Effectors of Plant Necrotrophic Fungi

Plant diseases caused by necrotrophic fungal pathogens result in large economic losses in field crop production worldwide. Effectors are important players of plant-pathogen interaction and deployed by pathogens to facilitate plant colonization and nutrient acquisition. Compared to biotrophic and hemibiotrophic fungal pathogens, effector biology is poorly understood for necrotrophic fungal pathogens. Recent bioinformatics advances have accelerated the prediction and discovery of effectors from necrotrophic fungi, and their functional context is currently being clarified. In this review we examine effectors utilized by necrotrophic fungi and hemibiotrophic fungi in the latter stages of disease development, including plant cell death manipulation. We define “effectors” as secreted proteins and other molecules that affect plant physiology in ways that contribute to disease establishment and progression. Studying and understanding the mechanisms of necrotrophic effectors is critical for identifying avenues of genetic intervention that could lead to improved resistance to these pathogens in plants.

Monitoring of Natural Occurrence and Severity of Leaf and Glume Blotch Diseases of Winter Wheat and Winter Triticale Incited by Necrotrophic Fungi Parastagonospora spp. and Zymoseptoria tritici

The occurrence of necrotrophic winter wheat and triticale pathogens in eight geographical regions of Poland was studied between 2015 and 2020. Over a period of six years, the incidence of the following pathogens was monitored: Parastagonospora nodorum, Parastagonospora avenae and Zymoseptoria tritici. The significant effect of meteorological factors on the incidence of pathogens was determined. The relationship between late-season and early-season factors associated with temperature and precipitation on the severity of diseases incited by the pathogens was statistically significant. Statistical models estimating the natural occurrence and severity of diseases caused by the pathogens were developed with the random forest (RF) algorithm based on 10,412 cases of the diseases. The data were randomly divided into training and test datasets and the accuracy of models was determined by the root mean squared error (RMSE) and Pearson correlation coefficient (r). The most promising model was developed for Z. tritici with the following test metrics: RMSE = 57.5 and r = 0.862. The model can be used to link disease severity to weather and predict low severity years and high severity years. Over the period of 2015–2020, the most significant winter wheat pathogen showed to be Z. tritici, while on winter triticale P. nodorum incited disease symptoms on the largest number of leaves. The occurrence of P. avenae f. sp. triticea on winter wheat and winter triticale was the least frequent and on average was below the economic threshold.

Differential Expression of Fungal Genes Determines the Lifestyle of Plectosphaerella Strains During Arabidopsis thaliana Colonization

The fungal genus Plectosphaerella comprises species and strains with different lifestyles on plants, such as P. cucumerina, which has served as model for the characterization of Arabidopsis thaliana basal and nonhost resistance to necrotrophic fungi. We have sequenced, annotated, and compared the genomes and transcriptomes of three Plectosphaerella strains with different lifestyles on A. thaliana, namely, PcBMM, a natural pathogen of wild-type plants (Col-0), Pc2127, a nonpathogenic strain on Col-0 but pathogenic on the immunocompromised cyp79B2 cyp79B3 mutant, and P0831, which was isolated from a natural population of A. thaliana and is shown here to be nonpathogenic and to grow epiphytically on Col-0 and cyp79B2 cyp79B3 plants. The genomes of these Plectosphaerella strains are very similar and do not differ in the number of genes with pathogenesis-related functions, with the exception of secreted carbohydrate-active enzymes (CAZymes), which are up to five times more abundant in the pathogenic strain PcBMM. Analysis of the fungal transcriptomes in inoculated Col-0 and cyp79B2 cyp79B3 plants at initial colonization stages confirm the key role of secreted CAZymes in the necrotrophic interaction, since PcBMM expresses more genes encoding secreted CAZymes than Pc2127 and P0831. We also show that P0831 epiphytic growth on A. thaliana involves the transcription of specific repertoires of fungal genes, which might be necessary for epiphytic growth adaptation. Overall, these results suggest that in-planta expression of specific sets of fungal genes at early stages of colonization determine the diverse lifestyles and pathogenicity of Plectosphaerella strains.

Oxalic Acid Metabolism Contributes to Full Virulence and Pycnidial Development in the Poplar Canker Fungus Cytospora chrysosperma

Poplar Cytospora canker, which is mainly caused by Cytospora chrysosperma, is one of the most destructive and widespread tree diseases worldwide. Although oxalic acid (OA) is demonstrated as an important virulence determinant in several necrotrophic fungi, specific functions of OA during pathogenesis remain controversial. Here, we identified three genes (CcOah, CcOdc1, and CcOdc2) directly involved in OA biosynthesis and catabolism in C. chrysosperma. We demonstrated that CcOah is required for OA biogenesis. All three genes were found to be highly upregulated during early infection stages of the poplar stem. The deletion of any of the three genes led to an obvious reduction of pycnidial production but no abnormality of hyphal growth and morphology. Furthermore, the individual deletion strain exhibited significantly limited lesion sizes on poplar twigs and leaves. Exogenous application of OA or citric acid can complement the virulence defects of ΔCcOah and ΔCcOdc1 strains. We further found that the ΔCcOah strain strongly promoted reactive oxygen species burst of poplar leaves during infection. Finally, induced secretion of OA was observed by monitoring color change of the plates after poplar stem extracts were added in the cultures; however, we failed to quantify OA concentration by high-performance liquid chromatography. Taken together, the present results provide insights into the function of OA acting as an important virulence factor of C. chrysosperma.

Rhizophagus irregularis and Rhizoctonia solani Differentially Elicit Systemic Transcriptional Expression of Polyphenol Biosynthetic Pathways Genes in Sunflower

Plant roots are exposed to penetration by different biotrophic and necrotrophic fungi. However, plant immune responses vary, depending on the root-penetrating fungus. Using qRT-PCR, changes over time in the systemic transcriptional expression of the polyphenol biosynthesis-related genes were investigated in sunflower plants in response to colonization with Rhizophagus irregularis and/or infection with Rhizoctonia solani. The results demonstrated that both fungi systemically induced the transcriptional expression of most of the addressed genes at varying degrees. However, the inducing effect differed according to the treatment type, plant organ, targeted gene, and time stage. The inducing effect of R. irregularis was more prevalent than R. solani in the early stages. In general, the dual treatment showed a superior inducing effect over the single treatments at most of the time. The hierarchical clustering analysis showed that cinnamate-4-hydroxylase was the master expressed gene along the studied time period. The cell wall lignification was the main plant-defensive-mechanism induced. In addition, accumulations of chlorogenic acid, flavonoids, and anthocyanins were also triggered. Moreover, colonization with R. irregularis improved the plant growth and reduced the disease severity. We can conclude that the proactive, rather than curative, colonization with R. irregularis is of great importance, owing to their protective and growth-promoting roles, even if no infection occurred.

The impact of Pseudomonas syringae bacteria on the plant pathogenic fungi and cherry plants

The investigations are made of cherry biological peculiarities and disease excitants composition. The domination of necrotrophic fungi and Pseudomonas syringae van Hall bacteria in pathocenosis is shown. The fact of microbiota associative lesion of plants is established. The antagonist action of isolated bacterium in relation to Cytospora leucostoma (Pers.) Sacc., Alternaria alternata Keissl., Fusarium oxysporum Schlecht., Monillia cinerea Bonord is detected. The character of isolated bacterium metabolites, pathogenic fungi and their association effect on a host-plant is established. The ability of bacterium to lower the activity of phytotoxins secretion in fungi when cultivated together is identified. It has been found that it is the fungal pathogens causing the development of die-back in cherry in investigated gardens, whereas P. syringae bacterium, when in latent state, plays a part of protector and permits to suppress the development of fungal infection. Taking into account the fact that this bacterium is pathogenic, the works of advanced breeding for resistance of the horticultural crops to bacterial blight are recommended.