Cytological Studies On The Infection Mechanism Of Ustilaginoidea Virens, The Pathogen Of Rice False Smut

Rice false smut is a worldwide fungal disease in rice caused by Ustilaginodea virens (Cooke) Tak, which harmed the panicles of rice. It was historically a minor rice disease, just occurred sporadically in the middle and late paddy field. As the extension of high-yield rice varieties with big and compact panicles and higher level of fertility and irrigation, rice false smut has been become one of the most severe diseases in rice in our country, especially in middle and lower reaches of the Yangtze River areas. The disease occurrence not only influents the yield and quality of rice, but also caused severe food safety problem due to large amounts of mycotoxin, ustilotoxins, which inhibt cell division in animals and plants.In this study the infection process of Ustilaginoidea virens in rice spikelets, and the germination characteristics of asexual spores of Ustilaginoidea virens in different media were analyzed with cytological methods. The main results are as follows:1. The symptoms of Ustilaginoidea virens was examined by artificial inoculation. The results showed that the pathogen hyphae appeared firstly on the surfaces of filaments, anthers and stigma3days after inoculation (dai); the pistil and stamens were covered by a thin layer of the fungal hyphae, and the anthers and stigma were faintly visible5dai; the intact spikelets were filled with fungal hyphae10dai; fungal hyphae grew out of spikelets and formed white balls15dai; several days later the color of the mature balls were turning dark with a thick yellow outer layer of chlamydospores.2. Serial transverse sections were cut throughout the intact infected spikelets and the mature false smut balls collected from the rice fields. It was found that the disease was caused by infection of rice filament tissues at earlier rice booting stage. The primary infection sites for the pathogen were the upper parts of the three stamen filaments located between the ovary and the lodicules. The other three filaments could not be infected. The pathogen just invaded and extended into the filament intercellularly, and mass of hyphae and chlamydospores grew outside of the filaments forming yellow or dark green ball-like colonies. During the infection process, the pathogen did not penetrate host cell walls directly like the necrotrophic pathogens and did not produce haustoria like the typical biotrophic pathogens. The pathogen could not infect the ovary and anthers, but the secondary hyphae could infect the outer layers of cells on lodicules and stigmas occasionally. The pathogen could not penetrate the rigid rice lemmas. The pathogen does not kill the host cell during its infection process, indicating to be a typical of a biotrophic parasite.3. The germination characteristics of the two asexual spores of Ustilaginodea virens in different media were analyzed. The results showed that conidia and chlamydospores could germinate faster in solid media than in liquid media. In liquid media, both conidia and chlamydospores formed secondary conidia with similar morphological characteristics as conidia. On the surface of solid media, the conidia always clumped together and produced hyphae in one or two ends when geminated. Clamydospores produced secondary conidia or/and hyphae on solid media when geminated. On2%agar surface, chlamydospores preferred to form conidia with72.5%of the germinated. While in PSA,74.2%of chlamydospores germinated with hyphae. This suggested that nutrition was in favor of chlamydospores to form hyphae and the farther infection.