Structural network of arabinogalactan proteins (AGPs) and pectins in apple fruit during ripening and senescence processes

Low oxygen environment effect on the tomato cell wall composition during the fruit ripening process

BACKGROUND

Oxygen concentration is a key characteristic of the fruit storage environment determining shelf life and fruit quality. The aim of the work was to identify cell wall components that are related to the response to low oxygen conditions in fruit and to determine the effects of such conditions on the ripening process. Tomato (Solanum lycopersicum) fruits at different stages of the ripening process were stored in an anoxic and hypoxic environment, at 0% and 5% oxygen concentrations, respectively. We used comprehensive and comparative methods: from microscopic immunolabelling and estimation of enzymatic activities to detailed molecular approaches. Changes in the composition of extensin, arabinogalactan proteins, rhamnogalacturonan-I, low methyl-esterified homogalacturonan, and high methyl-esterified homogalacturonan were analysed.

RESULTS

In-depth molecular analyses showed that low oxygen stress affected the cell wall composition, i.e. changes in protein content, a significantly modified in situ distribution of low methyl-esterified homogalacturonan, appearance of callose deposits, disturbed native activities of β-1,3-glucanase, endo-β-1,4-glucanase, and guaiacol peroxidase (GPX), and disruptions in molecular parameters of single cell wall components. Taken together, the data obtained indicate that less significant changes were observed in fruit in the breaker stage than in the case of the red ripe stage. The first symptoms of changes were noted after 24 h, but only after 72 h, more crucial deviations were visible. The 5% oxygen concentration slows down the ripening process and 0% oxygen accelerates the changes taking place during ripening.

CONCLUSIONS

The observed molecular reset occurring in tomato cell walls in hypoxic and anoxic conditions seems to be a result of regulatory and protective mechanisms modulating ripening processes.

The modified activity of prolyl 4 hydroxylases reveals the effect of arabinogalactan proteins on changes in the cell wall during the tomato ripening process

Arabinogalactan proteins (AGPs) are proteoglycans with an unusual molecular structure characterised by the presence of a protein part and carbohydrate chains. Their specific properties at different stages of the fruit ripening programme make AGPs unique markers of this process. An important function of AGPs is to co-form an amorphous extracellular matrix in the cell wall-plasma membrane continuum; thus, changes in the structure of these molecules can determine the presence and distribution of other components. The aim of the current work was to characterise the molecular structure and localisation of AGPs during the fruit ripening process in transgenic lines with silencing and overexpression of SlP4H3 genes (prolyl 4 hydroxylase 3). The objective was accomplished through comprehensive and comparative in situ and ex situ analyses of AGPs from the fruit of transgenic lines and wild-type plants at specific stages of ripening. The experiment showed that changes in prolyl 4 hydroxylases (P4H3) activity affected the content of AGPs and the progress in their modifications in the ongoing ripening process. The analysis of the transgenic lines confirmed the presence of AGPs with high molecular weights (120-60 kDa) at all the examined stages, but a changed pattern of the molecular features of AGPs was found in the last ripening stages, compared to WT. In addition to the AGP molecular changes, morphological modifications of fruit tissue and alterations in the spatio-temporal pattern of AGP distribution at the subcellular level were detected in the transgenic lines with the progression of the ripening process. The work highlights the impact of AGPs and their alterations on the fruit cell wall and changes in AGPs associated with the progression of the ripening process.

A practical guide to in situ and ex situ characterisation of arabinogalactan proteins (AGPs) in fruits

BACKGROUND

Arabinogalactan proteins (AGPs) are plant cell components found in the extracellular matrix that play crucial roles in fruit growth and development. AGPs demonstrate structural diversity due to the presence of a protein domain and an expanded carbohydrate moiety. Considering their molecular structure, the modification of glycosylation is a primary factor contributing to the functional variety of AGPs.

MAIN BODY

Immunocytochemical methods are used for qualitative and quantitative analyses of AGPs in fruit tissues. These include in situ techniques such as immunofluorescence and immunogold labelling for visualising AGP distribution at different cellular levels and ex situ methods such as Western blotting and enzyme-linked immunoenzymatic assays (ELISA) for molecular characterisation and quantitative detection of isolated AGPs. The presented techniques were modified by considering the structure of AGPs and the changes that occur in fruit tissues during the development and ripening processes. These methods are based on antibodies that recognise carbohydrate chains, which are the only commercially available highly AGP-specific tools. These probes recognise AGP epitopes and identify structural modifications and changes in spatio-temporal distribution, shedding light on their functions in fruit.

CONCLUSION

This paper provides a concise overview of AGP research methods, emphasising their use in fruit tissue analysis and demonstrating the accessibility gaps in other tools used in such research (e.g. antibodies against protein moieties). It underscores fruit tissue as a valuable source of AGPs and emphasises the potential for future research to understand of AGP synthesis, degradation, and their roles in various physiological processes. Moreover, the application of advanced probes for AGP visualisation is a milestone in obtaining more detailed insights into the localisation and function of these proteins within fruit.

Potential role of cell wall pectin polysaccharides, water state, and cellular structure on twice "increase-decrease" texture changes during kohlrabi pickling process

Pickled kohlrabi is a traditional and favored vegetable product in China. During pickling, the hardness, springiness, and chewiness of kohlrabi all experienced a typical change with twice "increase-decrease" trend. However, little is known about its mechanism. In this study, in situ analysis including immunofluorescence, low field nuclear magnetic, and transmission electron microscopy were used to explore the effects of cell wall pectin, water state, and cellular structure on kohlrabi texture changes during pickling. Results revealed that at the early stage, due to the rapid loss of water after three times salting, the cells shrank and the interstitial space reduced, resulting in the first increase on kohlrabi texture. Subsequently, the dehydration-rehydration caused by the first brine processing resulted in the first decrease on kohlrabi texture. Then under the action of PME enzyme, more low-esterified pectin was produced, and chelate-soluble pectin with more branched structure was further formed, leading to another elevation of the sample texture. As the pickling continued, under the combined action of PG and PME, the molecular weight of pectin was decreased and the rigidity of the cell tissue was destroyed, caused kohlrabi texture continued to decline. These researches could provide important information and guidance for better maintaining the texture of pickled vegetables during processing.

An Integrative Transcriptomics and Proteomics Approach to Identify Putative Genes Underlying Fruit Ripening in Tomato near Isogenic Lines with Long Shelf Life

The elucidation of the ripening pathways of climacteric fruits helps to reduce postharvest losses and improve fruit quality. Here, we report an integrative study on tomato ripening for two near-isogenic lines (NIL115 and NIL080) with Solanum pimpinellifolium LA0722 introgressions. A comprehensive analysis using phenotyping, molecular, transcript, and protein data were performed. Both NILs show improved fruit firmness and NIL115 also has longer shelf life compared to the cultivated parent. NIL115 differentially expressed a transcript from the APETALA2 ethylene response transcription factor family (AP2/ERF) with a potential role in fruit ripening. E4, another ERF, showed an upregulated expression in NIL115 as well as in the wild parent, and it was located physically close to a wild introgression. Other proteins whose expression levels changed significantly during ripening were identified, including an ethylene biosynthetic enzyme (ACO3) and a pectate lyase (PL) in NIL115, and an alpha-1,4 glucan phosphorylase (Pho1a) in NIL080. In this study, we provide insights into the effects of several genes underlying tomato ripening with potential impact on fruit shelf life. Data integration contributed to unraveling ripening-related genes, providing opportunities for assisted breeding.

Working towards arabinogalactan proteins (AGPs) from fruit: carbohydrate composition and impact on fungal growth

BACKGROUND

Arabinogalactan proteins (AGPs) are extracellular matrix constituents involved in plant response to fungal infection. The aim of the current study was to investigate the antifungal effect of AGPs ex situ and to determine the structural features of AGPs that may have an influence on this activity. The features of AGPs isolated from fruit were investigated with molecular tools based on specific monoclonal antibodies recognizing carbohydrate AGP epitopes. The Antifungal (well-diffusion) Susceptibility Test and the Agar Invasion Test were used to assess the impact of AGPs on Penicillium notatum culture.

RESULTS

The results definitely ruled out the influence of AGPs on fungal growth. The immunochemical analyses revealed that AGPs consist mainly of carbohydrate chains composed of β-linked glucuronosyl residues recognized by LM2 and GlcA-β(1 → 3)-GalA-α(1 → 2) Rha recognized by JIM13, which do not have the same functional properties outside the plant cell in in vitro experimental conditions.

CONCLUSIONS

The action of a single cell wall component does not elicit any influence ex situ. The extensive accumulation of glycan chains of AGPs in infected tissue as a result of a complex mechanism occurring in the cell wall emphasizes the importance of dependencies between particular components of the extracellular matrix in response to fungal attack.

Not Just a Simple Sugar: Arabinose Metabolism and Function in Plants

Growth, development, structure as well as dynamic adaptations and remodeling processes in plants are largely controlled by properties of their cell walls. These intricate wall structures are mostly made up of different sugars connected through specific glycosidic linkages but also contain many glycosylated proteins. A key plant sugar that is present throughout the plantae, even before the divergence of the land plant lineage, but is not found in animals, is l-arabinose (l-Ara). Here, we summarize and discuss the processes and proteins involved in l-Ara de novo synthesis, l-Ara interconversion, and the assembly and recycling of l-Ara-containing cell wall polymers and proteins. We also discuss the biological function of l-Ara in a context-focused manner, mainly addressing cell wall-related functions that are conferred by the basic physical properties of arabinose-containing polymers/compounds. In this article we explore these processes with the goal of directing future research efforts to the many exciting yet unanswered questions in this research area.

Evaluation of the Changes in Optical Properties of Peaches with Different Maturity Levels during Bruising

The main objective was to measure the optical coefficients of peaches after bruising at different maturity levels and detect bruises. A spatially resolved method was used to acquire absorption coefficient (μ_a) and the reduced scattering coefficient (µ_s') spectra from 550 to 1000 nm, and a total of 12 groups (3 maturity levels * 4 detection times) were used to assess changes in µ_a and µ_s' resulting from bruising. Maturation and bruising both caused a decrease in µ_s' and an increase in µ_a, and the optical properties of immature peaches changed more after bruising than the optical properties of ripe peaches. Four hours after bruising, the optical properties of most samples were significantly different from those of intact peaches (p < 0.05), and the optical properties showed damage to tissue earlier than the appearance symptoms observed with the naked eye. The classification results of the Support Vector Machine model for bruised peaches showed that μ_a had the best classification accuracy compared to μ_s' and their combinations (µ_a × µ_s', µ_eff). Overall, based on μ_a, the average detection accuracies for peaches after bruising of 0 h, 4 h, and 24 h were increased.

Investigations of changes in the arabinogalactan proteins (AGPs) structure, size and composition during the fruit ripening process

Arabinogalactan proteins (AGPs) are ubiquitous cell wall and plasma membrane components and are characterised by extensive glycosylation and heterogeneity of their carbohydrate and protein units. The aim of the study was to evaluate the structural features of AGPs present in apple fruits at different stages of the ripening process. AGPs were extracted using the Yariv reagent and examined using SDS-PAGE, immunoblotting, FT-IR, and AFM. In situ analysis, immunofluorescence (CLSM) and immunogold-labelling (TEM), were performed. We demonstrated that AGPs were indeed present in apple fruits at the different stages of the ripening process. The changes in the amount (1.52–2.08 mg g⁻¹), diameter (152.73–75.05 nm), molecular mass (50–250 kDa), and distribution in the cell of AGPs demonstrate their variable presence and changeable structure during the ripening process. We propose specific wavenumbers, i.e. 1265 cm⁻¹, 1117 cm⁻¹, and 960 cm⁻¹, which could be assigned to AGPs. The immunofluorescence and immunogold-labelling results indicate that the JIM13 antibody is the most characteristic for AGPs in apple fruits. This study quantitatively demonstrated for the first time that AGP accumulation occurs in ripe fruits, which is supported by the highest AGPs content, the highest molecular mass, and the appearance of a specific distribution pattern at the cellular level.

The role of arabinogalactan proteins (AGPs) in fruit ripening-a review

Arabinogalactan proteins (AGPs) are proteoglycans challenging researchers for decades. However, despite the extremely interesting polydispersity of their structure and essential application potential, studies of AGPs in fruit are limited, and only a few groups deal with this scientific subject. Here, we summarise the results of pioneering studies on AGPs in fruit tissue with their structure, specific localization pattern, stress factors influencing their presence, and a focus on recent advances. We discuss the properties of AGPs, i.e., binding calcium ions, ability to aggregate, adhesive nature, and crosslinking with other cell wall components that may also be implicated in fruit metabolism. The aim of this review is an attempt to associate well-known features and properties of AGPs with their putative roles in fruit ripening. The putative physiological significance of AGPs might provide additional targets of regulation for fruit developmental programme. A comprehensive understanding of the AGP expression, structure, and untypical features may give new information for agronomic, horticulture, and renewable biomaterial applications.

Assessment of the optical properties of peaches with fungal infection using spatially-resolved diffuse reflectance technique and their relationships with tissue structural and biochemical properties

This research measured the optical absorption (µ_a) and reduced scattering (μ_s') properties in peaches during quality deterioration, and determine the relationships of the optical parameters with select structural and biochemical parameters. Spatially resolved reflectance was measured for healthy and fungal infected peaches, followed by physical (the size and tissue color), structural [membrane permeability and SEM], and biochemical (Vc, soluble sugar, titratable acid, chlorophyll, total phenolic content) measurements. Both µ_a and µ_s' were correlated well with the cellulosic structural and biochemical parameters of peaches, and they had the best correlations with those quality parameters at 675 nm. The correlation of μ_s' with membrane permeability was the highest from -0.962-0.743, while μ_a had the best correlation with the chlorophyll content at 675 nm which is an indicator of plant maturation and senescence. These findings would be useful for further development of an effective optical technique for early disease detection of peach fruit.

Properties of Arabinogalactan Proteins (AGPs) in Apple (Malus × Domestica) Fruit at Different Stages of Ripening

Arabinogalactan proteins (AGPs) are constituents of the cell wall–plasma membrane continuum in fruit tissue. The aim of the study was to characterise AGPs contained in fruit by determination of their chemical structure and morphological properties. The results were obtained from in and ex situ investigations and a comparative analysis of AGPs present in Malus × domestica fruit at different stages of ripening from green fruit through the mature stage to over-ripening during fruit storage. The HPLC and colorimetric methods were used for analyses of the composition of monosaccharides and proteins in AGPs extracted from fruit. We have found that AGPs from fruit mainly consists of carbohydrate chains composed predominantly of arabinose, galactose, glucose, galacturonic acid, and xylose. The protein moiety accounts for 3.15–4.58%, which depends on the various phases of ripening. Taken together, our results show that the structural and morphological properties of AGPs and calcium concentration in AGPs are related to the progress of ripening, which is correlated with proper fruit cell wall assembly. In line with the existing knowledge, our data confirmed the typical carbohydrate composition of AGPs and may be the basis for studies regarding their presumed properties of binding calcium ions.

An acidic heteropolysaccharide from Lycii fructus: Purification, characterization, neurotrophic and neuroprotective activities in vitro

Regeneration of neurites network constitutes a neurotrophic and therapeutic strategy for Parkinson's disease (PD). Increasing evidence is supporting the potential application of natural polysaccharides in prevention or treatment of PD. In this study, an acidic heteropolysaccharide LFP-1 was isolated from Lycii fructus, and purified by ion-exchange and gel filtration chromatography. Structural features of LFP-1 were analyzed with molecular weight (MW) distribution, monosaccharide composition, methylation and nuclear magnetic resonance (NMR) spectra. LFP-1 was a complicated structured polysaccharide with an average MW of 1.78 × 10⁴ Da and composed of highly branched arabinogalactans, homogalacturonan and rhamnogalacturonan moieties. LFP-1 promoted neuronal differentiation and neurite outgrowth in vitro in PC12 cell models. Furthermore, LFP-1 had a significantly protective effect against 1-methyl-4-phenylpyridiniumion (MPP+)-induced neurotoxicity in PD model PC12 cells. These observations unambiguously indicated the neurotrophic and neuroprotective activities of LFP-1, which may be developed for prevention or treatment of neurodegeneration in PD.

Linkage Mapping and Comparative Transcriptome Analysis of Firmness in Watermelon (Citrullus lanatus)

Watermelon fruit texture and quality are determined by flesh firmness. As a quality trait, flesh firmness is controlled by multigenes. Defining the key regulatory factors of watermelon flesh firmness is of great significance for watermelon genetic breeding. In this study, the hard-flesh egusi seed watermelon PI186490 was used as the male parent, the soft-flesh cultivated watermelon W1-1 was used as the female parent, and 175 F₂ generations were obtained from selfing F₁. Primary mapping of the major genes controlling center flesh firmness was achieved by bulked-segregant analysis (BSA)-Seq analysis and molecular marker technology. Finally, major genes were delimited in the physical interval between 6,210,787 and 7,742,559 bp on chromosome 2 and between 207,553 and 403,137 bp on chromosome 8. The content of each cell wall component and hormone was measured, and comparative transcriptome analysis was performed during fruit development in watermelon. The protopectin, cellulose, hemicellulose, indole-3-acetic acid (IAA) and abscisic acid (ABA) contents were measured, and paraffin sections were made during the three fruit developmental stages. The results revealed that protopectin, celluloses, and hemicelluloses exhibited similar trends for flesh firmness, while the IAA and ABA concentrations continued to decrease with fruit ripening. Paraffin sections showed that PI186490 cells were more numerous, were more tightly packed, had clearer cell wall edges and had thicker cell walls than W1-1 cells at every developmental stage. Comparative transcriptome analysis was conducted on RNA samples of flesh during fruit development and ripening in W1-1 and PI186490. The results from the localization interval transcriptome analysis showed that Cla016033 (DUF579 family member), which may influence the cell wall component contents to adjust the flesh firmness in watermelon fruit, was different in W1-1 and PI186490 and that Cla012507 (MADS-box transcription factor) may be involved in the regulation of fruit ripening and affect the hardness of watermelon fruit.

Immunocytochemical studies on the distribution of arabinogalactan proteins (AGPs) as a response to fungal infection in Malus x domestica fruit

Arabinogalactan proteins (AGPs) are cell components implicated in plant-microbe interactions. Despite the significance of AGPs in response to stress factors, their distribution during development of fungal disease in fruit is unknown. In our work, in situ analysis of AGP arrangement in fruit inoculated with Penicillium spinulosum during the consecutive days of infection development was carried out. For immunolocalization of AGPs, samples were incubated with JIM13, MAC207, LM2, and LM14 antibodies recognizing the AGP carbohydrate moieties. To analyse cell walls without proper action of AGP, an experiment with β-glucosyl Yariv reagent specifically binding AGPs was performed. The results showed an increase of signal fluorescence in the fruit after 16 days of fungal disease. Higher amounts of the examined epitopes were observed in the infection-altered sites of the fruit, in close vicinity to a surface filled by fungal spores. The results indicate that the Yariv reagent treatment induced progress of the fungal disease. Changes in the AGP presence during the fungal disease confirmed their involvement in defence against pathogen attack in fruit.

Gene expression and localization of arabinogalactan proteins during the development of anther, ovule, and embryo in rice

Arabinogalactan proteins (AGPs) are hyperglycosylated members of the hydroxyproline-rich glycoprotein (HRGP) superfamily and are widely distributed throughout the plant kingdom. In Oryza sativa (rice), the gene expression and biological function of AGPs only have received minimal research attention. Here, we used qRT-PCR to detect the expression patterns of OsAGP genes in various organs, and found that six genes were preferentially expressed in panicles, three genes were specifically expressed in anthers, and one gene in the stigma. Furthermore, using four specific monoclonal antibodies (JIM8, JIM13, LM2, MAC207), we observed the distribution of AGPs in rice anthers, ovules, and embryos. In anthers, the strong fluorescence signals of AGPs were present in tapetum cells, pollen mother cells, and mature pollens, suggesting that AGPs might be related to the development of anther and pollen. In ovules, signals of AGPs were specifically distributed in the three micropylar megaspores of the tetrad, and with intense signals in the egg cell and synergid cells in the mature embryo sac. This suggests that AGPs may be involved in megaspore determination and double fertilization. In embryos, the immunological signals of AGPs appeared in peripheral and inner cells at the early stage, and in the scutellum, plumule, and radicle at the late stage, indicating that AGPs may be associated with organ differentiation and maturation of embryos. In this study, we revealed that AGPs were widely distributed in rice anthers, ovules, and embryos, which lays a foundation for the functional investigation of AGPs in various processes of sexual reproduction.

Tracking polysaccharides during white winemaking using glycan microarrays reveals glycoprotein-rich sediments

Winemaking results in a significant amount of sediments that are formed in the tanks, the vats and in the bottles before and after fermentation. Little is known about the biochemical composition of these sediments apart from the fact that they are assumed to be derived in large part from the grape matrix. Glycan microarray technology offers a relatively rapid means to track the polysaccharides from their origin in the grape material and throughout the various steps in the winemaking process. In this study Comprehensive Microarray Polymer Profiling (CoMPP) was used to investigate the glycan-rich composition of particularly white grapes during winemaking and then investigate the effects of recombinant and commercial enzyme formulations on wine sediment compositions. The gross lees or sediments produced in the absence of enzymes were found to be composed of an abundance of homogalacturonans, rhamnogalacturonans, arabinans and galactans in addition to an abundance of extensins and arabinogalactan proteins. The addition of enzymes was shown to strip off the homogalacturonan and much of the rhamnogalacturonan with its side chains revealing a sediment layer composed almost exclusively of extensins and arabinogalactan proteins. The effect of winemaking techniques was shown to have an effect on the glycan-rich wine sediment compositions and holds implications for the management of gross lees in a winery environment.

Enzymes and vitamin C as factors influencing the presence of arabinogalactan proteins (AGPs) in Solanum lycopersicum fruit

Arabinogalactan proteins (AGPs) are ubiquitous components of the amorphous plant extracellular matrix. They are characterized by a high proportion of sugar moieties, heterogeneity of their protein backbone and carbohydrate chains. It is known that AGPs form a complex network with other basic constituents in cell wall thus it may also play a role in softening process of fruit. The use of enzymatic degradation and cell wall polysaccharide directed probes are valid analytical tools for the study of developmental modification of the fruit structure. However, it is unknown whether pectolytic enzymes affect AGPs. Thus, the aim of the current work is to detect AGP epitopes in situ to understand the impact of selected degradation enzymes on various carbohydrate moieties of AGPs. Secondly, there are no data with clarification of the impact of vitamin C on fruit ripening processes at the cellular level; hence, we also focused on the effect of vitamin C on the arrangement of AGPs as important constituents of the polysaccharide-proteoglycan network in the fruit cell wall. The results indicate that the distribution of the examined AGP carbohydrate moieties differs, which are related to changes in tissue architecture. The absence of glycan chains causes disruption in establishment of correlations between cell wall constituents and rearrangement in the cell wall structure. The induced modifications of cell walls are not comparable to alterations occurring in naturally ripening fruit, which allows a conclusion that the synergistic action of a wide variety of factors influences ripening.