Regulation of loquat fruit low temperature response and lignification involves interaction of heat shock factors and genes associated with lignin biosynthesis

An AP2/ERF member LlERF012 confers thermotolerance via activation of HSF pathway in lily

Heat stress transcription factors (HSFs) are core factors of plants in response to heat stress (HS), but their regulatory network is complicated and remains elusive in a large part, especially HSFBs. In this study, we reported that the LlERF012-LlHSFA1 module participates in heat stress response (HSR) by directly regulating HSF pathway in lily (Lilium longiflorum). LlHSFB1 was confirmed as a positive regulator in lily thermotolerance and a heat-inducible AP2/ERF member LlERF012 (Ethylene Response Factor 012) was further identified to be a direct trans-activator of LlHSFB1. Overexpression of LlERF012 elevated the thermotolerance of transgenic Arabidopsis and lily, but silencing LlERF012 reduced thermotolerance in lily. Further analysis showed LlERF012 interacted with LlHSFA1, which led to enhanced transactivation activity and DNA-binding capability of LlERF012. In addition, LlERF012 also directly activated the expression of LlHSFA1 by binding its promoter. As expected, we found that LlERF012 bound the promoters of LlHSFA2, LlHSFA3A, and LlHSFA3B to stimulate their expression, and LlERF012-LlHSFA1 interaction enhanced these activation effects. Overall, our data suggested that LlERF012 was a key factor for lily thermotolerance and the LlERF012-LlHSFA1 interaction synergistically regulated the activity of the HSF pathway including the class A and B members, which might be of great significance for coordinating the functions of different HSFs.

Genome-wide identification and role of HSFs in antioxidant response of hot water treated zucchini fruit during cold storage

Zucchini squashes are cold-sensitive and vulnerable to chilling injury (CI) resulting from reactive oxygen species (ROS) and hot water (HW) immersing effectively reduce CI symptoms during cold storage. However, mechanism involved in reduced ROS due to HW treatment has not been characterized well. In this study, tender green zucchini fruit were treated with HW for 15 min at 45 ± 1 °C and stored for 15 d at 4 ± 1 °C and above 90 % relative humidity. Results showed substantial reduction in CI index, electrolyte leakage, malonaldehyde (MDA) contents and ROS accumulation along with increased activity of ROS-scavenging enzymes due to HW treatment. To gain insight into the molecular mechanism involved in antioxidant defense system, transcriptomic analysis revealed that heat shock factors (HSF) accumulated due to HW treatment regulated the ROS pathway during cold stress. CpHSFA4a was one of the highly expressed transcription factors (TF) due to HW treatment that regulated the transcription of ROS enzymes related genes. CpHSFA4a bind actively with heat shock element (HSE) in promoter regions of CpSOD, CpCAT, CpAPX1, CpAPX2, and CpAPX3, activated and increased the expression of these genes. In conclusion, HW treatment alleviated the CI by maintaining ROS homeostasis through CpHSFA4a mediated ROS pathway in zucchini squashes during cold storage.

Genome-Wide Identification and Characterization of Lignin Synthesis Genes in Maize

Lignin is a crucial substance in the formation of the secondary cell wall in plants. It is widely distributed in various plant tissues and plays a significant role in various biological processes. However, the number of copies, characteristics, and expression patterns of genes involved in lignin biosynthesis in maize are not fully understood. In this study, bioinformatic analysis and gene expression analysis were used to discover the lignin synthetic genes, and two representative maize inbred lines were used for stem strength phenotypic analysis and gene identification. Finally, 10 gene families harboring 117 related genes involved in the lignin synthesis pathway were retrieved in the maize genome. These genes have a high number of copies and are typically clustered on chromosomes. By examining the lignin content of stems and the expression patterns of stem-specific genes in two representative maize inbred lines, we identified three potential stem lodging resistance genes and their interactions with transcription factors. This study provides a foundation for further research on the regulation of lignin biosynthesis and maize lodging resistance genes.

MaHsf24, a novel negative modulator, regulates cold tolerance in banana fruits by repressing the expression of HSPs and antioxidant enzyme genes

Transcriptional regulation mechanisms underlying chilling injury (CI) development have been widely investigated in model plants and cold-sensitive fruits, such as banana (Musa acuminata). However, unlike the well-known NAC and WRKY transcription factors (TFs), the function and deciphering mechanism of heat shock factors (HSFs) involving in cold response are still fragmented. Here, we showed that hot water treatment (HWT) alleviated CI in harvested banana fruits accomplishing with reduced reactive oxygen species (ROS) accumulation and increased antioxidant enzyme activities. A cold-inducible but HWT-inhibited HSF, MaHsf24, was identified. Using DNA affinity purification sequencing (DAP-seq) combined with RNA-seq analyses, we found three heat shock protein (HSP) genes (MaHSP23.6, MaHSP70-1.1 and MaHSP70-1.2) and three antioxidant enzyme genes (MaAPX1, MaMDAR4 and MaGSTZ1) were the potential targets of MaHsf24. Subsequent electrophoretic mobility shift assay (EMSA), chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) and dual-luciferase reporter (DLR) analyses demonstrated that MaHsf24 repressed the transcription of these six targets via directly binding to their promoters. Moreover, stably overexpressing MaHsf24 in tomatoes increased cold sensitivity by suppressing the expressions of HSPs and antioxidant enzyme genes, while HWT could recover cold tolerance, maintaining higher levels of HSPs and antioxidant enzyme genes, and activities of antioxidant enzymes. In contrast, transiently silencing MaHsf24 by virus-induced gene silencing (VIGS) in banana peels conferred cold resistance with the upregulation of MaHSPs and antioxidant enzyme genes. Collectively, our findings support the negative role of MaHsf24 in cold tolerance, and unravel a novel regulatory network controlling bananas CI occurrence, concerning MaHsf24-exerted inhibition of MaHSPs and antioxidant enzyme genes.

MaC2H2-IDD regulates fruit softening and involved in softening disorder induced by cold stress in banana

Chilling stress causes banana fruit softening disorder and severely impairs fruit quality. Various factors, such as transcription factors, regulate fruit softening. Herein, we identified a novel regulator, MaC2H2-IDD, whose expression is closely associated with fruit ripening and softening disorder. MaC2H2-IDD is a transcriptional activator located in the nucleus. The transient and ectopic overexpression of MaC2H2-IDD promoted "Fenjiao" banana and tomato fruit ripening. However, transient silencing of MaC2H2-IDD repressed "Fenjiao" banana fruit ripening. MaC2H2-IDD modulates fruit softening by activating the promoter activity of starch (MaBAM3, MaBAM6, MaBAM8, MaAMY3, and MaISA2) and cell wall (MaEXP-A2, MaEXP-A8, MaSUR14-like, and MaGLU22-like) degradation genes. DLR, Y1H, EMSA, and ChIP-qPCR assays validated the expression regulation. MaC2H2-IDD interacts with MaEBF1, enhancing the regulation of MaC2H2-IDD to MaAMY3, MaEXP-A2, and MaGLU22-like. Overexpressing/silencing MaC2H2-IDD in banana and tomato fruit altered the transcript levels of the cell wall and starch (CWS) degradation genes. Several differentially expressed genes (DEGs) were authenticated between the overexpression and control fruit. The DEGs mainly enriched biosynthesis of secondary metabolism, amino sugar and nucleotide sugar metabolism, fructose and mannose metabolism, starch and sucrose metabolism, and plant hormones signal transduction. Overexpressing MaC2H2-IDD also upregulated protein levels of MaEBF1. MaEBF1 does not ubiquitinate or degrade MaC2H2-IDD. These data indicate that MaC2H2-IDD is a new regulator of CWS degradation in "Fenjiao" banana and cooperates with MaEBF1 to modulate fruit softening, which also involves the cold softening disorder.

Differential CaKAN3-CaHSF8 associations underlie distinct immune and heat responses under high temperature and high humidity conditions

High temperature and high humidity (HTHH) conditions increase plant susceptibility to a variety of diseases, including bacterial wilt in solanaceous plants. Some solanaceous plant cultivars have evolved mechanisms to activate HTHH-specific immunity to cope with bacterial wilt disease. However, the underlying mechanisms remain poorly understood. Here we find that CaKAN3 and CaHSF8 upregulate and physically interact with each other in nuclei under HTHH conditions without inoculation or early after inoculation with R. solanacearum in pepper. Consequently, CaKAN3 and CaHSF8 synergistically confer immunity against R. solanacearum via activating a subset of NLRs which initiates immune signaling upon perception of unidentified pathogen effectors. Intriguingly, when HTHH conditions are prolonged without pathogen attack or the temperature goes higher, CaHSF8 no longer interacts with CaKAN3. Instead, it directly upregulates a subset of HSP genes thus activating thermotolerance. Our findings highlight mechanisms controlling context-specific activation of high-temperature-specific pepper immunity and thermotolerance mediated by differential CaKAN3-CaHSF8 associations.

Effect of ipt Gene Induction in Transgenic Tobacco Plants on Hydraulic Conductance, Formation of Apoplastic Barriers and Aquaporin Activity under Heat Shock

Cytokinins are known to keep stomata open, which supports gas exchange and correlates with increased photosynthesis. However, keeping the stomata open can be detrimental if the increased transpiration is not compensated for by water supply to the shoots. In this study, we traced the effect of ipt (isopentenyl transferase) gene induction, which increases the concentration of cytokinins in transgenic tobacco plants, on transpiration and hydraulic conductivity. Since water flow depends on the conductivity of the apoplast, the deposition of lignin and suberin in the apoplast was studied by staining with berberine. The effect of an increased concentration of cytokinins on the flow of water through aquaporins (AQPs) was revealed by inhibition of AQPs with HgCl₂. It was shown that an elevated concentration of cytokinins in ipt-transgenic plants increases hydraulic conductivity by enhancing the activity of aquaporins and reducing the formation of apoplastic barriers. The simultaneous effect of cytokinins on both stomatal and hydraulic conductivity makes it possible to coordinate the evaporation of water from leaves and its flow from roots to leaves, thereby maintaining the water balance and leaf hydration.

Postharvest Biology and Technology of Loquat (Eriobotrya japonica Lindl.)

Loquat (Eriobotrya japonica Lindl.) fruit is a rich source of carotenoids, flavonoids, phenolics, sugars, and organic acids. Although it is classified as a non-climacteric fruit, susceptibility to mechanical and physical bruising causes its rapid deterioration by moisture loss and postharvest decay caused by pathogens. Anthracnose, canker, and purple spot are the most prevalent postharvest diseases of loquat fruit. Cold storage has been used for quality management of loquat fruit, but the susceptibility of some cultivars to chilling injury (CI) consequently leads to browning and other disorders. Various techniques, including cold storage, controlled atmosphere storage, hypobaric storage, modified atmosphere packaging, low-temperature conditioning, heat treatment, edible coatings, and postharvest chemical application, have been tested to extend shelf life, mitigate chilling injury, and quality preservation. This review comprehensively focuses on the recent advances in the postharvest physiology and technology of loquat fruit, such as harvest maturity, fruit ripening physiology, postharvest storage techniques, and physiological disorders and diseases.

Lignin Biosynthesis in Postharvest Water Bamboo (Zizania latifolia) Shoots during Cold Storage Is Regulated by RBOH-Mediated Reactive Oxygen Species Signaling

Lignification is a major cause of senescence in fresh shoots of water bamboo (Zizania latifolia), which is a popular vegetable in southeast Asia; however, its physiological and molecular mechanisms is less understood. In the present study, lignin content and transcriptome change in postharvested water bamboo shoots under cold storage were investigated. We found that lignin significantly accumulated in the epidermis of the shoots with the increase of firmness. In the cold storage shoots, the major up-regulated genes were involved in phenylpropanoid biosynthesis, plant-pathogen interactions, and starch and sucrose metabolism. The lignin biosynthesis genes PAL, 4CL, C4H, CCoAOMT, CCR, F5H, CAD, and POD family were up-regulated during cold storage, while HCT and C3H were down-regulated. The MAPK signaling pathway was also up-regulated and respiratory burst oxidase homologue (RBOH) genes were strongly up-regulated. Therefore, we investigated the RBOH gene family and their expression profile in water bamboo shoots. The results indicated that 10 ZlRBOHs were up-regulated in cold storage shoots. Diphenyleneiodonium chloride (DPI), an inhibitor of RBOH oxidase, significantly inhibited the expression of genes involved in lignin deposition and biosynthesis, while H2O2 enhanced these processes. These results suggest that lignification of water bamboo shoots is regulated by RBOH-mediated ROS signaling.

The Regulatory Mechanisms and Control Technologies of Chilling Injury and Fungal Diseases of Postharvest Loquat Fruit

Loquat is a popular fruit widely cultivated all over the world. It is rich in minerals and carotenoids and has high commercial value. At room temperature, loquat fruit is impressionable to water and nutritional losses, physical damage, and microbial decay, resulting in a short postharvest life. Low-temperature storage is routinely used to prolong the shelf life of loquat fruit; however, cold storage can also lead to lignification of flesh tissue, which is one of the major symptoms of chilling injury (CI), reducing the quality and economic value of the fruit. In addition, fruit decay caused by microbial infection is another important reason for postharvest losses of loquat. To reduce quality deterioration and optimize the postharvest storage strategies of loquat fruit, considerable progress has been made in the physiological and molecular biological studies of CI, microbial decay, and preservation technologies of loquat fruit during the postharvest phase in recent decades. This review summarizes the current research progress and provides a reference for the improvement of loquat fruit quality.

Climate-responsive DNA methylation is involved in the biosynthesis of lignin in birch

Lignin is one of the most important secondary metabolites and essential to the formation of cell walls. Changes in lignin biosynthesis have been reported to be associated with environmental variations and can influence plant fitness and their adaptation to abiotic stresses. However, the molecular mechanisms underlying this association remain unclear. In this study, we evaluated the relations between the lignin biosynthesis and environmental factors and explored the role of epigenetic modification (DNA methylation) in contributing to these relations if any in natural birch. Significantly negative correlations were observed between the lignin content and temperature ranges. Analyzing the transcriptomes of birches in two habitats with different temperature ranges showed that the expressions of genes and transcription factors (TFs) involving lignin biosynthesis were significantly reduced at higher temperature ranges. Whole-genome bisulfite sequencing revealed that promoter DNA methylation of two NAC-domain TFs, BpNST1/2 and BpSND1, may be involved in the inhibition of these gene expressions, and thereby reduced the content of lignin. Based on these results we proposed a DNA methylation-mediated lignin biosynthesis model which responds to environmental factors. Overall, this study suggests the possibility of environmental signals to induce epigenetic variations that result in changes in lignin content, which can aid to develop resilient plants to combat ongoing climate changes or to manipulate secondary metabolite biosynthesis for agricultural, medicinal, or industrial values.

The Effect of Short-Term Temperature Pretreatments on Sugars, Organic Acids, and Amino Acids Metabolism in Valencia Orange Fruit

Temperature pretreatment is one of the most important factors which significantly affects the postharvest quality of citrus fruit. In this study, late-ripening Valencia orange (citrus sinensis) fruits were used to investigate the effect of short-term treatment at low (6°C), room (20°C), and high (40°C) temperatures on fruit quality. Our results revealed that both low and room-temperature treatments maintained the content of sugars and organic acids, whereas high-temperature treatments elevated the accumulation of sugars but decreased the content of citric acid. In fruit peel (flavedo and albedo), the accumulation of sugars and organic acids responding to temperatures was diverse and mostly different from that in the pulp. Meanwhile, GABA and several amino acids were upregulated under short-term high-temperature treatment but downregulated in response to low-temperature treatment in both peel and pulp. Furthermore, PCA and correlation analysis revealed that the short-term temperature treatments changed the metabolic flow, and GABA was positively correlated with sugars and organic acids. Our study analyzed the metabolic changes of fruit peel and pulp in response to short-term temperature treatments and revealed that GABA may act as a signaling molecular involved in temperature-controlled quality changes.

Heat shock-induced cold acclimation in cucumber through CsHSFA1d-activated JA biosynthesis and signaling

Cucumber (Cucumis sativus) originated in tropical areas and is very sensitive to low temperatures. Cold acclimation is a universal strategy that improves plant resistance to cold stress. In this study, we report that heat shock induces cold acclimation in cucumber seedlings, via a process involving the heat-shock transcription factor HSFA1d. CsHSFA1d expression was improved by both heat shock and cold treatment. Moreover, CsHSFA1d transcripts accumulated more under cold treatment after a heat-shock pre-treatment than with either heat shock or cold treatment alone. After exposure to cold, cucumber lines overexpressing CsHSFA1d displayed stronger tolerance for cold stress than the wild type, whereas CsHSFA1d knockdown lines obtained by RNA interference were more sensitive to cold stress. Furthermore, both the overexpression of CsHSFA1d and heat-shock pre-treatment increased the endogenous jasmonic acid (JA) content in cucumber seedlings after cold treatment. Exogenous application of JA rescued the cold-sensitive phenotype of CsHSFA1d knockdown lines, underscoring that JA biosynthesis is key for CsHSFA1d-mediated cold tolerance. Higher JA content is likely to lead to the degradation of CsJAZ5, a repressor protein of the JA pathway. We also established that CsJAZ5 interacts with CsICE1. JA-induced degradation of CsJAZ5 would be expected to release CsICE1, which would then activate the ICE-CBF-COR pathway. After cold treatment, the relative expression levels of ICE-CBF-COR signaling pathway genes, such as CsICE1, CsCBF1, CsCBF2 and CsCOR1, in CsHSFA1d overexpression lines were significantly higher than in the wild type and knockdown lines. Taken together, our results help to reveal the mechanism underlying heat shock-induced cold acclimation in cucumber.

Construction of a high-density bin-map and identification of fruit quality-related quantitative trait loci and functional genes in pear

Pear (Pyrus spp.) is one of the most common fruit crops grown in temperate regions worldwide. Genetic enhancement of fruit quality is a fundamental goal of pear breeding programs. The genetic control of pear fruit quality traits is highly quantitative, and development of high-density genetic maps can facilitate fine-mapping of quantitative trait loci (QTLs) and gene identification. Bin-mapping is a powerful method of constructing high-resolution genetic maps from large-scale genotyping datasets. We performed whole-genome sequencing of pear cultivars 'Niitaka' and 'Hongxiangsu' and their 176 F ₁ progeny to identify genome-wide single-nucleotide polymorphism (SNP) markers for constructing a high-density bin-map of pear. This analysis yielded a total of 1.93 million SNPs and a genetic bin-map of 3190 markers spanning 1358.5 cM, with an average adjacent interval of 0.43 cM. This bin-map, along with other high-density genetic maps in pear, improved the reference genome assembly from 75.5 to 83.7% by re-anchoring the scaffolds. A quantitative genetic analysis identified 148 QTLs for 18 fruit-related traits; among them, QTLs for stone cell content, several key monosaccharides, and fruit pulp acids were identified for the first time in pear. A gene expression analysis of six pear cultivars identified 399 candidates in the identified QTL regions, which showed expression specific to fruit developmental stages in pear. Finally, we confirmed the function of PbrtMT1, a tonoplast monosaccharide transporter-related gene responsible for the enhancement of fructose accumulation in pear fruit on linkage group 16, in a transient transformation experiment. This study provides genomic and genetic resources as well as potential candidate genes for fruit quality improvement in pear.

Influence of Climate Change on Metabolism and Biological Characteristics in Perennial Woody Fruit Crops in the Mediterranean Environment

The changes in the state of the climate have a high impact on perennial fruit crops thus threatening food availability. Indeed, climatic factors affect several plant aspects, such as phenological stages, physiological processes, disease-pest frequency, yield, and qualitative composition of the plant tissues and derived products. To mitigate the effects of climatic parameters variability, plants implement several strategies of defense, by changing phenological trends, altering physiology, increasing carbon sequestration, and metabolites synthesis. This review was divided into two sections. The first provides data on climate change in the last years and a general consideration on their impact, mitigation, and resilience in the production of food crops. The second section reviews the consequences of climate change on the industry of two woody fruit crops models (evergreen and deciduous trees). The research focused on, citrus, olive, and loquat as evergreen trees examples; while grape, apple, pear, cherry, apricot, almond, peach, kiwi, fig, and persimmon as deciduous species. Perennial fruit crops originated by a complex of decisions valuable in a long period and involving economic and technical problems that farmers may quickly change in the case of annual crops. However, the low flexibility of woody crops is balanced by resilience in the long-life cycle.

An EjbHLH14-EjHB1-EjPRX12 module is involved in methyl jasmonate alleviation of chilling-induced lignin deposition in loquat fruit

Loquat fruit are susceptible to chilling injuries induced by postharvest storage at low temperature. The major symptoms are increased lignin content and flesh firmness, which cause a leathery texture. Pretreatment with methyl jasmonate (MeJA) can alleviate this low-temperature-induced lignification, but the mechanism is not understood. In this study, we characterized a novel class III peroxidase, EjPRX12, and studied its relationship to lignification. Transcript levels of EjPRX12 were attenuated following MeJA pretreatment, consistent with the reduced lignin content in fruit. In vitro enzyme activity assay indicated that EjPRX12 polymerized sinapyl alcohol, and overexpression of EjPRX12 in Arabidopsis promoted lignin accumulation, indicating that it plays a functional role in lignin polymerization. We also identified an HD-ZIP transcription factor, EjHB1, repressed by MeJA pretreatment, which directly bound to and significantly activated the EjPRX12 promoter. Overexpression of EjHB1 in Arabidopsis promoted lignin accumulation with induced expression of lignin-related genes, especially AtPRX64. Furthermore, a JAZ-interacting repressor, EjbHLH14, was characterized, and it is proposed that MeJA pretreatment caused EjbHLH14 to be released to repress the expression of EjHB1. These results identified a novel regulatory pathway involving EjbHLH14-EjHB1-EjPRX12 and revealed the molecular mechanism whereby MeJA alleviated lignification of loquat fruit at low temperature.

Citrus heat shock transcription factor CitHsfA7-mediated citric acid degradation in response to heat stress

Heat stress is a major abiotic stress for plants, which can generate a range of biochemical and genetic responses. In 'Ponkan' mandarin fruit, hot air treatment (HAT) accelerates the degradation of citric acid. However, the transcriptional regulatory mechanisms of citrate degradation in response to HAT remain to be elucidated. Here, 17 heat shock transcription factor sequences were isolated, and dual-luciferase assays were employed to investigate whether the encoded proteins that could trans-activate the promoters of key genes in the GABA shunt, involved in citrate metabolism. We identified four heat shock transcription factors (CitHsfA7, CitHsfA3, CitHsfA4b and CitHsfA8) that showed trans-activation effects on CitAco3, CitIDH3 and CitGAD4, respectively. Transient expression of the CitHsfs in citrus fruits indicated that CitHsfA7 was the only factor that resulted in a significant lowering of the citric acid content, and these results were confirmed by a virus-induced gene silencing system (VIGS). Sub-cellar localization showed that CitHsfA7 is located in the nucleus and is capable of binding directly to a putative HSE in the CitAco3 promoter and enhance its expression. We proposed that the induction of CitHsfA7 transcript level contributes to citric acid degradation in citrus fruit, via modulation of CitAco3 in response to HAT.

RNA Sequencing Reveals Phenylpropanoid Biosynthesis Genes and Transcription Factors for Hevea brasiliensis Reaction Wood Formation

Given the importance of wood in many industrial applications, much research has focused on wood formation, especially lignin biosynthesis. However, the mechanisms governing the regulation of lignin biosynthesis in the rubber tree (Hevea brasiliensis) remain to be elucidated. Here, we gained insight into the mechanisms of rubber tree lignin biosynthesis using reaction wood (wood with abnormal tissue structure induced by gravity or artificial mechanical treatment) as an experimental model. We performed transcriptome analysis of rubber tree mature xylem from tension wood (TW), opposite wood (OW), and normal wood (NW) using RNA sequencing (RNA-seq). A total of 214, 1,280, and 32 differentially expressed genes (DEGs) were identified in TW vs. NW, OW vs. NW, and TW vs. OW, respectively. GO and KEGG enrichment analysis of DEGs from different comparison groups showed that zeatin biosynthesis, plant hormone signal transduction, phenylpropanoid biosynthesis, and plant-pathogen interaction pathways may play important roles in reaction wood formation. Sixteen transcripts involved in phenylpropanoid biosynthesis and 129 transcripts encoding transcription factors (TFs) were used to construct a TF-gene regulatory network for rubber tree lignin biosynthesis. Among them, MYB, C2H2, and NAC TFs could regulate all the DEGs involved in phenylpropanoid biosynthesis. Overall, this study identified candidate genes and TFs likely involved in phenylpropanoid biosynthesis and provides novel insights into the mechanisms regulating rubber tree lignin biosynthesis.

A systems genetics approach reveals PbrNSC as a regulator of lignin and cellulose biosynthesis in stone cells of pear fruit

BACKGROUND

Stone cells in fruits of pear (Pyrus pyrifolia) negatively influence fruit quality because their lignified cell walls impart a coarse and granular texture to the fruit flesh.

RESULTS

We generate RNA-seq data from the developing fruits of 206 pear cultivars with a wide range of stone cell contents and use a systems genetics approach to integrate co-expression networks and expression quantitative trait loci (eQTLs) to characterize the regulatory mechanisms controlling lignocellulose formation in the stone cells of pear fruits. Our data with a total of 35,897 expressed genes and 974,404 SNPs support the identification of seven stone cell formation modules and the detection of 139,515 eQTLs for 3229 genes in these modules. Focusing on regulatory factors and using a co-expression network comprising 39 structural genes, we identify PbrNSC as a candidate regulator of stone cell formation. We then verify the function of PbrNSC in regulating lignocellulose formation using both pear fruit and Arabidopsis plants and further show that PbrNSC can transcriptionally activate multiple target genes involved in secondary cell wall formation.

CONCLUSIONS

This study generates a large resource for studying stone cell formation and provides insights into gene regulatory networks controlling the formation of stone cell and lignocellulose.

Identification, expression, and functional analysis of Hsf and Hsp20 gene families in Brachypodium distachyon under heat stress

Background

The heat shock factor (Hsf) and small heat shock protein (sHsp, also called Hsp20) complex has been identified as a primary component in the protection of plant cells from ubiquitous stresses, particularly heat stress. Our study aimed to characterize and analyze the Hsf and Hsp genes in Brachypodium distachyon, an annual temperate grass and model plant in cereal and grass studies.

Results

We identified 24 Hsf and 18 Hsp20 genes in B. distachyon and explored their evolution in gene organization, sequence features, chromosomal localization, and gene duplication. Our phylogenetic analysis showed that BdHsfs could be divided into three categories and BdHsp20s into ten subfamilies. Further analysis showed that the 3’UTR length of BdHsp20 genes had a negative relationship with their expression under heat stress. Expression analyses indicated that BdHsp20s and BdHsfs were strongly and rapidly induced by high-temperature treatment. Additionally, we constructed a complex regulatory network based on their expression patterns under heat stress. Morphological analysis suggested that the overexpression of five BdHsp20 genes enhanced the seed germination rate and decreased cell death under high temperatures.

Conclusion

Ultimately, our study provided important evolutionary and functional characterizations for future research on the regulatory mechanisms of BdHsp20s and BdHsfs in herbaceous plants under environmental stress.

Genomic and transcriptomic analyses of Citrus sinensis varieties provide insights into Valencia orange fruit mastication trait formation

Valencia orange (Citrus sinensis Osbeck) (VO) is a type of late-ripening sweet orange whose ripening occurs 4 to 5 months later than that of the mid-ripening common sweet orange (CO). Notably, the mastication trait of VO fruit is inferior to that of CO fruit. To date, how inferior pulp mastication trait forms in VO has not been determined. In this study, 13 VO varieties and 12 CO varieties were subjected to whole-genome resequencing. A total of 2.98 million SNPs were identified from 25 varieties, and a SNP molecular marker was developed to distinguish VO and CO. Moreover, 144 and 141 genes identified by selective sweep analysis were selected during VO and CO evolution, respectively. Based on gene functional enrichment analysis, most of the selected VO genes were related to the stress response and lignin biosynthesis. Simultaneously, we comparatively analyzed the transcriptome profiles of peel and pulp tissues among three VO varieties and three CO varieties, and the results demonstrated differences in lignin biosynthesis between VO and CO fruits. Furthermore, coexpression network analysis was performed to identify hub genes of lignin-related and variety-specific networks, which included CsERF74, CsNAC25, CsHSFB3, CsSPL4/13, etc. Overall, this study provides important insights into the mastication trait formation of Valencia orange fruit.

Effects of CaCl2 Treatment Alleviates Chilling Injury of Loquat Fruit (Eribotrya japonica) by Modulating ROS Homeostasis

The effects of calcium chloride (CaCl₂) treatment on chilling injury (CI), reactive oxygen species (ROS) metabolism, and ascorbate-glutathione (AsA-GSH) cycle in loquat fruit at 1 °C storage for 35 d were investigated. The results indicated that CaCl₂ treatment remarkably suppressed the increase in browning index and firmness as well as the decrease in extractable juice rate. CaCl₂ treatment also decreased the production of superoxide radical (O2•-), hydrogen peroxide (H₂O₂) content, but increased the 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl radical (OH•) scavenging ability, the activities of superoxide dismutase (SOD), catalase (CAT), and their gene expressions. Moreover, compared to the control loquat fruit, CaCl₂-treated fruit maintained higher contents of AsA, GSH, higher levels of activities of ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR) and expressions of EjAPX, EjGR, EjMDHAR, and EjDHAR, but exhibited lower glutathione disulfide (GSSG) content. These results suggested that CaCl₂ treatment alleviated CI in loquat fruit through enhancing antioxidant enzymes activities and AsA-GSH cycle system to quench ROS.

The MADS-Box Transcription Factor EjAGL65 Controls Loquat Flesh Lignification via Direct Transcriptional Inhibition of EjMYB8

Loquat fruit accumulates lignin in its flesh when undergoing chilling injury during postharvest storage, making it a suitable model for the study of flesh lignification. Transcriptional regulation of lignin biosynthesis is principally controlled by the NAC-MYB transcriptional cascade in model plants. Previous research has demonstrated that EjMYB8 activates lignin biosynthesis through direct interaction with the promoter of Ej4CL1. However, the classic NAC-MYB gene regulation network has not been established. Here, the MADS-box gene EjAGL65 was discovered by screening a cDNA library using the EjMYB8 promoter as bait in yeast. A phylogenetic analysis and structural comparisons revealed that EjAGL65 belongs to the Mδ subgroup of the MADS-box family, whose members have not been reported as being involved in the regulation of lignin deposition. EjAGL65 transcription was downregulated at 0°C compared to 5°C, indicating a negative correlation with the change of lignin content. A dual-luciferase assay indicated that EjAGL65 is capable of inhibiting the promoter activity of EjMYB8 in vivo. These results showed that the Mδ MADS-box gene EjAGL65 transcriptionally regulates EjMYB8 during postharvest chilling induced flesh lignification, which differs from the classical regulation model of lignin biosynthesis that has been illustrated for developmental lignin accumulation.

Exogenous melatonin positively regulates lignin biosynthesis in Camellia sinensis

Melatonin (MT) is a bioactive molecule that can regulate various developmental processes. Changes in lignin content play important roles in plant growth and development. Herein, quantitative analysis and histochemical staining showed that lignin content significantly increased over time, and melatonin treatment triggered the lignification at 8 and 16 d in tea leaves. The POD activity participated in lignin formation had also been significantly improved. The effect of melatonin on the increase of lignin content was attenuation over time. Sequencing results based on transcriptome at 8 and 16 d showed that 5273 and 3019 differentially expressed genes (DEGs) were identified in CK1 vs. MT1 and CK2 vs. MT2, respectively. A total of 67 DEGs were annotated to lignin biosynthesis, and 38 and 9 genes were significantly up-regulated under melatonin treatment, respectively. Some transcription factor genes such as MYB were also identified among the two pairwise comparisons, which might relate to lignin metabolism. Melatonin increased the degree of lignification in tea leaves by modifying the enzyme genes expression involved in lignin synthesis pathway. These results provide a reference for further study on the molecular mechanism of the dynamic changes of lignin content induced by melatonin treatment in tea plants.

Genome-wide identification and characterization of Lectin receptor-like kinase (LecRLK) genes in mungbean (Vigna radiata L. Wilczek)

Lectins are a diverse group of proteins found throughout plant species. Numerous lectins are involved in many important processes such as organogenesis, defense mechanism, signaling, and stress response. Although the mungbean whole genome sequence has been published, distribution, diversification, and gene structure of lectin genes in mungbean are still unknown. A total of 73 putative lectin genes with kinase domain have been identified through BLAST and HMM profiling. Furthermore, these sequences could be classified into three families, such as G-type, L-type, and C-type VrLecRLKs. 59 out of 73 VrLecRLKs were distributed on to 11 chromosomes, whereas rest could not be anchored onto any specific chromosome. Gene structure analysis revealed a varying number of exons in 73 VrLecRLK genes. Gene ontology annotations were grouped into three categories like biological processes, cellular components and molecular functions, which were associated with signaling pathways, defense responses, transferase activity, binding activity, and kinase activity. The comprehensive and systematic studies of LecRLK genes family provides a reference and foundation for further functional analysis of VrLecRLK genes in mungbean.

Comparison of different analytical methods to evaluate the heat shock protein (HSP) response in fruits. Application to tomatoes subjected to stress treatments

Heat shock proteins (HSP) are synthesized in living tissues exposed to transient increase in temperature and play a central role in the protective response against heat and other stresses. In fruits, this response to heat treatment provides resistance to a physiological alteration known as chilling injury. Despite the physiological importance of this group of proteins, publications comparing different methodological alternatives for their analysis are rather scarce. In the present paper, we conducted a comparative study using different electrophoretic and immunological techniques to evaluate the HSP response in fruits. Proteins were extracted from tomato fruit exposed to an HSP-inducing temperature (38 °C) for different times (0, 3, 20, and 27 h). Different alternatives of analysis (SDS-PAGE, SDS-PAGE followed by IEF, Western blot, and dot blot) were performed, and their potential application discussed. The study was complemented with a practical application, in which tomatoes were subjected to heat and anaerobic treatments and then stored in a chill-inducing temperature. This application evidences the relevance of knowing the level of proteins attained by stress treatments which correlates with the acquired tolerance.

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HSP evaluation can be used for practical purposes.

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To assess the HSP response in fruits, different complementary methods should be used.

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A simple method (dot blot) can quantify HSP induced in fruits by heat exposure.

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HSP level induced by stress treatments correlates with acquired physiological tolerance.

Feeding on tea GH19 chitinase enhances tea defense responses induced by regurgitant derived from Ectropis grisescens

Multiple isoforms of chitinases participate in plant defense against outside invaders. However, the functions of hydrolase family 19 (GH19) chitinases on pest control remain largely unknown. Here we reported the isolation and functional analysis of a gene CsChi19, which encodes a GH19 endochitinase protein of 332 amino acid residues from tea plant (Camellia sinensis). CsChi19 expression levels were upregulated in response to mechanical wounding, infestation by two important pests: the tea geometrid Ectropis grisescens and the tea green leafhopper Empoasca (Matsumurasca) onukii, a fungal pathogen Colletotrichum fructicola, and treatment with two phytohormones: jasmonic acid (JA) and salicylic acid. CsChi19 was heterologously expressed in Escherichia coli, and its catalytic function was further elucidated. The protein could hydrolyze colloidal chitin, and the optimum temperature and pH for its activity was 40°C and pH 5.0. CsChi19 were found to be toxic to tea pests when they were fed on artificial diets containing this protein. Interestingly, the regurgitant derived from E. grisescens fed with artificial diets containing CsChi19 protein induced stronger expression of CsMPK3, more JA burst, more accumulation of defense-related secondary metabolites, and more emission of volatiles than the regurgitant derived from E. grisescens fed only with artificial diets. Our results provide first evidence that CsChi19 is involved in mediating a novel defense mechanism of tea plant through altering the composition of the regurgitant.

ETHYLENE RESPONSE FACTOR39-MYB8 complex regulates low-temperature-induced lignification of loquat fruit

Flesh lignification is a specific chilling response that causes deterioration in the quality of stored red-fleshed loquat fruit (Eribotrya japonica) and is one aspect of wider chilling injury. APETALA2/ETHLENE RESPONSIVE FACTOR (AP2/ERF) transcription factors are important regulators of plant low-temperature responses and lignin biosynthesis. In this study, the expression and action of 27 AP2/ERF genes from the red-fleshed loquat cultivar 'Luoyangqing' were investigated in order to identify transcription factors regulating low-temperature-induced lignification. EjERF27, EjERF30, EjERF36, and EjERF39 were significantly induced by storage at 0 °C but inhibited by a low-temperature conditioning treatment (pre-storage at 5 °C for 6 days before storage at 0 °C, which reduces low-temperature-induced lignification), and their transcript levels positively correlated with flesh lignification. A dual-luciferase assay indicated that EjERF39 could transactivate the promoter of the lignin biosynthetic gene Ej4CL1, and an electrophoretic mobility shift assay confirmed that EjERF39 recognizes the DRE element in the promoter region of Ej4CL1. Furthermore, the combination of EjERF39 and the previously characterized EjMYB8 synergistically transactivated the Ej4CL1 promoter, and both transcription factors showed expression patterns correlated with lignification in postharvest treatments and red-fleshed 'Luoyangqing' and white-fleshed 'Ninghaibai' cultivars with different lignification responses. Bimolecular fluorescence complementation and luciferase complementation imaging assays confirmed direct protein-protein interaction between EjERF39 and EjMYB8. These data indicate that EjERF39 is a novel cold-responsive transcriptional activator of Ej4CL1 that forms a synergistic activator complex with EjMYB8 and contributes to loquat fruit lignification at low temperatures.

Transcriptome-wide identification and characterization of the Rab GTPase family in mango

The Rab GTPase family plays a vital role in several plant physiological processes including fruit ripening. Fruit softening during ripening involves trafficking of cell wall polymers and enzymes between cellular compartments. Mango, an economically important fruit crop, is known for its delicious taste, exotic flavour and nutritional value. So far, there is a paucity of information on the mango Rab GTPase family. In this study, 23 genes encoding Rab proteins were identified in mango by a comprehensive in silico approach. Sequence alignment and similarity tree analysis with the model plant Arabidopsis as a reference enabled the bona fide assignment of the deduced mango proteins to classify into eight subfamilies. Expression analysis by RNA-Sequencing (RNA-Seq) showed that the Rab genes were differentially expressed in ripe and unripe mangoes suggesting the involvement of vesicle trafficking during ripening. Interaction analysis showed that the proteins involved in vesicle trafficking and cell wall softening were interconnected providing further evidence of the involvement of the Rab GTPases in fruit softening. Correlation analyses showed a significant relationship between the expression level of the RabA3 and RabA4 genes and fruit firmness at the unripe stage of the mango varieties suggesting that the differences in gene expression level might be associated with the contrasting firmness of these varieties. This study will not only provide new insights into the complexity of the ripening-regulated molecular mechanism but also facilitate the identification of potential Rab GTPases to address excessive fruit softening.

(E)-Nerolidol is a volatile signal that induces defenses against insects and pathogens in tea plants

Plants release large amounts of volatile organic compounds (VOCs) in response to attackers. Several VOCs can serve as volatile signals to elicit defense responses in undamaged tissues and neighboring plants, but many questions about the ecological functions of VOCs remain unanswered. Tea plants are impacted by two harmful invaders, the piercing herbivore Empoasca (Matsumurasca) onukii Matsuda and the pathogen Colletotrichum fructicola. To determine the VOC signals in tea, we confirmed CsOPR3 as a marker gene and set up a rapid screening method based on a 1.51 kb CsOPR3 promoter fused with a β-glucuronidase (GUS) reporter construct (OPR3p::GUS) in Arabidopsis. Using this screening system, a terpenoid volatile (E)-nerolidol was identified as a potent signal that elicits plant defenses. The early responses triggered by (E)-nerolidol included the activation of a mitogen-activated protein kinase and WRKY, an H2O2 burst, and the induction of jasmonic acid and abscisic acid signaling. The induced plants accumulated high levels of defense-related chemicals, which possessed broad-spectrum anti-herbivore or anti-pathogen properties, and ultimately triggered resistance against Empoasca onukii and Colletotrichum fructicola in tea. We propose that these findings can supply an environmentally friendly management strategy for controlling an insect pest and a disease of tea plants.

Deep-sequencing of Solanum commersonii small RNA libraries reveals riboregulators involved in cold stress response

Among wild species used in potato breeding, Solanum commersonii displays the highest tolerance to low temperatures under both acclimated (ACC) and non-acclimated (NACC) conditions. It is also the first wild potato relative with a known whole genome sequence. Recent studies have shown that abiotic stresses induce changes in the expression of many small non-coding RNA (sncRNA). We determined the small non-coding RNA (sncRNAome) of two clones of S. commersonii contrasting in their cold response phenotypes via smRNAseq. Differential analysis provided evidence that expression of several miRNAs changed in response to cold stress conditions. Conserved miR408a and miR408b changed their expression under NACC conditions, whereas miR156 and miR169 were differentially expressed only under ACC conditions. We also report changes in tasiRNA and secondary siRNA expression under both stress conditions. Our results reveal possible roles of sncRNA in the regulatory networks associated with tolerance to low temperatures and provide useful information for a more strategic use of genomic resources in potato breeding.

A Prunus persica genome-wide RNA-seq approach uncovers major differences in the transcriptome among chilling injury sensitive and non-sensitive varieties

Chilling injury represents a major constrain for crops productivity. Prunus persica, one of the most relevant rosacea crops, have early season varieties that are resistant to chilling injury, in contrast to late season varieties, which display chilling symptoms such as mealiness (dry, sandy fruit mesocarp) after prolonged storage at chilling temperatures. To uncover the molecular processes related to the ability of early varieties to withstand mealiness, postharvest and genome-wide RNA-seq assessments were performed in two early and two late varieties. Differences in juice content and ethylene biosynthesis were detected among early and late season fruits that became mealy after exposed to prolonged chilling. Principal component and data distribution analysis revealed that cold-stored late variety fruit displayed an exacerbated and unique transcriptome profile when compared to any other postharvest condition. A differential expression analysis performed using an empirical Bayes mixture modeling approach followed by co-expression and functional enrichment analysis uncover processes related to ethylene, lipids, cell wall, carotenoids and DNA metabolism, light response, and plastid homeostasis associated to the susceptibility or resistance of P. persica varieties to chilling stress. Several of the genes related to these processes are in quantitative trait loci (QTL) associated to mealiness in P. persica. Together, these analyses exemplify how P. persica can be used as a model for studying chilling stress in plants.

Label-free visualization of lignin deposition in loquats using complementary stimulated and spontaneous Raman microscopy

The lignification triggered by biotic or abiotic stresses hardens fruits and vegetables and eventually influences their consumer appeal. Extensive prior efforts have been made to unveil the underlying mechanism of flesh lignification, primarily focused on its physicochemical and molecular biological properties. Nevertheless, most of these studies used destroyed and homogenized bulk tissues as analytes; as a result, potentially valuable spatial information was lost. In this study, the deposition of lignin in loquat flesh during lignification was visualized from the tissue level to the single-cell level by combining the advantages of stimulated Raman scattering (SRS) and spontaneous Raman microscopy using label-free in situ molecular imaging. SRS has the advantages of being fast and providing large-area chemical imaging to reveal the spatial heterogeneity of lignin and cell wall polysaccharide distribution in loquat flesh. After 2 days of storage at 0 °C, increased lignins were observed by large-area SRS imaging. In addition, microscopic SRS images of the flesh cells indicated that the increased lignins were trapped in the cell corner (CC) and middle lamella (ML). Furthermore, the compositional and structural features of lignified cells (LCs), CC and ML of loquat flesh were investigated by spontaneous Raman microscopy, and the results showed that the LCs were a combination of lignin, cellulose, and hemicellulose, whereas CC and ML showed only deposited lignin and pectin without cross-linked cellulose and hemicellulose. This result further suggests that the lignins in the CC and ML regions of loquats were later synthesized alone during postharvest storage. This innovative combination of SRS and spontaneous Raman microscopy allows the label-free macroscale and fine chemical imaging of plant cell walls and will enhance our fundamental understanding of the structures and functions of the plant cell wall.

EjHAT1 Participates in Heat Alleviation of Loquat Fruit Lignification by Suppressing the Promoter Activity of Key Lignin Monomer Synthesis Gene EjCAD5

Texture attributes such as firmness and lignification are important for fruit quality. Lignification has been widely studied in model plants and energy crops, but fruit lignification has rarely been investigated, despite having an adverse effect on fruit quality and consumer preference. Chilling-induced loquat fruit lignification that occurs after harvest can be alleviated by heat treatment (HT) applied prior to low temperature storage. Enzyme activity assay showed that HT treatment could retard the low temperature-induced increase in cinnamyl alcohol dehydrogenase (CAD) activity. Transcript analysis and substrate activity assays of recombinant CAD proteins highlighted the key role of EjCAD5 in chilling-induced lignin biosynthesis. A novel homeobox-leucine zipper protein ( EjHAT1) was identified as a negative regulator of EjCAD5. Therefore, the effect of HT treatment on lignification may be partially due to the suppression of the EjCAD5 promoter activity by EjHAT1.

Transcriptome analysis provides insights into the regulation of metabolic processes during postharvest cold storage of loquat (Eriobotrya japonica) fruit

Loquat (Eriobotrya japonica) fruit accumulates lignin during postharvest storage under chilling conditions (0 °C), while low-temperature conditioning (LTC; 5 °C for 6 days followed by transfer to 0 °C) or heat treatment (HT; 40 °C for 4 h followed by transfer to 0 °C) can alleviate lignification. Here we compared transcriptome profiles of loquat fruit samples under LTC or HT to those stored at 0 °C at five time points from day 1 to day 8 after treatment. High-throughput transcriptome sequences were de novo assembled into 53,319 unique transcripts with an N50 length of 1306 bp. A total of 2235 differentially expressed genes were identified in LTC, and 1020 were identified in HT compared to 0 °C. Key genes in the lignin biosynthetic pathway, including EjPAL2, EjCAD1, EjCAD3, 4CL, COMT, and HCT, were responsive to LTC or HT treatment, but they showed different expression patterns during the treatments, indicating that different structural genes could regulate lignification at different treatment stages. Coexpression network analysis showed that these candidate biosynthetic genes were associated with a number of transcription factors, including those belonging to the AP2, MYB, and NAC families. Gene ontology (GO) enrichment analysis of differentially expressed genes indicated that biological processes such as stress responses, cell wall and lignin metabolism, hormone metabolism, and metal ion transport were significantly affected under LTC or HT treatment when compared to 0 °C. Our analyses provide insights into transcriptome responses to postharvest treatments in loquat fruit.

Molecular Characterization and Expression Profile Analysis of Heat Shock Transcription Factors in Mungbean

Heat shock transcription factors (Hsfs) are essential elements in plant signal transduction pathways that mediate gene expression in response to various abiotic stresses. Mungbean (Vigna radiata) is an important crop worldwide. The emergence of a genome database now allows for functional analysis of mungbean genes. In this study, we dissect the mungbean Hsfs using genome-wide identification and expression profiles. We characterized a total of 24 VrHsf genes and classified them into three groups (A, B, and C) based on their phylogeny and conserved domain structures. All VrHsf genes exhibit highly conserved exon-intron organization, with two exons and one intron. In addition, all VrHsf proteins contain 16 distinct motifs. Chromosome location analysis revealed that VrHsf genes are located on 8 of the 11 mungbean chromosomes, and that seven duplicated gene pairs had formed among them. Moreover, transcription patterns of VrHsf genes varied in different tissues, indicating their different roles in plant growth and development. We identified multiple stress related cis-elements in VrHsf promoter regions 2 kb upstream of the translation initiation codons, and the expression of most VrHsf genes was altered under different stress conditions, suggesting their potential functions in stress resistance pathways. These molecular characterization and expression profile analyses of VrHsf genes provide essential information for further function investigation.

PbrmiR397a regulates lignification during stone cell development in pear fruit

Lignified stone cells substantially reduce fruit quality. Therefore, it is desirable to inhibit stone cell development using genetic technologies. However, the molecular mechanisms regulating lignification are poorly understood in fruit stone cells. In this study, we have shown that microRNA (miR) miR397a regulates fruit cell lignification by inhibiting laccase (LAC) genes that encode key lignin biosynthesis enzymes. Transient overexpression of PbrmiR397a, which is the miR397a of Chinese pear (Pyrus bretschneideri), and simultaneous silencing of three LAC genes reduced the lignin content and stone cell number in pear fruit. A single nucleotide polymorphism (SNP) identified in the promoter of the PbrmiR397a gene was found to associate with low levels of fruit lignin, after analysis of the genome sequences of sixty pear varieties. This SNP created a TCA element that responded to salicylic acid to induce gene expression as confirmed using a cell-based assay system. Furthermore, stable overexpression of PbrmiR397a in transgenic tobacco plants reduced the expression of target LAC genes and decreased the content of lignin but did not change the ratio of syringyl- and guaiacyl-lignin monomers. Consistent with reduction in lignin content, the transgenic plants showed fewer numbers of vessel elements and thinner secondary walls in the remaining elements compared to wild-type control plants. This study has advanced our understanding of the regulation of lignin biosynthesis and provided useful molecular genetic information for improving pear fruit quality.

Metabolic Dynamics During Loquat Fruit Ripening and Postharvest Technologies

Loquat is an important fruit widely cultivated worldwide with high commercial value. During loquat fruit development, ripening, and storage, many important metabolites undergo dramatic changes, resulting in accumulation of a diverse mixture of nutrients. Given the value of loquat fruit, significant progresses have been achieved in understanding the metabolic changes during fruit ripening and storage, as well as postharvest technologies applied in loquat fruit in recent years. The objective of the present review is to summarize currently available knowledge and provide new references for improving loquat fruit quality.

De Novo Analysis Reveals Transcriptomic Responses in Eriobotrya japonica Fruits during Postharvest Cold Storage

Cold storage is the primary preservation method of postharvest loquat fruits. However, cold storage also results in many chilling injury physiological disorders called lignification, which decreases the quality and economic value of the fruits. Few studies to date have focused on the transcriptomic responses associated with lignification except lignin synthesis pathways. This study aimed to explore the changes of loquat transcriptome during long-term cold storage. Our results showed that the gene expression patterns were differed among the five stages. The differentially expressed genes (DEGs) in response to cold storage were more intense and complex in earlier stage. The membrane-related genes preferentially responded to low temperature and were followed by intracellular-located genes. The cold-induced pathways were mainly concerned with signal transduction and secondary metabolism (i.e., lignin, pectin, cellulose, terpenoid, carotenoid, steroid) in the first three stages and were chiefly related to primary metabolism in the later two stages, especially energy metabolism. Further investigation suggested that 503 protein kinases, 106 protein phosphatases, and 40 Ca2+ signal components were involved in the cold signal transduction of postharvest loquat fruits. We predicted a pathway including 649 encoding genes of 49 enzymes, which displayed the metabolisms of major sugars and polysaccharides in cold-stored loquat fruits. The coordinated expression patterns of these genes might contribute to the changes of saccharides in the pathway. These results provide new insight into the transcriptomic changes of postharvest loquat fruits in response to cold storage environment, which may be helpful for improving the postharvest life of loquat in the future.

Lignins: Biosynthesis and Biological Functions in Plants

Lignin is one of the main components of plant cell wall and it is a natural phenolic polymer with high molecular weight, complex composition and structure. Lignin biosynthesis extensively contributes to plant growth, tissue/organ development, lodging resistance and the responses to a variety of biotic and abiotic stresses. In the present review, we systematically introduce the biosynthesis of lignin and its regulation by genetic modification and summarize the main biological functions of lignin in plants and their applications. We hope this review will give an in-depth understanding of the important roles of lignin biosynthesis in various plants’ biological processes and provide a theoretical basis for the genetic improvement of lignin content and composition in energy plants and crops.

Label-free visualization of fruit lignification: Raman molecular imaging of loquat lignified cells

BACKGROUND

Flesh lignification, leading to increased fruit firmness, has been reported in several kinds of fruit. Understanding the mechanisms underlying fruit lignification is important to optimize the postharvest storage strategies and reduce the quality deterioration of postharvest fruit. Especially cellular level investigation of lignin deposition in fruits provides novel insight for deciphering the mechanisms underlying fruit lignification. The primary objective of this study was to establish a procedure of using Raman microspectroscopy technique to depict fruit lignification at the cell level.

RESULTS

Lignified cells, a special kind of cells contained high lignin content, were found abundantly scattered in red-fleshed 'Luoyangqing' loquat. Whereas these special lignified cells were barely detected in 'Baisha' loquat flesh. Dominant Raman bands of lignified cells were found primarily attributed to lignin (1664, 1628, 1603, 1467, and 1272 cm^-1), cellulose (1383, 1124 and 1098 cm^-1) and pectin (852 and 1740 cm^-1). The band intensity correlation analysis indicated the peak at 1335 cm^-1 assigned to either lignin or cellulose in previous works was related to lignin for the lignified cells. Multi-peaks Gaussian fitting successfully resolved the overlapped fingerprint peaks of lignin in 1550-1700 cm^-1 into three independent peaks, which were assigned to different functional groups of lignin. Furthermore, the spatially resolved Raman images of lignified cells were generated, indicating that lignin and cellulose saturated the whole lignified cells, pectin mainly located in the cell corner, and the parenchyma cells contained little lignin. In addition, both phloroglucinol-HCl staining and autofluorescence analysis confirmed the results of lignin distribution of Raman microscopic analysis.

CONCLUSIONS

A procedure for the simultaneous visualization of the main components of the flesh cells without labeling by high-resolution Raman microspectroscopy has been established. With Raman microscopic imaging technique, we can add a microscopic level to cell compositions, essential for a detailed molecular understanding of loquat lignification. Such method can be further used to chemically monitor the textural changes during the ripening process or postharvest storage of other fruits and vegetables.

Transcriptomic and metabolic analyses provide new insights into chilling injury in peach fruit

Low temperature conditioning (LTC) alleviates peach fruit chilling injury but the underlying molecular basis is poorly understood. Here, changes in transcriptome, ethylene production, flesh softening, internal browning and membrane lipids were compared in fruit maintained in constant 0 °C and LTC (pre-storage at 8 °C for 5 d before storage at 0 °C). Low temperature conditioning resulted in a higher rate of ethylene production and a more rapid flesh softening as a result of higher expression of ethylene biosynthetic genes and a series of cell wall hydrolases. Reduced internal browning of fruit was observed in LTC, with lower transcript levels of polyphenol oxidase and peroxidase, but higher lipoxygenase. Low temperature conditioning fruit also showed enhanced fatty acid content, increased desaturation, higher levels of phospholipids and a preferential biosynthesis of glucosylceramide. Genes encoding cell wall hydrolases and lipid metabolism enzymes were coexpressed with differentially expressed ethylene response factors (ERFs) and contained ERF binding elements in their promoters. In conclusion, LTC is a special case of cold acclimation which increases ethylene production and, operating through ERFs, promotes both softening and changes in lipid composition and desaturation, which may modulate membrane stability, reducing browning and contributing to alleviation of peach fruit chilling injury.

EjODO1, a MYB Transcription Factor, Regulating Lignin Biosynthesis in Developing Loquat (Eriobotrya japonica) Fruit

Lignin is important for plant secondary cell wall formation and participates in resistance to various biotic and abiotic stresses. Loquat undergoes lignification not only in vegetative tissues but also in flesh of postharvest fruit, which adversely affects consumer acceptance. Thus, researches on lignin biosynthesis and regulation are important to understand loquat fruit lignification. In loquat, a gene encoding an enzyme in the lignin biosynthesis pathway, Ej4CL1, was reported to be regulated by transcription factors, including EjMYB1, EjMYB2, EjMYB8, and EjAP2-1, knowledge of this process is still limited. With the aim of identifying novel transcriptional factors controlling lignin biosynthesis in loquat, the promoter of Ej4CL1 was utilized to screen a cDNA library by yeast one hybrid assay. A novel R2R3 MYB, named EjODO1, was identified. Real-time PCR analyses indicated that EjODO1 is highly expressed in lignified stems and roots. During fruit development, expression of EjODO1 decreased along with the reduction of lignin content and became undetectable in mature ripe fruit. Thus, EjODO1 is likely to be involved in lignification of vegetative organs and early fruit development but not in mature fruit or postharvest lignification. Dual-luciferase assay indicated that EjODO1 could trans-activate promoters of lignin biosynthesis genes, such as EjPAL1, Ej4CL1, and Ej4CL5 and transient overexpression of EjODO1 triggered lignin biosynthesis. These results indicate a role for EjODO1 in regulating lignin biosynthesis in loquat which is different from the previously characterized transcription factors.