Cytological studies on the infection of rice root by Ustilaginoidea virens

In recent years, false smut disease of rice has been one of the most important diseases of cultivated rice in China. Ustilaginoidea virens is an ascomycete fungal pathogen that causes false smut in rice. There is always controversy about whether the pathogen can infect the rice root and cause the occurrence of false smut, mainly due to lack direct cytological evidence. In our study, we observed the cytological structure of rice root invaded by U. virens. The results showed that U. virens could attach to the surface of young roots and penetrate into the intercellular space of the root epidermis. The cellulose microfibrils in root epidermal cell wall are very loose and soft, and their structural features are similar to filaments of rice. After the fungus infected the roots, a large number of fungal secretions were accumulated outside of the cell walls. At 40 days, the fungus began to degrade, but pathogens still had not infected the sclerenchyma, in which the cells are arranged densely and the cell walls are thicker. U. virens could not cross the sclerenchyma layer into the endodermis and phloem of the root. To some extent, the U. virens infection affected the leaf and root growth of the rice. After inoculation, there was no fungal mycelium found in transverse sections of the rice young stem. These results suggested that root colonization of U. virens does not lead to systemic invasion in rice.