The Polyubiquitin Gene MrUBI4 Is Required for Conidiation, Conidial Germination, and Stress Tolerance in the Filamentous Fungus Metarhizium robertsii

Abstract

The polyubiquitin gene is a highly conserved open reading frame that encodes different numbers of tandem ubiquitin repeats from different species, which play important roles in different biological processes. <i>Metarhizium robertsii</i> is a fungal entomopathogen that is widely applied in the biological control of pest insects. However, it is unclear whether the polyubiquitin gene is required for fungal development, stress tolerance, and virulence in the entomopathogenic fungus. In the present study, the polyubiquitin gene (<i>MrUBI4</i>, MAA_02160) was functionally characterized via gene deletion in <i>M. robertsii.</i><b>Compared to the control strains, the <i>MrUBI4</i> deletion mutant showed delayed conidial germination and significantly decreased conidial yields (39% of the wild-type 14 days post-incubation). Correspondingly, the transcript levels of several genes from the central regulatory pathways associated with conidiation, including <i>brlA</i>, <i>abaA,</i> and <i>wetA</i>, were significantly downregulated, which indicated that <i>MrUBI4</i> played an important role in asexual sporulation. Deletion of <i>MrUBI4</i> especially resulted in increased sensitivity to ultraviolet (UV) and heat-shock stress based on conidial germination analysis between mutant and control strains. The significant increase in sensitivity to heat-shock was accompanied with reduced transcript levels of genes related to heat-shock protein (<i>hsp</i>), trehalose, and mannitol accumulation (<i>tps</i>, <i>tpp</i>, <i>nth,</i> and <i>mpd</i>) in the <i>MrUBI4</i> deletion mutant. Deletion of <i>MrUBI4</i> has no effect on fungal virulence. Altogether, <i>MrUBI4</i> is involved in the regulation of conidiation, conidial germination, UV stress, and heat-shock response in <i>M. robertsii</i>.</b>