Involvement of CsWRKY70 in salicylic acid-induced citrus fruit resistance against Penicillium digitatum

Comparative transcriptome and metabolome reveal the role of acidic electrolyzed oxidizing water in improving postharvest disease resistance of longan fruit

Acidic electrolyzed oxidizing water (AEOW) was applied to suppress disease development and maintain good quality of fresh fruit. However, the involvement of AEOW in improving disease resistance of fresh longan remains unknown. Here, transcriptomic and metabolic analyses were performed to compare non-treated and AEOW-treated longan during storage. The transcriptome analysis showed AEOW-induced genes associated with phenylpropanoid and flavonoid biosynthesis. The metabolome analysis found the contents of coumarin, phenolic acid, and tannin maintained higher levels in AEOW-treated longan than non-treated longan. Moreover, the weighted correlation network analysis (WGCNA) was performed to identify hub genes, and a gene-metabolite correlation network associated with AEOW-improved disease resistance in longan was constructed by the co-analysis of transcriptomics and metabolomics. These findings identified a series of important genes and metabolites involving in AEOW-induced disease resistance of longan fruit, expanding our knowledges on fruit disease resistance and quality maintenance at the transcript and metabolic levels.

Transcription factor CrWRKY42 coregulates chlorophyll degradation and carotenoid biosynthesis in citrus

Chlorophyll degradation and carotenoid biosynthesis, which occur almost simultaneously during fruit ripening, are essential for the coloration and nutritional value of fruits. However, the synergistic regulation of these 2 processes at the transcriptional level remains largely unknown. In this study, we identified a WRKY transcription factor, CrWRKY42, from the transcriptome data of the yellowish bud mutant "Jinlegan" ([Citrus unshiu × C. sinensis] × C. reticulata) tangor and its wild-type "Shiranui" tangor, which was involved in the transcriptional regulation of both chlorophyll degradation and carotenoid biosynthesis pathways. CrWRKY42 directly bound to the promoter of β-carotene hydroxylase 1 (CrBCH1) and activated its expression. The overexpression and interference of CrWRKY42 in citrus calli demonstrated that CrWRKY42 promoted carotenoid accumulation by inducing the expression of multiple carotenoid biosynthetic genes. Further assays confirmed that CrWRKY42 also directly bound to and activated the promoters of the genes involved in carotenoid biosynthesis, including phytoene desaturase (CrPDS) and lycopene β-cyclase 2 (CrLCYB2). In addition, CrWRKY42 could bind to the promoters of NONYELLOW COLORING (CrNYC) and STAY-GREEN (CrSGR) and activate their expression, thus promoting chlorophyll degradation. The overexpression and silencing of CrWRKY42 in citrus fruits indicated that CrWRKY42 positively regulated chlorophyll degradation and carotenoid biosynthesis by synergistically activating the expression of genes involved in both pathways. Our data revealed that CrWRKY42 acts as a positive regulator of chlorophyll degradation and carotenoid biosynthesis to alter the conversion of citrus fruit color. Our findings provide insight into the complex transcriptional regulation of chlorophyll and carotenoid metabolism during fruit ripening.

Comparative Transcriptomics Analysis Reveals the Differences in Transcription between Resistant and Susceptible Pepper (Capsicum annuum L.) Varieties in Response to Anthracnose

Pepper (Capsicum annuum L.) is a herbaceous plant species in the family Solanaceae. Capsicum anthracnose is caused by the genus Colletotrichum. spp., which decreases pepper production by about 50% each year due to anthracnose. In this study, we evaluated the resistance of red ripe fruits from 17 pepper varieties against anthracnose fungus Colletotrichum capsici. We assessed the size of the lesion diameter and conducted significance analysis to identify the resistant variety of B158 and susceptible variety of B161. We selected a resistant cultivar B158 and a susceptible cultivar B161 of pepper and used a transcription to investigate the molecular mechanisms underlying the plant's resistance to C. capsici, of which little is known. The inoculated fruit from these two varieties were used for the comparative transcription analysis, which revealed the anthracnose-induced differential transcription in the resistant and susceptible pepper samples. In the environment of an anthrax infection, we found that there were more differentially expressed genes in resistant varieties compared to susceptible varieties. Moreover, the response to stimulus and stress ability was stronger in the KANG. The transcription analysis revealed the activation of plant hormone signaling pathways, phenylpropanoid synthesis, and metabolic processes in the defense response of peppers against anthracnose. In addition, ARR-B, AP2-EREBP, bHLH, WRKY, and NAC are associated with disease resistance to anthracnose. Notably, WRKY and NAC were found to have a potentially positive regulatory role in the defense response against anthracnose. These findings contribute to a more comprehensive understanding of the resistance mechanisms of red pepper fruit to anthracnose infection, providing valuable molecular insights for further research on the resistance mechanisms and genetic regulations during this developmental stage of pepper.

Induced Resistance in Fruit and Vegetables: A Host Physiological Response Limiting Postharvest Disease Development

Harvested fruit and vegetables are perishable, subject to desiccation, show increased respiration during ripening, and are colonized by postharvest fungal pathogens. Induced resistance is a strategy to control diseases by eliciting biochemical processes in fruits and vegetables. This is accomplished by modulating the progress of ripening and senescence, which maintains the produce in a state of heightened resistance to decay-causing fungi. Utilization of induced resistance to protect produce has been improved by scientific tools that better characterize physiological changes in plants. Induced resistance slows the decline of innate immunity after harvest and increases the production of defensive responses that directly inhibit plant pathogens. This increase in defense response in fruits and vegetables contributes to higher amounts of phenols and antioxidant compounds, improving both the quality and appearance of the produce. This review summarizes mechanisms and treatments that induce resistance in harvested fruits and vegetables to suppress fungal colonization. Moreover, it highlights the importance of host maturity and stage of ripening as limiting conditions for the improved expression of induced-resistance processes.

Limonene anti-TMV activity and its mode of action

The main component of orange peel essential oil is limonene. Limonene is a natural active monoterpene with multiple functions, such as antibacterial, antiseptic and antitumor activity, and has important development value in agriculture. This study found that limonene exhibited excellent anti-tobacco mosaic virus (TMV) bioactivity, with results showing that its protection activity, inactivation activity, and curative activity at 800 μg/mL were 84.93%, 59.28%, and 58.89%, respectively-significantly higher than those of chito-oligosaccharides. A direct effect of limonene on TMV particles was not observed, but limonene triggered the hypersensitive response (HR) in tobacco. Further determination of the induction activity of limonene against TMV demonstrated that it displayed good induction activity at 800 μg/mL, with a value of 60.59%. The results of physiological and biochemical experiments showed that at different treatment days, 800 μg/mL limonene induced the enhancement of defense enzymes activity in tobacco, including of SOD, CAT, POD, and PAL, which respectively increased by 3.2, 4.67, 4.12, and 2.33 times compared with the control (POD and SOD activities reached highest on the seventh day, and PAL and CAT activities reached highest on the fifth day). Limonene also enhanced the relative expression levels of pathogenesis related (PR) genes, including NPR1, PR1, and PR5, which were upregulated 3.84-fold, 1.86-fold and 1.71-fold, respectively. Limonene induced the accumulation of salicylic acid (SA), and increased the relative expression levels of genes related to SA biosynthesis (PAL) and reactive oxygen species (ROS) burst (RBOHB), which respectively increased by 2.76 times and 4.23 times higher than the control. Systemic acquired resistance (SAR) is an important plant immune defense against pathogen infection. The observed accumulation of SA, the enhancement of defense enzymes activity and the high-level expression of defense-related genes suggested that limonene may induce resistance to TMV in tobacco by activating SAR mediated by the SA signaling pathway. Furthermore, the experimental results demonstrated that the expression level of the chlorophyll biosynthesis gene POR1 was increased 1.72-fold compared to the control in tobacco treated with 800 μg/mL limonene, indicating that limonene treatment may increase chlorophyll content in tobacco. The results of pot experiment showed that 800 μg/mL limonene induced plant resistance against Sclerotinia sclerotiorum (33.33%), Phytophthora capsici (54.55%), Botrytis cinerea (50.00%). The bioassay results indicated that limonene provided broad-spectrum and long-lasting resistance to pathogen infection. Therefore, limonene has good development and utilization value, and is expected to be developed into a new botanical-derived anti-virus agent and plant immunity activator in addition to insecticides and fungicides.

Functional characterization of two flavone synthase II members in citrus

Polymethoxylated flavones (PMFs), the main form of flavones in citrus, are derived from the flavone branch of the flavonoid biosynthesis pathway. Flavone synthases (FNSs) are enzymes that catalyze the synthesis of flavones from flavanones. However, the FNS in citrus has not been characterized yet. Here, we identified two type II FNSs, designated CitFNSII-1 and CitFNSII-2, based on phylogenetics and transcriptome analysis. Both recombinant CitFNSII-1 and CitFNSII-2 proteins directly converted naringenin, pinocembrin, and liquiritigenin to the corresponding flavones in yeast. In addition, transient overexpression of CitFNSII-1 and CitFNSII-2, respectively, in citrus peel significantly enhanced the accumulation of total PMFs, while virus-induced CitFNSII-1 and CitFNSII-2 genes silencing simultaneously significantly reduced the expression levels of both genes and total PMF content in citrus seedlings. CitFNSII-1 and CitFNSII-2 presented distinct expression patterns in different cultivars as well as different developmental stages. Methyl salicylate (MeSA) treatment reduced the CitFNSII-2 expression as well as the PMFs content in the peel of Citrus sinensis fruit but did not affect the CitFNSII-1 expression. These results indicated that both CitFNSII-1 and CitFNSII-2 participated in the flavone biosynthesis in citrus while the regulatory mechanism governing their expression might be specific. Our findings improved the understanding of the PMFs biosynthesis pathway in citrus and laid the foundation for further investigation on flavone synthesis regulation.

Transcriptional regulatory mechanism of a variant transcription factor CsWRKY23 in citrus fruit resistance to Penicillium digitatum

As the most productive fruit in the world, there is a lack of research on disease resistance mechanisms in citrus. WRKY transcription factors are mediators of disease resistance in plants. In this research, CsWRKY23, which could respond to exogenous salicylic acid (SA) was identified. As a variant WRKY, the transient overexpression of CsWRKY23 in citrus peel enhanced the resistance to Penicillium digitatum. The potential targets of CsWRKY23 were identified by using DNA affinity purification sequencing (DAP-seq). In combination with RNA sequencing (RNA-seq) data, 48 direct target genes of CsWRKY23 were identified. In addition, five genes for EMSA and dual-luciferase report system analysis were selected and the results showed that CsWRKY23 activate CsAAE12, CsRbohD, CsSARD1, CsWRKY22 and CsIQM6 expressions. Further analysis revealed that CsWRKY23 contributed to SA synthesis pathway, reactive oxygen species (ROS) accumulation, and cell wall strengthening. This study presented a possible regulatory network for disease resistance involving the CsWRKY23.

Transcriptomic and metabolic studies on the role of inorganic and organic iodine compounds in lettuce plants

Iodine (I) is considered a beneficial element or even micronutrient for plants. The aim of this study was to determine the molecular and physiological processes of uptake, transport, and metabolism of I applied to lettuce plants. KIO3, KIO3 + salicylic acid, 5-iodosalicylic acid and 3,5-diiodosalicylic acid were applied. RNA-sequencing was executed using 18 cDNA libraries constructed separately for leaves and roots from KIO3, SA and control plants. De novo transcriptome assembly generated 1937.76 million sequence reads resulting in 27,163 transcripts with N50 of 1638 bp. 329 differentially expressed genes (DEGs) in roots were detected after application of KIO3, out of which 252 genes were up-regulated, and 77 were down-regulated. In leaves, 9 genes revealed differential expression pattern. DEGs analysis indicated its involvement in such metabolic pathways and processes as: chloride transmembrane transport, phenylpropanoid metabolism, positive regulation of defense response and leaf abscission, and also ubiquinone and other terpenoid-quinone biosynthesis, protein processing in endoplasmic reticulum, circadian rhythm including flowering induction as well as a putative PDTHA (i.e. Plant Derived Thyroid Hormone Analogs) metabolic pathway. qRT-PCR of selected genes suggested their participation in the transport and metabolism of iodine compounds, biosynthesis of primary and secondary metabolites, PDTHA pathway and flowering induction.

CaWRKY50 Acts as a Negative Regulator in Response to Colletotrichum scovillei Infection in Pepper

Chili anthracnose is one of the most common and destructive fungal pathogens that affects the yield and quality of pepper. Although WRKY proteins play crucial roles in pepper resistance to a variety of pathogens, the mechanism of their resistance to anthracnose is still unknown. In this study, we found that CaWRKY50 expression was obviously induced by Colletotrichum scovillei infection and salicylic acid (SA) treatments. CaWRKY50-silencing enhanced pepper resistance to C. scovillei, while transient overexpression of CaWRKY50 in pepper increased susceptibility to C. scovillei. We further found that overexpression of CaWRKY50 in tomatoes significantly decreased resistance to C. scovillei by SA and reactive oxygen species (ROS) signaling pathways. Moreover, CaWRKY50 suppressed the expression of two SA-related genes, CaEDS1 (enhanced disease susceptibility 1) and CaSAMT1 (salicylate carboxymethyltransferase 1), by directly binding to the W-box motif in their promoters. Additionally, we demonstrated that CaWRKY50 interacts with CaWRKY42 and CaMIEL1 in the nucleus. Thus, our findings revealed that CaWRKY50 plays a negative role in pepper resistance to C. scovillei through the SA-mediated signaling pathway and the antioxidant defense system. These results provide a theoretical foundation for molecular breeding of pepper varieties resistant to anthracnose.

PIF8-WRKY42-mediated salicylic acid synthesis modulates red light induced powdery mildew resistance in oriental melon

Light signals and plant hormones are involved in regulating the growth, development and stress resistance of plants; however, it remains unclear whether light affects hormones and thus pathogen resistance in oriental melon. Here, we found that red light promoted salicylic acid (SA) accumulation and powdery mildew resistance by activating the transcription of CmICS, the key gene for SA biosynthesis, and silencing CmICS seriously weakened the induction effect of red light on powdery mildew resistance in oriental melon leaves. Further studies showed that red light induced the expression of CmWRKY42 under powdery mildew stress, and CmWRKY42 directly bound to the CmICS promoter to activate its expression and promote the accumulation of SA under red light. Furthermore, we found that PHYTOCHROME INTERACTING FACTOR 8 (PIF8), as a negative regulator of SA biosynthesis, inhibits CmWRKY42 transcriptional activation by binding to the CmWRKY42 promoter, and thus inhibits transcriptional activation of CmICS by CmWRKY42. Also, CmPIF8 binds to the CmICS promoter and directly inhibits its transcription. In conclusion, our study revealed a new molecular mechanism of the relationship between red light-SA-powdery mildew resistance and provided a theoretical basis for resistance breeding of oriental melon.

Biocontrol Microneedle Patch: A Promising Agent for Protecting Citrus Fruits from Postharvest Infection

With increasing human awareness of food safety, the replacement of highly toxic pesticides with biocompatible antimicrobials has become a trend. This study proposes a biocontrol microneedle (BMN) to expand the application of the food-grade preservative epsilon-poly-L-lysine (ε-PL) in fruit preservatives by utilizing a dissolving microneedle system. The macromolecular polymer ε-PL not only possesses broad-spectrum antimicrobial activity but also exhibits good mechanical properties. With the addition of a small amount of polyvinyl alcohol, the mechanical strength of the ε-PL-based microneedle patch could be further improved to achieve an enhanced failure force of needles at 1.6 N/needle and induce an approximately 96% insertion rate in citrus fruit pericarps. An ex vivo insertion test revealed that the microneedle tips could be effectively inserted into the citrus fruit pericarp, rapidly dissolve within 3 min, and produce inconspicuous needle holes. Moreover, the high drug loading capacity of BMN was observed to reach approximately 1890 μg/patch, which is essential for enhancing the concentration-dependent antifungal activity of ε-PL. The drug distribution study has confirmed the feasibility of mediating the local diffusion of EPL in the pericarp through BMN. Therefore, BMN has great potential to reduce the incidence of invasive fungal infections in local areas of citrus fruit pericarp.

Physiological and Metabolic Changes in Tamarillo (Solanum betaceum) during Fruit Ripening

Physiological and metabolic profiles in tamarillo were investigated to reveal the molecular changes during fruit maturation. The firmness, ethylene production, soluble sugar contents, and metabolomic analysis were determined in tamarillo fruit at different maturity stages. The firmness of tamarillo fruit gradually decreased during fruit ripening with increasing fructose and glucose accumulation. The rapid increase in ethylene production was found in mature fruit. Based on the untargeted metabolomic analysis, we found that amino acids, phospholipids, monosaccharides, and vitamin-related metabolites were identified as being changed during ripening. The contents of malic acid and citric acid were significantly decreased in mature fruits. Metabolites involved in phenylpropanoid biosynthesis, phenylalanine metabolism, caffeine metabolism, monoterpenoid biosynthesis, and thiamine metabolism pathways showed high abundance in mature fruits. However, we also found that most of the mature-enhanced metabolites showed reduced abundance in over-mature fruits. These results reveal the molecular profiles during tamarillo fruit maturing and suggest tamarillos have potential benefits with high nutrition and health function.

Cellobiose elicits immunity in lettuce conferring resistance to Botrytis cinerea

Cellobiose is the primary product of cellulose hydrolysis and is expected to function as a type of pathogen/damage-associated molecular pattern in evoking plant innate immunity. In this study, cellobiose was demonstrated to be a positive regulator in the immune response of lettuce, but halted autoimmunity when lettuce was exposed to concentrations of cellobiose >60 mg l-1. When lettuce plants were infected by Botrytis cinerea, cellobiose endowed plants with enhanced pre-invasion resistance by activating high β-1,3-glucanase and antioxidative enzyme activities at the initial stage of pathogen infection. Cellobiose-activated core regulatory factors such as EDS1, PTI6, and WRKY70, as well as salicylic acid signaling, played an indispensable role in modulating plant growth-defense trade-offs. Transcriptomics data further suggested that the cellobiose-activated plant-pathogen pathways are involved in microbe/pathogen-associated molecular pattern-triggered immune responses. Genes encoding receptor-like kinases, transcription factors, and redox homeostasis, phytohormone signal transduction, and pathogenesis-related proteins were also up- or down-regulated by cellobiose. Taken together, the findings of this study demonstrated that cellobiose serves as an elicitor to directly activate disease-resistance-related cellular functions. In addition, multiple genes have been identified as potential modulators of the cellobiose-induced immune response, which could aid understanding of underlying molecular events.

Characterization of three tandem-duplicated calcium binding protein (CaBP) genes and promoters reveals their roles in the phytohormone and wounding responses in citrus

Accumulated evidences have revealed the critical roles of calcium binding protein (CaBP) in growth and stress responses of plants. However, its function in woody plants is poorly understood. In this study, we cloned the CDS, gDNA and promoter sequences of three tandem-duplicated CaBPs (CsCaBP1, CsCaBP2 and CsCaBP3) from Citrus sinensis, analyzed their sequence characteristics, and investigated their gene expression patterns and promoter activities under treatments of CaCl₂, several phytohormones and wounding. Results showed that the three CsCaBPs have high sequence similarity. Their expression was strongly induced by CaCl₂, ethylene, jasmonic acid, salicylic acid and wounding, and the promoting effect of wounding on their expression was found to be partially ethylene-dependent. Consistently, we identified many phytohormone-related cis-acting elements in their promoters, and their promoter activity could be induced significantly by ethylene, jasmonic acid, salicylic acid and wounding. All the three CsCaBPs can interact with WRKY40, whose encoding gene showed a similar expression pattern to CsCaBPs under phytohormone and wounding treatments. In addition, CsERF14, CsERF21, CsERF3 and CsERF2 could bind to their promoters. The results obtained in this study indicated that the three duplicated CsCaBPs were functionally redundant and played similar roles in the phytohormone and wounding responses of C. sinensis.

A Global Transcriptome and Co-expression Analysis Reveals Robust Host Defense Pathway Reprogramming and Identifies Key Regulators of Early Phases of Cicer-Ascochyta Interactions

Ascochyta blight (AB) caused by the filamentous fungus Ascochyta rabiei is a major threat to global chickpea production. The mechanisms underlying chickpea response to A. rabiei remain elusive to date. Here, we investigated the comparative transcriptional dynamics of AB-resistant and -susceptible chickpea genotypes upon A. rabiei infection, to understand the early host defense response. Our findings revealed that AB-resistant plants underwent rapid and extensive transcriptional reprogramming compared with a susceptible host. At the early stage (24 h postinoculation [hpi]), mainly cell-wall remodeling and secondary metabolite pathways were highly activated, while differentially expressed genes related to signaling components, such as protein kinases, transcription factors, and hormonal pathways, show a remarkable upsurge at 72 hpi, especially in the resistant genotype. Notably, our data suggest an imperative role of jasmonic acid, ethylene, and abscisic acid signaling in providing immunity against A. rabiei. Furthermore, gene co-expression networks and modules corroborated the importance of cell-wall remodeling, signal transduction, and phytohormone pathways. Hub genes such as MYB14, PRE6, and MADS-SOC1 discovered in these modules might be the master regulators governing chickpea immunity. Overall, we not only provide novel insights for comprehensive understanding of immune signaling components mediating AB resistance and susceptibility at early Cicer-Ascochyta interactions but, also, offer a valuable resource for developing AB-resistant chickpea. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Transcription factor FfMYB15 regulates the expression of cellulase gene FfCEL6B during mycelial growth of Flammulina filiformis

Background

Cellulose degradation can determine mycelial growth rate and affect yield during the growth of Flammulina filiformis. The degradation of cellulose requires the joint action of a variety of cellulases, and some cellulase-related genes have been detected in mushrooms. However, little is known about the transcriptional regulatory mechanisms of cellulose degradation.

Results

In this study, FfMYB15 that may regulate the expression of cellulase gene FfCEL6B in F. filiformis was identified. RNA interference (RNAi) showed that FfCEL6B positively regulated mycelial growth. Gene expression analyses indicated that the expression patterns of FfCEL6B and FfMYB15 in mycelia cultured on the 0.9% cellulose medium for different times were similar with a correlation coefficient of 0.953. Subcellular localization and transcriptional activity analyses implied that FfMYB15 was located in the nucleus and was a transcriptional activator. Electrophoretic mobility shift assay (EMSA) and dual-luciferase assays demonstrated that FfMYB15 could bind and activate FfCEL6B promoter by recognizing MYB cis-acting element.

Conclusions

This study indicated that FfCEL6B played an active role in mycelial growth of F. filiformis and was regulated by FfMYB15.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01932-z.

Transcriptome analysis provides insights into the bases of salicylic acid-induced resistance to anthracnose in sorghum

Key Message Transcriptome analysis of SA sensitive and tolerant lines indicates that SA enhances anthracnose resistance in sorghum by upregulating the expression of some immune-related genes and pathways.Abstract Anthracnose caused by the hemibiotrophic pathogen Colletotrichum sublineolum is one of the most destructive diseases of sorghum, the fifth most important cereal crop in the world. Salicylic acid (SA) is a phytohormone essential for plant immunity; however, the role of SA in sorghum resistance to anthracnose has not been well explored. In this study, we found that Colletotrichum sublineolum infection induced the expression of SA-responsive genes and that exogenous SA enhanced resistance to anthracnose in the sorghum line BTx623. To rule out the possibility that SA triggers anthracnose resistance in sorghum by its direct toxic function on pathogen, an SA-tolerant line, WHEATLAND, was identified, and we found that SA treatment could not induce anthracnose resistance in WHEATLAND. Then, SA-induced transcriptome changes during Colletotrichum sublineolum infection in BTx623 and WHEATLAND were analyzed to explore the molecular mechanism of SA-triggered resistance. SA pretreatment regulated the expression of 2125 genes in BTx623 but only 524 genes in WHEATLAND during Colletotrichum sublineolum infection. The cutin, suberine and wax biosynthesis pathway involved in the plant immune response and the flavonoid biosynthesis pathway involved in anthracnose resistance were enriched in BTx623-specifically upregulated genes. Additionally, some immune-related genes, including multiple resistance genes, were differentially expressed in BTx623 and WHEATLAND. Taken together, our results revealed the mechanisms of SA-induced anthracnose resistance in sorghum at the transcriptional level and shed light on the possibility of enhancing sorghum resistance to anthracnose by activating the SA signaling pathway by molecular breeding.

Lignin Biosynthesis Pathway and Redox Balance Act Synergistically in Conferring Resistance against Penicillium italicum Infection in 7-Demethoxytylophorine-Treated Navel Orange

7-Demethoxytylophorine (DEM), a natural water-soluble phenanthroindolizidine alkaloid, has a great potential for in vitro suppression of Penicillium italicum growth. In the present study, we investigated the ability of DEM to confer resistance against P. italicum in harvested "Newhall" navel orange and the underlying mechanism. Results from the in vivo experiment showed that DEM treatment delayed blue mold development. The water-soaked lesion diameter in 40 mg L^-1 DEM-treated fruit was 35.2% lower than that in the control after 96 h. Moreover, the decrease in peel firmness loss and increase in electrolyte leakage, superoxide anion (O₂^•-) production, and malondialdehyde (MDA) content were significantly inhibited by DEM treatment. Hydrogen peroxide (H₂O₂) burst in DEM-treated fruit at the early stage of P. italicum infection contributed to the conferred resistance by increasing the activities of lignin biosynthesis-related enzymes, along with the expressions of their encoding genes, resulting in lignin accumulation. The DEM-treated fruit maintained an elevated antioxidant capacity, as evidenced by high levels of ascorbic acid and glutathione content, and enhanced or upregulated the activities and gene expression levels of APX, GR, MDHAR, DHAR, GPX, and GST, thereby maintaining ROS homeostasis and reducing postharvest blue mold. Collectively, the results in the present study revealed a control mechanism in which DEM treatment conferred the resistance against P. italicum infection in harvested "Newhall" navel orange fruit by activating lignin biosynthesis and maintaining the redox balance.

Analyses of Botrytis cinerea-responsive LrWRKY genes from Lilium regale reveal distinct roles of two LrWRKY transcription factors in mediating responses to B. cinerea

Botrytis cinerea induced expression of 15 LrWRKY genes; overexpression of LrWRKY39 and LrWRKY41a increased resistance and susceptibility, respectively, to B. cinerea in a manner related to SA and JA signaling. WRKY transcription factors (TFs), a large family, play important roles in coping with biotic stresses. Lilium regale Wilson is a lily species with strong resistance to fungi and viruses; however, functional characterization of LrWRKY TFs remains very limited. Here, a total of 25 LrWRKY members were identified from the L. regale transcriptome, and 15 LrWRKY genes were significantly induced by Botrytis cinerea. Based on their structural features, B. cinerea-responsive LrWRKY genes could be classified into six subgroups (Groups I, IIa-d, and III), and sequence alignment showed that 12 LrWRKY proteins have a well-conserved WRKYGQK domain, while 3 LrWRKYs have a variant sequence (WRKYGKK or WRMYEQK). Quantitative RT-PCR analysis revealed tissue-specific expression of B. cinerea-responsive LrWRKY genes and their expression profiles in response to defense-related hormones salicylic acid (SA), methyl jasmonate (MeJA) and hydrogen peroxide. LrWRKY39 and LrWRKY41a, which encode two LrWRKY TFs with different three-dimensional (3D) models of the WRKY domain, were cloned, and both proteins were targeted to the nucleus. Overexpression of LrWRKY39 and LrWRKY41a in Arabidopsis thaliana increased the resistance and susceptibility to B. cinerea, respectively, compared to the wild type. Similar results were also observed in tobacco and lily (L. longiflorum 'Snow Queen') by transient transformation analyses. Their distinct roles may be related to changes in the transcript levels of SA-/JA-responsive genes. Our results provide new insights into B. cinerea-responsive LrWRKY members and the biological functions of two different 3D models of LrWRKY TFs in defense responses to B. cinerea infection.

CsNIP5;1 acts as a multifunctional regulator to confer water loss tolerance in citrus fruit

Plant aquaporins facilitate the transport of water across the inner membranes and play an important role in the response to water loss stress. A citrus NOD26-like intrinsic protein, CsNIP5;1, has been investigated to participate in the regulation of water permeability. In the present study, the expression profile indicated that CsNIP5;1 showed high transcription abundance in conducting tissues. Function analysis revealed that CsNIP5;1 reduced water loss of Arabidopsis rosette leaf, as well as promoted the seed germination under hyperosmotic stress. Besides, overexpression of CsNIP5;1 contributed to the alleviation of water loss in citrus fruit and citrus callus during storage. Further metabolomic profiling and RNA-seq analysis of transgenic citrus callus revealed that CsNIP5;1 may modulate the water loss by inducing the accumulation of osmotic adjustment substances and repressing the expression of other AQPs. Moreover, CsWRKY4 and CsWRKY28 were found to directly bind to the promoter and acted as opposite regulators of CsNIP5;1 during the postharvest period. These findings provide new insights into the regulatory mechanism of aquaporins in response to the water loss stress of citrus fruit during postharvest storage.

CsWRKY25 Improves Resistance of Citrus Fruit to Penicillium digitatum via Modulating Reactive Oxygen Species Production

WRKY transcription factors (TFs) play crucial roles in the regulation of biotic stress. Citrus is the most productive fruit in the world. It is of great value to investigate the regulatory molecular mechanism of WRKYs in improving disease resistance. In this research, the transcription level of CsWRKY25 was upregulated in P. digitatum infected citrus peel, and CsWRKY25 activated the expression of three target genes (RbohB, RbohD, and PR10). Besides, the Agrobacterium-mediated transient overexpression of CsWRKY25 has also been shown to enhance resistance to P. digitatum in citrus, and caused the accumulation of hydrogen peroxide and lignin. The accumulation of ROS also activated the antioxidant system, the catalase (CAT), peroxidase (POD), and cinnamyl alcohol dehydrogenase (CAD) genes were significant upregulated, leading to activation of antioxidant enzymes. In addition, the up-regulated expression of MPK5 and MPK6 genes suggested that the regulatory role of CsWRKY25 might be related to the phosphorylation process. In conclusion, CsWRKY25 could enhance the resistance to P. digitatum via modulating ROS production and PR genes in citrus peel.

Molecular mechanisms underlying multi-level defense responses of horticultural crops to fungal pathogens

The horticultural industry helps to enrich and improve the human diet while contributing to growth of the agricultural economy. However, fungal diseases of horticultural crops frequently occur during pre- and postharvest periods, reducing yields and crop quality and causing huge economic losses and wasted food. Outcomes of fungal diseases depend on both horticultural plant defense responses and fungal pathogenicity. Plant defense responses are highly sophisticated and are generally divided into preformed and induced defense responses. Preformed defense responses include both physical barriers and phytochemicals, which are the first line of protection. Induced defense responses, which include innate immunity (pattern-triggered immunity and effector-triggered immunity), local defense responses, and systemic defense signaling, are triggered to counterstrike fungal pathogens. Therefore, to develop regulatory strategies for horticultural plant resistance, a comprehensive understanding of defense responses and their underlying mechanisms is critical. Recently, integrated multi-omics analyses, CRISPR-Cas9-based gene editing, high-throughput sequencing, and data mining have greatly contributed to identification and functional determination of novel phytochemicals, regulatory factors, and signaling molecules and their signaling pathways in plant resistance. In this review, research progress on defense responses of horticultural crops to fungal pathogens and novel regulatory strategies to regulate induction of plant resistance are summarized, and then the problems, challenges, and future research directions are examined.

Identification of yield-related genes through genome-wide association: case study of weeping forsythia, an emerging medicinal crop

KEY MESSAGE

This study identified candidate genes related to fruit yield for an emerging medicinal crop, weeping forsythia.

BACKGROUND

The genetic basis of crop yield is an agricultural research hotspot. Identifying the genes related to yield traits is the key to increase the yield. Weeping forsythia is an emerging medicinal crop that currently lacks excellent varieties. The genes related to fruit yield in weeping forsythia have not been identified.

OBJECTIVE

Thus, we aimed to screen the candidate genes related to fruit yield of weeping forsythia by using genome-wide association analysis.

METHODS

Here, 60 samples from the same field and source of weeping forsythia were collected to identify its yield-related candidate genes. Association analysis was performed on the variant loci and the traits related to yield, i.e., fruit length, width, thickness, and weight.

RESULTS

Results from admixture, neighbor-joining, and kinship matrix analyses supported the non-significant genetic differentiation of these samples. Significant association was found between 2 variant loci and fruit length, 8 loci and fruit width, 24 loci and fruit thickness, and 13 loci and fruit weight. Further search on the 20 kb up/downstream of these variant loci revealed 1 gene related to fruit length, 16 genes related to fruit width, 12 genes related to fruit thickness, and 13 genes related to fruit weight. Among which, 4 genes, namely, WRKY transcription factor 35, salicylic acid-binding protein, auxin response factor 6, and alpha-mannosidase were highly related to the fruit development of weeping forsythia.

CONCLUSION

This study identify four candidate genes related to fruit development, which will provide useful information for the subsequent molecular-assisted and genetic breeding of weeping forsythia.

CsGSTU8, a Glutathione S-Transferase From Camellia sinensis, Is Regulated by CsWRKY48 and Plays a Positive Role in Drought Tolerance

Glutathione S-transferases (GSTs) constitute a large family of enzymes with a wide range of cellular functions. Recently, plant GSTs have gained a great deal of attention due to their involvement in the detoxification of electrophilic xenobiotics and peroxides under adverse environmental conditions, such as salt, cold, UV-B and drought stress. A previous study reported that a GST gene (CsGSTU8) in tea plant was distinctly induced in response to drought, suggesting this gene plays a critical role in the drought stress response. In this study, by using quantitative real-time PCR (qRT-PCR) and β-glucuronidase (GUS) reporter lines, we further demonstrated that CsGSTU8 was upregulated in response to drought stress and exogenous abscisic acid (ABA) treatments. Overexpression of CsGSTU8 in Arabidopsis resulted in enhanced drought tolerance as indicated by the improved scavenging of excess amounts of reactive oxygen species (ROS) under drought conditions. Furthermore, we found that CsWRKY48 acts as a transcriptional activator and that its expression is induced in response to drought stress and ABA treatment. Electrophoretic mobility shift assays (EMSAs), dual-luciferase (LUC) assays and transient expression assays in tea plant leaves revealed that CsWRKY48 directly binds to the W-box elements in the promoter of CsGSTU8 and activates its expression. Taken together, our results provide additional knowledge of drought stress responses in tea plant.

A salicylic acid inducible mulberry WRKY transcription factor, MiWRKY53 is involved in plant defence response

MiWRKY53 is expressed in response to various stresses and hormones. Although it is localized in the nucleus, it shows no transcriptional activation. Role of SA-mediated plant defence response is demonstrated. WRKY transcription factors are one the largest gene families in plants involved in almost every process in plants including development, physiological processes, and stress response. Salicylic acid (SA) is key regulator of biotic stress against various pathogens in plants acting via its multiple mechanisms to induce defence response. Herein, we have identified and functionally validated WRKY53 from mulberry (Morus indica var. K2). MiWRKY53 expressed differentially in response to different stress and hormonal treatments. MiWRKY53 belongs to group III of WKRY gene family, localized in nucleus, and lacks transcriptional activation activity in yeast. Hormone responsive behaviour of MiWRKY53 Arabidopsis overexpression (OE) transgenics preferentially was noted in root growth assay in response to Salicylic acid (SA). Arabidopsis overexpression plants also displayed alteration in leaf phenotype having wider leaves than the wild-type plants. PR-1 transcripts were higher in MiWRKY53 Arabidopsis OE plants and they displayed resistance towards biotrophic pathogen Pseudomonas syringae PstDC3000. MiWRKY53 Mulberry OE transgenics also depicted SA-responsive behaviour. Several hormones and stress-related cis-acting elements were also identified in the 1.2-Kb upstream regulatory region (URR) of MiWRKY53. Functional characterization of full-length promoter region revealed that it is induced by SA and further analysis of deletion constructs helped in the identification of minimal promoter responsible for its inducibility by SA. Altogether, the findings from this study point towards the SA preferential behaviour of MiWRKY53 and its function as regulator of plant defence response through SA-mediated mechanisms.

CsMYB96 enhances citrus fruit resistance against fungal pathogen by activating salicylic acid biosynthesis and facilitating defense metabolite accumulation

Citrus fruit are generally confronted with various fungal diseases that cause fruit deterioration and economic loss. Salicylic acid (SA), a plant hormone, is an important signal molecule required for stimulating the disease resistance of plants. However, there has been limited information about the molecular mechanism of SA biosynthesis involving biotic stress response in citrus fruit. In the present study, an R2R3 MYB transcription factor (CsMYB96) was identified to mediate SA signaling in response to fungal diseases. The transient overexpression assay revealed that CsMYB96 contributed to the strong tolerance of citrus fruit to Penicillium italicum along with an increase in SA content; meanwhile, CsMYB96 conferred resistance to Botrytis cinerea in Arabidopsis plants. Further metabolomic profiling of stable transgenic Arabidopsis revealed that CsMYB96 participated in the regulation of various metabolism pathways and enhanced the accumulation of phenolic acids. RNA-seq analysis confirmed that overexpression of CsMYB96 activated the expression of genes involved in plant-pathogen interaction, phenylpropanoid biosynthesis, and SA signaling. Besides, CsMBY96 directly activated the transcription of calmodulin binding protein 60g (CsCBP60g), a predominant transcription factor required for the activation of SA signaling. In summary, our results reveal that CsMYB96 promotes SA biosynthesis and the accumulation of defense metabolites to enhance the fungal pathogen resistance of citrus fruit and Arabidopsis and provide new insights into the regulation of disease response.

Transcription Factor CsWRKY65 Participates in the Establishment of Disease Resistance of Citrus Fruits to Penicillium digitatum

Penicillium digitatum is the primary pathogen that causes serious yield losses worldwide. In our previous study, CsWRKY transcription factors (TFs) and some genes associated with immunity were identified in citrus fruits after P. digitatum infection, but little information is available in the literature on the mechanisms of TFs in citrus disease resistance. In this study, the possible mechanisms of CsWRKY65 participating in the establishment of disease resistance were investigated. Results show that CsWRKY65 was a transcriptional activator in the nucleus. The dual-luciferase transient assays and electrophoretic mobility shift assays showed that CsWRKY65 bound with CsRbohB, CsRbohD, CsCDPK33, and CsPR10 promoters to activate gene transcription. Besides, the transient overexpression of CsWRKY65 induced reactive oxygen species accumulation and increased PR gene expression in Nicotiana benthamiana leaves. The transient overexpression of CsWRKY65 in the citrus peel enhanced the disease resistance against P. digitatum. In conclusion, CsWRKY65 is likely to be involved in regulating the disease resistance to P. digitatum of citrus fruits by directly activating the expressions of CsRbohB, CsRbohD, CsCDPK33, and CsPR10. This study provides new information for the mechanism of citrus WRKY TFs participating in the establishment of disease resistance.