Dihydroxyacid dehydratase (DHAD), encoded by ILV3, catalyses
the third step in the biosynthetic pathway of branchedchain
amino acids (BCAAs), which include isoleucine (Ile),
leucine (Leu), and valine (Val). Enzymes involved in BCAA
biosynthesis exist in bacteria, plants, and fungi but not in
mammals and are therefore attractive targets for antimicrobial
or herbicide development. In this study, three paralogous
ILV3 genes (FgILV3A, FgILV3B, and FgILV3C) were identified
in the genome of Fusarium graminearum, the causal
agent of Fusarium head blight (FHB). Deletion of FgILV3A
alone or combined with FgILV3B or FgILV3C indicated an
important role for FgILV3A in BCAA biosynthesis. FgILV3A
deletion mutants lost the ability to grow on medium lacking
amino acids. Exogenous supplementation of 1 mM Ile and
Val rescued the auxotrophy of ΔFgIlv3A, though 5 mM was
required to recover the growth defects in ΔFgIlv3AB and
ΔFgIlv3AC strains, indicating that FgIlv3b and FgIlv3c exhibit
redundant but accessory roles with FgIlv3a in BCAA
biosynthesis. The auxotrophy of ΔFgIlv3A resulted in pleiotropic
defects in aerial hyphal growth, in conidial formation
and germination, and in aurofusarin accumulation. In addition,
the mutants showed reduced virulence and deoxynivalenol
production. Overall, our study demonstrates that
FgIlv3a is crucial for BCAA biosynthesis in F. graminearum and a candidate fungicide target for FHB management.
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