Silicon influences the localization and expression of Phytophthora sojae effectors in interaction with soybean

A rapid molecular diagnostic tool to discriminate alleles of avirulence genes and haplotypes of Phytophthora sojae using high-resolution melting analysis

Effectors encoded by avirulence genes (Avr) interact with the Phytophthora sojae resistance gene (Rps) products to generate incompatible interactions. The virulence profile of P. sojae is rapidly evolving as a result of the large-scale deployment of Rps genes in soybean. For a successful exploitation of Rps genes, it is recommended that soybean growers use cultivars containing the Rps genes corresponding to Avr genes present in P. sojae populations present in their fields. Determination of the virulence profile of P. sojae isolates is critical for the selection of soybean cultivars. High-resolution melting curve (HRM) analysis is a powerful tool, first applied in medicine, for detecting mutations with potential applications in different biological fields. Here, we report the development of an HRM protocol, as an original approach to discriminate effectors, to differentiate P. sojae haplotypes for six Avr genes. An HRM assay was performed on 24 P. sojae isolates with different haplotypes collected from soybean fields across Canada. The results clearly confirmed that the HRM assay discriminated different virulence genotypes. Moreover, the HRM assay was able to differentiate multiple haplotypes representing small allelic variations. HRM-based prediction was validated by phenotyping assays. This HRM assay provides a unique, cost-effective and efficient tool to predict virulence pathotypes associated with six different Avr (1b, 1c, 1d, 1k, 3a and 6) genes from P. sojae, which can be applied in the deployment of appropriate Rps genes in soybean fields.

Progress and Challenges in Elucidating the Functional Role of Effectors in the Soybean-Phytophthora sojae Interaction

Phytophthora sojae, the agent responsible for stem and root rot, is one of the most damaging plant pathogens of soybean. To establish a compatible-interaction, P. sojae secretes a wide array of effector proteins into the host cell. These effectors have been shown to act either in the apoplastic area or the cytoplasm of the cell to manipulate the host cellular processes in favor of the development of the pathogen. Deciphering effector-plant interactions is important for understanding the role of P. sojae effectors in disease progression and developing approaches to prevent infection. Here, we review the subcellular localization, the host proteins, and the processes associated with P. sojae effectors. We also discuss the emerging topic of effectors in the context of effector-resistance genes interaction, as well as model systems and recent developments in resources and techniques that may provide a better understanding of the soybean-P. sojae interaction.

Mapping of partial resistance to Phytophthora sojae in soybean PIs using whole-genome sequencing reveals a major QTL

In the last decade, more than 70 quantitative trait loci (QTL) related to soybean [Glycine max (L.) Merr.] partial resistance (PR) against Phytophthora sojae have been identified by genome-wide association studies (GWAS). However, most of them have either a minor effect on the resistance level or are specific to a single phenotypic variable or one isolate, thereby limiting their use in breeding programs. In this study, we have used an analytical approach combining (a) the phenotypic characterization of a diverse panel of 357 soybean accessions for resistance to P. sojae captured through a single variable, corrected dry weight; (b) a new hydroponic assay allowing the inoculation of a combination of P. sojae isolates covering the spectrum of commercially relevant Rps genes; and (c) exhaustive genotyping through whole-genome resequencing (WGS). This led to the identification of a novel P. sojae resistance QTL with a relatively major effect compared with the previously reported QTL. The QTL interval, spanning ∼500 kb on chromosome (Chr) 15, does not colocalize with previously reported QTL for P. sojae resistance. Plants carrying the favorable allele at this QTL were 60% more resistant. Eight genes were found to reside in the linkage disequilibrium (LD) block containing the peak single-nucleotide polymorphism (SNP) including Glyma.15G217100, which encodes a major latex protein (MLP)-like protein, with a functional annotation related to pathogen resistance. Expression analysis of Glyma.15G217100 indicated that it was nearly eight times more highly expressed in a group of plant introductions (PIs) carrying the resistant (R) allele compared with those carrying the susceptible (S) allele within a short period after inoculation. These results offer new and valuable options to develop improved soybean cultivars with broad resistance to P. sojae through marker-assisted selection.

Role of silicon in elevating resistance against sheath blight and blast diseases in rice (Oryza sativa L.)

Rice blast caused by Magnaporthe oryzae and sheath blight caused by Rhizoctonia solani, are the two major diseases of rice that cause enormous losses in rice production worldwide. Identification and utilization of broad-spectrum resistance resources have been considered sustainable and effective strategies. However, the majority of the resistance genes and QTLs identified have often been found to be race-specific, and their resistance is frequently broken down due to continuous exposure to the pathogen. Therefore, integrated approaches to improve plant resistance against such devastating pathogen have great importance. Silicon (Si), a beneficial element for plant growth, has shown to provide a prophylactic effect against many pathogens. The application of Si helps the plants to combat the disease-causing pathogens, either through its deposition in different parts of the plant or through modulation/induction of specific defense genes by yet an unknown mechanism. Some reports have shown that Si imparts resistance to rice blast and sheath blight. The present review summarizes the mechanism of Si transport and deposition and its effect on rice growth and development. A special emphasis has been given to explore the existing evidence showing Si mediated blast and sheath blight resistance and the mechanism involved in resistance. This review will help to understand the prophylactic effects of Si against sheath blight and blast disease at the mechanical, physiological, and genetic levels. The information provided here will help develop a strategy to explore Si derived benefits for sustainable rice production.

Mechanisms of silicon-induced fungal disease resistance in plants

Silicon (Si) acts as a beneficial element for plant growth and provides protection against abiotic and biotic stresses. Despite numerous reports on the beneficial role of Si in enhancing plant resistance to fungal pathogens, the underlying mechanisms remain largely unclear. Silicon shows antifungal activity; however, Si-induced improved disease resistance is partly manifested by the formation of Si polymerized mechanical obstruction under the cuticle and in cell walls, which prevents fungal ingress. Moreover, rapid production of defense compounds through secondary metabolic pathways is thought to be a key mechanism of Si-induced chemical defense against fungal pathogens beyond the physical barrier. Besides, improved mineral nutrition assures the healthy status of Si-supplied plants and a healthy plant exhibits better photosynthetic potential, antioxidant capacity and disease resistance. Multiple plant hormones and their crosstalk mediate the Si-induced basal as well as induced resistance; nonetheless, how root uptake of Si systemically modulates resistance to foliar diseases in low Si accumulating plants, needs in-depth investigation. Recent studies also indicate that Si influences effector-triggered immunity by affecting host recognition and/or limiting receptor-effector interactions. Here we review the role of Si in plant response to fungal pathogens. We also discuss and propose potential mechanisms of Si-induced enhanced disease resistance in plants. Finally, we identify some limitations of research approaches in addressing the beneficial roles of Si in biotic stress management.