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Alternaria brassicicola‐induced ROS accumulation during black spot disease differentially affects antioxidant efficiency, phenolic content and susceptibility of Brassica species

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Alternaria brassicicola-induced ROS accumulation during black spot disease differentially affects antioxidant efficiency, phenolic content and susceptibility of Brassica species

Alternaria brassicicola triggers reactive oxygen species generation, causing black spot disease, following diverse patterns of antioxidative enzyme activity in susceptible Brassica juncea, B. napus and B. oleracea.


Abstract

Early-stage responses of Brassica juncea, B. napus and B. oleracea during black spot disease and the development of Alternaria brassicicola were evaluated. Infection of plant cells by the fungus occurred mainly through direct penetration or by appressoria and, rarely, stomata in B. juncea and B. oleracea; in B. napus, penetration was mostly direct. The process of conidial germination, germ tubes and appressoria formation on a leaf surface was correlated with the post-inoculation time and host species (p < 0.05). Changes in the leaf surface were observed at successful infection sites as bright, wax-deprived areas. Significant differences in progression of infection between Brassica species were observed, with B. oleracea being the most susceptible and B. napus the least. Accumulation of superoxide anion radicals and hydrogen peroxide was not only observed in plant cells, but also in A. brassicicola germ tubes and appressoria at 8 and 12 h post-inoculation (hpi). Enhanced production of reactive oxygen species (ROS) and lipid peroxidation during infection triggered diverse patterns of enzymatic and non-enzymatic antioxidant activities and phenolic compound contents in response to A. brassicicola. In each Brassica species, a different antioxidant was the most active. With their antioxidant properties, phenolic compounds also played an essential role in these interactions. This phenomenon could be related to the disparate levels of susceptibility of the Brassica species to A. brassicicola; for example, the most susceptible, B. oleracea, showed high ROS accumulation with decreasing tendency during disease progression, unchanged phenolics content, and the highest catalase activity, differentiating it from B. juncea and B. napus.

Annual species’ experimental germination responses to light and temperature do not correspond with their microhabitat associations in the field

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Annual species' experimental germination responses to light and temperature do not correspond with their microhabitat associations in the field

Species may “select” microsites via germination cues that predict future habitat quality. We investigated germination responses to light and temperature as drivers of species' associations with tree and litter cover in a guild of winter annuals in Western Australia. Species responded differently to experimental light and temperature treatments, but these responses did not explain their microsite associations in the field.


Abstract

Questions

Annual species have evolved sets of germination cues that are thought to be predictive of the post-germination environment. In naturally patchy environments, germination microsites often vary considerably in the amount of light they receive and in the diurnal temperature fluctuations they experience. However, whether species' differential germination responses to light and temperature are associated with their spatial patterns of occurrence remains largely untested.

Location

Mediterranean-climate woodlands in Southwest Western Australia.

Methods

We surveyed species' occurrences in annual plant communities in 150 quadrats across gradients of canopy cover and litter cover. Nineteen species recorded in this survey were then included in a germination experiment that manipulated (1) Light vs Dark (12 h light or continuous dark) approximating seeds near the soil surface vs those covered by litter and (2) Cold vs Warm temperature regimes (7/18°C and 7/24°C) approximating diurnal fluctuations experienced in shaded vs sun-exposed microsites, respectively.

Results

In the germination experiment, six species had highest germination probabilities in the Light treatment (regardless of temperature), five in Cold + Light, one in Warm + Light, two were indifferent to the treatments, and four did not germinate at all. Binomial linear mixed-effects models showed that species' maximum responses to light and temperature did not explain their spatial distributions along canopy cover and litter cover gradients, contrary to theoretical expectations of germination being a strong driver of species' occurrences.

Conclusions

Despite variation in species' responses to experimental treatments, no association was found with their field microsite associations. Germination strategies in our system were wider than expected for Mediterranean systems. Our results support that germination cues are not strong drivers of microhabitat associations in this system.

Hunting for sources of durable resistance in crop cultivar evaluation data: The case of wheat yellow rust in France

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Hunting for sources of durable resistance in crop cultivar evaluation data: The case of wheat yellow rust in France

Trajectories of field adult plant resistance ratings to yellow rust in 719 French wheat cultivars between 1963 and 2018 suggest the presence of effector-triggered and/or quantitative resistance.


Abstract

Cultivar resistance is a major asset for the management of crop diseases and can play an important role in agroecological transition. However, the wide deployment of a reduced number of resistance genes can lead to a rapid adaptation of pathogen populations and to a loss of resistance efficiency. The objective of this study was to characterize and discuss different trajectories of adult plant ratings for resistance to yellow rust in French wheat cultivars between 1963 and 2018. Among 719 cultivars assessed for at least 2 years, 590 cultivars showed no variation in their resistance scores, despite a mean of 4.3 years and up to 33 years of assessment. A set of descriptive variables was computed in order to compare the evolution of resistance score of 129 cultivars that experienced resistance variation. We applied a principal component analysis and a hierarchical clustering on principal components to this subdataset to constitute clusters corresponding to different cultivar profiles. Clusters C1 and C2 had small resistance variations (1–2 points on a 1–9 scale); Cluster C3 had long assessment durations and several small drops in resistance score and could be associated with quantitative resistance erosion; Cluster C4 included major drops in resistance score (4–5 points), often associated with known breakdowns of major resistance genes. Cases of limited drops in resistance score as a known resistance gene was broken down suggest the presence of efficient adult plant resistance. We discuss the use of information extracted from this dataset and methods to further explore sources of resistance to yellow rust present in French cultivars.

Ergot of cereals: Toxins, pathogens and management

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Ergot of cereals: Toxins, pathogens and management

This review article focuses on recent progress in understanding the toxins, pathogens and management of ergot, an important disease of cereals that affects food safety.


Abstract

Ergot is a fungal disease of many plants but is perhaps most commonly associated with domesticated grasses or cereals, such as rye, wheat, barley, oat, sorghum, millet, maize and rice. Ergot is of historical significance, having been reported for several millennia, but is also of concern in modern agricultural production systems. Caused by many different species within the genus Claviceps, the fungi cause the production of sclerotia, which are typically dark in colour, in place of healthy grain. The sclerotia contain toxins that can make the grain unsafe for consumption by humans or livestock. Ergot can be managed both preharvest as well as postharvest to minimize the presence of sclerotia and their associated toxins in food and feed systems. In this review, we provide a detailed update on our current knowledge of ergot on cereals, with a focus on recent advances in our understanding of fungal toxins and their regulation, pathogen biology and disease management.

Predicting airborne ascospores of Sclerotinia sclerotiorum through machine learning and statistical methods

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Predicting airborne ascospores of Sclerotinia sclerotiorum through machine learning and statistical methods

Across all statistical and machine learning models, the greatest predictor of ascospores released on a given day was the number of ascospores released on the previous day.


Abstract

A main biological constraint of dry bean (Phaseolus vulgaris) production in Canada is white mould, caused by the fungal pathogen Sclerotinia sclerotiorum. The primary infectious propagules of S. sclerotiorum are airborne ascospores and monitoring the air for inoculum levels could help predict the severity of white mould in bean fields. Daily air samples were collected in commercial dry bean fields in Alberta, Manitoba and Ontario and ascospores were quantified using quantitative PCR. Daily weather data was obtained from in-field weather stations. The number of ascospores on a given day was modelled using 63 different environmental variables and several modelling methods, both regression and classification approaches, were implemented with machine learning (ML) (random forests, logistic regression and support vector machines) and statistical (generalized linear models) approaches. Across all years and provinces, ascospores were most highly correlated with ascospore release from the previous day (r ranged from 0.15 to 0.6). This variable was also the only variable included in all models and had the greatest weight in all models. Models without this variable had much poorer performance than those with it. Correlations of ascospores with other environmental variables varied by province and sometimes by year. A comparison of ML and statistical models revealed that they both performed similarly, but that the statistical models were easier to interpret. However, the precise relationship between airborne ascospore levels and in-field disease severity remains unclear, and spore sampling methods will require further development before they can be deployed as a disease management tool.

Clavibacter tessellarius causing bacterial mosaic of wheat establishes in the Old World

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Clavibacter tessellarius causing bacterial mosaic of wheat establishes in the Old World

Using field surveys and samplings we confirmed widespread occurrence of the wheat pathogen Clavibacter tessellarius in southern Iran and establishment of the bacterium in the Old World.


Abstract

The genus Clavibacter (family Microbacteriaceae) includes gram-positive actinobacterial species infecting diverse agricultural crops such as alfalfa, maize, pepper, potato, tomato and wheat. Clavibacter tessellarius, causing bacterial mosaic of wheat, was reported for the first time in 1976 in Nebraska (United States). Since then, the pathogen was thought to be restricted to North America (Canada and the United States). While bacterial strains suspected to be C. tessellarius have occasionally been isolated from wheat samples outside North America, occurrence of the pathogen in the Old World has not yet been confirmed by the plant protection authorities. In spring 2020 (January to May) dozens of wheat fields with young plants showing leaf chlorosis and mild mosaic symptoms were surveyed and sampled in southern Iran. Among 192 surveyed wheat fields, gram-positive, orange-pigmented actinobacterial strains were isolated from samples of 53 fields. Altogether, 61 bacterial strains were identified as C. tessellarius based on the phenotypic features, PCR-based molecular tests and pathogenicity assays. Multilocus sequence analysis of five housekeeping genes (atpD, dnaK, gyrB, ppk and rpoB) using 10 representative strains from diverse geographic areas in southern Iran confirmed the phylogenetic status of the strains within C. tessellarius. Our data indicate that the bacterial mosaic pathogen occurs in four southern Iranian provinces including Bushehr, Fars, Kerman and Khuzestan. Thus, occurrence of C. tessellarius outside North America is confirmed. Potential risks associated with the emergence of the pathogen in the Old World, and the need for strict quarantine measures in the area, is further discussed.

Survey of Phyllosticta citricarpa mating type in the São Paulo citrus belt and optimization of in vitro ascospore production

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Survey of Phyllosticta citricarpa mating type in the São Paulo citrus belt and optimization of in vitro ascospore production

The two mating types of Phyllosticta citricarpa occurred in similar ratio in the São Paulo citrus belt; ascospores were produced in potato dextrose agar (PDA), ½ PDA and malt extract agar (MEA) at 15, 20 and 25°C, especially in MEA at 25°C.


Abstract

Phyllosticta citricarpa, the citrus black spot causal agent, requires both mating types, designated as MAT1-1 and MAT1-2, to produce ascospores. Mating-type ratio in some citrus-growing areas has been characterized but remains little known in the São Paulo (SP) citrus belt, the largest sweet orange producer worldwide. A protocol for in vitro ascospore production was developed recently; however, there are few studies regarding the optimal conditions for their production. Thus, this study aimed to determine the mating-type ratio in SP and quantify ascospores produced under different conditions in vitro. The mating types were identified by duplex PCR, and ascospore production was assessed in five culture media: potato dextrose agar (PDA), ½ PDA, malt extract agar (MEA), oatmeal agar (OMA) and citrus agar (CA), and at temperatures from 10 to 30°C. The ratio of MAT1-1 and MAT1-2 was similar to 1:1. Ascospore production reached peaks of 8.0, 7.0 and 3.0 × 103 ascospores/mL in MEA, PDA and ½ PDA, respectively. There were no ascospores observed in OMA and CA. P. citricarpa ascospores were observed only at 15, 20 and 25°C in PDA, with peaks of up to 2.0 × 103 ascospores/mL at 42, 35 and 21 days, respectively. There were no ascospores observed at 10 and 30°C. Both mating types are similarly distributed in the SP citrus belt, and in vitro ascospore production may be optimized by using MEA at 25°C. These data provide a better understanding of P. citricarpa sexual reproduction and a background for further studies with ascospores.

Genome‐wide association study of adult plant resistance to spot blotch in an elite Canadian two‐row barley germplasm collection

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Genome-wide association study of adult plant resistance to spot blotch in an elite Canadian two-row barley germplasm collection

Using elite Canadian two-row barley germplasm, GWAS identified several single-nucleotide polymorphism (SNP) molecular markers of multiple chromosomes that were associated with adult-plant resistance to Bipolaris sorokiniana.


Abstract

Spot blotch, caused by Bipolaris sorokiniana, is a growing concern for barley (Hordeum vulgare) production in most humid, temperate growing regions of the world. Under epidemic conditions, this disease can reduce yields significantly. Reduction of kernel plumpness is associated with lower malt extract and consequently grain quality. Enhanced resistance to this disease is a major goal of western Canadian two-row barley breeding programmes; however, two-row barley is considered to be more susceptible than six-row barley. Plant–pathogen interactions observed for Canadian germplasm are polygenic, where quantitative breeding methods could be beneficial for the development of resistant germplasm. A germplasm panel consisting of 200 two-row, spring barley genotypes with differential reaction to spot blotch was evaluated at Brandon, Manitoba (MB), and Melfort, Saskatchewan (SK), over eight site years. Genome-wide association study was conducted using a 50k single-nucleotide polymorphism (SNP) Illumina iSelect genotyping array. The most-resistant germplasm was found to carry Midwest Six-rowed Durable Resistant Haplotype (MSDRH) alleles at the loci located on chromosomes 1H (Rcs-qtl-1H-11_10764), 3H (Rcs-qtl-3H-11_10565) and 7H (Rcs-qtl-7H-11_20162, Rcs5 locus), where the 3H locus was found to be the most under-represented in Canadian germplasm. Additional marker-trait associations within nurseries were identified on chromosomes 1H, 2H and 4H. Several of the SNP markers identified were found to be polymorphic within the Canadian two-row germplasm panel and thus could be useful for enhancing spot blotch resistance in two-row barley for development of resistant cultivars.

Environmental drivers of wheat yield variability across China’s production regions: Insights from field experiments

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Abstract

Wheat crops underpin contemporary global food security. Predominant wheat production zones in China include the Huang-Huai-Hai-Plain and the Mid-Lower Reaches of the Yangtze River, although climatic effects on productive potential across these regions vary markedly in space and time. Here, we conducted field experiments during the wheat season of 2015–2018 to examine environmental effects on growth, with fertilization and irrigation provided at levels ensuring that nutrient and water stress exposure was minimal. Yields in Huang-Huai-Hai-Plain and the Mid-Lower Reaches of the Yangtze River averaged 8950 and 4818 kg ha−1, respectively. Yield variation across regions was primarily related to spike number per unit area and grain number per spike. Maturity biomass was higher in Huang-Huai-Hai-Plain; this translated into higher grain yields. Lower temperature and longer growing duration between emergence and jointing in Huang-Huai-Hai-Plain afforded higher tillering and spike numbers, whereas higher growth rates from jointing to maturity resulted in higher biomass production in Huang-Huai-Hai-Plain compare with the Mid-Lower Reaches of Yangtze River. Growth rate, grain numbers and yield were positively correlated with the ratio of daily intercepted solar radiation to mean temperature during jointing to anthesis, termed photothermal quotient. Collectively, our results suggest that growth rate accounted for more variation in biomass production compared with growth duration, and the photothermal conditions in the Mid-Lower Reaches of the Yangtze River were restrictive for spike development and yield formation. Our results help disentangle drivers of crop growth through the development of agro-environmental conceptual frameworks, enabling a better understanding of yield variability in space and time.

Shared quantitative trait loci underlying root biomass and phenology in wheat (Triticum aestivum L.)

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Abstract

In this study, we investigated the genetic mapping of root biomass and root/shoot ratio. We utilized a large (n = 345) bi-parental recombinant inbred line (RIL) population from the ‘Penny’ × ‘Yecora-Rojo’ cross to investigate the partitioning of biomass above- and belowground and to identify the quantitative trait loci (QTL) that influence root biomass and root/shoot ratio. Genotyping of 345 RILs by using genotyping by sequencing produced 2918 single-nucleotide polymorphism markers by which a genome-wide map of 3507 cM was constructed. Phenotyping was conducted in an augmented design with large pots in controlled environment. We identified two significant QTL regions, QRt.peye-5A and QRt.peye-5B, which control root biomass and the root/shoot ratio. QRt.peye-5A, marking a 3.15 Mbp region on chromosome 5A, explained 11% of variations in root biomass and 9.5% of variations in root/shoot ratio, with the narrow region harbouring 28 genes. QRt.peye-5B, marking a 12.2 Mbp region on chromosome 5B, explained 7% of variations in root/shoot ratio and harbours 104 genes. The root/shoot ratio enhancing alleles at QRt.peye-5A and QRt.peye-5B come from ‘Penny’ and ‘Yecora-Rojo’ respectively. These QTL regions contains genes such as the two MADS box transcription factors on the 5A QTL that are candidate genes for Vrn1 locus, and other genes previously postulated for root traits such as a COBRA-like COBL2 and landmark hormonal responses genes such as IAA16, IAA4 and BRI1, DREB2A-INTERACTING PROTEIN2 (DRIP2) and bHLH92 which has a role in amelioration of stress conditions.