Impairment of the cellulose degradation machinery enhances fungal virulence but limits reproductive fitness

Fungal endophytes grow in the apoplastic space, in constant contact with the plant cell wall (CW) that hinders microbe progression, while representing a source of nutrients. Although numerous fungal CW modifying proteins have been identified, their role during host colonization remains underexplored. Here we show that the root-infecting plant pathogen Fusarium oxysporum (Fo) does not require its complete arsenal of cellulases to infect the host plant. Quite the opposite, Fo mutants impaired in cellulose degradation become hypervirulent by enhancing the secretion of virulence factors. On the other hand, the reduction on cellulase activity had a severe negative effect on saprophytic growth and microconidia production during the final stages of the Fo infection cycle. These findings enhance our understanding on the function of plant CW degradation on the outcome of host-microbe interactions and reveal an unexpected role of cellulose degradation in a pathogen’s evolutionary success. Teaser Unexpectedly, fungi compromised in their capacity to degrade plant cellulose are hypervirulent but impaired in sporulation.