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While the impacts of arsenic (As) and Magnaporthe oryzae on rice have been well-studied, a dearth of knowledge exists on how rice responds to their combined stress. Moreover, increasing exogenous silicon (Si) can alleviate M. oryzae infection and As uptake, but how increasing exogenous Si affects the combined stress of M. oryzae and As is unknown. We grew three cultivars of rice that varied in their susceptibility to As and M. oryzae under low (50 μM, Si L ) and high (1500 μM, Si H ) Si with and without As (4 μM, 80/20 As (III)/As(V)) and with or without M. oryzae infection and examined the impacts of treatments on plant As and Si concentrations, severity of disease by M. oryzae, and stress via targeted gene expression. Si H treatments generally decreased shoot As concentrations by 20-70% compared to Si L treatments depending on cultivar and M. oryzae exposure. There was no effect of Si or As treatments on percent of leaf diseased in the As-tolerant cultivar M206, but in the As-sensitive cultivar IR66, Si H treatment decreased percent of leaf diseased in the absence of As and had no impact when As was present. In the M. oryzae-susceptible Sariceltik, plants receiving Si H had significantly fewer lesions than those receiving Si L and plants with the fewest lesions were in the Si H  + As treatments. Plants that were exposed to As + M. oryzae were the most stressed when grown under Si L , but this stress response was lowered by Si H treatments. A separate pathogenicity assay with Sariceltik showed that in contrast to our hypothesis, As exposure decreased lesion growth, particularly under Si H treatments, and lessened the impact of M. oryzae on rice. These results suggest that rice grown under replete Si will be able to withstand combined stressors of M. oryzae and As, but will be highly stressed under Si deficient scenarios., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Background: The natural interactions between plant roots and their rhizospheric microbiome are vital to plant fitness, modulating both growth promotion and disease suppression. In rice (Oryza sativa), a globally important food crop, as much as 30% of yields are lost due to blast disease caused by fungal pathogen Magnaporthe oryzae. Capitalizing on the abilities of naturally occurring rice soil bacteria to reduce M. oryzae infections could provide a sustainable solution to reduce the amount of crops lost to blast disease., Results: Naturally occurring root-associated rhizospheric bacteria were isolated from California field grown rice plants (M-104), eleven of which were taxonomically identified by 16S rRNA gene sequencing and fatty acid methyl ester (FAME) analysis. Bacterial isolates were tested for biocontrol activity against the devastating foliar rice fungal pathogen, M. oryzae pathovar 70-15. In vitro, a Pseudomonas isolate, EA105, displayed antibiosis through reducing appressoria formation by nearly 90% as well as directly inhibiting fungal growth by 76%. Although hydrogen cyanide (HCN) is a volatile commonly produced by biocontrol pseudomonads, the activity of EA105 seems to be independent of its HCN production. During in planta experiments, EA105 reduced the number of blast lesions formed by 33% and Pantoea agglomerans isolate, EA106 by 46%. Our data also show both EA105 and EA106 trigger jasmonic acid (JA) and ethylene (ET) dependent induced systemic resistance (ISR) response in rice., Conclusions: Out of 11 bacteria isolated from rice soil, pseudomonad EA105 most effectively inhibited the growth and appressoria formation of M. oryzae through a mechanism that is independent of cyanide production. In addition to direct antagonism, EA105 also appears to trigger ISR in rice plants through a mechanism that is dependent on JA and ET signaling, ultimately resulting in fewer blast lesions. The application of native bacteria as biocontrol agents in combination with current disease protection strategies could aid in global food security.

Efficient regulation of nitrogen metabolism likely plays a role in the ability of fungi to exploit ecological niches. To learn about regulation of nitrogen metabolism in the rice blast pathogen Magnaporthe grisea, we undertook a genome-wide analysis of gene expression under nitrogen-limiting conditions. Five hundred and twenty genes showed increased transcript levels at 12 and 48 h after shifting the fungus to media lacking nitrate as a nitrogen source. Thirty-nine of these genes have putative functions in amino acid metabolism and uptake, and include the global nitrogen regulator in M. grisea, NUT1. Evaluation of seven nitrogen starvation-induced genes revealed that all were expressed during rice infection. Targeted gene replacement on one such gene, the vacuolar serine protease, SPM1, resulted in decreased sporulation and appressorial development as well as a greatly attenuated ability to cause disease. Data are discussed in the context of nitrogen metabolism under starvation conditions, as well as conditions potentially encountered during invasive growth in planta.

Magnaporthe grisea is the most destructive pathogen of rice worldwide and the principal model organism for elucidating the molecular basis of fungal disease of plants. Here, we report the draft sequence of the M. grisea genome. Analysis of the gene set provides an insight into the adaptations required by a fungus to cause disease. The genome encodes a large and diverse set of secreted proteins, including those defined by unusual carbohydrate-binding domains. This fungus also possesses an expanded family of G-protein-coupled receptors, several new virulence-associated genes and large suites of enzymes involved in secondary metabolism. Consistent with a role in fungal pathogenesis, the expression of several of these genes is upregulated during the early stages of infection-related development. The M. grisea genome has been subject to invasion and proliferation of active transposable elements, reflecting the clonal nature of this fungus imposed by widespread rice cultivation.