False Smut of Rice

False smut of rice, caused by Ustilaginoidea virens (Cooke) Takahashi (Teleomorph: Villosiclava virens), has become a common disease in most major rice growing regions throughout the world. Considerable confusion exists regarding the infection process and the disease cycle. Therefore, a clearer understanding of pathogenesis caused by Ustilaginoidea virens is critical for future efforts to develop genetic and chemical tools to manage false smut in Arkansas and other regions of the world. The overall goal of this research was to clarify the infection process underlying false smut, with emphasis on comparing and contrasting the histological basis of root and foliar infections. Therefore, a green fluorescent protein (GFP)-expressing U. virens strain was developed to address a critical need in histological investigations of the infection process as well as future investigations in functional genomics. Transformants of U. virens constitutively expressed GFP at high levels throughout all developmental stages and tissues, were stable after repeated sub-culturing, and were also confirmed to be pathogenic. Thus, the GFP-expressing strain was used in root and inflorescence inoculations of rice, and infectious development was analyzed via confocal microscopy. For comparative purposes, all root inoculation experiments were performed in parallel with a GFP-expressing strain of Magnaporthe oryzae, a known root-infecting pathogen of rice. Injections of the flag leaf sheath at booting consistently led to the formation of spore balls in a small number of infected plants. Confocal microscopy showed no evidence of systemic infection in these experiments, but rather infection was consistent with the fungus gaining access through the gap in the lemma as reported elsewhere. In root inoculations, U. virens failed to penetrate the epidermis, whereas M. oryzae frequently colonized roots extensively. Taken together, these results indicated that systemic infection of rice during false smut is not likely to originate from root infection, and the limited evidence for systemic infection calls into question whether it is a component of the false smut disease cycle.