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

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.Metarhizium robertsiiis 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 (MrUBI4, MAA_02160) was functionally characterized via gene deletion inM. robertsii.Compared to the control strains, theMrUBI4deletion 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, includingbrlA,abaA,andwetA, were significantly downregulated, which indicated thatMrUBI4played an important role in asexual sporulation. Deletion ofMrUBI4especially 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 (hsp), trehalose, and mannitol accumulation (tps,tpp,nth,andmpd) in theMrUBI4deletion mutant. Deletion ofMrUBI4has no effect on fungal virulence. Altogether,MrUBI4is involved in the regulation of conidiation, conidial germination, UV stress, and heat-shock response inM. robertsii.Keywords:Metarhizium robertsii;polyubiquitin;conidiation;conidial germination;stress tolerance