Fruit softening: evidence for rhamnogalacturonan lyase action in vivo in ripe fruit cell walls

BACKGROUND AND AIMS

The softening of ripening fruit involves partial depolymerization of cell-wall pectin by three types of reaction: enzymic hydrolysis, enzymic elimination (lyase-catalysed) and non-enzymic oxidative scission. Two known lyase activities are pectate lyase and rhamnogalacturonan lyase (RGL), potentially causing mid-chain cleavage of homogalacturonan and rhamnogalacturonan-I (RG-I) domains of pectin respectively. However, the important biological question of whether RGL exhibits action in vivo had not been tested.

METHODS

We developed a method for specifically and sensitively detecting in-vivo RGL products, based on Driselase digestion of cell walls and detection of a characteristic unsaturated 'fingerprint' product (tetrasaccharide) of RGL action.

KEY RESULTS

In model experiments, potato RG-I that had been partially cleaved in vitro by commercial RGL was digested by Driselase, releasing an unsaturated tetrasaccharide ('ΔUA-Rha-GalA-Rha'), taken as diagnostic of RGL action. This highly acidic fingerprint compound was separated from monosaccharides (galacturonate, galactose, rhamnose, etc.) by electrophoresis at pH 2, then separated from ΔUA-GalA (the fingerprint of pectate lyase action) by thin-layer chromatography. The 'ΔUA-Rha-GalA-Rha' was confirmed as 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→2)-l-rhamnosyl-(1→4)-d-galacturonosyl-(1→2)-l-rhamnose by mass spectrometry and acid hydrolysis. Driselase digestion of cell walls from diverse ripe fruits [date, sea buckthorn, cranberry, yew (arils), mango, plum, blackberry, apple, pear and strawberry] yielded the same fingerprint compound, demonstrating that RGL had been acting in vivo in these fruits prior to harvest. The 'fingerprint' : (galacturonate + rhamnose) ratio in digests from ripe dates was approximately 1 : 72 (mol/mol), indicating that ~1.4 % of the backbone Rha→GalA bonds in endogenous RG-I had been cleaved by in-vivo RGL action.

CONCLUSIONS

The results provide the first demonstration that RGL, previously known from studies of fruit gene expression, proteomic studies and in-vitro enzyme activity, exhibits enzyme action in the walls of soft fruits and may thus be proposed to contribute to fruit softening.

The apple transcription factor MdZF-HD11 regulates fruit softening by promoting Mdβ-GAL18 expression

Fruit ripening and the associated softening are major determinants of fruit quality and post-harvest shelf life. Although the mechanisms underlying fruit softening have been intensively studied, there are limited reports on the regulation of fruit softening in apples (Malus domestica). Here, we identified a zinc finger homeodomain transcription factor MdZF-HD11that trans-activates the promoter of Mdβ-GAL18, which encodes a pectin-degradation enzyme associated with cell wall metabolism. Both MdZF-HD11 and Mdβ-GAL18 genes were up-regulated by exogenous ethylene treatment and repressed by 1-methylcyclopropene treatment. Further experiments revealed that MdZF-HD11 binds directly to the Mdβ-GAL18 promoter and up-regulates its transcription. Moreover, using transgenic apple fruit calli, we found that overexpression of Mdβ-GAL18 or MdZF-HD11 significantly enhanced β-galactosidase activity, and overexpression of MdZF-HD11 induced the expression of Mdβ-GAL18. We also discovered that transient overexpression of Mdβ-GAL18 or MdZF-HD11 in 'Golden Delicious' apple significantly increased the release of ethylene, reduced fruit firmness, promoted the transformation of skin color from green to yellow, and accelerated ripening and softening of the fruit. Finally, the overexpression of MdZF-HD11 in tomato also promoted fruit softening. Collectively, these results indicate that ethylene-induced MdZF-HD11 interacts with Mdβ-GAL18 to promote the post-harvest softening of apple.

Genome-Wide Identification of Expansins in Rubus chingii and Profiling Analysis during Fruit Ripening and Softening

Improving fruit size or weight, firmness, and shelf life is a major target for horticultural crop breeding. It is associated with the depolymerization and rearrangement of cell components, including pectin, hemicellulose, cellulose, and other structural (glyco)proteins. Expansins are structural proteins to loosen plant cell wall polysaccharides in a pH-dependent manner and play pivotal roles in the process of fruit development, ripening, and softening. Rubus chingii Hu, a unique Chinese red raspberry, is a prestigious pharmaceutical and nutraceutical dual-function food with great economic value. Thirty-three RchEXPs were predicted by genome-wide identification in this study, containing twenty-seven α-expansins (EXPAs), three β-expansins (EXPBs), one expansin-like A (EXPLA), and two expansin-like B (EXPLBs). Subsequently, molecular characteristics, gene structure and motif compositions, phylogenetic relationships, chromosomal location, collinearity, and regulatory elements were further profiled. Furthermore, transcriptome sequencing (RNA-seq) and real-time quantitative PCR assays of fruits from different developmental stages and lineages showed that the group of RchEXPA5, RchEXPA7, and RchEXPA15 were synergistically involved in fruit expanding and ripening, while another group of RchEXPA6 and RchEXPA26 might be essential for fruit ripening and softening. They were regulated by both abscisic acid and ethylene and were collinear with phylogenetic relationships in the same group. Our new findings laid the molecular foundation for improving the fruit texture and shelf life of R. chingii medicinal and edible fruit.

Cell wall composition and enzyme-related activities in eggplant as affected by hot water, 1-MCP and calcium chloride treatments

The effects of postharvest applications of hot water (HWT) (45, 50, and 55 °C), 1-MCP (1, 5, and 10 μL/L), and CaCl2 (1, 2, and 3%) on polygalacturonase (PG), pectin methylesterase (PME), α-galactosidase (α-Gal), β-galactosidase (β-Gal) and β-1,4-glucanase (β-1,4-Glu) activities, and the fruit firmness and cell wall composition of eggplant fruit were investigated. The results showed that the decrease in the eggplants firmness was delayed by HWT, 1-MCP, and CaCl2 treatments during storage compared with the control. However, HWTs were less effective than the 1-MCP and CaCl2 treatments. The results show that 1-MCP and CaCl2 treatments inhibited the depolymerization of water (WSP), CDTA (CSP), and sodium carbonate (SSP) soluble polyuronides. The results suggest that 1-MCP (5 and 10 μL/L) and CaCl2 (1, 2, and 3%) could prevent eggplant softening by inhibiting hydrolase enzymes and reducing the disintegration of the polysaccharides. In addition, 1-MCP and CaCl2 were more effective than hot water treatment in extending postharvest storage life. There is a significantly high correlation between firmness, polyuronide content and cell wall enzyme activity.

Comprehensive Analysis of the Pectate Lyase Gene Family and the Role of FaPL1 in Strawberry Softening

Fruit softening is a crucial factor that controls shelf life and commercial value. Pectate lyase (PL) has a major role in strawberry fruit softening. However, the PL gene family in strawberry has not been comprehensively analyzed. In this study, 65 FaPL genes were identified in the octoploid strawberry genome. Subcellular localization prediction indicated that FaPLs are mostly localized to the extracellular and cytoplasmic spaces. Duplication event analysis suggested that FaPL gene family expansion is mainly driven by whole genome or segmental duplication. The FaPL family members were classified into six groups according to the phylogenetic analysis. Among them, FaPL1, 3, 5, 20, 25, 42, and 57 had gradually increased expressions during strawberry fruit development and ripening and higher expression levels in the fruits with less firmness than that in firmer fruit. This result suggested that these members are involved in strawberry softening. Furthermore, overexpression of FaPL1 significantly reduced the fruit firmness, ascorbic acid (AsA), and malondialdehyde (MDA) content but obviously increased the anthocyanins, soluble proteins, and titratable acidity (TA), while it had no apparent effects on flavonoids, phenolics, and soluble sugar content. These findings provide basic information on the FaPL gene family for further functional research and indicate that FaPL1 plays a vital role in strawberry fruit softening.

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.

Comparative physiological and transcriptomic analyses provide insights into fruit softening in Chinese cherry [Cerasus pseudocerasus (Lindl.) G.Don]

Fruit softening is a complex, genetically programmed and environmentally regulated process, which undergoes biochemical and physiological changes during fruit development. The molecular mechanisms that determine these changes in Chinese cherry [Cerasus peseudocerasus (Lindl.) G.Don] fruits are still unknown. In the present study, fruits of hard-fleshed 'Hongfei' and soft-fleshed 'Pengzhoubai' varieties of Chinese cherry were selected to illustrate the fruit softening at different developmental stages. We analyzed physiological characteristics and transcriptome profiles to identify key cell wall components and candidate genes related to fruit softening and construct the co-expression networks. The dynamic changes of cell wall components (cellulose, hemicellulose, pectin, and lignin), the degrading enzyme activities, and the microstructure were closely related to the fruit firmness during fruit softening. A total of 6,757 and 3,998 differentially expressed genes (DEGs) were screened between stages and varieties, respectively. Comprehensive functional enrichment analysis supported that cell wall metabolism and plant hormone signal transduction pathways were involved in fruit softening. The majority of structural genes were significantly increased with fruit ripening in both varieties, but mainly down-regulated in Hongfei fruits compared with Pengzhoubai, especially DEGs related to cellulose and hemicellulose metabolism. The expression levels of genes involving lignin biosynthesis were decreased with fruit ripening, while mainly up-regulated in Hongfei fruits at red stage. These obvious differences might delay the cell all degrading and loosening, and enhance the cell wall stiffing in Hongfei fruits, which maintained a higher level of fruit firmness than Pengzhoubai. Co-expressed network analysis showed that the key structural genes were correlated with plant hormone signal genes (such as abscisic acid, auxin, and jasmonic acid) and transcription factors (MADS, bHLH, MYB, ERF, NAC, and WRKY). The RNA-seq results were supported using RT-qPCR by 25 selected DEGs that involved in cell wall metabolism, hormone signal pathways and TF genes. These results provide important basis for the molecular mechanism of fruit softening in Chinese cherry.

CRISRP/Cas9-Mediated Targeted Mutagenesis of Tomato Polygalacturonase Gene (SlPG) Delays Fruit Softening

Polygalacturonase (PG) gene has been documented as a key candidate for the improvement of fruit firmness, which is a target trait for tomato production because it facilitates transportation and storage. To reduce the expression of the PG gene, most of the elite commercial tomato varieties were obtained by RNA interference technology. However, this approach of producing commercialized tomatoes by integration of the exogenous gene is controversial. In this work, CRISPR/Cas9 technology was used to induce the targeted mutagenesis of the SlPG gene to delay the softening of tomato fruit. Results showed that the SlPG gene was frameshift mutated by 4 bp deletion, 10 bp deletion, and 1 bp insertion, which generated premature translation termination codons. Compared with wild-type (WT), homozygous T₁-generation tomato plants exhibited late fruit softening under natural conditions. Consistent with this phenomenon, the firmness value of WT fruit was lower in slpg mutant fruit, and the physiological loss of water was higher. Collectively, these data demonstrate that the mutation of the SlPG gene delays tomato fruit softening. More importantly, 8 out of 20 transgene-free tomato plants, which were homozygous for null alleles of SlPG, were separated in the T₃-generation of line slpgT₂-#2. This transgene-free slpg may provide materials for more in-depth research of SlPG functions and the molecular mechanism of fruit softening in tomatoes.

Fungus Polygalacturonase-Generated Oligogalacturonide Restrains Fruit Softening in Ripening Tomato

Fruit softening exacerbates mechanical damage incurred during shipping and handling and the increase in pathogen susceptibility. Here, oligogalacturonides (OGs) produced by fungal polygalacturonase (PG) delayed fruit softening in tomato and maintained fruit firmness at 8.37 ± 0.45 N at 13 d of storage, which was consistent with the fruit firmness level of 5 d in the control groups. From RNA sequencing data in line production of phytohormones, we confirmed ethylene and jasmonic acid signals, the MAPK signaling cascade, and calmodulin involved in the OG-mediated firmness response of whole fruit. SlPG2, SlPL3, and SlPL5 were the major contributing factors for fruit softening, and their expression decreased continuously upon OG application. Suppression of the expression of ethylene response factors using a virus-induced gene-silencing strategy revealed that SlERF6 was negatively involved in OG-restrained fruit softening. Taken together, these results indicated that fungal PG-generated OGs have potential application value in controlling tomato fruit softening.

FvMYB79 Positively Regulates Strawberry Fruit Softening via Transcriptional Activation of FvPME38

Strawberry is a soft fruit with short postharvest life, due to a rapid loss of firmness. Pectin methylesterase (PME)-mediated cell wall remodeling is important to determine fruit firmness and softening. Previously, we have verified the essential role of FvPME38 in regulation of PME-mediated strawberry fruit softening. However, the regulatory network involved in PME-mediated fruit softening is still largely unknown. Here, we identified an R2R3-type MYB transcription factor FvMYB79, which activates the expression level of FvPME38, thereby accelerating fruit softening. During fruit development, FvMYB79 co-expressed with FvPME38, and this co-expression pattern was opposite to the change of fruit firmness in the fruit of 'Ruegen' which significantly decreased during fruit developmental stages and suddenly became very low after the color turning stage. Via transient transformation, FvMYB79 could significantly increase the transcriptional level of FvPME38, leading to a decrease of firmness and acceleration of fruit ripening. In addition, silencing of FvMYB79 showed an insensitivity to ABA-induced fruit ripening, suggesting a possible involvement of FvMYB79 in the ABA-dependent fruit softening process. Our findings suggest FvMYB79 acts as a novel regulator during strawberry ripening via transcriptional activation of FvPME38, which provides a novel mechanism for improvement of strawberry fruit firmness.

Genome-Wide Identification of the Xyloglucan endotransglucosylase/Hydrolase (XTH) and Polygalacturonase (PG) Genes and Characterization of Their Role in Fruit Softening of Sweet Cherry

Fruit firmness is an important economical trait in sweet cherry (Prunus avium L.) where the change of this trait is related to cell wall degradation. Xyloglucan endotransglycosylase/hydrolase (XTH) and polygalacturonases (PGs) are critical cell-wall-modifying enzymes that occupy a crucial position in fruit ripening and softening. Herein, we identified 18 XTHs and 45 PGs designated PavXTH1-18 and PavPG1-45 based on their locations in the genome of sweet cherry. We provided a systematical overview of PavXTHs and PavPGs, including phylogenetic relationships, conserved motifs, and expression profiling of these genes. The results showed that PavXTH14, PavXTH15 and PavPG38 were most likely to participated in fruit softening owing to the substantial increment in expression during fruit development and ripening. Furthermore, the phytohormone ABA, MeJA, and ethephon significantly elevated the expression of PavPG38 and PavXTH15, and thus promoted fruit softening. Importantly, transient expression PavXTH14, PavXTH15 and PavPG38 in cherry fruits significantly reduced the fruit firmness, and the content of various cell wall components including hemicellulose and pectin significantly changed correspondingly in the transgenic fruit. Taken together, these results present an extensive analysis of XTHs and PGs in sweet cherry and provide potential targets for breeding softening-resistant sweet cherry cultivars via manipulating cell wall-associated genes.

Molecular Insights of Fruit Quality Traits in Peaches, Prunus persica

Fleshy fruits are the most demanded fruits because of their organoleptic qualities and nutritional values. The genus Prunus is a rich source of diversified stone/drupe fruits such as almonds, apricots, plums, sweet cherries, peaches, and nectarines. The fruit-ripening process in Prunus involves coordinated biochemical and physiological changes resulting in changes in fruit texture, aroma gain, color change in the pericarp, sugar/organic acid balance, fruit growth, and weight gain. There are different varieties of peaches with unique palatable qualities and gaining knowledge in the genetics behind these quality traits helps in seedling selection for breeding programs. In addition, peaches have shorter post-harvest life due to excessive softening, resulting in fruit quality reduction and market loss. Many studies have been executed to understand the softening process at the molecular level to find the genetic basis. To summarize, this review focused on the molecular aspects of peach fruit quality attributes and their related genetics to understand the underlying mechanisms.

Fruit softening: evidence for pectate lyase action in vivo in date (Phoenix dactylifera) and rosaceous fruit cell walls

BACKGROUND AND AIMS

The programmed softening occurring during fruit development requires scission of cell wall polysaccharides, especially pectin. Proposed mechanisms include the action of wall enzymes or hydroxyl radicals. Enzyme activities found in fruit extracts include pectate lyase (PL) and endo-polygalacturonase (EPG), which, in vitro, cleave de-esterified homogalacturonan in mid-chain by β-elimination and hydrolysis, respectively. However, the important biological question of whether PL exhibits action in vivo had not been tested.

METHODS

We developed a method for specifically and sensitively detecting in-vivo PL products, based on Driselase digestion of cell wall polysaccharides and detection of the characteristic unsaturated product of PL action.

KEY RESULTS

In model in-vitro experiments, pectic homogalacturonan that had been partially cleaved by commercial PL was digested to completion with Driselase, releasing an unsaturated disaccharide ('ΔUA-GalA'), taken as diagnostic of PL action. ΔUA-GalA was separated from saturated oligogalacturonides (EPG products) by electrophoresis, then subjected to thin-layer chromatography (TLC), resolving ΔUA-GalA from higher homologues. The ΔUA-GalA was confirmed as 4-deoxy-β-l-threo-hex-4-enopyranuronosyl-(1→4)-d-galacturonic acid by NMR spectroscopy. Driselase digestion of cell walls from ripe fruits of date (Phoenix dactylifera), pear (Pyrus communis), rowan (Sorbus aucuparia) and apple (Malus pumila) yielded ΔUA-GalA, demonstrating that PL had been acting in vivo in these fruits prior to harvest. Date-derived ΔUA-GalA was verified by negative-mode mass spectrometry, including collision-induced dissociation (CID) fragmentation. The ΔUA-GalA:GalA ratio from ripe dates was roughly 1:20 (mol mol-1), indicating that approx. 5 % of the bonds in endogenous homogalacturonan had been cleaved by in-vivo PL action.

CONCLUSIONS

The results provide the first demonstration that PL, previously known from studies of fruit gene expression, proteomic studies and in-vitro enzyme activity, exhibits enzyme action in the walls of soft fruits and may thus be proposed to contribute to fruit softening.

Transcriptome Responses of Ripe Cherry Tomato Fruit Exposed to Chilling and Rewarming Identify Reversible and Irreversible Gene Expression Changes

Tomato fruit stored below 12°C lose quality and can develop chilling injury upon subsequent transfer to a shelf temperature of 20°C. The more severe symptoms of altered fruit softening, uneven ripening and susceptibility to rots can cause postharvest losses. We compared the effects of exposure to mild (10°C) and severe chilling (4°C) on the fruit quality and transcriptome of 'Angelle', a cherry-type tomato, harvested at the red ripe stage. Storage at 4°C (but not at 10°C) for 27 days plus an additional 6 days at 20°C caused accelerated softening and the development of mealiness, both of which are commonly related to cell wall metabolism. Transcriptome analysis using RNA-Seq identified a range of transcripts encoding enzymes putatively involved in cell wall disassembly whose expression was strongly down-regulated at both 10 and 4°C, suggesting that accelerated softening at 4°C was due to factors unrelated to cell wall disassembly, such as reductions in turgor. In fruit exposed to severe chilling, the reduced transcript abundances of genes related to cell wall modification were predominantly irreversible and only partially restored upon rewarming of the fruit. Within 1 day of exposure to 4°C, large increases occurred in the expression of alternative oxidase, superoxide dismutase and several glutathione S-transferases, enzymes that protect cell contents from oxidative damage. Numerous heat shock proteins and chaperonins also showed large increases in expression, with genes showing peak transcript accumulation after different times of chilling exposure. These changes in transcript abundance were not induced at 10°C, and were reversible upon transfer of the fruit from 4 to 20°C. The data show that genes involved in cell wall modification and cellular protection have differential sensitivity to chilling temperatures, and exhibit different capacities for recovery upon rewarming of the fruit.

Comparative Analysis of Akebia trifoliata Fruit Softening at Different Flesh Ripening Stages Using Tandem Mass Tag Technology

Owing to its medicinal and high nutritional values, Akebia trifoliata can be considered as a new type of medicinal and edible homologous resources, and it has begun to be widely cultivated in many areas of China. Over-softening of fruit would affect the sensorial quality, utilization rate, and consumer acceptance of the fruit postharvest. However, fruit softening has not been characterized and the molecular mechanism underlying A. trifoliata fruit softening during ripening remains unclear. A comparative proteomic analysis was performed on the fruit at three developmental stages using tandem mass tag technology. In total, 2,839 proteins and 302 differentially abundant proteins (DAPs) were identified. Bioinformatics analysis indicated that most DAPs were implicated in oxidoreductase activity, protein domain-specific binding and pyruvate metabolism. Moreover, 29 DAPs associated with cell wall metabolism, plant hormone, and stress and defense response pathways were validated using quantitative PCR. Notably, pectinesterase, pectate lyase, and β-galactosidase, which are involved in cell wall degradation, as well as gibberellin regulated protein, cysteine protease, thaumatin-like protein and heat shock proteins which is involved in plant hormone, and stress and defense response, were significantly up-regulated in softening fruit compared with the levels in non-softening fruit. This indicated that they might play key roles in A. trifoliata fruit softening. Our findings will provide new insights into potential genes influencing fruit softening traits of A. trifoliata, which will help to develop strategies to improve fruit quality and reduce softening-related losses.

Expression profiling of endo-xylanases during ripening of strawberry cultivars with contrasting softening rates. Influence of postharvest and hormonal treatments

BACKGROUND

Softening is one of the main features that determine fruit quality during strawberry (Fragaria x ananassa, Duch.) ripening and storage. Being closely related to textural changes, the molecular and biochemical bases underlying strawberry cell-wall metabolism is a matter of interest. Here we investigated the abundance of transcripts encoding putative strawberry endo-xylanases in plant tissues, during fruit ripening and under postharvest and hormonal treatments. Total xylanase activity and expression of related genes in strawberry varieties with contrasting firmness were analyzed.

RESULTS

FaXynA and FaXynC mRNA abundance was significantly higher than FaXynB in each plant tissue studied. Higher total xylanase activity was detected at the end of the ripening of the softer cultivar ('Toyonoka') in comparison with the firmer one ('Camarosa'), correlating with the abundance of FaXynA and FaXynC transcripts. Postharvest 1-methylcyclopropene treatment up-regulated FaXynA and FaXynC expressions. FaXynC mRNA abundance decreased with heat treatment but the opposite was observed for FaXynA. Calcium chloride treatment down-regulated FaXynA and FaXynC expression. Both genes responded differently to plant growth regulators' exposure. FaXynC expression was down-regulated by auxins and gibberellins treatment and up-regulated by abscisic acid. FaXynA was up-regulated by auxins, while no changes in mRNA levels were evident by abscisic acid and gibberellins treatment. Ethephon exposure did not change FaXynA and FaXynC expressions.

CONCLUSION

New knowledge about the presence of xylanases in ripening strawberry fruit and their response to postharvest and hormonal treatments is provided. Our findings suggest a role for endo-xylanases in hemicelluloses depolymerization and possibly in strawberry fruit softening. © 2020 Society of Chemical Industry.

Elucidating the role of polygalacturonase genes in strawberry fruit softening

To disentangle the role of polygalacturonase (PG) genes in strawberry softening, the two PG genes most expressed in ripe receptacles, FaPG1 and FaPG2, were down-regulated. Transgenic ripe fruits were firmer than those of the wild type when PG genes were silenced individually. Simultaneous silencing of both PG genes by transgene stacking did not result in an additional increase in firmness. Cell walls from ripe fruits were characterized by a carbohydrate microarray. Higher signals of homogalacturonan and rhamnogalacturonan I pectin epitopes in polysaccharide fractions tightly bound to the cell wall were observed in the transgenic genotypes, suggesting a lower pectin solubilization. At the transcriptomic level, the suppression of FaPG1 or FaPG2 alone induced few transcriptomic changes in the ripe receptacle, but the amount of differentially expressed genes increased notably when both genes were silenced. Many genes encoding cell wall-modifying enzymes were down-regulated. The expression of a putative high affinity potassium transporter was induced in all transgenic genotypes, indicating that cell wall weakening and loss of cell turgor could be linked. These results suggest that, besides the disassembly of pectins tightly linked to the cell wall, PGs could play other roles in strawberry softening, such as the release of oligogalacturonides exerting a positive feedback in softening.

Sensing when the wall comes tumbling down

This article comments on:

Paniagua C, Ric-Varas P, Garcia-Gago JA, López-Casado G, Blanco-Portales R, Muñoz-Blanco J, Schückel J, Knox JP, Matas AJ, Quesada MA, Posé S, Mercado JA. 2020. Elucidating the role of polygalacturonase genes in strawberry fruit softening. Journal of Experimental Botany 71, 7103–7117.

Exploring the Use of Fruit Callus Culture as a Model System to Study Color Development and Cell Wall Remodeling during Strawberry Fruit Ripening

Cell cultures derived from strawberry fruit at different developmental stages have been obtained to evaluate their potential use to study different aspects of strawberry ripening. Callus from leaf and cortical tissue of unripe-green, white, and mature-red strawberry fruits were induced in a medium supplemented with 11.3 µM 2,4-dichlorophenoxyacetic acid (2,4-D) under darkness. The transfer of the established callus from darkness to light induced the production of anthocyanin. The replacement of 2,4-D by abscisic acid (ABA) noticeably increased anthocyanin accumulation in green-fruit callus. Cell walls were isolated from the different fruit cell lines and from fruit receptacles at equivalent developmental stages and sequentially fractionated to obtain fractions enriched in soluble pectins, ester bound pectins, xyloglucans (XG), and matrix glycans tightly associated with cellulose microfibrils. These fractions were analyzed by cell wall carbohydrate microarrays. In fruit receptacle samples, pectins were abundant in all fractions, including those enriched in matrix glycans. The amount of pectin increased from green to white stage, and later these carbohydrates were solubilized in red fruit. Apparently, XG content was similar in white and red fruit, but the proportion of galactosylated XG increased in red fruit. Cell wall fractions from callus cultures were enriched in extensin and displayed a minor amount of pectins. Stronger signals of extensin Abs were detected in sodium carbonate fraction, suggesting that these proteins could be linked to pectins. Overall, the results obtained suggest that fruit cell lines could be used to analyze hormonal regulation of color development in strawberry but that the cell wall remodeling process associated with fruit softening might be masked by the high presence of extensin in callus cultures.

Inhibition of fruit softening by cold plasma treatments: affecting factors and applications

Softening is a common phenomenon of texture changes associated with plant cell walls, inducing a decrease in the quality of fruit. Inhibiting the softening is effective to extend the shelf life of fruit. Cold plasma (CP), as a novel nonthermal technology, has been applied to keep the freshness of the fruit. This review centers on applying cold plasma treatments to the inhibition of fruit softening. Different pathways for inhibiting fruit softening by CP treatments, including maintenance of fruit firmness, reduction in the activities of enzymes, inactivation of fungal pathogens and lowering of respiration rates, are discussed. The biochemistry of fruit softening and the fundamental of cold plasma are also presented. In general, among all postharvest technologies, cold plasma is a promising method with many advantages, showing great potential in maintaining the quality and inhibiting the softening of the fruit. Future work should focus on process optimization to achieve better results in maintaining fruit freshness, and commercial applications of cold plasma technology should also be explored.

Characterization of the Tomato (Solanum lycopersicum) Pectin Methylesterases: Evolution, Activity of Isoforms and Expression During Fruit Ripening

Pectin methylesterase (PME, EC 3.1.1.11) is a hydrolytic enzyme of pectin that plays multiple roles in different plant development processes and responses to biotic stress. To characterize the molecular evolution and functional divergence of the PME gene family, a genome-wide analysis of the PME gene family in the tomato was performed. In total, 57 non-redundant PME genes were identified, and these PME genes were divided into five groups based on their phylogeneny; their classification was supported by similar gene structures and domain distributions. The PME genes were found to be unevenly distributed among 12 chromosomes of the tomato. In addition, 11 segmental duplication and 11 tandem duplication events occurred in these PME genes, implying that both contributed to the expansion of the tomato PME gene family. Non-synonymous/synonymous mutation ratio analysis revealed that positive selection played a key role in the functional divergence of PME genes. Interspecific collinear analysis indicated a large divergence in the PME gene family after the divergence of monocot and dicot plants in ancient times. Gene expression pattern analysis suggested that PMEs plays roles in the different parts of the tomato plant, including the fruit. Three newly identified candidate genes (Solyc03g083360, Solyc07g071600, and Solyc12g098340) may have functions during fruit ripening. Immunoassays suggested that the tomato isoform PE1 and PE2 may change pectin structure at cell junctions, which could be associated with fruit softening. In addition, our analysis indicate that two undescribed PE isoforms might be active in leaves and fruits. This study increases our understanding of the PME gene family in the tomato and may facilitate further functional analyses to elucidate PME function, especially during fruit ripening.

Analysis of the Xyloglucan Endotransglucosylase/Hydrolase Gene Family during Apple Fruit Ripening and Softening

Ethylene and xyloglucan endotransglucosylase/hydrolase (XTH) genes were important for fruit ripening and softening in 'Taishanzaoxia' apple. In this study, we found it was ACS1-1/-1 homozygotes in 'Taishanzaoxia' apple, which determined the higher transcription activity of ACS1. XTH1, XTH3, XTH4, XTH5, and XTH9 were mainly involved in the early fruit softening independent of ethylene, while XTH2, XTH6, XTH7, XTH8, XTH10, and XTH11 were predominantly involved in the late fruit softening dependent on ethylene. Overexpression of XTH2 and XTH10 in tomato resulted in the elevated expression of genes involved in ethylene biosynthesis (ACS2, ACO1), signal transduction (ERF2), and fruit softening (XTHs, PG2A, Cel2, and TBG4). In summary, the burst of ethylene in 'Taishanzaoxia' apple was predominantly determined by ACS1-1/-1 genotype, and the differential expression of XTH genes dependent on and independent of ethylene played critical roles in the fruit ripening and softening. XTH2 and XTH10 may act as a signal switch in the feedback regulation of ethylene signaling and fruit softening.

Physiological Degradation of Pectin in Papaya Cell Walls: Release of Long Chains Galacturonans Derived from Insoluble Fractions during Postharvest Fruit Ripening

Papaya (Carica papaya L.) is a fleshy fruit that presents a rapid pulp softening during ripening. However, the timeline on how papaya pectinases act in polysaccharide solubilization and the consequent modification of the cell wall fractions during ripening is still not clear. In this work, the gene expression correlations between, on one hand, 16 enzymes potentially acting during papaya cell wall disassembling and, on the other hand, the monosaccharide composition of cell wall fractions during papaya ripening were evaluated. In order to explain differences in the ripening of papaya samplings, the molecular mass distribution of polysaccharides from water-soluble and oxalate-soluble fractions (WSF and OSF, respectively), as well as the oligosaccharide profiling from the WSF fraction, were evaluated by high performance size exclusion chromatography coupled to a refractive index detector and high performance anion-exchange chromatography coupled to pulse amperometric detection analyses, respectively. Results showed that up-regulated polygalacturonase and β-galactosidase genes were positively correlated with some monosaccharide profiles. In addition, an overall increase in the retention time of high molecular weight (HMW) and low molecular weight (LMW) polysaccharides in WSF and OSF was shown. The apparent disappearance of one HMW peak of the OSF may result from the conversion of pectin that were crosslinked with calcium into more soluble forms through the action of PGs, which would increase the solubilization of polysaccharides by lowering their molecular weight. Thus, the results allowed us to propose a detailed process of papaya cell wall disassembling that would affect sensorial properties and post-harvesting losses of this commercially important fruit.

Structural and functional analysis of tomato β-galactosidase 4: insight into the substrate specificity of the fruit softening-related enzyme

Plant β-galactosidases hydrolyze cell wall β-(1,4)-galactans to play important roles in cell wall expansion and degradation, and turnover of signaling molecules, during ripening. Tomato β-galactosidase 4 (TBG4) is an enzyme responsible for fruit softening through the degradation of β-(1,4)-galactan in the pericarp cell wall. TBG4 is the only enzyme among TBGs 1-7 that belongs to the β-galactosidase/exo-β-(1,4)-galactanase subfamily. The enzyme can hydrolyze a wide range of plant-derived (1,4)- or 4-linked polysaccharides, and shows a strong ability to attack β-(1,4)-galactan. To gain structural insight into its substrate specificity, we determined crystal structures of TBG4 and its complex with β-d-galactose. TBG4 comprises a catalytic TIM barrel domain followed by three β-sandwich domains. Three aromatic residues in the catalytic site that are thought to be important for substrate specificity are conserved in GH35 β-galactosidases derived from bacteria, fungi and animals; however, the crystal structures of TBG4 revealed that the enzyme has a valine residue (V548) replacing one of the conserved aromatic residues. The V548W mutant of TBG4 showed a roughly sixfold increase in activity towards β-(1,6)-galactobiose, and ~0.6-fold activity towards β-(1,4)-galactobiose, compared with wild-type TBG4. Amino acid residues corresponding to V548 of TBG4 thus appear to determine the substrate specificities of plant β-galactosidases towards β-1,4 and β-1,6 linkages.

Antisense down-regulation of the strawberry β-galactosidase gene FaβGal4 increases cell wall galactose levels and reduces fruit softening

Strawberry softening is characterized by an increase in the solubilization and depolymerization of pectins from cell walls. Galactose release from pectin side chains by β-galactosidase enzymes has been proposed as one reason for the increase in soluble pectins. A putative β-galactosidase gene, FaβGal4, has been identified using a custom-made oligonucleotide-based strawberry microarray platform. FaβGal4 was expressed mainly in the receptacle during fruit ripening, and was positively regulated by abscisic acid and negatively regulated by auxins. To ascertain the role of FaβGal4 in strawberry softening, transgenic plants containing an antisense sequence of this gene under the control of the CaMV35S promoter were generated. Phenotypic analyses were carried out in transgenic plants during three consecutive growing seasons, using non-transformed plants as control. Two out of nine independent transgenic lines yielded fruits that were 30% firmer than control at the ripe stage. FaβGal4 mRNA levels were reduced by 70% in ripe fruits from these selected transgenic lines, but they also showed significant silencing of FaβGal1, although the genes did not share significant similarity. These two transgenic lines also showed an increase in pectin covalently bound to the cell wall, extracted using Na2CO3. The amount of galactose in cell walls from transgenic fruits was 30% higher than in control; notably, the galactose increase was larger in the 1 M KOH fraction, which is enriched in hemicellulose. These results suggest that FaβGal4 participates in the solubilization of covalently bound pectins during ripening, reducing strawberry fruit firmness.

The impact of sodium nitroprusside and ozone in kiwifruit ripening physiology: a combined gene and protein expression profiling approach

BACKGROUND AND AIMS

Despite their importance in many aspects of plant physiology, information about the function of oxidative and, particularly, of nitrosative signalling in fruit biology is limited. This study examined the possible implications of O3 and sodium nitroprusside (SNP) in kiwifruit ripening, and their interacting effects. It also aimed to investigate changes in the kiwifruit proteome in response to SNP and O3 treatments, together with selected transcript analysis, as a way to enhance our understanding of the fruit ripening syndrome.

METHODS

Kiwifruits following harvest were pre-treated with 100 μm SNP, then cold-stored (0 °C, relative humidity 95 %) for either 2 or 6 months in the absence or in the presence of O3 (0·3 μL L(-1)), and subsequently were allowed to ripen at 20 °C. The ripening behaviour of fruit was characterized using several approaches: together with ethylene production, several genes, enzymes and metabolites involved in ethylene biosynthesis were analysed. Kiwifruit proteins were identified using 2-D electrophoresis coupled with nanoliquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Expression patterns of kiwifruit ripening-related genes were also analysed using real-time quantitative reverse transcription-PCR (RT-qPCR).

KEY RESULTS

O3 treatment markedly delayed fruit softening and depressed the ethylene biosynthetic mechanism. Although SNP alone was relatively ineffective in regulating ripening, SNP treatment prior to O3 exposure attenuated the O3-induced ripening inhibition. Proteomic analysis revealed a considerable overlap between proteins affected by both SNP and O3. Consistent with this, the temporal dynamics in the expression of selected kiwifruit ripening-related genes were noticeably different between individual O3 and combined SNP and O3 treatments.

CONCLUSIONS

This study demonstrates that O3-induced ripening inhibition could be reversed by SNP and provides insights into the interaction between oxidative and nitrosative signalling in climacteric fruit ripening.

The impact of sodium nitroprusside and ozone in kiwifruit ripening physiology: a combined gene and protein expression profiling approach

BACKGROUND AND AIMS

Despite their importance in many aspects of plant physiology, information about the function of oxidative and, particularly, of nitrosative signalling in fruit biology is limited. This study examined the possible implications of O3 and sodium nitroprusside (SNP) in kiwifruit ripening, and their interacting effects. It also aimed to investigate changes in the kiwifruit proteome in response to SNP and O3 treatments, together with selected transcript analysis, as a way to enhance our understanding of the fruit ripening syndrome.

METHODS

Kiwifruits following harvest were pre-treated with 100 μm SNP, then cold-stored (0 °C, relative humidity 95 %) for either 2 or 6 months in the absence or in the presence of O3 (0·3 μL L(-1)), and subsequently were allowed to ripen at 20 °C. The ripening behaviour of fruit was characterized using several approaches: together with ethylene production, several genes, enzymes and metabolites involved in ethylene biosynthesis were analysed. Kiwifruit proteins were identified using 2-D electrophoresis coupled with nanoliquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Expression patterns of kiwifruit ripening-related genes were also analysed using real-time quantitative reverse transcription-PCR (RT-qPCR).

KEY RESULTS

O3 treatment markedly delayed fruit softening and depressed the ethylene biosynthetic mechanism. Although SNP alone was relatively ineffective in regulating ripening, SNP treatment prior to O3 exposure attenuated the O3-induced ripening inhibition. Proteomic analysis revealed a considerable overlap between proteins affected by both SNP and O3. Consistent with this, the temporal dynamics in the expression of selected kiwifruit ripening-related genes were noticeably different between individual O3 and combined SNP and O3 treatments.

CONCLUSIONS

This study demonstrates that O3-induced ripening inhibition could be reversed by SNP and provides insights into the interaction between oxidative and nitrosative signalling in climacteric fruit ripening.

Fruit softening and pectin disassembly: an overview of nanostructural pectin modifications assessed by atomic force microscopy

BACKGROUND

One of the main factors that reduce fruit quality and lead to economically important losses is oversoftening. Textural changes during fruit ripening are mainly due to the dissolution of the middle lamella, the reduction of cell-to-cell adhesion and the weakening of parenchyma cell walls as a result of the action of cell wall modifying enzymes. Pectins, major components of fruit cell walls, are extensively modified during ripening. These changes include solubilization, depolymerization and the loss of neutral side chains. Recent evidence in strawberry and apple, fruits with a soft or crisp texture at ripening, suggests that pectin disassembly is a key factor in textural changes. In both these fruits, softening was reduced as result of antisense downregulation of polygalacturonase genes. Changes in pectic polymer size, composition and structure have traditionally been studied by conventional techniques, most of them relying on bulk analysis of a population of polysaccharides, and studies focusing on modifications at the nanostructural level are scarce. Atomic force microscopy (AFM) allows the study of individual polymers at high magnification and with minimal sample preparation; however, AFM has rarely been employed to analyse pectin disassembly during fruit ripening.

SCOPE

In this review, the main features of the pectin disassembly process during fruit ripening are first discussed, and then the nanostructural characterization of fruit pectins by AFM and its relationship with texture and postharvest fruit shelf life is reviewed. In general, fruit pectins are visualized under AFM as linear chains, a few of which show long branches, and aggregates. Number- and weight-average values obtained from these images are in good agreement with chromatographic analyses. Most AFM studies indicate reductions in the length of individual pectin chains and the frequency of aggregates as the fruits ripen. Pectins extracted with sodium carbonate, supposedly located within the primary cell wall, are the most affected.

Effect of silencing the two major tomato fruit pectin methylesterase isoforms on cell wall pectin metabolism

Post-harvest storage is largely limited by fruit softening, a result of cell wall degradation. Pectin methylesterase (PE) (EC 3.1.1.11) is a major hydrolase responsible for pectin de-esterification in the cell wall, a response to fruit ripening. Two major PE isoforms, PE1 and PE2, have been isolated from tomato (Solanum lycopersicon) pericarp tissue and both have previously been down-regulated using antisense suppression. In this paper, PE1 and PE2 double antisense tomato plants were successfully generated through crossing the two single antisense lines. In the double antisense fruit, approximately 10% of normal PE activity remained and ripening associated pectin de-esterification was almost completely blocked. However, double antisense fruit softened normally during ripening. In tomato fruit, the PE1 isoform was found to contribute little to total PE activity and have little effect on the degree of esterification of pectin. In contrast, the other dominant fruit isoform, PE2, has a major impact on de-esterification of total pectin. PE2 appears to act on non-CDTA-soluble pectin during ripening and on CDTA-soluble pectin before the start of ripening in a potentially block-wise fashion.

Insights into the effects of polygalacturonase FaPG1 gene silencing on pectin matrix disassembly, enhanced tissue integrity, and firmness in ripe strawberry fruits

Antisense-mediated down-regulation of the fruit-specific polygalacturonase (PG) gene FaPG1 in strawberries (Fragaria×ananassa Duch.) has been previously demonstrated to reduce fruit softening and to extend post-harvest shelf life, despite the low PG activity detected in this fruit. The improved fruit traits were suggested to be attributable to a reduced cell wall disassembly due to FaPG1 silencing. This research provides empirical evidence that supports this assumption at the biochemical, cellular, and tissue levels. Cell wall modifications of two independent transgenic antisense lines that demonstrated a >90% reduction in FaPG1 transcript levels were analysed. Sequential extraction of cell wall fractions from control and ripe fruits exhibited a 42% decrease in pectin solubilization in transgenic fruits. A detailed chromatographic analysis of the gel filtration pectin profiles of the different cell wall fractions revealed a diminished depolymerization of the more tightly bound pectins in transgenic fruits, which were solubilized with both a chelating agent and sodium carbonate. The cell wall extracts from antisense FaPG1 fruits also displayed less severe in vitro swelling. A histological analysis revealed more extended cell-cell adhesion areas and an enhanced tissue integrity in transgenic ripe fruits. An immunohistological analysis of fruit sections using the JIM5 antibody against low methyl-esterified pectins demonstrated a higher labelling in transgenic fruit sections, whereas minor differences were observed with JIM7, an antibody that recognizes highly methyl-esterified pectins. These results support that the increased firmness of transgenic antisense FaPG1 strawberry fruits is predominantly due to a decrease in pectin solubilization and depolymerization that correlates with more tightly attached cell wall-bound pectins. This limited disassembly in the transgenic lines indicates that these pectin fractions could play a key role in tissue integrity maintenance that results in firmer ripe fruit.

Down-regulation of POLYGALACTURONASE1 alters firmness, tensile strength and water loss in apple (Malus x domestica) fruit

BACKGROUND

While there is now a significant body of research correlating apple (Malus x domestica) fruit softening with the cell wall hydrolase ENDO-POLYGALACTURONASE1 (PG1), there is currently little knowledge of its physiological effects in planta. This study examined the effect of down regulation of PG1 expression in 'Royal Gala' apples, a cultivar that typically has high levels of PG1, and softens during fruit ripening.

RESULTS

PG1-suppressed 'Royal Gala' apples harvested from multiple seasons were firmer than controls after ripening, and intercellular adhesion was higher. Cell wall analyses indicated changes in yield and composition of pectin, and a higher molecular weight distribution of CDTA-soluble pectin. Structural analyses revealed more ruptured cells and free juice in pulled apart sections, suggesting improved integrity of intercellular connections and consequent cell rupture due to failure of the primary cell walls under stress. PG1-suppressed lines also had reduced expansion of cells in the hypodermis of ripe apples, resulting in more densely packed cells in this layer. This change in morphology appears to be linked with reduced transpirational water loss in the fruit.

CONCLUSIONS

These findings confirm PG1's role in apple fruit softening and suggests that this is achieved in part by reducing cellular adhesion. This is consistent with previous studies carried out in strawberry but not with those performed in tomato. In apple PG1 also appears to influence other fruit texture characters such as juiciness and water loss.

The N-glycan processing enzymes alpha-mannosidase and beta-D-N-acetylhexosaminidase are involved in ripening-associated softening in the non-climacteric fruits of capsicum

Excessive softening of fruits during the ripening process leads to deterioration. This is of significant global importance as softening-mediated deterioration leads to huge postharvest losses. N-glycan processing enzymes are reported to play an important role during climacteric fruit softening: however, to date these enzymes have not been characterized in non-climacteric fruit. Two ripening-specific N-glycan processing enzymes, α-mannosidase (α-Man) and β-D-N-acetylhexosaminidase (β-Hex), have been identified and targeted to enhance the shelf life in non-climacteric fruits such as capsicum (Capsicum annuum). The purification, cloning, and functional characterization of α-Man and β-Hex from capsicum, which belong to glycosyl hydrolase (GH) families 38 and 20, respectively, are described here. α-Man and β-Hex are cell wall glycoproteins that are able to cleave terminal α-mannose and β-D-N-acetylglucosamine residues of N-glycans, respectively. α-Man and β-Hex transcripts as well as enzyme activity increase with the ripening and/or softening of capsicum. The function of α-Man and β-Hex in capsicum softening is investigated through RNA interference (RNAi) in fruits. α-Man and β-Hex RNAi fruits were approximately two times firmer compared with the control and fruit deterioration was delayed by approximately 7 d. It is shown that silencing of α-Man and β-Hex enhances fruit shelf life due to the reduced degradation of N-glycoproteins which resulted in delayed softening. Altogether, the results provide evidence for the involvement of N-glycan processing in non-climacteric fruit softening. In conclusion, genetic engineering of N-glycan processing can be a common strategy in both climacteric and non-climacteric species to reduce the post-harvest crop losses.

Enhancement of fruit shelf life by suppressing N-glycan processing enzymes

In a globalized economy, the control of fruit ripening is of strategic importance because excessive softening limits shelf life. Efforts have been made to reduce fruit softening in transgenic tomato through the suppression of genes encoding cell wall-degrading proteins. However, these have met with very limited success. N-glycans are reported to play an important role during fruit ripening, although the role of any particular enzyme is yet unknown. We have identified and targeted two ripening-specific N-glycoprotein modifying enzymes, alpha-mannosidase (alpha-Man) and beta-D-N-acetylhexosaminidase (beta-Hex). We show that their suppression enhances fruit shelf life, owing to the reduced rate of softening. Analysis of transgenic tomatoes revealed approximately 2.5- and approximately 2-fold firmer fruits in the alpha-Man and beta-Hex RNAi lines, respectively, and approximately 30 days of enhanced shelf life. Overexpression of alpha-Man or beta-Hex resulted in excessive fruit softening. Expression of alpha-Man and beta-Hex is induced by the ripening hormone ethylene and is modulated by a regulator of ripening, rin (ripening inhibitor). Furthermore, transcriptomic comparative studies demonstrate the down-regulation of cell wall degradation- and ripening-related genes in RNAi fruits. It is evident from these results that N-glycan processing is involved in ripening-associated fruit softening. Genetic manipulation of N-glycan processing can be of strategic importance to enhance fruit shelf life, without any negative effect on phenotype, including yield.