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

Abstract

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 extends upto several nodes and internodes into the stem. Foliar SDS symptom is characterized by interveinal chlorosis and necrosis in leaves which finally curl and fall off, and in severe cases by flower, pod abscission and immature seed formation. A major toxin involved in initiating foliar SDS has been identified. Nothing is known about how root necrosis develops. In order to unravel the mechanisms used by the pathogen to cause root necrosis, the transcriptome of the pathogen in infected soybean root tissues of a susceptible cultivar (Essex) was investigated. The transcriptomes of the germinating conidia and mycelia were also examined. 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 several 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 an important role in the transitioning of the pathogen from biotrophic to necrotrophic phase. Expression of a large number of genes encoding enzymes with catalytic and hydrolytic activities during late infection stage suggests cell wall degradation by some of these enzymes could be involved in root necrosis and establishing the necrotrophic phase in this pathogen.