The Strawberry FaWRKY1 Transcription Factor Negatively Regulates Resistance to Colletotrichum acutatum in Fruit Upon Infection

Ripening and rot: How ripening processes influence disease susceptibility in fleshy fruits

Fleshy fruits become more susceptible to pathogen infection when they ripen; for example, changes in cell wall properties related to softening make it easier for pathogens to infect fruits. The need for high-quality fruit has driven extensive research on improving pathogen resistance in important fruit crops such as tomato (Solanum lycopersicum). In this review, we summarize current progress in understanding how changes in fruit properties during ripening affect infection by pathogens. These changes affect physical barriers that limit pathogen entry, such as the fruit epidermis and its cuticle, along with other defenses that limit pathogen growth, such as preformed and induced defense compounds. The plant immune system also protects ripening fruit by recognizing pathogens and initiating defense responses involving reactive oxygen species production, mitogen-activated protein kinase signaling cascades, and jasmonic acid, salicylic acid, ethylene, and abscisic acid signaling. These phytohormones regulate an intricate web of transcription factors (TFs) that activate resistance mechanisms, including the expression of pathogenesis-related genes. In tomato, ripening regulators, such as RIPENING INHIBITOR and NON_RIPENING, not only regulate ripening but also influence fruit defenses against pathogens. Moreover, members of the ETHYLENE RESPONSE FACTOR (ERF) family play pivotal and distinct roles in ripening and defense, with different members being regulated by different phytohormones. We also discuss the interaction of ripening-related and defense-related TFs with the Mediator transcription complex. As the ripening processes in climacteric and non-climacteric fruits share many similarities, these processes have broad applications across fruiting crops. Further research on the individual contributions of ERFs and other TFs will inform efforts to diminish disease susceptibility in ripe fruit, satisfy the growing demand for high-quality fruit and decrease food waste and related economic losses.

Genome-Wide Analysis of the WRKY Transcription Factor Family in Roses and Their Putative Role in Defence Signalling in the Rose-Blackspot Interaction

WRKY transcription factors are important players in plant regulatory networks, where they control and integrate various physiological processes and responses to biotic and abiotic stresses. Here, we analysed six rose genomes of 5 different species (Rosa chinensis, R. multiflora, R. roxburghii, R. sterilis, and R. rugosa) and extracted a set of 68 putative WRKY genes, extending a previously published set of 58 WRKY sequences based on the R. chinensis genome. Analysis of the promoter regions revealed numerous motifs related to induction by abiotic and, in some cases, biotic stressors. Transcriptomic data from leaves of two rose genotypes inoculated with the hemibiotrophic rose black spot fungus Diplocarpon rosae revealed the upregulation of 18 and downregulation of 9 of these WRKY genes after contact with the fungus. Notably, the resistant genotype exhibited the regulation of 25 of these genes (16 upregulated and 9 downregulated), while the susceptible genotype exhibited the regulation of 20 genes (15 upregulated and 5 downregulated). A detailed RT-qPCR analysis of RcWRKY37, an orthologue of AtWRKY75 and FaWRKY1, revealed induction patterns similar to those of the pathogenesis-related (PR) genes induced in salicylic acid (SA)-dependent defence pathways in black spot inoculation experiments. However, the overexpression of RcWRKY37 in rose petals did not induce the expression of any of the PR genes upon contact with black spot. However, wounding significantly induced the expression of RcWRKY37, while heat, cold, or drought did not have a significant effect. This study provides the first evidence for the role of RcWRKY37 in rose signalling cascades and highlights the differences between RcWRKY37 and AtWRKY75. These results improve our understanding of the regulatory function of WRKY transcription factors in plant responses to stress factors. Additionally, they provide foundational data for further studies.

The multifaceted roles of R2R3 transcription factor HlMYB7 in the regulation of flavonoid and bitter acids biosynthesis, development and biotic stress tolerance in hop (Humulus lupulus L.)

Hop (Humulus lupulus) biosynthesizes the highly economically valuable secondary metabolites, which include flavonoids, bitter acids, polyphenols and essential oils. These compounds have important pharmacological properties and are widely implicated in the brewing industry owing to bittering flavor, floral aroma and preservative activity. Our previous studies documented that ternary MYB-bHLH-WD40 (MBW) and binary WRKY1-WD40 (WW) protein complexes transcriptionally regulate the accumulation of bitter acid (BA) and prenylflavonoids (PF). In the present study, we investigated the regulatory functions of the R2R3-MYB repressor HlMYB7 transcription factor, which contains a conserved N-terminal domain along with the repressive motif EAR, in regulating the PF- and BA-biosynthetic pathway and their accumulation in hop. Constitutive expression of HlMYB7 resulted in transcriptional repression of structural genes involved in the terminal steps of biosynthesis of PF and BA, as well as stunted growth, delayed flowering, and reduced tolerance to viroid infection in hop. Furthermore, yeast two-hybrid and transient reporter assays revealed that HlMYB7 targets both PF and BA pathway genes and suppresses MBW and WW protein complexes. Heterologous expression of HlMYB7 leads to down-regulation of structural genes of flavonoid pathway in Arabidopsis thaliana, including a decrease in anthocyanin content in Nicotiana tabacum. The combined results from functional and transcriptomic analyses highlight the important role of HlMYB7 in fine-tuning and balancing the accumulation of secondary metabolites at the transcriptional level, thus offer a plausible target for metabolic engineering in hop.

WHIRLY protein functions in plants

Environmental stresses pose a significant threat to food security. Understanding the function of proteins that regulate plant responses to biotic and abiotic stresses is therefore pivotal in developing strategies for crop improvement. The WHIRLY (WHY) family of DNA-binding proteins are important in this regard because they fulfil a portfolio of important functions in organelles and nuclei. The WHY1 and WHY2 proteins function as transcription factors in the nucleus regulating phytohormone synthesis and associated growth and stress responses, as well as fulfilling crucial roles in DNA and RNA metabolism in plastids and mitochondria. WHY1, WHY2 (and WHY3 proteins in Arabidopsis) maintain organelle genome stability and serve as auxiliary factors for homologous recombination and double-strand break repair. Our understanding of WHY protein functions has greatly increased in recent years, as has our knowledge of the flexibility of their localization and overlap of functions but there is no review of the topic in the literature. Our aim in this review was therefore to provide a comprehensive overview of the topic, discussing WHY protein functions in nuclei and organelles and highlighting roles in plant development and stress responses. In particular, we consider areas of uncertainty such as the flexible localization of WHY proteins in terms of retrograde signalling connecting mitochondria, plastids, and the nucleus. Moreover, we identify WHY proteins as important targets in plant breeding programmes designed to increase stress tolerance and the sustainability of crop yield in a changing climate.

Management of Post-Harvest Anthracnose: Current Approaches and Future Perspectives

Anthracnose is a severe disease caused by Colletotrichum spp. on several crop species. Fungal infections can occur both in the field and at the post-harvest stage causing severe lesions on fruits and economic losses. Physical treatments and synthetic fungicides have traditionally been the preferred means to control anthracnose adverse effects; however, the urgent need to decrease the use of toxic chemicals led to the investigation of innovative and sustainable protection techniques. Evidence for the efficacy of biological agents and vegetal derivates has been reported; however, their introduction into actual crop protection strategies requires the solutions of several critical issues. Biotechnology-based approaches have also been explored, revealing the opportunity to develop innovative and safe methods for anthracnose management through genome editing and RNA interference technologies. Nevertheless, besides the number of advantages related to their use, e.g., the putative absence of adverse effects due to their high specificity, a number of aspects remain to be clarified to enable their introduction into Integrated Pest Management (IPM) protocols against Colletotrichum spp. disease.

A Comprehensive Study of the WRKY Transcription Factor Family in Strawberry

WRKY transcription factors play critical roles in plant growth and development or stress responses. Using up-to-date genomic data, a total of 64 and 257 WRKY genes have been identified in the diploid woodland strawberry, Fragaria vesca, and the more complex allo-octoploid commercial strawberry, Fragaria × ananassa cv. Camarosa, respectively. The completeness of the new genomes and annotations has enabled us to perform a more detailed evolutionary and functional study of the strawberry WRKY family members, particularly in the case of the cultivated hybrid, in which homoeologous and paralogous FaWRKY genes have been characterized. Analysis of the available expression profiles has revealed that many strawberry WRKY genes show preferential or tissue-specific expression. Furthermore, significant differential expression of several FaWRKY genes has been clearly detected in fruit receptacles and achenes during the ripening process and pathogen challenged, supporting a precise functional role of these strawberry genes in such processes. Further, an extensive analysis of predicted development, stress and hormone-responsive cis-acting elements in the strawberry WRKY family is shown. Our results provide a deeper and more comprehensive knowledge of the WRKY gene family in strawberry.

Lilium regale Wilson WRKY3 modulates an antimicrobial peptide gene, LrDef1, during response to Fusarium oxysporum

BACKGROUND

WRKY transcription factors (TFs) play vital roles in plant growth and development, secondary metabolite synthesis, and response to biotic and abiotic stresses. In a previous transcriptome sequencing analysis of Lilium regale Wilson, we identified multiple WRKY TFs that respond to exogenous methyl jasmonate treatment and lily Fusarium wilt (Fusarium oxysporum).

RESULTS

In the present study, the WRKY TF LrWRKY3 was further analyzed to reveal its function in defense response to F. oxysporum. The LrWRKY3 protein was localized in the plant cell nucleus, and LrWRKY3 transgenic tobacco lines showed higher resistance to F. oxysporum compared with wild-type (WT) tobacco. In addition, some genes related to jasmonic acid (JA) biosynthesis, salicylic acid (SA) signal transduction, and disease resistance had higher transcriptional levels in the LrWRKY3 transgenic tobacco lines than in the WT. On the contrary, L. regale scales transiently expressing LrWRKY3 RNA interference fragments showed higher sensitivity to F. oxysporum infection. Moreover, a F. oxysporum-induced defensin gene, Def1, was isolated from L. regale, and the recombinant protein LrDef1 isolated and purified from Escherichia coli possessed antifungal activity to several phytopathogens, including F. oxysporum. Furthermore, co-expression of LrWRKY3 and the LrDef1 promoter in tobacco enhanced the LrDef1 promoter-driven expression activity.

CONCLUSIONS

These results clearly indicate that LrWRKY3 is an important positive regulator in response to F. oxysporum infection, and one of its targets is the antimicrobial peptide gene LrDef1.

The Intragenesis and Synthetic Biology Approach towards Accelerating Genetic Gains on Strawberry: Development of New Tools to Improve Fruit Quality and Resistance to Pathogens

Under climate change, the spread of pests and pathogens into new environments has a dramatic effect on crop protection control. Strawberry (Fragaria spp.) is one the most profitable crops of the Rosaceae family worldwide, but more than 50 different genera of pathogens affect this species. Therefore, accelerating the improvement of fruit quality and pathogen resistance in strawberry represents an important objective for breeding and reducing the usage of pesticides. New genome sequencing data and bioinformatics tools has provided important resources to expand the use of synthetic biology-assisted intragenesis strategies as a powerful tool to accelerate genetic gains in strawberry. In this paper, we took advantage of these innovative approaches to create four RNAi intragenic silencing cassettes by combining specific strawberry new promoters and pathogen defense-related candidate DNA sequences to increase strawberry fruit quality and resistance by silencing their corresponding endogenous genes, mainly during fruit ripening stages, thus avoiding any unwanted effect on plant growth and development. Using a fruit transient assay, GUS expression was detected by the two synthetic FvAAT2 and FvDOF2 promoters, both by histochemical assay and qPCR analysis of GUS transcript levels, thus ensuring the ability of the same to drive the expression of the silencing cassettes in this strawberry tissue. The approaches described here represent valuable new tools for the rapid development of improved strawberry lines.

MdWRKY75e enhances resistance to Alternaria alternata in Malus domestica

The Alternaria alternata apple pathotype adversely affects apple (Malus domestica Borkh.) cultivation. However, the molecular mechanisms underlying enhanced resistance to this pathogen in apple remain poorly understood. We have previously reported that MdWRKY75 expression is upregulated by A. alternata infection in 'Sushuai' apples. In this study, we discovered that overexpression of MdWRKY75e increased the resistance of transgenic apple lines to A. alternata infection, whereas silencing this gene enhanced susceptibility to A. alternata infection. Furthermore, we found that MdWRKY75e directly binds to the MdLAC7 promoter to regulate the biosynthesis of laccase and increase the biosynthesis of lignin during A. alternata infection. Moreover, the thickening of the cell wall enhanced the mechanical defense capabilities of apple. In addition, we found that jasmonic acid remarkably induced MdWRKY75e expression, and its levels in transgenic apple lines were elevated. These results indicate that MdWRKY75e confers resistance to the A. alternata apple pathotype mainly via the jasmonic acid pathway and that pathogenesis-related genes and antioxidant-related enzyme activity are involved in the disease resistance of MdWRKY75e transgenic plants. In conclusion, our findings provide insights into the importance of MdWRKY75e for resistance to A. alternata infection in apples.

WRKY Transcription Factors Actively Respond to Fusarium oxysporum in Lilium regale

The WRKY transcription factors form a plant-specific superfamily important for regulating plant development, stress responses, and hormone signal transduction. In this study, many WRKY genes (LrWRKY1-35) were identified in Lilium regale, which is a wild lily species highly resistant to Fusarium wilt. These WRKY genes were divided into three classes (I to III) based on a phylogenetic analysis. The Class-II WRKY transcription factors were further divided into five subclasses (IIa, IIb, IIc, IId, and IIe). Moreover, the gene expression patterns based on a quantitative real-time PCR analysis revealed the WRKY genes were differentially expressed in the L. regale roots, stems, leaves, and flowers. Additionally, the expression of the WRKY genes was affected by an infection by Fusarium oxysporum as well as by salicylic acid, methyl jasmonate, ethephon, and hydrogen peroxide treatments. Moreover, the LrWRKY1 protein was localized to the nucleus of onion epidermal cells. The recombinant LrWRKY1 protein purified from Escherichia coli bound specifically to DNA fragments containing the W-box sequence, and a yeast one-hybrid assay indicated that LrWRKY1 can activate transcription. A co-expression assay in tobacco (Nicotiana tabacum) confirmed LrWRKY1 regulates the expression of LrPR10-5. Furthermore, the overexpression of LrWRKY1 in tobacco and the Oriental hybrid 'Siberia' (susceptible to F. oxysporum) increased the resistance of the transgenic plants to F. oxysporum. Overall, LrWRKY1 regulates the expression of the resistance gene LrPR10-5 and is involved in the defense response of L. regale to F. oxysporum. This study provides valuable information regarding the expression and functional characteristics of L. regale WRKY genes.

Down-regulation of Fra a 1.02 in strawberry fruits causes transcriptomic and metabolic changes compatible with an altered defense response

The strawberry Fra a 1 proteins belong to the class 10 Pathogenesis-Related (PR-10) superfamily. In strawberry, a large number of members have been identified, but only a limited number is expressed in the fruits. In this organ, Fra a 1.01 and Fra a 1.02 are the most abundant Fra proteins in the green and red fruits, respectively, however, their function remains unknown. To know the function of Fra a 1.02 we have generated transgenic lines that silence this gene, and performed metabolomics, RNA-Seq, and hormonal assays. Previous studies associated Fra a 1.02 to strawberry fruit color, but the analysis of anthocyanins in the ripe fruits showed no diminution in their content in the silenced lines. Gene ontology (GO) analysis of the genes differentially expressed indicated that oxidation/reduction was the most represented biological process. Redox state was not apparently altered since no changes were found in ascorbic acid and glutathione (GSH) reduced/oxidized ratio, but GSH content was reduced in the silenced fruits. In addition, a number of glutathione-S-transferases (GST) were down-regulated as result of Fra a 1.02-silencing. Another highly represented GO category was transport which included a number of ABC and MATE transporters. Among the regulatory genes differentially expressed WRKY33.1 and WRKY33.2 were down-regulated, which had previously been assigned a role in strawberry plant defense. A reduced expression of the VQ23 gene and a diminished content of the hormones JA, SA, and IAA were also found. These data might indicate that Fra a 1.02 participates in the defense against pathogens in the ripe strawberry fruits.

Early transcriptional response of terpenoid metabolism to Colletotrichum gloeosporioides in a resistant wild strawberry Fragaria nilgerrensis

Modern strawberry production is often threatened by microbe pathogens. Anthracnose is among the most prominent fungal disease caused mainly by Colletotrichum gloeosporioides and leads to large-scale losses both in quality and yield. Little is known regarding the mechanisms underlying the genetics in the strawberry-C. gloeosporioides interaction. In the current research, a wild accession 'Fragaria nilgerrensis' is used as a resistant model to study the roles of terpenoid and terpene genes in leaf response to C. gloeosporioides. We found that several terpenoids and terpene genes were up-regulated at early time points after challenged with C. gloeosporioides. Among the metabolites detected, sesquiterpenes were the most significantly accumulated compounds, increasing up to ~12-fold at 18 h post infection (hpi), followed by monoterpenes which showed a slight increase upon infection. Consistently, the time-resolved transcriptome data revealed that genes pertaining to terpenoid metabolism were rapidly up-regulated and co-expressed with signaling pathway genes relevant to defense response. Notably, quantitative real-time PCR confirmed that the expression of five terpene synthase genes (TPS) were greatly enhanced, by a factor of one to three orders of magnitude at 3-6 hpi. Our results reveal a possible link between rapidly induced terpenoid metabolism and the autoimmunity underlying anthracnose resistance in a wild strawberry species.

Strawberry FaWRKY25 Transcription Factor Negatively Regulated the Resistance of Strawberry Fruits to Botrytis cinerea

WRKY genes and jasmonic acid (JA) play a crucial role in plants’ responses against biotic and abiotic stress. However, the regulating mechanism of WRKY genes on strawberry fruits’ resistance against Botrytis cinerea is largely unknown, and few studies have been performed on their effect on the JA-mediated defense mechanism against B. cinerea. This study explored the effect of FaWRKY25 on the JA-mediated strawberry resistance against B. cinerea. Results showed that the JA content decreased significantly as the fruits matured, whereas the FaWRKY25 expression rose substantially, which led to heightened susceptibility to B. cinerea and in strawberries. External JA treatment significantly increased the JA content in strawberries and reduced the FaWRKY25 expression, thereby enhancing the fruits’ resistance against B. cinerea. FaWRKY25 overexpression significantly lowered the fruits’ resistance against B. cinerea, whereas FaWRKY25 silencing significantly increased resistance. Moreover, FaWRKY25 overexpression significantly lowered the JA content, whereas FaWRKY25 silencing significantly increased it. FaWRKY25 expression level substantially affects the expression levels of genes related to JA biosynthesis and metabolism, other members of the WRKY family, and defense genes. Accordingly, FaWRKY25 plays a crucial role in regulating strawberries’ resistance against B. cinerea and may negatively regulate their JA-mediated resistance mechanism against B. cinerea.

Genome-Wide Identification and Expression Analysis of the Strawberry FvbZIP Gene Family and the Role of Key Gene FabZIP46 in Fruit Resistance to Gray Mold

A total of 54 FvbZIP genes were identified from the strawberry genome. These genes were found to be unevenly distributed on seven different chromosomes, and two of the genes had no matching chromosomal localization. FvbZIP genes were divided into 10 subfamilies according to protein sequence, and the structures of these genes were found to be highly conserved. Based on the bioinformatics analysis of FvbZIP genes, the expression of FabZIP genes changed during different stages of its growth and of its infection with gray mold disease. FabZIP46 was substantially upregulated, and its expression remained relatively high. FabZIP46 was cloned from cultivated strawberries by homologous cloning. The results of a transient transgenic assay revealed that the damage to the fruit tissue was markedly alleviated in strawberries overexpressing FabZIP46, with the incidence rate being substantially lower than that in the control group. By contrast, a brief silencing of FabZIP46 had the opposite effect. The results revealed that FabZIP46 played a positive role in the resistance of strawberries to Botrytis cinerea. The study findings provide valuable insights into the role of bZIP transcription factors as well as a theoretical reference for the regulation of resistance to gray mold disease in strawberry fruit.

Genome-Wide Analysis of Members of the WRKY Gene Family and Their Cold Stress Response in Prunus mume

Prunus mume, which is a rosaceous arbor with very high ornamental, edible and medical values, has a distribution that is mainly restricted by low temperature. WRKY transcription factor genes play crucial roles in the growth, development, and stress responses of plants. However, the WRKY gene family has not been characterised in P. mume. There were 58 PmWRKYs identified from genome of P. mume. They were anchored onto eight link groups and categorised into three broad groups. The gene structure and motif composition were reasonably conservative in each group. Investigation of gene duplication indicated that nine and seven PmWRKYs were arranged in tandem and segmental duplications, respectively. PmWRKYs were discriminately expressed in different tissues (i.e., roots, stems, leaves, flowers and fruits) in P. mume. The 17 cold-related candidate genes were selected based on RNA-seq data. Further, to investigate the function of PmWRKYs in low temperatures, the expression patterns under artificial cold treatments were analysed. The results showed that the expression levels of the 12 PmWRKYs genes significantly and 5 genes slightly changed in stems. In particular, the expression level of PmWRKY18 was up-regulated after ABA treatment. In addition, the spatiotemporal expression patterns of 17 PmWRKYs were analysed in winter. These results indicated that 17 PmWRKYs were potential transcription factors regulating cold resistance in P. mume.