UV-C irradiation delays strawberry fruit softening and modifies the expression of genes involved in cell wall degradation

Changes and postharvest regulation of activity and gene expression of enzymes related to cell wall degradation in ripening apple fruit

γ-Aminobutyric acid delays fruit softening in postharvest kiwifruit by inhibiting starch and cell wall degradation

Nitric oxide delays tomato fruit softening by inhibiting SlNAP2 (NAC-like, activated by apetala3/pistillata2) transcription factor-activated transcription of soften-related genes.

Fruit softening is an important change during the ripening process that affects fruit quality and postharvest shelf life. Cell wall degrading enzymes are considered to be the main factor responsible for cell wall disassembly and texture changes in fruit. To elucidate the effect of hot water treatment (HWT, 50 oC, 10 min) on the softening of ‘Hom Thong’ banana (Musa acuminata, AAA group), firmness, cell wall composition, the activities of cell wall degrading enzymes, and the expression of related genes were investigated in the peel. The decrease in firmness was delayed in hot water treated fruit as compare to control fruit during storage at 25 oC. Application of hot water could delay the increase of water-soluble pectin correlated to fruit firmness during storage. A higher level of HCl-soluble pectin was detected in treated fruits, indicating the reduction of cell wall polymer solubilization. Loss of hemicellulose was also slowed down in treated fruits. HWT did not affect the amount of EDTA-soluble pectin and cellulose. For enzyme activity and gene expression, treatment of intact fruit with hot water reduced activities of pectate lyase (PL) and β-galactosidase (β-Gal) in the peel, and also decreased the accumulation of their mRNAs (MaPL1 and MaGAL, respectively). Polygalacturonase (PG) enzyme activity was inhibited by heat treatment, and the increase in MaPG1 expression was delayed. HWT had little effect on pectin methylesterase (PME) or β-1,3 glucanase (Glu). After treatment, reduced PME activity was measured, but activity returned to levels similar to that of control fruits. PME activity varied during storage and did not correlate well with MaPME1 expression. There was no significant difference in Glu enzyme activity or MaGLU expression between treated and control fruit. Further investigation in cell wall structural changes following HWT through microscopic observation showed that structural alterations in peel of hot water treated banana fruits being slower as compared to untreated fruits. The reduction of peel thickness and number of cell layer occurred during storage was retarded by exposure to hot water. The formation of air space in ground tissue of hot water treated fruit was slower than that of control fruits. These results suggest that postharvest HWT was effective in preventing cell wall degradation of ‘Hom Thong’ banana fruit and reduced activities and gene expression of some enzymes associated with cell wall modification leading to maintainence of firmness in ‘Hom Thong’ banana.

Fruit ripening has been reported to be related to calcium (Ca), but the underlying mechanisms by which Ca regulates this process remain largely unknown. In order to study the changes of proteins and enriched phosphopeptides, we conducted TMT labeling, bio-material-based PTM enrichment based on mass spectrometry in Ca-treated ‘Golden Delicious’ (GD) apple fruit (Malus × domestica). This dataset presents a comprehensive overview of the critical pathways involved in fruit ripening. A total of 47 proteins and 124 phosphoproteins significantly changed in Ca-treated fruit, which are crucial for regulating the cell wall and cytoskeleton, Ca-mediated signaling and transport, ethylene production, protein fate, especially ubiquitination-based protein degradation, and primary and secondary metabolisms. Our results indicated that Ca inhibited the abundance of polygalacturonase (PG) activity and increased the phosphorylation level of CSLD3. PG and phosphorylation were involved in cell wall degradation, thereby delaying fruit softening. As a secondary messenger, Ca-mediated signaling subsequently triggered downstream mitogen-activated protein kinases (MAPK) cascades and activated the membrane, transport, and ROS signaling. Moreover, MdEIN2, a key enzyme involved in the ubiquitin of protein modification, increased at Ser753 and Ser758 in Ca-treated fruit. Furthermore, diverse primary and secondary metabolisms including glycolysis, fatty acid metabolism, and oxidation respiratory chain were modulated to prevent fruit softening. These results provide basic information from protein and phosphorylation levels for apple fruit ripening during storage, which may be helpful for apple fruit storage control.

Melatonin treatment inhibits mango fruit (Cv. ‘Guiqi’) softening by maintaining cell wall and reactive oxygen metabolisms during cold storage

This study aimed to elucidate whether 1-methylcyclopropene (1-MCP) treatment delays the fruit softening mechanism associated with the fruit quality of the newly released apple cultivars “Summer King” and “Green Ball” during cold storage. For both cultivars, the fruit treated with 1-MCP exhibited lower internal ethylene concentration, higher firmness, and higher titratable acidity relative to the control fruit, in association with less fruit softening. In addition, the treated fruit significantly delayed fresh weight loss and reduction of soluble solids content, especially in “Green Ball.” Moreover, slower degradation of cell wall components (water-soluble pectin, sodium carbonate-soluble pectin, hemicellulose, and cellulose) was also observed in the treated fruit in comparison to the control fruit. Similarly, the enzymatic activities (of polygalacturonase, pectin methylesterase, cellulase, β-galactosidase, and α-L-arabinofuranosidase) that cause cell wall degradation were relatively lower in the treated fruit than in the control fruit for both cultivars, which was further proved by transcriptional analysis of the genes encoding the enzymes. Overall, the results suggested that the usage of 1-MCP is useful to delay fruit softening of the two cultivars during cold storage by delaying the degradation of cell wall components and enzymatic activities of cell wall hydrolysis.

Effect of ozone treatment on the decay and cell wall metabolism during the postharvest storage of cantaloupe

SummaryPostharvest deterioration is among the major challenges for the fruit industry. Regulation of the fruit softening rate is an effective strategy for extending shelf‐life and reducing the economic losses due postharvest deterioration. The tomato myoinositol monophosphatase 3 gene SlIMP3, which showed highest expression level in fruit, was expressed and purified. SlIMP3 demonstrated high affinity with the L‐Gal 1‐P and D‐Ins 3‐P, and acted as a bifunctional enzyme in the biosynthesis of AsA and myoinositol. Overexpression of SlIMP3 not only improved AsA and myoinositol content, but also increased cell wall thickness, improved fruit firmness, delayed fruit softening, decreased water loss, and extended shelf‐life. Overexpression of SlIMP3 also increased uronic acid, rhamnose, xylose, mannose, and galactose content in cell wall of fruit. Treating fruit with myoinositol obtained similar fruit phenotypes of SlIMP3‐overexpressed fruit, with increased cell wall thickness and delayed fruit softening. Meanwhile, overexpression of SlIMP3 conferred tomato fruit tolerance to Botrytis cinerea. The function of SlIMP3 in cell wall biogenesis and fruit softening were also verified using another tomato species, Ailsa Craig (AC). Overexpression of SlDHAR in fruit increased AsA content, but did not affect the cell wall thickness or fruit firmness and softening. The results support a critical role for SlIMP3 in AsA biosynthesis and cell wall biogenesis, and provide a new method of delaying tomato fruit softening, and insight into the link between AsA and cell wall metabolism.

Texture is a critical quality attribute of strawberry fruit, and phytohormones play a pivotal role in fruit softening, which mainly results from cell wall metabolism, which is governed by genes and enzymes. To gain further insights into strawberry (Fragaria × ananassa, Duch. cv. Akihime ) softening, our study investigated changes across five stages in fruits in their firmness, soluble solids content (SSC), cell microstructure, cell wall materials, activities of cell wall-modifying enzymes, gene expression, endogenous phytohormone levels, and their correlation. During strawberry ripening, firmness decreased, while SSC, intercellular space, and separation of the cell wall from the plasma membrane increased. Meanwhile, the contents of ionic pectin (ISP) and cellulose (CE), pectin methylesterase (PME) activity, FaPME expression, and the levels of zeatin (Z) and strigolactone (SL) decreased, showing a positive correlation with firmness. In contrast, the activities of pectate lyase (PL) and cellulase (Cx), the expression of FaPL and FaCx, and the contents of gibberellin A4 (GA4), GA9, and abscisic acid (ABA) increased during ripening, and these were negatively correlated with firmness. These results suggest that Z and SL are associated with the maintenance of cell wall integrity and firmness, whereas increases in GA4, GA9, and ABA are linked to enhanced cell wall disassembly and fruit softening.

Excessive softening is the main factor limiting fruit shelf life and storage. Transgenic plants modified in the expression of cell wall modifying proteins have been used to investigate the role of particular activities in fruit softening during ripening, and in the manufacture of processed fruit products. Transgenic experiments show that polygalacturonase (PG) activity is largely responsible for pectin depolymerization and solubilization, but that PG-mediated pectin depolymerization requires pectin to be de-methyl-esterified by pectin methylesterase (PME), and that the PG beta-subunit protein plays a role in limiting pectin solubilization. Suppression of PG activity only slightly reduces fruit softening (but extends fruit shelf life), suppression of PME activity does not affect firmness during normal ripening, and suppression of beta-subunit protein accumulation increases softening. All these pectin-modifying proteins affect the integrity of the middle lamella, which controls cell-to-cell adhesion and thus influences fruit texture. Diminished accumulation of either PG or PME activity considerably increases the viscosity of tomato juice or paste, which is correlated with reduced polyuronide depolymerization during processing. In contrast, suppression of beta-galactosidase activity early in ripening significantly reduces fruit softening, suggesting that the removal of pectic galactan side-chains is an important factor in the cell wall changes leading to ripening-related firmness loss. Suppression or overexpression of endo-(1-->4)beta-D-glucanase activity has no detectable effect on fruit softening or the depolymerization of matrix glycans, and neither the substrate nor the function for this enzyme has been determined. The role of xyloglucan endotransglycosylase activity in softening is also obscure, and the activity responsible for xyloglucan depolymerization during ripening, a major contributor to softening, has not yet been identified. However, ripening-related expansin protein abundance is directly correlated with fruit softening and has additional indirect effects on pectin depolymerization, showing that this protein is intimately involved in the softening process. Transgenic work has shown that the cell wall changes leading to fruit softening and textural changes are complex, and involve the coordinated and interdependent activities of a range of cell wall-modifying proteins. It is suggested that the cell wall changes caused early in ripening by the activities of some enzymes, notably beta-galactosidase and ripening-related expansin, may restrict or control the activities of other ripening-related enzymes necessary for the fruit softening process.

Summary Harvested banana undergoes rapid softening, which limits its shelf life. To delay softening, banana fruit was treated with hydrogen sulfide (H 2 S) emitted by 0.6 m m sodium hydrosulfide (NaHS) and stored at 15 °C for 16 days. The effects of H 2 S on cell wall degradation and other physiological characteristics were investigated. In comparison to control fruits, H 2 S‐treated bananas retained a higher level of fruit firmness, but lower hue values. Ethylene metabolism analysis suggested that H 2 S inhibited the production of ethylene and the accumulation of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) and decreased the activities of ACC synthase (ACS) and ACC oxidase (ACO). Cell wall polysaccharides determination assays suggested that H 2 S treatment increased the proportion of HCl‐soluble pectin (HSP), NaOH‐soluble pectin (NSP), cellulose and hemicellulose but decreased water‐soluble pectin (WSP) in fruits. H 2 S treatment also inhibited the activities of polysaccharide degrading enzymes such as polygalacturonase (PG), pectin methylesterase (PME), pectin lyase (PL), cellulase (CL) and β ‐galactosidase ( β ‐Gal). Transmission electron microscopy analysis suggested that H 2 S protected cell walls from structural breakdown. These results indicated that H 2 S is effective in delaying banana softening by regulating cell wall polysaccharide metabolism.

Delayed fruit softening in tomato (Solanum lycopersicum) is highly desirable for extending shelf life, facilitating long-distance transportation, and reducing post-harvest losses caused by mechanical damage. Fruit softening is a natural ripening process characterized by the increased expression of genes involved in cell wall modification, leading to the breakdown of cell wall polysaccharides and the gradual disintegration of cellular structure. The yft1 mutant (yellow-fruited tomato 1, originally designated n3122) exhibits inhibited ethylene production, preventing normal ripening and resulting in firmer fruit. Concurrently, yft1 shows significant downregulation of several genes associated with cell wall degradation, including endoglucanase SlCEL2 and EXPANSIN SlEXP1. Both genes exhibit similar expression patterns, peaking during ripening, suggesting their importance in fruit softening. To investigate this further, RNAi silencing lines targeting SlCEL2 and SlEXP1 were generated. The double mutant, slcel2 slexp1, displayed increased firmness at the red ripe stage (54 days post-anthesis, dpa), whereas the single mutants showed similar softening to the wild-type M82. Anatomical analysis at 54 dpa revealed enhanced cell wall structure, slightly increased cuticle thickness, and significantly higher pericarp cellulose content in slcel2 slexp1 compared to M82, slcel2, and slexp1. Furthermore, this study found that SlEXP1 expression was significantly upregulated in slcel2 fruit, compared to M82 (wild type), at 54 dpa. This suggests a compensatory transcriptional regulation between these two genes in tomato fruit, potentially aimed at maintaining normal softening during ripening. These findings demonstrate that SlCEL2 and SlEXP1 act synergistically in cellulose degradation during tomato ripening, and promoting fruit softening.

Summary‘Qingnai’ plums are popular as “green” fruit throughout south-east Asia. However, the fruit soften rapidly after harvest, limiting commercial exports from China. Traditionally, the crop is sold mostly to the local population. We evaluated the potential of hot-air treatment (HAT) to delay ripening of ‘Qingnai’ plum fruit as a potential technology to expand the marketing of “green” plums. Fruit were stored at 20°C after being exposed to 20°C (control), 40°C (HAT40), 45°C (HAT45), or 50°C (HAT50) air for 5 h. Information was collected on fruit firmness, peel colour, chlorophyll contents, chlorophyllase activities, soluble solids contents (SSC), titratable acidities (TA), the rates of respiration and ethylene production, and the activities of cell wall hydrolysing enzymes. Fruit softening was accompanied by a progressive decrease in hue angle (h°), chlorophyll content, and TA, and increases in chlorophyllase, pectin methylesterase (PME) and polygalacturonase (PG) activities. Hot-air treatment delayed the onset of the climacteric peaks of CO2 and ethylene production, delayed fruit softening, and delayed the peaks of PME and PG activities. Heat-treated fruit changed from green-to-yellow at a slower rate than control fruit, with a delay in the peak of chlorophyllase activity. Heat-treated fruit could be stored for up to 18 d, and non-treated fruit for only 12 d. These results suggest that HAT can extend the post-harvest life of ‘Qingnai’ plums by up to 6 d.

Integrated analysis of postharvest storage characteristics of seven apple cultivars and transcriptome data identifies MdBBX25 as a negative regulator of fruit softening during storage in apples