Subcellular localization of six thioredoxins and their antioxidant activity and contributions to biological control potential in Beauveria bassiana.

Thioredoxins (Trx) can detoxify sulfide or act as electron donors in the reduction of disulfide and dithiol to protect yeast cells from ROS damage but remain poorly explored in filamentous fungi. Here we show more Trx homologs in Beauveria bassiana than in many other fungi and examine their functions. This filamentous entomopathogen has six Trx homologs, including four (Txr1-4) evidently localized in cytoplasm, one (Trx5) in nuclear membrane and another (Trx6) in mitochondria. Deletion of each trx had no effect on radial growth on rich or minimal medium but resulted in remarkable transcriptional up-regulation of other partners for compensation. Compared with wild-type, only Δtrx2 was significantly more sensitive to menadione whereas none of six Δtrx mutants was responsive to other oxidants including H2O2. Intriguingly, Δtrx2 showed uniquely a significant increase in total Trx activity in normal cultures but a remarkable decrease in total SOD activity in the cultures grown normally or co-cultivated with menadione. The ratio of reduced/oxidized glutathione accumulated in hyphal cells stressed with menadione decreased to only 0.4 in Δtrx2 from ∼1.0 observed in wild-type and other mutants. The six Δtrx mutants displayed one or more phenotypic changes associated with the fungal biocontrol potential, including conidiation, and germination, thermotolerance, UV-B resistance and virulence of their conidia. All the changes were restored by trx complementation. Taken together, the greater Trx diversity evolutionarily gained by B. bassiana could help it to maintain cellular redox homeostasis and infect insect hosts in diverse habitats.