Grain Characteristics, Moisture, and Specific Peptides Produced by Ustilaginoidea virens Contribute to False Smut Disease in Rice (Oryza sativa L.)

The fungus Ustilaginoidea virens, the causative agent of false smut in rice (Oryza sativa L.), is responsible for one of the severe grain diseases that lead to significant losses worldwide. In this research, microscopic and proteomic analyses were performed by comparing U. virens infected and non-infected grains of the susceptible and resistant rice varieties to provide insights into the molecular and ultrastructural factors involved in false smut formation. Prominent differentially expressed peptide bands and spots were detected due to false smut formation as revealed by sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and two-dimensional gel electrophoresis (2-DE) SDS-PAGE profiles and were identified using liquid chromatography-mass spectrometry (LC-MS/MS). The proteins identified from the resistant grains were involved in diverse biological processes such as cell redox homeostasis, energy, stress tolerance, enzymatic activities, and metabolic pathways. It was found that U. virens produces diverse degrading enzymes such as β-1, 3-endoglucanase, subtilisin-like protease, putative nuclease S1, transaldolase, putative palmitoyl-protein thioesterase, adenosine kinase, and DNase 1 that could discretely alter the host morphophysiology resulting in false smut. The fungus also produced superoxide dismutase, small secreted proteins, and peroxidases during the smut formation. This study revealed that the dimension of rice grain spikes, their elemental composition, moisture content, and the specific peptides produced by the grains and the fungi U. virens play a vital role in the formation of false smut.

Efficient Agrobacterium tumefaciens-mediated transformation system of Diaporthe caulivora

This is the first report describing the genetic transformation of Diaporthe caulivora, the soybean stem canker fungus. A simple and 100% efficient protocol of Agrobacterium tumefaciens-mediated transformation used mycelium as starting material and the hygromycin B resistance and green fluorescent protein (GFP) as a selection and reporter agents, respectively. All transgenic isolates were mitotically stable in two independent experiments and polymerase chain reaction with hygromycin B resistance primers confirmed successful T-DNA integration into the fungal genome. Plant-fungus interaction studies, including pathogenicity, latency, and endophytism, as well as further studies of random and targeted mutagenesis will be possible with GFP-expressing isolates of D. caulivora and other species in the Diaporthe / Phomopsis complex.

Development of rice conidiation media for Ustilaginoidea virens

Rice false smut, caused by the ascomycete Ustilaginoidea virens, is a serious disease of rice worldwide. Conidia are very important infectious propagules of U. virens, but the ability of pathogenic isolates to produce conidia frequently decreases in culture, which influences pathogenicity testing. Here, we developed tissue media with rice leaves or panicles that stimulate conidiation of U. virens. Among the tested media, 0.10 g/ml panicle medium was most efficient for conidiation. Whereas, some rice leaf media more effectively increased conidiation than panicle media except 0.10 g/ml panicle medium, and certain non-filtered tissue media were better than their filtered counterparts. Although the conidia induced in rice tissue media were smaller, they were able to germinate on potato sucrose agar medium and infect rice normally. The rice tissue medium is also workable in inducing conidia for conidiation-defective isolates. This method provides a foundation for the production of conidia by U. virens that will be widely applicable in pathogenicity testing as well as in genetic analyses for false smut resistance in rice cultivars.

Characterization of propiconazole field-resistant isolates of Ustilaginoidea virens

The plant-pathogenic fungus Ustilaginoidea virens (Cooke) Takah causes rice false smut (RFS), which is responsible for significant quantitative and qualitative losses in rice industry. Propiconazole is a triazole fungicide which belongs to Demethylation inhibitors (DMIs). It is used to control RFS in China. We previously screened 158 isolates of U. virens collected in the fields in 2015 in Jiangsu province of China, and found two of them were highly resistant to propiconazole (named 82 and 88, respectively). In this study, we have analyzed the physiological and biochemical characters of six field-sensitive isolates and the two field-resistant isolates, including mycelial growth and cell wall integrity. We found there was cross-resistance between different DMIs fungicides, but was no cross-resistance between DMIs and QoIs fungicides. We also analyzed the fitness, and found the pathogenicity in 88 was stronger than the field-sensitive isolates, but was completely lost in 82. Sequence analyses of CYP51 and the 1000-bp upstream of CYP51 coding region showed no mutation in 82 compared to the field-sensitive strains, but two more bases CC were identified at 154-bp upstream of the coding region in the field-resistant isolate 88. Moreover, the expression of CYP51 gene in all tested isolates was significantly induced by propiconazole. However, the up-regulation expression level in both 82 and 88 was much higher than that in the field-sensitive isolates. We also found propiconazole could inhibit the ergosterol biosynthesis in the field-sensitive isolates, but stimulated it in both field-resistant isolates 82 and 88. Given the high level of U. virens developing propiconazole resistance and the good fitness of the field-resistant isolate 88, the resistance of U. virens to DMIs must be monitored and managed in rice fields.

Arabidopsis thaliana: A Model Host Plant to Study Plant-Pathogen Interaction Using Rice False Smut Isolates of Ustilaginoidea virens

Rice false smut fungus which is a biotrophic fungal pathogen causes an important rice disease and brings a severe damage where rice is cultivated. We established a new fungal-plant pathosystem where Ustilaginoidea virens was able to interact compatibly with the model plant Arabidopsis thaliana. Disease symptoms were apparent on the leaves of the plants after 6 days of post inoculation in the form of chlorosis. Cytological studies showed that U. virens caused a heavy infestation inside the cells of the chlorotic tissues. Development and colonization of aerial mycelia in association with floral organ, particularly on anther and stigma of the flowers after 3 weeks of post inoculation was evident which finally caused infection on the developing seeds and pod tissues. The fungus adopts a uniquely biotrophic infection strategy in roots and spreads without causing a loss of host cell viability. We have also demonstrated that U. virens isolates infect Arabidopsis and the plant subsequently activates different defense response mechanisms which are witnessed by the expression of pathogenesis-related genes, PR-1, PR-2, PR-5, PDF1.1, and PDF1.2. The established A. thaliana-U. virens pathosystem will now permit various follow-up molecular genetics and gene expression experiments to be performed to identify the defense signals and responses that restrict fungal hyphae colonization in planta and also provide initial evidence for tissue-adapted fungal infection strategies.