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

MdWRKY31-MdNAC7 regulatory network: orchestrating fruit softening by modulating cell wall-modifying enzyme MdXTH2 in response to ethylene signalling

Softening in fruit adversely impacts their edible quality and commercial value, leading to substantial economic losses during fruit ripening, long-term storage, long-distance transportation, and marketing. As the apple fruit demonstrates climacteric respiration, its firmness decreases with increasing ethylene release rate during fruit ripening and postharvest storage. However, the molecular mechanisms underlying ethylene-mediated regulation of fruit softening in apple remain poorly understood. In this study, we identified a WRKY transcription factor (TF) MdWRKY31, which is repressed by ethylene treatment. Using transgenic approaches, we found that overexpression of MdWRKY31 delays softening by negatively regulating xyloglucan endotransglucosylase/hydrolases 2 (MdXTH2) expression. Yeast one-hybrid (Y1H), electrophoretic mobility shift (EMSA), and dual-luciferase assays further suggested that MdWRKY31 directly binds to the MdXTH2 promoter via a W-box element and represses its transcription. Transient overexpression of ethylene-induced MdNAC7, a NAC TF, in apple fruit promoted softening by decreasing cellulose content and increasing water-soluble pectin content in fruit. MdNAC7 interacted with MdWRKY31 to form a protein complex, and their interaction decreased the transcriptional repression of MdWRKY31 on MdXTH2. Furthermore, MdNAC7 does not directly regulate MdXTH2 expression, but the protein complex formed with MdWRKY31 hinders MdWRKY31 from binding to the promoter of MdXTH2. Our findings underscore the significance of the regulatory complex NAC7-WRKY31 in ethylene-responsive signalling, connecting the ethylene signal to XTH2 expression to promote fruit softening. This sheds light on the intricate mechanisms governing apple fruit firmness and opens avenues for enhancing fruit quality and reducing economic losses associated with softening.

Haplotype-resolved genome assembly for tetraploid Chinese cherry (Prunus pseudocerasus) offers insights into fruit firmness

Chinese cherry (Prunus pseudocerasus) holds considerable importance as one of the primary stone fruit crops in China. However, artificially improving its traits and genetic analysis are challenging due to lack of high-quality genomic resources, which mainly result from difficulties associated with resolving its tetraploid and highly heterozygous genome. Herein, we assembled a chromosome-level, haplotype-resolved genome of the cultivar 'Zhuji Duanbing', comprising 993.69 Mb assembled into 32 pseudochromosomes using PacBio HiFi, Oxford Nanopore, and Hi-C. Intra-haplotype comparative analyses revealed extensive intra-genomic sequence and expression consistency. Phylogenetic and comparative genomic analyses demonstrated that P. pseudocerasus was a stable autotetraploid species, closely related to wild P. pusilliflora, with the two diverging ~18.34 million years ago. Similar to other Prunus species, P. pseudocerasus underwent a common whole-genome duplication event that occurred ~139.96 million years ago. Because of its low fruit firmness, P. pseudocerasus is unsuitable for long-distance transportation, thereby restricting its rapid development throughout China. At the ripe fruit stage, P. pseudocerasus cv. 'Zhuji Duanbing' was significantly less firm than P. avium cv. 'Heizhenzhu'. The difference in firmness is attributed to the degree of alteration in pectin, cellulose, and hemicellulose contents. In addition, comparative transcriptomic analyses identified GalAK-like and Stv1, two genes involved in pectin biosynthesis, which potentially caused the difference in firmness between 'Zhuji Duanbing' and 'Heizhenzhu'. Transient transformations of PpsGalAK-like and PpsStv1 increase protopectin content and thereby enhance fruit firmness. Our study lays a solid foundation for functional genomic studies and the enhancement of important horticultural traits in Chinese cherries.

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.

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.

Comparative transcriptomic analysis provides novel insights into the difference in textural alteration between mealy and crisp apple patterns

Mealiness is a common textural deterioration of several fruit after harvest. To unravel the underlying mechanism involved in mealiness, biochemical characterization and global transcriptomic profiling were comparatively performed between mealy 'Hongjiangjun' (HJJ) and crisp 'Fuji' apples. Sensory evaluation and SEM-based microstructure observation showed that HJJ apples appeared to be mealy in only 3 d at 23 ± 1 °C, while Fuji apples did not appear to be mealy even after 28 d of storage. Textural deterioration and ethylene burst occurred more sharply in HJJ apples than in Fuji apples during storage. The results obtained from the dimensional RNA-sequencing analysis showed that a much stronger upregulation of the transcription of genes encoding polygalacturonase (PG), pectin acetylesterase (PAE), pectinesterase (PE), β-galactosidase (GAL), α-l-arabinofuranase (AF), and expansin (EXP) was observed in the pair of mealy HJJ apples vs. harvest than in the pair of Fuji apples after 28 d vs. harvest. The gene expression of ethylene responsive factor (ERF) was found to be strongly upregulated in HJJ apples compared with Fuji apples, which may mediate the regulation of downstream genes encoding cell wall-modifying enzymes. Weighted gene co-expression network analysis showed that the transcription factors MdbHLH63 and MdERF-like, and a constructure gene of MdGAL had strong connectivity with mealiness. Validation by qRT-PCR further confirmed the main findings obtained by RNA-sequencing. The occurrence of apple mealiness involves altered expression patterns of cell wall-modifying enzymes as well as MdbHLH63 and MdERF-like, which are core genes regulating the mealiness process. The above findings provide global insight into the difference in textural alteration between mealy and crisp apple patterns.

Synergistic Effects of Tragacanth and Anti-ethylene Treatments on Postharvest Quality Maintenance of Mango (Mangifera indica L.)

Mango (Mangifera indica L.) is one of the most popular tropical fruits grown in Egypt and several other countries, making it a potential export commodity. Excessive deterioration after harvest requires various treatments to maintain fruit quality. We evaluated the treatments effects of melatonin (MT) as an anti-ethylene agent and tragacanth gum (TRG) as an edible coating individually and together (MT-TRG) before storing mangoes at 12 °C for 32 days under 85-90% relative humidity. Compared with control, all treatments were significantly effective in preserving fruit quality. Fruits treated with MT-TRG showed significantly lower decay values, respiration rates, ethylene production, and weight loss than untreated fruits. MT-TRG treatment significantly enhanced fruit quality, thereby maintaining fruit appearance, flesh color, firmness, total soluble solids and phenolic contents, and pectin methyl esterase, polyphenol oxidase, and peroxidase activities during the storage period. We propose 200 µM MT + 1% TRG as a safe postharvest treatment to reduce the deterioration of mangoes and maintain fruit quality.

Transcriptomic, metabolomic, and ATAC-seq analysis reveal the regulatory mechanism of senescence of post-harvest tomato fruit

Several physiological changes occur during fruit storage, which include the regulation of genes, metabolisms and transcription factors. In this study, we compared 'JF308' (a normal tomato cultivar) and 'YS006' (a storable tomato cultivar) to determine the difference in accumulated metabolites, gene expression, and accessible chromatin regions through metabolome, transcriptome, and ATAC-seq analysis. A total of 1006 metabolites were identified in two cultivars. During storage time, sugars, alcohols and flavonoids were found to be more abundant in 'YS006' compared to 'JF308' on day 7, 14, and 21, respectively. Differentially expressed genes, which involved in starch and sucrose biosynthesis were observed higher in 'YS006'. 'YS006' had lower expression levels of CesA (cellulose synthase), PL (pectate lyase), EXPA (expansin) and XTH (xyglucan endoglutransglucosylase/hydrolase) than 'JF308'. The results showed that phenylpropanoid pathway, carbohydrate metabolism and cell wall metabolism play important roles in prolonging the shelf life of tomato (Solanum lycopersicum) fruit. The ATAC-seq analysis revealed that the most significantly up-regulated transcription factors during storage were TCP 2,3,4,5, and 24 in 'YS006' compared to 'JF308' on day 21. This information on the molecular regulatory mechanisms and metabolic pathways of post-harvest quality changes in tomato fruit provides a theoretical foundation for slowing post-harvest decay and loss, and has theoretical importance and application value in breeding for longer shelf life cultivars.

Physicochemical properties and transcriptional changes underlying the quality of 'Gala' apples (Malus × domestica Borkh.) under atmosphere manipulation in long-term storage

BACKGROUND

The year-round availability of apples (Malus × domestica Borkh.) depends on post-harvest technologies, which are essential for the retention of fruit sensory and chemical properties by delaying senescence. The effectiveness of strategies for preserving the quality of apples depends on complex interactions between the storage environment and endogenous biological factors. In the current work, we integrated instrumental, sensory, and transcriptional data to determine the role of conservation technologies cold storage, controlled atmosphere, and 1-methylcyclopropene-mediated ethylene blockage on the long-term conservation of apples.

RESULTS

The results demonstrated that inhibition of the consumer's perception of the apples' ethylene content is essential for long-term cold storage, and such quality conservation can be achieved by reducing oxygen pressure. Overall appreciation of apples after storage was determined mainly by their texture, with crispness and juiciness contributing favorably, and mealiness contributing negatively. Reduced oxygen pressure and inhibition of ethylene perception exerted distinct effects on the transcription of candidate genes associated with ripening in apple. Hexose and cell-wall carbohydrate metabolism genes exhibit distinct expression patterns under storage.

CONCLUSION

Inhibition of ethylene perception and reduction of relative oxygen pressure under cold storage both promote similar conservation of apple sensory traits under long-term cold storage. Texture was the main contributor to global appreciation of apples subjected to long-term storage. The conditions that were investigated were able to delay, but not fully prevent, senescence, as evidenced by physicochemical and gene expression analyses. The expression of gene-encoding enzymes involved in hexose metabolism was mainly developmentally regulated, whereas storage conditions exerted a stronger effect on the expression of genes associated with cell-wall metabolism. © 2022 Society of Chemical Industry.

Abscisic acid-responsive transcription factors PavDof2/6/15 mediate fruit softening in sweet cherry

Softening is a key step during fruit ripening that is modulated by the interplay between multiple phytohormones. The antagonistic action of abscisic acid (ABA) and auxin determines the rate of fruit ripening and softening. However, the transcription factors that integrate ABA and auxin signals to regulate fruit softening remain to be determined. In this study, we identified several DNA-binding with One Finger (Dof) transcription factors essential for ABA-promoted fruit softening, based on transcriptome analysis of two sweet cherry (Prunus avium L.) varieties with different fruit firmness. We show that PavDof6 directly binds to the promoters of genes encoding cell wall-modifying enzymes to activate their transcription, while PavDof2/15 directly repress their transcription. Transient overexpression of PavDof6 and PavDof2/15 in sweet cherry fruits resulted in precocious and delayed softening, respectively. In addition, we show that the auxin response factor PavARF8, the expression of whose encoding gene is repressed by ABA, activates PavDof2/15 transcription. Furthermore, PavDof2/6/15 and PavARF8 directly bind to the 9-cis-epoxycarotenoid dioxygenase 1 (PavNCED1) promoter and regulate its expression, forming a feedback mechanism for ABA-mediated fruit softening. These findings unveil the physiological framework of fruit softening and establish a direct functional link between the ABA-PavARF8-PavDofs module and cell-wall-modifying genes in mediating fruit softening.

PpePL1 and PpePL15 Are the Core Members of the Pectate Lyase Gene Family Involved in Peach Fruit Ripening and Softening

Pectin is the major component in the primary cell wall and middle lamella, maintaining the physical stability and mechanical strength of the cell wall. Pectate lyase (PL), a cell wall modification enzyme, has a major influence on the structure of pectin. However, little information and no comprehensive analysis is available on the PL gene family in peach (Prunus persica L. Batsch). In this study, 20 PpePL genes were identified in peach. We characterized their physicochemical characteristics, sequence alignments, chromosomal locations, and gene structures. The PpePL family members were classified into five groups based on their phylogenetic relationships. Among those, PpePL1, 9, 10, 15, and 18 had the higher expression abundance in ripe fruit, and PpePL1, 15, and 18 were upregulated during storage. Detailed RT-qPCR analysis revealed that PpePL1 and PpePL15 were responsive to ETH treatment (1 g L^-1 ethephon) with an abundant transcript accumulation, which suggested these genes were involved in peach ripening and softening. In addition, virus-induced gene silencing (VIGS) technology was used to identify the roles of PpePL1 and PpePL15. Compared to controls, the RNAi fruit maintained greater firmness in the early storage stage, increased acid-soluble pectin (ASP), and reduced water-soluble pectin (WSP). Moreover, transmission electron microscopy (TEM) showed that cell wall degradation was reduced in the fruit of RNAi-1 and RNAi-15, which indicated that softening of the RNAi fruit has been delayed. Our results indicated that PpePL1 and PpePL15 play an important role in peach softening by depolymerizing pectin and degrading cell wall.

Synergistic Effects of Plant Hormones on Spontaneous Late-ripening Mutant of ‘Jinghong’ Peach Detected by Transcriptome Analysis

Objectives

Peach (Prunus persica L.) is an ancient fruit tree that originated from China. It is the climacteric fruit which belongs to genus Prunus in family Rosaceae. Ethylene, which is produced during ripening, accelerates fruit softening, and therefore peaches cannot be stored for a long time.

Materials and Methods

To study the mechanism of fruit late ripening, transcriptome analysis of the fruit of a late-ripening mutant of ‘Jinghong’ peach was performed to identify genes and pathways involved in fruit late ripening.

Results

A total of 1,805, 1,511, and 2,309 genes were found to be differentially expressed in W2_vs_M1, W3_vs_M2, and W3_vs_M3, respectively. Functional enrichment analysis of the differentially expressed genes showed they were related to carotenoid biosynthesis, starch and sucrose metabolism plant hormone signal transduction, flavonoid biosynthesis, and photosynthesis. The expression trends of ripening-related genes that encode transcription factors and plant hormone signal transduction-related genes that encode enzymes were similar.

Conclusions

It will help to elucidate the transcriptional regulatory network of fruit development in the spontaneous late-ripening mutant of ‘Jinghong’ peach and provide a theoretical basis for understanding the molecular regulatory mechanism of fruit ripening.

Could Causal Discovery in Proteogenomics Assist in Understanding Gene-Protein Relations? A Perennial Fruit Tree Case Study Using Sweet Cherry as a Model

Genome-wide transcriptome analysis is a method that produces important data on plant biology at a systemic level. The lack of understanding of the relationships between proteins and genes in plants necessitates a further thorough analysis at the proteogenomic level. Recently, our group generated a quantitative proteogenomic atlas of 15 sweet cherry (Prunus avium L.) cv. ‘Tragana Edessis’ tissues represented by 29,247 genes and 7584 proteins. The aim of the current study was to perform a targeted analysis at the gene/protein level to assess the structure of their relation, and the biological implications. Weighted correlation network analysis and causal modeling were employed to, respectively, cluster the gene/protein pairs, and reveal their cause–effect relations, aiming to assess the associated biological functions. To the best of our knowledge, this is the first time that causal modeling has been employed within the proteogenomics concept in plants. The analysis revealed the complex nature of causal relations among genes/proteins that are important for traits of interest in perennial fruit trees, particularly regarding the fruit softening and ripening process in sweet cherry. Causal discovery could be used to highlight persistent relations at the gene/protein level, stimulating biological interpretation and facilitating further study of the proteogenomic atlas in plants.

Effect of Chitosan Coatings with Cinnamon Essential Oil on Postharvest Quality of Mangoes

Mango (Mangifera indica Linn.) is a famous climacteric fruit containing abundant flavor and nutrients in the tropics, but it is prone to decay without suitable postharvest preservation measures. In this study, the chitosan (CH)-cinnamon essential oil (CEO) Pickering emulsion (CH-PE) coating was prepared, with cellulose nanocrystals as the emulsifier, and applied to harvested mangoes at the green stage of maturity. It was compared with a pure CH coating and a CH-CEO emulsion (CH-E) coating, prepared with the emulsifier Tween 80. Results showed that the CH-PE coating had a lower water solubility and water vapor permeability than the other coatings, which was mainly due to electrostatic interactions, and had a better sustained-release performance for CEO than the CH-E coating. During mango storage, the CH-PE coating effectively improved the appearance of mangoes at 25 °C for 12 d by reducing yellowing and dark spots, and delayed water loss. Hardness was maintained and membrane lipid peroxidation was reduced by regulating the activities of pectin methyl esterase, polygalacturonase, and peroxidase. In addition, the nutrient quality was improved by the CH-PE coating, with higher contents of total soluble solid, titratable acid, and ascorbic acid. Therefore, the CH-PE coating is promising to comprehensively maintain the postharvest quality of mangoes, due to its enhanced physical and sustained-release properties.

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.

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.

Analysis of the postharvest storage characteristics of the new red-fleshed apple cultivar 'meihong'

This study examined the effects of postharvest storage conditions on the fruit quality of a new red-fleshed apple cultivar ('Meihong'). Mature 'Meihong' and 'Golden delicious' apples were exposed to room temperature, low temperature, and low temperature and 1-MCP, after which several fruit characteristics were evaluated (i.e., firmness, ethylene release rate, relative content of aroma components, phenolic compounds and antioxidant capacity, fruit softening-related enzyme activities, and related gene expression). Both 'Meihong' and 'Golden delicious' were ACS1-1/-2 heterozygotes, but the ethylene release rate in 'Meihong' fruits was lower than that in 'Golden delicious' fruits during storage. Therefore, 'Meihong' fruits are more conducive to storage. The low temperature storage with and without 1-MCP delayed fruit softening, decreased the ethylene release rate and ester aroma component content, and maintained total flavonoid and anthocyanin contents. Therefore, storage at low temperatures with 1-MCP or other preservatives may be useful for maintaining the 'Meihong' fruit quality.

The MdXTHB gene is involved in fruit softening in 'Golden Del. Reinders' (Malus pumila)

BACKGROUND

Fruit softening is a major determinant of commercial value and shelf life. A transcriptomic analysis of 'Golden Delicious' and 'Golden Del. Reinders' (a bud mutation of 'Golden Delicious' that readily softens) apple fruit was conducted during storage.

RESULTS

A comparative analysis of the obtained expression profiles of fruit between two cultivars identified 1345 upregulated and 3475 downregulated differentially expressed genes (DEGs). The DEGs identified were associated with cellular processes and carbohydrate metabolism and were especially enriched in cell-wall-related genes. Among the cell-wall-related genes, the xyloglucan endotransglucosylase/hydrolases (XTH) gene MdXTHB was significantly upregulated and exhibited high expression levels in 'Golden Del. Reinders' fruit, which had a lower level of firmness relative to 'Golden Delicious'. Overexpression of MdXTHB in both 'Golden Delicious' and 'Fuji', which typically maintain high levels of firmness in storage, exhibited faster rates of softening and an earlier peak of ethylene production than empty-vector-infiltrated fruit did.

CONCLUSION

The results of this study indicate that MdXTHB potentially promotes apple fruit softening by degrading the fruit cell wall. This result is also useful to designing further experiments on the molecular regulation of fruit softening in apple. © 2020 Society of Chemical Industry.

Transcriptome analysis of strawberry fruit in response to exogenous arginine

Transcriptome and physiological analysis showed that exogenous arginine can delay the ripening process of postharvest strawberry fruit. Arginine (Arg) plays an important role in the growth and development of plants, but its growth and development regulatory mechanisms in strawberry fruit are unknown. In this study, we found that the content of Arg decreased after the onset of fruit coloration and exogenous Arg inhibited fruit coloration. We comprehensively analyzed the transcriptome of 'Sweet Charlie' strawberry fruit with or without Arg treatment and identified a large number of differential genes and metabolites. Based on the transcriptome data, we also found that Arg inhibited ripening, which coincided with changes in several physiological parameters and their corresponding gene transcripts, including firmness, anthocyanin content, sugar content, Arg content, indole-acetic acid (IAA) content, abscisic acid (ABA) content, and ethylene emissions. We also found that Arg induced the expression of heat-shock proteins (HSPs) and antioxidant enzyme genes, which improved strawberry stress resistance. This study elucidated the molecular mechanism by which exogenous Arg delays strawberry fruit ripening, providing some genetic information to help guide the future improvement and cultivation of strawberry.

Technical benefit on apple fruit of controlled atmosphere influenced by 1-MCP at molecular levels

The apple is a highly perishable fruit after harvesting and, therefore, several storage technologies have been studied to provide the consumer market with a quality product with a longer shelf life. However, little is known about the apple genome that is submitted to the storage, and even less with the application of ripening inhibitors. Due to these factors, this study sought to elucidate the transcriptional profile of apple cultivate Gala stored in a controlled atmosphere (AC) treated and not treated with 1-methyl cyclopropene (1-MCP). Through the genetic mapping of the apple, applying the microarray technique, it was possible to verify the action of treatments on transcripts related to photosynthesis, carbohydrate metabolism, response to hormonal stimuli, nucleic acid metabolism, reduction of oxidation, regulation of transcription and metabolism of cell wall and lipids. The results showed that the transcriptional profile in the entire genome of the fruit showed significant differences in the relative expression of the gene, this in response to CA in the presence and absence of 1-MCP. It should be noted that the transcription genes involved in the anabolic pathway were only maintained after six months in fruits treated with 1-MCP. The data in this work suggests that the apple in the absence of 1-MCP begins to prepare its metabolism to mature, even during the storage period in AC. Meanwhile, in the presence of the inhibitor, the transcriptional profile of the fruit is similar to that at the time of harvest. It was also found that a set of genes that code for ethylene receptors, auxin homeostasis, MADS Box, and NAC transcription factors may be involved in the regulation of post-harvest ripening after storage and in the absence of 1-MCP.

Transcriptome analysis reveals delaying of the ripening and cell-wall degradation of kiwifruit by hydrogen sulfide

BACKGROUND

Hydrogen sulfide (H₂ S) is a known signaling molecule in plants, which has the ability to delay fruit ripening. Our previous studies have shown that H₂ S treatment could delay the maturation of kiwifruits by inhibiting ethylene production, improving protective enzyme activities, and decreasing the accumulation of reactive oxygen species to protect the cell membrane during storage. The mechanism related to the way in which H₂ S affected kiwifruit maturation was still unclear. We performed transcriptome sequencing to explore the influences of H₂ S on the softening of kiwifruit.

RESULTS

The firmness and the soluble solids content (SSC) of the kiwifruit were significantly better maintained with H₂ S treatment compared to the control during the storage period (P < 0.05). Transmission electron microscopy (TEM) showed that degradation of the cell wall was inhibited after H₂ S treatment. Based on transcriptome data analysis and quantitative real-time polymerase chain reaction (qRT-PCR), expression levels of endo-1,4-β-glucanase (β-glu), β-galactosidase (β-gal) and pectinesterase (PME) decreased whereas pectinesterase inhibitor (PMEI) significantly increased in response to H₂ S. The members of the signal transduction pathway involved in ethylene were also identified. Hydrogen sulfide inhibited the expression of ethylene receptor 2 (ETR2), ERF003, ERF5, and ERF016, and increased the expression of ethylene-responsive transcription factor 4 (ERF4) and ERF113.

CONCLUSION

Hydrogen sulfide could delay the ripening and senescence of kiwifruit by regulating the cell-wall degrading enzyme genes and affecting ethylene signal transduction pathway genes. Our results revealed the effect of H₂ S treatment on the softening of kiwifruit at the transcription level, laying a foundation for further research. © 2020 Society of Chemical Industry.

Isolation and characterization of Populus xyloglucan endotransglycosylase/hydrolase (XTH) involved in osmotic stress responses

Xyloglucan endotransglycosylase/hydrolase (XTH) belongs to the GH16 subfamily of the glycoside hydrolases of carbohydrate active enzymes and plays an important role in the structure and function of plant cell walls. In this study, 11 members of the XTH gene family were cloned from Populus tomentosa. A bioinformatics analysis revealed that 11 PtoXTHs could be classified into three groups, where PtoXTH27 and PtoXTH34 were most likely to exhibit XTH activity. Biochemical analyses of purified PtoXTHs demonstrated that PtoXTH27 and PtoXTH34 had detectable xyloglucan endotransglucosylase (XET) activity, while the others did not exhibit XET or XEH activity. Moreover, enzymatic assays revealed that the optimum reaction temperature of both PtoXTH27 and PtoXTH34 was 37 °C, while their optimum pH values differed, such that PtoXTH27 was 6.0 and PtoXTH34 was 5.0. Enzyme kinetic parameters indicated that PtoXTH34 had higher affinity for the receptor substrate, XXXG, implying that PtoXTH34 and PtoXTH27 in plants have different substrate structure specificity. Finally, heterologous expression of XTH significantly increased intracellular total sugar content and osmotolerance of yeast cells, indicating that PtoXTH27 and PtoXTH34 are potentially involved in osmotic stress responses. These results clearly demonstrate the enzymatic characteristics and putative role of XTH in osmotic stress responses.

Transcriptome analysis of acerola fruit ripening: insights into ascorbate, ethylene, respiration, and softening metabolisms

The first transcriptome coupled to metabolite analyses reveals major trends during acerola fruit ripening and shed lights on ascorbate, ethylene signalling, cellular respiration, sugar accumulation, and softening key regulatory genes. Acerola is a fast growing and ripening fruit that exhibits high amounts of ascorbate. During ripening, the fruit experience high respiratory rates leading to ascorbate depletion and a quickly fragile and perishable state. Despite its growing economic importance, understanding of its developmental metabolism remains obscure due to the absence of genomic and transcriptomic data. We performed an acerola transcriptome sequencing that generated over 600 million reads, 40,830 contigs, and provided the annotation of 25,298 unique transcripts. Overall, this study revealed the main metabolic changes that occur in the acerola ripening. This transcriptional profile linked to metabolite measurements, allowed us to focus on ascorbate, ethylene, respiration, sugar, and firmness, the major metabolism indicators for acerola quality. Our results suggest a cooperative role of several genes involved in AsA biosynthesis (PMM, GMP1 and 3, GME1 and 2, GGP1 and 2), translocation (NAT3, 4, 6 and 6-like) and recycling (MDHAR2 and DHAR1) pathways for AsA accumulation in unripe fruits. Moreover, the association of metabolites with transcript profiles provided a comprehensive understanding of ethylene signalling, respiration, sugar accumulation and softening of acerola, shedding light on promising key regulatory genes. Overall, this study provides a foundation for further examination of the functional significance of these genes to improve fruit quality traits.

Functionally redundant LNG3 and LNG4 genes regulate turgor-driven polar cell elongation through activation of XTH17 and XTH24

In this work, we genetically characterized the function of Arabidopsis thaliana, LONGIFOLIA (LNG1), LNG2, LNG3, LNG4, their contribution to regulate vegetative architecture in plant. We used molecular and biophysical approaches to elucidate a gene function that regulates vegetative architecture, as revealed by the leaf phenotype and later effects on flowering patterns in Arabidopsis loss-of-function mutants. As a result, LNG genes play an important role in polar cell elongation by turgor pressure controlling the activation of XTH17 and XTH24. Plant vegetative architecture is related to important traits that later influence the floral architecture involved in seed production. Leaf morphology is the primary key trait to compose plant vegetative architecture. However, molecular mechanism on leaf shape determination is not fully understood even in the model plant A. thaliana. We previously showed that LONGIFOLIA (LNG1) and LONGIFOLIA2 (LNG2) genes regulate leaf morphology by promoting longitudinal cell elongation in Arabidopsis. In this study, we further characterized two homologs of LNG1, LNG3, and LNG4, using genetic, biophysical, and molecular approaches. Single loss-of-function mutants, lng3 and lng4, do not show any phenotypic difference, but mutants of lng quadruple (lngq), and lng1/2/3 and lng1/2/4 triples, display reduced leaf length, compared to wild type. Using the paradermal analysis, we conclude that the reduced leaf size of lngq is due to decreased cell elongation in the direction of longitudinal leaf growth, and not decreased cell proliferation. This data indicate that LNG1/2/3/4 are functionally redundant, and are involved in polar cell elongation in Arabidopsis leaf. Using a biophysical approach, we show that the LNGs contribute to maintain high turgor pressure, thus regulating turgor pressure-dependent polar cell elongation. In addition, gene expression analysis showed that LNGs positively regulate the expression of the cell wall modifying enzyme encoded by a multi-gene family, xyloglucan endotransglucosylase/hydrolase (XTH). Taking all of these together, we propose that LNG related genes play an important role in polar cell elongation by changing turgor pressure and controlling the activation of XTH17 and XTH24.

Contrasting Transcriptional Programs Control Postharvest Development of Apples (Malus x domestica Borkh.) Submitted to Cold Storage and Ethylene Blockage

Apple is commercially important worldwide. Favorable genomic contexts and postharvest technologies allow year-round availability. Although ripening is considered a unidirectional developmental process toward senescence, storage at low temperatures, alone or in combination with ethylene blockage, is effective in preserving apple properties. Quality traits and genome wide expression were integrated to investigate the mechanisms underlying postharvest changes. Development and conservation techniques were responsible for transcriptional reprogramming and distinct programs associated with quality traits. A large portion of the differentially regulated genes constitutes a program involved in ripening and senescence, whereas a smaller module consists of genes associated with reestablishment and maintenance of juvenile traits after harvest. Ethylene inhibition was associated with a reversal of ripening by transcriptional induction of anabolic pathways. Our results demonstrate that the blockage of ethylene perception and signaling leads to upregulation of genes in anabolic pathways. We also associated complex phenotypes to subsets of differentially regulated genes.