The Intermediates in Branched-Chain Amino Acid Biosynthesis Are Indispensable for Conidial Germination of the Insect-Pathogenic Fungus Metarhizium <i>robertsii</i>

Abstract

<i>Metarhizium</i> spp. are well-known biocontrol agents used worldwide to control different insect pests. Keto-acid reductoisomerase (ILVC) is a key enzyme for branched-chain amino acid (BCAA) biosynthesis, and it regulates many physiological activities. However, its functions in insect-pathogenic fungi are poorly understood. In this work, we identified <i>MrilvC</i> in <i>M. robertsii</i> and dissected its roles in fungal growth, conidiation, germination, destruxin biosynthesis, environmental stress response, and insecticidal virulence. BCAA metabolism affects conidial yields and germination. However, BCAAs cannot recover the conidial germination of an <i>MrilvC</i>-deficient strain. Further feeding assays with intermediates showed that some conidia of the Δ<i>MrilvC</i> mutant start to germinate. Therefore, it is the germination defect that causes the complete failures of conidial penetration and pathogenicity in the Δ<i>MrilvC</i> mutant. In conclusion, we found intermediates in BCAA biosynthesis are indispensable for <i>Metarhizium robertsii</i> conidial germination. This study will advance our understanding of the fungal germination mechanism.<b>IMPORTANCE</b> Branched-chain amino acid (BCAA) metabolism plays a significant role in many biological activities beyond protein synthesis. Spore germination initiates the first stage of vegetative growth, which is critical for the virulence of pathogenic fungi. In this study, we demonstrated that the keto-acid reductoisomerase MrILVC, a key enzyme for BCAA biosynthesis, from the insect-pathogenic fungus <i>Metarhizium robertsii</i> is associated with conidial germination and fungal pathogenicity. Surprisingly, the germination of the Δ<i>MrilvC</i> mutant was restored when supplemented with the intermediates of BCAA metabolism rather than three BCAAs. The result was significantly different from that of plant-pathogenic fungi. Therefore, this report highlights that the intermediates in BCAA biosynthesis are indispensable for conidial germination of <i>M. robertsii</i>.