An In Vitro Method for Studying the Three-Way Interaction between Soybean, Rhizophagus irregularis and the Soil-Borne Pathogen Fusarium virguliforme

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.

Trichoderma Isolates Inhibit Fusarium virguliforme Growth, Reduce Root Rot, and Induce Defense-Related Genes on Soybean Seedlings

Sudden death syndrome (SDS) caused by Fusarium virguliforme is among the most important diseases affecting soybean in the United States. The use of biological control agents (BCAs) such as Trichoderma spp. can be a valuable resource to suppress F. virguliforme populations. Therefore, this research focused on screening possible BCAs against F. virguliforme and evaluating mycoparasitism and the induction of systemic resistance as mechanisms underlying the antagonistic activity of selected BCAs against F. virguliforme. In total, 47 potential BCAs, including 41 Trichoderma isolates and 6 Mortierella isolates, were screened in a dual-plate assay. The most effective isolates belonged to the Trichoderma harzianum species and were able to inhibit F. virguliforme radial growth by up to 92%. Selected Trichoderma isolates were tested in the greenhouse and in a microplot study. They reduced root rot caused by F. virguliforme when the plants were coinoculated with the pathogen and the BCA. The tested BCA's ability to reduce F. virguliforme growth may be related to several mechanisms of action, including mycoparasitism and induction of defense-related genes in plants, as revealed by monitoring the expression of defense-related genes in soybean. Our results highlight the potential of native Trichoderma isolates to inhibit F. virguliforme growth and reduce SDS severity, providing the basis for future implementation of biological control in soybean production. More efforts are needed to implement the use of these approaches in production fields, and to deepen the current knowledge on the biology of these highly antagonistic isolates.

Reduction of Sudden Death Syndrome Foliar Symptoms and Fusarium virguliforme DNA in Roots Inoculated With Rhizophagus intraradices

There is increasing interest in incorporating arbuscular mycorrhizal fungi (AMF) into agricultural production because of the benefits they provide, including protection against pathogens and pests. Sudden death syndrome (SDS) of soybean is a devastating disease caused by the soilborne pathogen Fusarium virguliforme. Multiple management methods are needed to control SDS. The relationship between F. virguliforme and AMF is not well documented. The goal of this study was to determine whether soybean plants co-inoculated with F. virguliforme and the AMF species Rhizophagus intraradices showed reduced SDS foliar symptom severity and reduced relative F. virguliforme DNA quantities in soybean roots. Six soybean genotypes were inoculated with F. virguliforme alone or with R. intraradices in a greenhouse experiment. Averaged over the six soybean genotypes, area under the disease progress curve values and relative F. virguliforme DNA quantities were 45 and 28% lower (P < 0.05), respectively, in roots co-inoculated with R. intraradices compared with roots of control plants inoculated with F. virguliforme only. Weight of roots co-inoculated with R. intraradices were 58% higher (P < 0.05) compared with roots of plants not inoculated with R. intraradices. Nutrient analysis showed higher boron, phosphorus, potassium, sodium, and sulfur concentrations in root tissues of plants co-inoculated with R. intraradices compared with plants inoculated with F. virguliforme (P < 0.05). Overall, this study showed that R. intraradices reduced SDS severity and relative F. virguliforme DNA quantities while simultaneously increasing growth and nutrient uptake of plants. Further testing of AMF inoculants in the field will indicate whether incorporating them into soybean SDS management practices will reduce the impact of SDS on soybean production.

Transcriptional Changes in Mycorrhizal and Nonmycorrhizal Soybean Plants upon Infection with the Fungal Pathogen Macrophomina phaseolina

Macrophomina phaseolina is a soil-borne fungal pathogen with a wide host range that causes charcoal rot in soybean [Glycine max (L.) Merr.]. Control of the disease is a challenge, due to the absence of genetic resistance and effective chemical control. Alternative or complementary measures are needed, such as the use of biological control agents, in an integrated approach. Several studies have demonstrated the role of arbuscular mycorrhizal fungi (AMF) in enhancing plant resistance or tolerance to biotic stresses, decreasing the symptoms and pressure caused by various pests and diseases, including M. phaseolina in soybean. However, the specific contribution of AMF in the regulation of the plant response to M. phaseolina remains unclear. Therefore, the objective of the present study was to investigate, under strict in-vitro culture conditions, the global transcriptional changes in roots of premycorrhized soybean plantlets challenged by M. phaseolina (+AMF+Mp) as compared with nonmycorrhizal soybean plantlets (-AMF+Mp). MapMan software was used to distinguish transcriptional changes, with special emphasis on those related to plant defense responses. Soybean genes identified as strongly upregulated during infection by the pathogen included pathogenesis-related proteins, disease-resistance proteins, transcription factors, and secondary metabolism-related genes, as well as those encoding for signaling hormones. Remarkably, the +AMF+Mp treatment displayed a lower number of upregulated genes as compared with the -AMF+Mp treatment. AMF seemed to counteract or balance costs upon M. phaseolina infection, which could be associated to a negative impact on biomass and seed production. These detailed insights in soybean-AMF interaction help us to understand the complex underlying mechanisms involved in AMF-mediated biocontrol and support the importance of preserving and stimulating the existing plant-AMF associates, via adequate agricultural practices, to optimize their agro-ecological potential.