The Velvet Proteins VosA and VelB Play Different Roles in Conidiation, Trap Formation, and Pathogenicity in the Nematode-Trapping Fungus Arthrobotrys oligospora

Abstract

The velvet family proteins VosA and VelB are involved in growth regulation and differentiation in the model fungus <i>Aspergillus nidulans</i> and other filamentous fungi. In this study, the orthologs of VosA and VelB, AoVosA, and AoVelB, respectively, were characterized in the nematode-trapping fungus <i>Arthrobotrys oligospora</i>, which captures nematodes by producing trapping devices (traps). Deletion of the <i>AovelB</i> gene resulted in growth defects in different media, and the aerial hyphae from the Δ<i>AovelB</i> mutant lines were fewer in number and their colonies were less dense than those from the wild-type (WT) strain. The Δ<i>AovelB</i> mutants each displayed serious sporulation defects, and the transcripts of several sporulation-related genes (e.g., <i>abaA</i>, <i>flbC</i>, <i>rodA</i>, and <i>vosA</i>) were significantly down-regulated compared to those from the WT strain. Furthermore, the Δ<i>AovelB</i> mutant strains became more sensitive to chemical reagents, including sodium dodecyl sulfate and H₂O₂. Importantly, the Δ<i>AovelB</i> mutants were unable to produce nematode-capturing traps. Similarly, extracellular proteolytic activity was also lower in the Δ<i>AovelB</i> mutants than in the WT strain. In contrast, the Δ<i>AovosA</i> mutants displayed no obvious differences from the WT strain in these phenotypic traits, whereas conidial germination was lower in the Δ<i>AovosA</i> mutants, which became more sensitive to heat shock stress. Our results demonstrate that the velvet protein AoVelB is essential for conidiation, trap formation, and pathogenicity in <i>A. oligospora</i>, while AoVosA plays a role in the regulation of conidial germination and heat shock stress.