Analysis of the Fusarium virguliforme Transcriptomes Induced During Infection of Soybean Roots Suggests That Enzymes With Hydrolytic Activities Could Play a Key Role in Root Necrosis

Sudden death syndrome (SDS) caused by the fungal pathogen, Fusarium virguliforme, is a major threat to soybean production in North America. There are two major components of this disease: (i) root necrosis and (ii) foliar SDS. Root symptoms consist of root necrosis with vascular discoloration that outspreads upto numerous nodes and internodes into the stem. Foliar SDS symptom is characterized by interveinal chlorosis and necrosis in leaves which lastly curl and fall off, and in severe cases by flower, pod abscission and immature seed formation. Nothing is known about how root necrosis develops. In order to unravel the root necrosis mechanisms by F. virguliforme, the transcriptome of the pathogen in infected soybean root tissues of a susceptible cultivar (Essex) was investigated. Of the 14,845 predicted F. virguliforme genes, we observed that 12,017 (81%) were expressed in germinating conidial spores and 12,208 (82%) in mycelia and 10,626 (72%) in infected soybean roots. Of the 10,626 genes induced in infected roots, 224 were transcribed only following infection. Expression of numerous infection-induced genes encoding enzymes with oxidation-reduction properties suggests that degradation of antimicrobial compounds such as the phytoalexin, glyceollin could be important in establishing the biotrophic phase. Enzymes with hydrolytic and catalytic activities could play a vital role in the transitioning of the pathogen from biotrophic to necrotrophic phase. We assume that genes encoding enzymes with catalytic and hydrolytic activities through late infection stage for cell wall degradation could be involved in root necrosis and establishing the necrotrophic phase in this pathogen.