RXLR effector gene Avr3a from Phytophthora sojae is recognized by Rps8 in soybean.

The use of resistance genes in elite soybean cultivars is one of the most widely used methods to manage Phytophthora sojae. This method relies on effector-triggered immunity, where a Resistant to P. sojae (Rps) gene product from the plant recognizes a specific effector from the pathogen, encoded by an avirulence (Avr) gene. Many Avr genes from P. sojae have been identified in the last decade, allowing a better exploitation of this type of resistance. The objective of the present study was to identify the Avr gene triggering immunity derived from the soybean resistance gene Rps8. The analysis of a segregating F2 progeny coupled with a genotyping-by-sequencing approach led to the identification of a putative Avr8 locus. The investigation of this locus using whole-genome sequencing data from 31 isolates of P. sojae identified Avr3a as the likely candidate for Avr8. Long-read sequencing also revealed that P. sojae isolates can carry up to five copies of the Avr3a gene, compared to the four previously reported. Haplotype and transcriptional analyses showed that amino acid changes and absence of Avr3a transcripts from P. sojae isolates caused changes in virulence towards Rps8. Functional analyses using CRISPR/Cas9 knockout and constitutive expression demonstrated that Rps8 interacted with Avr3a. We also showed that a specific allele of Avr3a is recognized by Rps3a but not Rps8. While Rps3a and Rps8 have been previously described as closely linked, this is the first report of a clear distinction hitherto undefined between these two resistance genes.

New Insights into the Fungal Diversity of Cranberry Fruit Rot in Québec Farms Through a Large-Scale Molecular Analysis.

Cranberry fruit rot (CFR) pathogens are widely reported in the literature, but performing large-scale analysis of their presence inside fruit has always been challenging. In this study, a new molecular diagnostic tool, capable of identifying simultaneously 12 potential fungal species causing CFR, was used to better define the impact of CFR across cranberry fields in Québec. For this purpose, 126 fields and 7,825 fruits were sampled in three cranberry farms distributed throughout the province and subjected to comparative analyses of fungal presence and abundance according to cultural practices, sampling times, and cranberry cultivars. All 12 pathogens were detected throughout the study, but as a first major finding, the analyses revealed that four species, Godronia cassandrae, Colletotrichum fructivorum, Allantophomopsis cytisporea, and Coleophoma empetri, were consistently predominant regardless of the parameters studied. Comparison of conventional and organic productions showed a significant reduction in fungal richness and relative abundance. Interestingly, Monilinia oxycocci was found almost exclusively in organic productions, indicating that fungicides had a strong and persistent effect on its population. Surprisingly, there were no significant differences in fungal relative abundance or species richness between fruit sampled at harvest or in storage, suggesting that there may not exist a clear distinction between field and storage rot, as was previously thought. Comparative analysis of fungal species found on eight different cranberry cultivars indicated that they were all infected by the same fungi but could not rule out differences in genetic resistance. This large-scale analysis allows us to draw an exhaustive picture of CFR in Québec and provides new information with respect to its management.

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.