formation of ¹O₂ by RCC in the plant, but has other crucial reasons mainly.

A Comparative Study of the Effect of Field Retting Time on the Properties of Hemp Fibres Harvested at Different Growth Stages

In this study, the comparison of field retting of hemp fibres harvested at different growth stages (beginning and end of flowering, seed maturity) was studied. Regardless of the harvest period, identical evolution of the fibres’ properties was observed during retting. The main difference is the kinetics of this transformation, which depend on weather conditions and the initial state of the fibres after harvesting. Retting leads to a change in colour of the stems and fibres, an increase of the cellulose fraction and a gradual improvement of the fibres’ thermal stability, in relation with a decrease in the non-cellulosic materials. This process induces fibre bundle separation into elementary fibres. A long period (5 weeks) is required for getting the highest mechanical properties of fibres harvested at the beginning and the end of flowering. However, the retting of fibres harvested at seed maturity has to be performed in a short period (1 week) in order to avoid over-retting treatment. If the fibres are over-retted, their quality decreases in terms of structure and mechanical properties.

Evolution of temporal dynamic of volatile organic compounds (VOCs) and odors of hemp stem during field retting

New non-destructive approach to evaluate the retting process was investigated. Increase of retting duration led to a decrease of VOCs emitted by plants and change of color and plant odor. The variation of VOCs and odor could be used as indicators for the degree of retting. In the hemp industry, retting is an upstream bioprocessing applied to the plants to facilitate the decortication of fibres from the central woody part of the stem. This treatment is currently carried out in an empirical way on the ground which leads to variability in the hemp stems quality, and thus to the hemp fibres quality. Therefore, controlling retting treatment is a crucial step for high-performance hemp fibre. In this study, a new approach is used to assess the retting degree by following the evolution of VOCs emitted by plants during different retting durations. Either harvest time or retting induces a change in VOCs released by plants. During plant maturity, volatile compounds emitted decreased with a factor of about 2, in relation to VOCs released at the end of flowering. Regardless of the harvest period, the majority of VOCs and odor concentrations, monitored by olfactometric analysis, decrease gradually until some of them disappear at the end of retting. Likewise, the green plant odor disappears during retting with an increase of dry plants odor and an appearance of fermented odor at the end of retting. Following the evolution of VOCs emitted by plants during retting could be a tool for farmers to improve the retting management.

Detecting the onset of autumn leaf senescence in deciduous forest trees of the temperate zone

Information on the onset of leaf senescence in temperate deciduous trees and comparisons on its assessment methods are limited, hampering our understanding of autumn dynamics. We compare five field proxies, five remote sensing proxies and two data analysis approaches to assess leaf senescence onset at one main beech stand, two stands of oak and birch, and three ancillary stands of the same species in Belgium during 2017 and 2018. Across species and sites, onset of leaf senescence was not significantly different for the field proxies based on Chl leaf content and canopy coloration, except for an advanced canopy coloration during the extremely dry and warm 2018. Two remote sensing indices provided results fully consistent with the field data. A significant lag emerged between leaf senescence onset and leaf fall, and when a threshold of 50% change in the seasonal variable under study (e.g. Chl content) was used to derive the leaf senescence onset. Our results provide unprecedented information on the quality and applicability of different proxies to assess leaf senescence onset in temperate deciduous trees. In addition, a sound base is offered to select the most suited methods for the different disciplines that need this type of data.

Yellow Dioxobilin-Type Tetrapyrroles from Chlorophyll Breakdown in Higher Plants-A New Class of Colored Phyllobilins

In senescent leaves chlorophyll (Chl) catabolites typically accumulate as colorless tetrapyrroles, classified as formyloxobilin-type (or type-I) or dioxobilin-type (type-II) phyllobilins (PBs). Yellow type-I Chl catabolites (YCCs) also occur in some senescent leaves, in which they are generated by oxidation of colorless type-I PBs. A yellow type-II PB was recently proposed to occur in extracts of fall leaves of grapevine (Vitis vinifera), tentatively identified by its mass and UV/Vis absorption characteristics. Here, the first synthesis of a yellow type-II Chl catabolite (DYCC) from its presumed natural colorless type-II precursor is reported. A homogenate of a Spatiphyllum wallisii leaf was used as "green" means of effective and selective oxidation. The synthetic DYCC was fully characterized and identified with the yellow grapevine leaf pigment. As related yellow type-I PBs do, the DYCC functions as a reversible photoswitch by undergoing selective photo-induced Z/E isomerization of its C15=C16 bond.

Synchrony in fall leaf drop: chlorophyll degradation, color change, and abscission layer formation in three temperate deciduous tree species

PREMISE OF THE STUDY

Deciduous tree species remove some nutrients from their leaves during fall leaf senescence through retranslocation. Retranslocation impacts the timeline of leaf fall, amount of active chlorophyll, and overall leaf nitrogen content as fall color change occurs. Our objective was to identify interspecific differences in the timing of abscission layer formation, leaf color change, and the level of chlorophyll degradation of young trees during fall senescence.

METHODS

Leaf relative chlorophyll content for three tree species was measured during fall 2015 by a greenness meter. These measurements were calibrated for each species through spectrophotometric determination of leaf chlorophyll concentration. Abscission layer formation was tracked using light microscopy of sampled leaves. Excised leaves were photographed on a flat white surface to track species leaf color through time.

KEY RESULTS

All three species had different chlorophyll declination rates throughout the fall season. The maple species started with less chlorophyll and began abscission layer formation earlier. The other two species had a similar starting chlorophyll level and onset timing of abscission layer formation. Visible leaf color change was not associated with a threshold in either chlorophyll degradation or abscission layer formation across species.

CONCLUSIONS

Maple species degraded less chlorophyll on average, in the fall, than did the oak and beech species. The rate of chlorophyll degradation in coordination with abscission layer formation varied by species. Color change was not a good predictor of level of chlorophyll degradation in leaves across species.

Novel Types of Hypermodified Fluorescent Phyllobilins from Breakdown of Chlorophyll in Senescent Leaves of Grapevine (Vitis vinifera)

The tetrapyrrolic chlorophyll catabolites (or phyllobilins, PBs) were analyzed in yellow fall leaves of the grape Chardonnay, a common Vitis vinifera white wine cultivar. The major fractions in leaf extracts of V. vinifera, tentatively assigned to PBs, were isolated and their structures elucidated. The dominant fraction is a dioxobilin-type non-fluorescent Chl-catabolite of a previously observed type. Two less polar fluorescent PBs were characterized as a novel dioxobilin-type fluorescent Chl-catabolite with a bicyclo-1',6'-glycosyl architecture, and its new fluorescent formyloxobilin-type analogue. The discovery of persistent hypermodified fluorescent PBs with the architecture of bicyclo-[17.3.1]-PBs (bcPBs), suggests the activity of an unknown enzyme that forges the 20-membered macroring at the tetrapyrrolic core of a fluorescent PB. bcPBs may play specific physiological roles in grapevine plants and represent endogenous anti-infective agents, as found similarly for other organic bicyclo-[n.3.1]-1',6'-glycosyl derivatives.

Chlorophyll Breakdown in a Fern-Discovery of Phyllobilin Isomers with a Rearranged Carbon Skeleton

All structure-based information on chlorophyll (Chl) breakdown in the higher plants relies on studies with angiosperms. Herein, the first investigation of a fern is reported, revealing a novel type of Chl catabolites (phyllobilins) in leaves of this large division of the vascular plants, and providing structural insights into an astounding metabolic process of the higher plants that appears to have played a role even in early phases of plant evolution. The tetrapyrrolic Chl catabolites in the cosmopolitan bracken fern were discovered to be phyllobilin isomers with an unprecedented skeleton, proposed to be the striking result of a rearrangement of a hypothetical phyllobilin precursor.

Categorizing natural color distributions

The natural objects that we are surrounded with virtually always contain many different shades of color, yet the visual system usually categorizes them into a single color category. We examined various image statistics and their role in categorizing the color of leaves. Our subjects categorized photographs of autumn leaves and versions that were manipulated, including: randomly repositioned pixels, leaves uniformly colored with their mean color, leaves that were made by reflecting the original leaves' chromaticity distribution about their mean ("flipped leaves"), and simple patches colored with the mean colors of the original leaves. We trained a linear classifier with a set of image statistics in order to predict the category that each object was assigned to. Our results show that the mean hue of an object is highly predictive of the natural object's color category (>90% accuracy) and observers' choices are consistent with their use of unique yellow as a decision boundary for classification. The flipped leaves produced consistent changes in color categorization that are possibly explained by an interaction between the color distributions and the texture of the leaves.

Reversible colour change in leaves enhances pollinator attraction and reproductive success in Saururus chinensis (Saururaceae)

Background and Aims

Although there has been much experimental work on leaf colour change associated with selection generated by abiotic environmental factors and antagonists, the role of leaf colour change in pollinator attraction has been largely ignored. We tested whether whitening of the apical leaves subtending the inflorescences of Saururus chinensis during flowering enhances pollinator attraction, and whether re-greening of the white leaves after flowering increases carbon assimilation and promotes seed development.

Methods

White leaves were removed or covered, and the effects of these manipulations on pollinator visitation and subsequent reproductive success were assessed. The net photosynthetic rates of leaves of different colour were measured and their photosynthetic contributions to seed development were evaluated.

Key Results

Saururus chinensis is able to self-pollinate autonomously, but depends largely on flies for pollination. White leaves had different reflectance spectra from green leaves, and white leaves attracted significantly more pollinators and led to significantly higher fruit and seed set. Although leaf whitening resulted in a reduction in photosynthetic capacity, it translated into only a small decrease in seed mass. When leaves had turned back from white to green after flowering their photosynthetic capacity was similar to that of 'normal' green leaves and promoted seed development.

Conclusions

The reversible leaf colour change in S. chinensis appears to be adaptive because it enhances pollination success during flowering, with a small photosynthetic cost, while re-greening of these leaves after flowering helps to meet the carbon requirements for seed development.

Identification and fine mapping of a stay-green gene (Brnye1) in pakchoi (Brassica campestris L. ssp. chinensis)

Using bulked segregant analysis combined with next-generation sequencing, we delimited the Brnye1 gene responsible for the stay-green trait of nye in pakchoi. Sequence analysis identified Bra019346 as the candidate gene. "Stay-green" refers to a plant trait whereby leaves remain green during senescence. This trait is useful in the cultivation of pakchoi (Brassica campestris L. ssp. chinensis), which is marketed as a green leaf product. This study aimed to identify the gene responsible for the stay-green trait in pakchoi. We identified a stay-green mutant in pakchoi, which we termed "nye". Genetic analysis revealed that the stay-green trait is controlled by a single recessive gene, Brnye1. Using the BSA-seq method, a 3.0-Mb candidate region was mapped on chromosome A03, which helped us localize Brnye1 to an 81.01-kb interval between SSR markers SSRWN27 and SSRWN30 via linkage analysis in an F₂ population. We identified 12 genes in this region, 11 of which were annotated based on the Brassica rapa annotation database, and one was a functionally unknown gene. An orthologous gene of the Arabidopsis gene AtNYE1, Bra019346, was identified as the potential candidate for Brnye1. Sequence analysis revealed a 40-bp insertion in the second exon of Bra019346 in nye, which generated the TAA stop codon. A candidate gene-specific Indel marker in 1561 F₂ individuals showed perfect cosegregation with Brnye1 in the nye mutant. These results provide a foundation for uncovering the molecular mechanism of the stay-green trait in pakchoi.

Pyro-Phyllobilins: Elusive Chlorophyll Catabolites Lacking a Critical Carboxylate Function of the Natural Chlorophylls

A β-keto ester grouping is a characteristic of ring E of the chlorophylls (Chls). Its presence has also reinforced the original identification of nonfluorescent Chl catabolites (NCCs) as colorless, amphiphilic phyllobilins (PBs). Polar NCCs were also detected in higher plants, in which a free carboxyl group replaced the ring E ester group. Such NCCs are surprisingly resistant to loss of this carboxyl unit, and NCCs lacking the latter, that is, pyro-NCCs (pyNCCs), have not been reported. Intrigued by the question of the natural occurrence of pyro-phyllobilins (pyPBs), we have prepared a representative pyNCC by decarboxylation of a natural NCC. We also converted the pyNCC into its yellow oxidation product, a pyro-YCC (pyYCC). The solution structures of pyNCC and of pyYCC, and a crystal structure of the pyYCC methyl ester (pyYCC-Me) were obtained. pyYCC-Me features the same remarkable H-bonded and π-stacked dimer structure as the corresponding natural yellow Chl catabolite (YCC) with the ring E methyl ester group. Indeed, the latter substituent has little effect on the structure, as well as on the unique self-assembly and photoswitch behavior of yellow PBs.

Comparative ultrastructure of fruit plastids in three genetically diverse genotypes of apple (Malus × domestica Borkh.) during development

Comparative ultrastructural developmental time-course analysis has identified discrete stages at which the fruit plastids undergo structural and consequently functional transitions to facilitate subsequent development-guided understanding of the complex plastid biology. Plastids are the defining organelle for a plant cell and are critical for myriad metabolic functions. The role of leaf plastid, chloroplast, is extensively documented; however, fruit plastids-chromoplasts-are poorly understood, especially in the context of the diverse metabolic processes operating in these diverse plant organs. Recently, in a comparative study of the predicted plastid-targeted proteomes across seven plant species, we reported that each plant species is predicted to harbor a unique set of plastid-targeted proteins. However, the temporal and developmental context of these processes remains unknown. In this study, an ultrastructural analysis approach was used to characterize fruit plastids in the epidermal and collenchymal cell layers at 11 developmental timepoints in three genotypes of apple (Malus × domestica Borkh.): chlorophyll-predominant 'Granny Smith', carotenoid-predominant 'Golden Delicious', and anthocyanin-predominant 'Top Red Delicious'. Plastids transitioned from a proplastid-like plastid to a chromoplast-like plastid in epidermis cells, while in the collenchyma cells, they transitioned from a chloroplast-like plastid to a chloro-chromo-amyloplast plastid. Plastids in the collenchyma cells of the three genotypes demonstrated a diverse array of structures and features. This study enabled the identification of discrete developmental stages during which specific functions are most likely being performed by the plastids as indicated by accumulation of plastoglobuli, starch granules, and other sub-organeller structures. Information regarding the metabolically active developmental stages is expected to facilitate biologically relevant omics studies to unravel the complex biochemistry of plastids in perennial non-model systems.

Biochemistry and cell ultrastructure changes during senescence of Beta vulgaris L. leaf

The comparative study of biochemical and ultrastructure features in senescing sugar beet (Beta vulgaris L.) leaves was carried out. One group of plants was grown under normal conditions in washed river sand and poured in turn with nitrate-containing mineral solution or water (N plants). Another group of plants, after 1 month of normal growth, was further grown with nitrate omitted in the nutritive solution (defN plants). The starting point of normal leaf senescence in N plants was identified by the maximal content of soluble protein. Soluble carbohydrate pools were statistically constant in senescing N plants, whereas glucose pools varied noticeably. A decrease in the contents of soluble protein and chlorophyll (a + b) in the course of senescing was typical for N plant leaves. The cell membrane in N plant leaves remained mostly intact; the central vacuoles in the leaf cells were large, and their membranes remained intact. The chloroplasts and mitochondria in senescing N plant leaves became swollen. The vesicles that were present in the cytoplasm of N plant leaves were especially large in the oldest leaves. It was concluded that senescing of sugar beet leaves at sufficient nitrate nutrition occurs according to a "vacuolar" scenario. In the case of nitrate deficiency, the content of soluble carbohydrates in defN leaves first reached maximum and then decreased in older leaves; the protein and chlorophyll (a + b) contents were totally lower than those in normal leaves and continuously decreased during the experiments. Chloroplasts in mesophyll cells of defN plant leaves became more rounded; starch grains in chloroplasts degraded and the number and size of lipid globules increased. The multitude of membrane impairments and lots of large vesicles-"crystals" appeared during the experiment. The results showed the controlling action of nitrogen nutrition in the senescing of sugar beet leaves.

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.

Carbon dynamics in aboveground biomass of co-dominant plant species in a temperate grassland ecosystem: same or different?

Understanding the role of individual organisms in whole-ecosystem carbon (C) fluxes is probably the biggest current challenge in C cycle research. Thus, it is unknown whether different plant community members share the same or different residence times in metabolic (τmetab ) and nonmetabolic (i.e. structural) (τnonmetab ) C pools of aboveground biomass and the fraction of fixed C allocated to aboveground nonmetabolic biomass (Anonmetab ). We assessed τmetab , τnonmetab and Anonmetab of co-dominant species from different functional groups (two bunchgrasses, a stoloniferous legume and a rosette dicot) in a temperate grassland community. Continuous, 14-16-d-long (13) C-labeling experiments were performed in September 2006, May 2007 and September 2007. A two-pool compartmental system, with a well-mixed metabolic and a nonmixed nonmetabolic pool, was the simplest biologically meaningful model that fitted the (13) C tracer kinetics in the whole-shoot biomass of all species. In all experimental periods, the species had similar τmetab (5-8 d), whereas τnonmetab ranged from 20 to 58 d (except for one outlier) and Anonmetab from 7 to 45%. Variations in τnonmetab and Anonmetab were not systematically associated with species or experimental periods, but exhibited relationships with leaf life span, particularly in the grasses. Similar pool kinetics of species suggested similar kinetics at the community level.

Transition metal complexes of phyllobilins - a new realm of bioinorganic chemistry

Phyllobilins may function as natural ligand molecules for biologically important transition metal ions, giving complexes with remarkable chemical and photophysical properties.

Natural cyclic tetrapyrroles feature outstanding capacity for binding transition metal ions, furnishing Nature with the important metallo-porphyrinoid ‘Pigments of Life’, such as heme, chlorophyll (Chl) and vitamin B₁₂. In contrast, linear tetrapyrroles are not generally ascribed a biologically relevant ability for metal-binding. Indeed, when heme or Chl are degraded to natural linear tetrapyrroles, their central Fe- or Mg-ions are set free. Some linear tetrapyrroles are, however, effective multi-dentate ligands and their transition metal complexes have remarkable chemical properties. The focus of this short review is centred on such complexes of the linear tetrapyrroles derived from natural Chl-breakdown, called phyllobilins. These natural bilin-type compounds are massively produced in Nature and in highly visible processes. Colourless non-fluorescing Chl-catabolites (NCCs) and the related dioxobilin-type NCCs, which typically accumulate in leaves as ‘final’ products of Chl-breakdown, show low affinity for transition metal-ions. However, NCCs are oxidized in leaves to give less saturated coloured phyllobilins, such as yellow or pink Chl-catabolites (YCCs or PiCCs). YCCs and PiCCs are ligands for various biologically relevant transition metal-ions, such as Zn(ii)-, Ni(ii)- and Cu(ii)-ions. Complexation of Zn(ii)- and Cd(ii)-ions by the effectively tridentate PiCC produces blue metal-complexes that exhibit an intense red fluorescence, thus providing a tool for the sensitive detection of these metal ions. Outlined here are fundamental aspects of structure and metal coordination of phyllobilins, including a comparison with the corresponding properties of bilins. This knowledge may be valuable in the quest of finding possible biological roles of the phyllobilins. Thanks to their capacity for metal-ion coordination, phyllobilins could, e.g., be involved in heavy-metal transport and detoxification, and some of their metal-complexes could act as sensitizers for singlet oxygen or as plant toxins against pathogens.

Early Autumn Senescence in Red Maple (Acer rubrum L.) Is Associated with High Leaf Anthocyanin Content

Several theories exist about the role of anthocyanins in senescing leaves. To elucidate factors contributing to variation in autumn leaf anthocyanin contents among individual trees, we analysed anthocyanins and other leaf traits in 27 individuals of red maple (Acer rubrum L.) over two growing seasons in the context of timing of leaf senescence. Red maple usually turns bright red in the autumn, but there is considerable variation among the trees. Leaf autumn anthocyanin contents were consistent between the two years of investigation. Autumn anthocyanin content strongly correlated with degree of chlorophyll degradation mid to late September, early senescing leaves having the highest concentrations of anthocyanins. It also correlated positively with leaf summer chlorophyll content and dry matter content, and negatively with specific leaf area. Time of leaf senescence and anthocyanin contents correlated with soil pH and with canopy openness. We conclude that the importance of anthocyanins in protection of leaf processes during senescence depends on the time of senescence. Rather than prolonging the growing season by enabling a delayed senescence, autumn anthocyanins in red maple in Ontario are important when senescence happens early, possibly due to the higher irradiance and greater danger of oxidative damage early in the season.

Autofluorescence: Biological functions and technical applications

Chlorophylls are the most remarkable examples of fluorophores, and their fluorescence has been intensively studied as a non-invasive tool for assessment of photosynthesis. Many other fluorophores occur in plants, such as alkaloids, phenolic compounds and porphyrins. Fluorescence could be more than just a physicochemical curiosity in the plant kingdom, as several functional roles in biocommunication occur or have been proposed. Besides, fluorescence emitted by secondary metabolites can convert damaging blue and UV into wavelengths potentially useful for photosynthesis. Detection of the fluorescence of some secondary phytochemicals may be a cue for some pollinators and/or seed dispersal organisms. Independently of their functions, plant fluorophores provide researchers with a tool that allows the visualization of some metabolites in plants and cells, complementing and overcoming some of the limitations of the use of fluorescent proteins and dyes to probe plant physiology and biochemistry. Some fluorophores are influenced by environmental interactions, allowing fluorescence to be also used as a specific stress indicator.

Senescence Meets Dedifferentiation

Senescence represents the final stage of leaf development but is often induced prematurely following exposure to biotic and abiotic stresses. Leaf senescence is manifested by color change from green to yellow (due to chlorophyll degradation) or to red (due to de novo synthesis of anthocyanins coupled with chlorophyll degradation) and frequently culminates in programmed death of leaves. However, the breakdown of chlorophyll and macromolecules such as proteins and RNAs that occurs during leaf senescence does not necessarily represent a one-way road to death but rather a reversible process whereby senescing leaves can, under certain conditions, re-green and regain their photosynthetic capacity. This phenomenon essentially distinguishes senescence from programmed cell death, leading researchers to hypothesize that changes occurring during senescence might represent a process of trans-differentiation, that is the conversion of one cell type to another. In this review, we highlight attributes common to senescence and dedifferentiation including chromatin structure and activation of transposable elements and provide further support to the notion that senescence is not merely a deterioration process leading to death but rather a unique developmental state resembling dedifferentiation.

Transcriptome analysis of a subtropical deciduous tree: autumn leaf senescence gene expression profile of Formosan gum

Autumn leaf senescence is a spectacular natural phenomenon; however, the regulation networks controlling autumnal colors and the leaf senescence program remain largely unelucidated. Whether regulation of leaf senescence is similar in subtropical deciduous plants and temperate deciduous plants is also unknown. In this study, the gene expression of a subtropical deciduous tree, Formosan gum (Liquidambar formosana Hance), was profiled. The transcriptomes of April leaves (green leaves, 'G') and December leaves (red leaves, 'R') were investigated by next-generation gene sequencing. Out of 58,402 de novo assembled contigs, 32,637 were annotated as putative genes. Furthermore, the L. formosana-specific microarray designed based on total contigs was used to extend the observation period throughout the growing seasons of 2011-2013. Network analysis from the gene expression profile focused on the genes up-regulated when autumn leaf senescence occurred. LfWRKY70, LfWRKY75, LfWRKY65, LfNAC1, LfSPL14, LfNAC100 and LfMYB113 were shown to be key regulators of leaf senescnece, and the genes regulated by LfWRKY75, LfNAC1 and LfMYB113 are candidates to link chlorophyll degradation and anthocyanin biosynthesis to senescence. In summary, the gene expression profiles over the entire year of the developing leaf from subtropical deciduous trees were used for in silico analysis and the putative gene regulation in autumn coloration and leaf senescence is discussed in this study.

Potato virus X-based expression vectors are stabilized for long-term production of proteins and larger inserts

Plus-strand RNA viruses such as Potato virus X (PVX) are often used as high-yielding expression vectors in plants, because they tolerate extra transgene insertion and expression without disrupting normal virus functions. However, sequence redundancy due to promoter duplication often leads to genetic instability. Although heterologous subgenomic promoter-like sequences (SGPs) have been successfully used in Tobacco mosaic virus vectors, only homologous SGP duplications have been used in PVX vectors. We stabilized PVX-based vectors by combining heterologous SGPs from related potexviruses with an N-terminal coat protein (CP) deletion. We selected two SGPs with core sequences homologous to PVX, from Bamboo mosaic virus (BaMV) and Cassava common mosaic virus, as well as a SGP with a heterologous core sequence from Foxtail mosaic virus (FoMV). We found that only the BaMV and CsCMV SGPs were utilized by the PVX replicase. However, the transgene remained unstable, due to the presence of an additional region with strong sequence similarity at the 5' end of the cp gene. The BaMV SGP combined with an N-terminal CP deletion achieved high PVX vector stability. This new expression vector is particularly useful for long-term production of proteins and for larger inserts. The improved PVX-based vectors are suitable for the systemic expression of any gene of interest in PVX host plants. The PVX-based vector can be advantageous for the overexpression of proteins, to analyze protein functions in planta or as a system for virus-induced gene silencing.

Phyllobilins – the abundant bilin-type tetrapyrrolic catabolites of the green plant pigment chlorophyll

Phyllobilins, the bilin-type chlorophyll catabolites, are a hardly known class of abundant linear tetrapyrroles in de-greening leaves and ripening fruit.

Photochemical studies of a fluorescent chlorophyll catabolite – source of bright blue fluorescence in plant tissue and efficient sensitizer of singlet oxygen

Hypermodified fluorescent chlorophyll catabolites, which accumulate in yellow banana peels, show strong blue fluorescence and generate singlet oxygen with high quantum efficiency.

Water-Use Efficiency Declines during Autumn Leaf Senescence in Three Deciduous Tree Species

During fall leaf senescence in deciduous species, photosynthesis nears completion due to chlorophyll breakdown and re-assimilation. However, several other processes such as leaf nutrient uptake, re-translocation, and storage, or tissue dehydration to avoid frost damage, may be important and dependent upon stomatal opening. We report here on measured changes in photosynthesis (A), leaf conductance to water vapor (g), and WUE (estimated by A/g) in three deciduous tree species (Acer saccharum, Cornus florida, and Ginkgo biloba) during the weeks of leaf senescence preceding abscission. Substantial decreases in A of 60 up to 80% were not matched quantitatively by similar declines in g (40 to 70%), resulting in corresponding decreases in WUE (estimated by A/g) from near 50% to over 300% among the three species. This shift to a lower WUE may reflect adaptive value in maintaining a higher g relative to A during the fall leaf senescence period.

Metabolic indicators of drought stress tolerance in wheat: glutamine synthetase isoenzymes and Rubisco

Drought stress has a considerable impact on the ecosystem and agriculture. Continuous water deficit induces early leaf senescence in plants. During this process, chloroplasts are degraded and photosynthesis drastically drops. The objective of this investigation was to look into the regulation of nitrogen and carbon metabolism during water deficit. Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) and the total protein contents inform us of the sink-source relation in plants. Glutamine synthetase (GS, EC 6.3.1.2) isoenzymes are good markers of plastid status (GS2) and the nitrogen metabolism (GS1). Tolerant and sensitive wheat (Triticum aestivum L.) genotypes were tested, which are widely used in agriculture. The amount of protein, Rubisco and GS isoforms in leaves were measured during the grain filling period, as indicative traits that ultimately determine the onset and stage of senescence. The symptoms of senescence first appeared on the oldest and finally on the youngest leaves. Drought stress disrupted the sequentiality of senescence in the sensitive varieties. An untimely senescence appeared in flag leaves, earlier than in the older leaves. Total protein and Rubisco contents decreased and the GS2 isoenzyme declined considerably in the youngest leaves. In the tolerant varieties, however, these physiological parameters did not change under drought, only the sequential senescence of leaf levels accelerated in some cases compared to the control, well-watered plants. Our results revealed that GS is a good indicator of drought stress, which can be applied for the characterization of wheat cultivars in terms of drought stress tolerance.

Extended leaf senescence promotes carbon gain and nutrient resorption: importance of maintaining winter photosynthesis in subtropical forests

The relative advantages of being deciduous or evergreen in subtropical forests and the relationship between leaf phenology and nutrient resorption efficiency are not well understood. The most successful deciduous species (Lyonia ovalifolia) in an evergreen-dominated subtropical montane cloud forest in southwest (SW) China maintains red senescing leaves throughout much of the winter. The aim of this study was to investigate whether red senescing leaves of this species were able to assimilate carbon in winter, to infer the importance of maintaining a positive winter carbon balance in subtropical forests, and to test whether an extended leaf life span is associated with enhanced nutrient resorption and yearly carbon gain. The red senescing leaves of L. ovalifolia assimilated considerable carbon during part of the winter, resulting in a higher yearly carbon gain than co-occurring deciduous species. Its leaf N and P resorption efficiency was higher than for co-occurring non-anthocyanic deciduous species that dropped leaves in autumn, supporting the hypothesis that anthocyanin accumulation and/or extended leaf senescence help in nutrient resorption. Substantial winter carbon gain and efficient nutrient resorption may partially explain the success of L. ovalifolia versus that of the other deciduous species in this subtropical forest. The importance of maintaining a positive carbon balance for ecological success in this forest also provides indirect evidence for the dominance of evergreen species in the subtropical forests of SW China.

NYC4, the rice ortholog of Arabidopsis THF1, is involved in the degradation of chlorophyll - protein complexes during leaf senescence

Yellowing/chlorophyll breakdown is a prominent phenomenon in leaf senescence, and is associated with the degradation of chlorophyll - protein complexes. From a rice mutant population generated by ionizing radiation, we isolated nyc4-1, a stay-green mutant with a defect in chlorophyll breakdown during leaf senescence. Using gene mapping, nyc4-1 was found to be linked to two chromosomal regions. We extracted Os07g0558500 as a candidate for NYC4 via gene expression microarray analysis, and concluded from further evidence that disruption of the gene by a translocation-related event causes the nyc4 phenotype. Os07g0558500 is thought to be the ortholog of THF1 in Arabidopsis thaliana. The thf1 mutant leaves show variegation in a light intensity-dependent manner. Surprisingly, the Fv /Fm value remained high in nyc4-1 during the dark incubation, suggesting that photosystem II retained its function. Western blot analysis revealed that, in nyc4-1, the PSII core subunits D1 and D2 were significantly retained during leaf senescence in comparison with wild-type and other non-functional stay-green mutants, including sgr-2, a mutant of the key regulator of chlorophyll degradation SGR. The role of NYC4 in degradation of chlorophyll and chlorophyll - protein complexes during leaf senescence is discussed.

Expression of enzymes involved in chlorophyll catabolism in Arabidopsis is light controlled

We found that the levels of mRNA of two enzymes involved in chlorophyll catabolism in Arabidopsis (Arabidopsis thaliana), products of two chlorophyllase genes, AtCLH1 and AtCLH2, dramatically increase (by almost 100- and 10-fold, respectively) upon illumination with white light. The measurements of photosystem II quantum efficiency in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-inhibited leaves show that their expression is not related to photosynthesis but mediated by photoreceptors. To identify the photoreceptors involved, we used various light treatments and Arabidopsis photoreceptor mutants (cry1, cry2, cry1cry2, phot1, phot2, phot1phot2, phyA phyB, phyAphyB). In wild-type Columbia, the amount of transcripts of both genes increase after white-light irradiation but their expression profile and the extent of regulation differ considerably. Blue and red light is active in the case of AtCLH1, whereas only blue light raises the AtCLH2 mRNA level. The fundamental difference is the extent of up-regulation, higher by one order of magnitude in AtCLH1. Both blue and red light is active in the induction of AtCLH1 expression in all mutants, pointing to a complex control network and redundancy between photoreceptors. The blue-specific up-regulation of the AtCLH2 transcript is mediated by cryptochromes and modulated by phototropin1 and phytochromes. Individually darkened leaves were used to test the effects of senescence on the expression of AtCLH1 and AtCLH2. The expression profile of AtCLH1 remains similar to that found in nonsenescing leaves up to 5 d after darkening. In contrast, the light induction of AtCLH2 mRNA declines during dark treatment. These results demonstrate that the expression of enzymes involved in chlorophyll catabolism is light controlled.

Chromoplast differentiation: current status and perspectives

Chromoplasts are carotenoid-accumulating plastids conferring color to many flowers and fruits as well as to some tubers and roots. Chromoplast differentiation proceeds from preexisting plastids, most often chloroplasts. One of the most prominent changes is remodeling of the internal membrane system associated with the formation of carotenoid-accumulating structures. During the differentiation process the plastid genome is essentially stable and transcriptional activity is restricted. The buildup of the chromoplast for specific metabolic characteristics is essentially dependent upon the transcriptional activity of the nucleus. Important progress has been made in terms of mediation of the chloroplast-to-chromoplast transition with the discovery of the crucial role of the Or gene. In this article we review recent developments in the structural, biochemical and molecular aspects of chromoplast differentiation and also consider the reverse differentiation of chromoplasts into chloroplast-like structures during the regreening process occurring in some fruit. Future perspectives toward a full understanding of chromoplast differentiation include in-depth knowledge of the changes occurring in the plastidial proteome during chromoplastogenesis, elucidation of the role of hormones and the search for signals that govern the dialog between the nuclear and the chromoplastic genome.