Deletion of Major Endoxylanase Genes in Fusarium graminearum Uncovers Distinct Virulence Phenotypes Influenced by the Genetic Background

ABSTRACT

Fusarium graminearum , the main causal agent of Fusarium head blight of wheat, secretes endoxylanases to hydrolyse xylan, a major component of hemicellulose. Mutants of the highly expressed FGSG_03624 ( xylB ) gene previously showed a disease severity level similar to wild‐type (WT) in wheat spikes. In this study, in contrast, three independent deletion mutants of FGSG_10999 ( xylA ), a second highly expressed endoxylanase gene, were significantly impaired in disease severity. Surprisingly, a knock‐out mutant of xylA in the ΔxylB genetic background (ΔΔxylB+A) exhibited virulence comparable to that of ΔxylB and WT. The ΔxylA mutant shared with ΔΔxylB+A a marked reduction in xylanase activity during in vitro growth, and also with ΔxylB a decreased fungal biomass and deoxynivalenol (DON) accumulation in infected spikes. In contrast to ΔΔxylB+A and ΔxylB, ΔxylA revealed down‐regulation of some endoxylanase and effector genes during spike infection. In addition, when grown on a wheat spike medium, the culture filtrate of ΔxylA showed a significant reduction in total protein secretion and a more severe impairment in necrotizing activity in Nicotiana benthamiana infiltrated leaves compared with ΔxylB and ΔΔxylB+A culture filtrates. When a deletion mutant of xylB was generated in the ΔxylA genetic background, the ΔΔxylA+B mutant displayed severely reduced virulence, similar to that of the parental ΔxylA. Altogether, these results demonstrate that xylA plays a critical role in F. graminearum pathogenicity through unexpected pleiotropic, network‐level alterations, as also supported by the partial restoration observed by complementing the xylA gene in ΔxylA, highlighting the strong influence of genetic background on phenotypic outcomes.