Induction of ethylene inhibits development of soybean sudden death syndrome by inducing defense-related genes and reducing Fusarium virguliforme growth

Ethylene: A Modulator of the Phytohormone-Mediated Insect Herbivory Network in Plants

Plants have evolved to establish insect herbivory defences by modulating their metabolism, growth, and development. Precise networks of phytohormones are essential to induce those herbivory defences. Gaseous phytohormone ET plays an important role in forming herbivory defences. Its role in insect herbivory is not fully understood, but previous studies have shown that it can both positively and negatively regulate herbivory. This review presents recent findings on crosstalk between ET and other phytohormones in herbivory responses. Additionally, the use of exogenous ETH treatment to induce ET in response to herbivory is discussed.

Comparative analysis of soybean transcriptional profiles reveals defense mechanisms involved in resistance against Diaporthe caulivora

Soybean stem canker (SSC) caused by the fungal pathogen Diaporthe caulivora is an important disease affecting soybean production worldwide. However, limited information related to the molecular mechanisms underlying soybean resistance to Diaporthe species is available. In the present work, we analyzed the defense responses to D. caulivora in the soybean genotypes Williams and Génesis 5601. The results showed that compared to Williams, Génesis 5601 is more resistant to fungal infection evidenced by significantly smaller lesion length, reduced disease severity and pathogen biomass. Transcriptional profiling was performed in untreated plants and in D. caulivora-inoculated and control-treated tissues at 8 and 48 h post inoculation (hpi). In total, 2.322 and 1.855 genes were differentially expressed in Génesis 5601 and Williams, respectively. Interestingly, Génesis 5601 exhibited a significantly higher number of upregulated genes compared to Williams at 8 hpi, 1.028 versus 434 genes. Resistance to D. caulivora was associated with defense activation through transcriptional reprogramming mediating perception of the pathogen by receptors, biosynthesis of phenylpropanoids, hormone signaling, small heat shock proteins and pathogenesis related (PR) genes. These findings provide novel insights into soybean defense mechanisms leading to host resistance against D. caulivora, and generate a foundation for the development of resistant SSC varieties within soybean breeding programs.

A Phakopsora pachyrhizi Effector Suppresses PAMP-Triggered Immunity and Interacts with a Soybean Glucan Endo-1,3-β-Glucosidase to Promote Virulence

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is one of the most important diseases affecting soybean production in tropical areas. During infection, P. pachyrhizi secretes proteins from haustoria that are transferred into plant cells to promote virulence. To date, only one candidate P. pachyrhizi effector protein has been characterized in detail to understand the mechanism by which it suppresses plant defenses to enhance infection. Here, we aimed to extend understanding of the pathogenic mechanisms of P. pachyrhizi based on the discovery of host proteins that interact with the effector candidate Phapa-7431740. We demonstrated that Phapa-7431740 suppresses pathogen-associated molecular pattern-triggered immunity (PTI) and that it interacts with a soybean glucan endo-1,3-β-glucosidase (GmβGLU), a pathogenesis-related (PR) protein belonging to the PR-2 family. Structural and phylogenetic characterization of the PR-2 protein family predicted in the soybean genome and comparison to PR-2 family members in Arabidopsis thaliana and cotton, demonstrated that GmβGLU is a type IV β-1,3-glucanase. Transcriptional profiling during an infection time course showed that the GmβGLU mRNA is highly induced during the initial hours after infection, coinciding with peak of expression of Phapa-7431740. The effector was able to interfere with the activity of GmβGLU in vitro, with a dose-dependent inhibition. Our results suggest that Phapa-7431740 may suppress PTI by interfering with glucan endo-1,3-β-glucosidase activity. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Phenotypic and Genetic Characterization of the Lentil Single Plant-Derived Core Collection for Resistance to Root Rot Caused by Fusarium avenaceum

Lentil (Lens culinaris) is a pulse crop grown for its amino acid profile, moderate drought tolerance, and ability to fix nitrogen. As the global demand for lentils expands and new production regions emerge so too have the complement of diseases that reduce yield, including the root rot complex. Although the predominant causal pathogen varies based on growing region, Fusarium avenaceum is often found to be an important contributor to disease. This study screened part of the lentil single plant-derived core collection for resistance to F. avenaceum in a greenhouse. Plants were phenotyped for disease severity using three scoring scales and the differences in biomass traits due to pathogen presence were measured. Lentil accessions varied in disease severity and differences in biomass traits were found to be correlated with each visual severity estimate (r = -0.37 to -0.63, P < 0.001), however, heritability estimates were low to moderate among traits (H² = 0.12 to 0.43). Results of a genome-wide association study (GWAS) using single nucleotide polymorphism (SNP) markers derived from genotyping-by-sequencing revealed 11 quantitative trait loci (QTL) across four chromosomes. Two pairs of QTL colocated for two traits and were found near putative orthologs that have been previously associated with plant disease resistance. The identification of lentil accessions that did not exhibit a difference in biomass traits may serve as parental material in breeding or in the development of biparental mapping populations to further validate and dissect the genetic control of resistance to root rot caused by F. avenaceum.

Soybean Hypocotyls Prevent Calonectria ilicicola Invasion by Multi-Layered Defenses

In plants, many pathogens infect a specific set of host organs to cause disease, yet the underlying mechanisms remain unclear. Here, we show that inoculation of soybean plants with Calonectria ilicicola, the soil-borne causal agent of soybean red crown rot, caused typical disease symptoms of root rot and leaf chlorosis and necrosis. However, the pathogen DNA was only detected in the roots and stem (hypocotyl) base but not other aerial parts of the plants. As we observed vigorous fungal growth in all culture media made of extracts from roots, stems, and leaves, differences in key components including available nutrients did not determine organ-specific infection and reproduction by C. ilicicola. Furthermore, inoculation of stems both with and without a surface wound showed that the stems resisted C. ilicicola infection via both the pre- and post-invasion defense layers. Transcriptomic comparison of roots and stems using RNA-seq analysis further revealed that upon C. ilicicola inoculation, a greater expression of genes involved in stress response was induced in the plant stems, including receptor-like kinase, AP2/ERF, MYB, and WRKY. In addition, pathways related to amino acid metabolism were also more upregulated in the stems in response to C. ilicicola infection. These results suggest that soybean stems provide C. ilicicola resistance, at least in part, by activating an organ-specific defense response.

Current recommendations and novel strategies for sustainable management of soybean sudden death syndrome

The increase in food production requires reduction of the damage caused by plant pathogens, minimizing the environmental impact of management practices. Soil-borne pathogens are among the most relevant pathogens that affect soybean crop yield. Soybean sudden death syndrome (SDS), caused by several distinct species of Fusarium, produces significant yield losses in the leading soybean-producing countries in North and South America. Current management strategies for SDS are scarce since there are no highly resistant cultivars and only a few fungicide seed treatments are available. Because of this, innovative approaches for SDS management need to be developed. Here, we summarize recently explored strategies based on plant nutrition, biological control, priming of plant defenses, host-induced gene silencing, and the development of new SDS-resistance cultivars using precision breeding techniques. Finally, sustainable management of SDS should also consider cultural control practices with minimal environmental impact. © 2021 Society of Chemical Industry.

Ethephon-Induced Ethylene Enhances Protein Degradation in Source Leaves, but Its High Endogenous Level Inhibits the Development of Regenerative Organs in Brassica napus

To investigate the regulatory role of ethylene in the source-sink relationship for nitrogen remobilization, short-term effects of treatment with different concentrations (0, 25, 50, and 75 ppm) of ethephon (2-chloroethylphosphonic acid, an ethylene inducing agent) for 10 days (EXP 1) and long-term effects at 20 days (Day 30) after treatment with 100 ppm for 10 days (EXP 2) on protein degradation and amino acid transport in foliar sprayed mature leaves of Brassica napus (cv. Mosa) were determined. In EXP 1, endogenous ethylene concentration gradually increased in response to the treated ethephon concentration, leading to the upregulation of senescence-associated gene 12 (SAG12) expression and downregulation of chlorophyll a/b-binding protein (CAB) expression. Further, the increase in ethylene concentration caused a reduction in protein, Rubisco, and amino acid contents in the mature leaves. However, the activity of protease and expression of amino acid transporter (AAP6), an amino acid transport gene, were not significantly affected or slightly suppressed between the treatments with 50 and 75 ppm. In EXP 2, the enhanced ethylene level reduced photosynthetic pigments, leading to an inhibition of flower development without any pod development. A significant increase in protease activity, confirmed using in-gel staining of protease, was also observed in the ethephon-treated mature leaves. Ethephon application enhanced the expression of four amino acid transporter genes (AAP1, AAP2, AAP4, and AAP6) and the phloem loading of amino acids. Significant correlations between ethylene level, induced by ethephon application, and the descriptive parameters of protein degradation and amino acid transport were revealed. These results indicated that an increase in ethylene upregulated nitrogen remobilization in the mature leaves (source), which was accompanied by an increase in proteolytic activity and amino acid transport, but had no benefit to pod (sink) development.

Exogenous ethylene reduces growth via alterations in central metabolism and cell wall composition in tomato (Solanum lycopersicum)

Ethylene is a gaseous hormone with a well-established role in the regulation of plant growth and development. However, its role in the modulation of carbon assimilation and central metabolism remains unclear. Here, we investigated the morphophysiological and biochemical responses of tomato plants (Solanum lycopersicum) following the application of ethylene in the form of ethephon (CEPA - 2-chloroethylphosphonic acid), forcing the classical triple response phenotype. CEPA-treated plants were characterized by growth inhibition, as revealed by significant reductions in both shoot and root dry weights, coupled with a reduced number of leaves and lower specific leaf area. Growth inhibition was associated with a reduction in carbon assimilation due to both lower photosynthesis rates and stomatal conductance, coupled with impairments in carbohydrate turnover. Furthermore, exogenous ethylene led to the accumulation of cell wall compounds (i.e., cellulose and lignin) and phenolics, indicating that exposure to exogenous ethylene also led to changes in specialized metabolism. Collectively, our findings demonstrate that exogenous ethylene disrupts plant growth and leaf structure by affecting both central and specialized metabolism, especially that involved in carbohydrate turnover and cell wall biosynthesis, ultimately leading to metabolic responses that mimic stress situations.

Contrasting transcriptional responses to Fusarium virguliforme colonization in symptomatic and asymptomatic hosts

The broad host range of Fusarium virguliforme represents a unique comparative system to identify and define differentially induced responses between an asymptomatic monocot host, maize (Zea mays), and a symptomatic eudicot host, soybean (Glycine max). Using a temporal, comparative transcriptome-based approach, we observed that early gene expression profiles of root tissue from infected maize suggest that pathogen tolerance coincides with the rapid induction of senescence dampening transcriptional regulators, including ANACs (Arabidopsis thaliana NAM/ATAF/CUC protein) and Ethylene-Responsive Factors. In contrast, the expression of senescence-associated processes in soybean was coincident with the appearance of disease symptom development, suggesting pathogen-induced senescence as a key pathway driving pathogen susceptibility in soybean. Based on the analyses described herein, we posit that root senescence is a primary contributing factor underlying colonization and disease progression in symptomatic versus asymptomatic host-fungal interactions. This process also supports the lifestyle and virulence of F. virguliforme during biotrophy to necrotrophy transitions. Further support for this hypothesis lies in comprehensive co-expression and comparative transcriptome analyses, and in total, supports the emerging concept of necrotrophy-activated senescence. We propose that F. virguliforme conditions an environment within symptomatic hosts, which favors susceptibility through transcriptomic reprogramming, and as described herein, the induction of pathways associated with senescence during the necrotrophic stage of fungal development.

Overexpression of a plasma membrane protein generated broad-spectrum immunity in soybean

Plants fight-off pathogens and pests by manifesting an array of defence responses using their innate immunity mechanisms. Here we report the identification of a novel soybean gene encoding a plasma membrane protein, transcription of which is suppressed following infection with the fungal pathogen, Fusarium virguliforme. Overexpression of the protein led to enhanced resistance against not only against F. virguliforme, but also against spider mites (Tetranychus urticae, Koch), soybean aphids (Aphis glycines, Matsumura) and soybean cyst nematode (Heterodera glycines). We, therefore, name this protein as Glycine max disease resistance 1 (GmDR1; Glyma.10g094800). The homologues of GmDR1 have been detected only in legumes, cocoa, jute and cotton. The deduced GmDR1 protein contains 73 amino acids. GmDR1 is predicted to contain an ecto- and two transmembrane domains. Transient expression of the green fluorescent protein fused GmDR1 protein in soybean leaves showed that it is a plasma membrane protein. We investigated if chitin, a pathogen-associated molecular pattern (PAMP), common to all pathogen and pests considered in this study, can significantly enhance defence pathways among the GmDR1-overexpressed transgenic soybean lines. Chitin induces marker genes of the salicylic- and jasmonic acid-mediated defence pathways, but suppresses the defence pathway regulated by ethylene. Chitin induced SA- and JA-regulated defence pathways may be one of the mechanisms involved in generating broad-spectrum resistance among the GmDR1-overexpressed transgenic soybean lines against two serious pathogens and two pests including spider mites, against which no known resistance genes have been identified in soybean and among the most other crop species.

Gene expression during development and overexpression after Cercospora kikuchii and salicylic acid challenging indicate defensive roles of the soybean toxin

KEY MESSAGE

SBTX has defensive role against C. kikuchii, and therefore, its constituent genes SBTX17 and SBTX27 are promising candidates to engineer pathogen resistant plants. Soybean (Glycine max [L.] Merr.) is economically the most important legume crop in the world. Its productivity is strongly affected by fungal diseases, which reduce soybean production and seed quality and cause losses of billions of dollars worldwide. SBTX is a protein that apparently takes part in the defensive chemical arsenal of soybean against pathogens. This current study provides data that reinforce this hypothesis. Indeed, SBTX inhibited in vitro the mycelial growth of Cercospora kikuchii, it is constitutively located in the epidermal region of the soybean seed cotyledons, and it is exuded from mature imbibed seeds. Moreover, RT-qPCR analysis of the SBTX associated genes, SBTX17 and SBTX27, which encode for the 17 and 27 kDa polypeptide chains, showed that both genes are expressed in all studied plant tissues during the soybean development, with the highest levels found in the mature seeds and unifoliate leaves. In addition, to assess a local response of the soybean secondary leaves from 35-day-old plants, they were inoculated with C. kikuchii and treated with salicylic acid. It was verified using RT-qPCR that SBTX17 and SBTX27 genes overexpressed in leaves compared to controls. These findings strongly suggest that SBTX has defensive roles against C. kikuchii. Therefore, SBTX17 and SBTX27 genes are promising candidates to engineer pathogen resistant plants.

Identification of tomato circular RNAs responsive to Phytophthora infestans

Circular RNAs (circRNAs), a novel branch of noncoding RNAs, are widespread in eukaryotic cells. Particularly, due to their abilities to bind microRNA (miRNA) and serve as "sponges", circRNAs can regulate gene expression and participate in multiple biological processes. To detect the function of the circRNAs in tomato resistance, in our study, high-throughput sequencing were used to detect the circRNAs in tomatoes before and after Phytophthora infestans (P. infestans) infection. A total of 68 circRNAs were identified, of them, 18 (26%) were termed as exonic circRNAs, 33 (49%) were termed as intergenic circRNAs, 17 (25%) were termed as intronic circRNAs. Thirty-six out of 68 circRNAs differentially expressed in tomatoes after infection, including 9 up- and 27 down-regulated. Among the up-regulated circRNAs, two exoinc circRNAs, circRNA45 and circRNA47 were annotated as whitefly-induced gp91-phox and ethylene-forming enzyme, respectively. Both of them could act as miR477-3p sponge. Transgenic plants transiently overexpressed circRNA45 and circRNA47 both displayed smaller lesion area than the control plants upon infection, accompanied by lower expression levels of miR477-3p. Furthermore, transiently overexpression of miR477-3p in tomatoes leading to a decline in their targeted disease related genes expression. Our results firstly identified circRNAs in tomato upon P. infestans infection and demonstrated that circRNA45 and circRNA47 may act as positive regulators in tomato resistance by regulating miRNA-mRNAs expression levels.