Lysyl-tRNA synthetase (Krs) acts a virulence factor of Beauveria bassiana by its vital role in conidial germination and dimorphic transition.

Krs is a class II lysyl-tRNA synthetase (KRS) that is involved in cytosolic protein synthesis in budding yeast but functionally has not been explored in filamentous fungi. Previous transcriptomic analysis has revealed that a Krs-coding gene is likely involved in pathogenesis of Beauveria bassiana, a classic insect pathogen as a global source of fungal insecticides. Here, we show that Krs is localized in the cytoplasm of hyphal cells and acts as a substantial virulence factor in B. bassiana. Deletion of krs resulted in 10-h delayed germination, decreased (15 %) thermotolerance, and lowered (46 %) UV-B resistance of aerial conidia despite limited impact on conidiation capacity and slight or inconspicuous influence on radial growth on rich and minimal media with different carbon (10 sugars/polyols) and nitrogen (17 amino acids) sources. The deletion mutant suffered 58 % reduction in submerged blastospore yield (an index of in vitro dimorphic transition rate) in a minimal medium, and the reduction increased to 71 % in another trehalose-based medium mimic to insect haemolymph. In standardized bioassays, median lethal actions of Δkrs against Galleria mellonella larvae through the infections passing and bypassing the insect cuticle were prolonged to 192 and 153 h from wild-type median lethal time (LT50) estimates of 119 and 109 h, respectively. Microscopic examination revealed 2-d delayed presence of in vivo formed hyphal bodies in the haemolymph of the larvae infected by Δkrs in either mode. These findings unveil a vital role of Krs in conidial germination and dimorphic transition and its contribution to the fungal potential against arthropod pests.