Abstract
Fusarium graminearum causes Fusarium head blight (FHB) on wheat, barley, and other grains. During infection, F. graminearum produces deoxynivalenol (DON), which contaminates grain and functions as a virulence factor to promote FHB spread throughout the wheat head. F. graminearum secretes hundreds of putative effectors, which can interfere with plant immunity to promote disease development. However, the function of most of these putative effectors remains unknown. In this study, we investigated the expression profiles of 23 F. graminearum effector-coding genes during the early stage of wheat head infection. Gene expression analyses revealed that three effectors, FGSG_01831, FGSG_03599, and FGSG_12160, respectively, were highly induced in both a FHB susceptible and a moderately resistant variety. We generated deletion mutants for these effector genes and performed FHB virulence assays on wheat head using point and dip inoculations to evaluate FHB spread and initial infection. No statistically significant difference in FHB spread was observed in the deletion mutants. However, deletion mutants Δ01831 displayed a significant reduction in initial infection, and thus resulted in less DON contamination. To investigate the potential mechanisms involved, these three effectors were transiently expressed in Nicotiana benthamiana leaves. N. benthamiana leaves expressing these individual effectors had significantly reduced production of reactive oxygen species induced by chitin, but not by flg22. Furthermore, FGSG_01831 and FGSG_03599 markedly suppressed Bax-induced cell death when co-expressed with Bax in N. benthamiana leaves. Our study provides new insights into the functions of these effectors and suggests they play collective or redundant roles that likely ensure the successful plant infection.
Highlights
Fusarium graminearum, the causal agent of Fusarium head blight (FHB) on wheat and barley, poses a great threat to food safety and food security
Since functional redundancy tends to mask the role of individual effectors in single-gene deletion mutants, we further investigated the potential function of these effector genes by transiently expressing individual effector genes with/without the signal peptide sequences in N. benthamiana via Agrobacterium-mediated transformation
We identified three F. graminearum effector genes that are highly induced during the first week of infection and revealed one of them, FGSG_01831, is involved in initial infection of wheat
Summary
The causal agent of Fusarium head blight (FHB) on wheat and barley, poses a great threat to food safety and food security. Disease symptoms were primarily restricted to the inoculated florets within the first 7 dpi (Brown et al, 2010) This period is considered the initial infection. F. graminearum mutants blocked in the production of DON are restricted to inoculated florets and cannot pass the rachis barrier to spread to the rest of the wheat head (Jansen et al, 2005). F. graminearum typically spreads along the surface of the rachis in barley because it lacks the ability to penetrate through the rachis node regardless of DON production (Maier et al, 2006). It remains unclear how F. graminearum initiates infection and overcomes host immunity to cause disease. To enhance FHB resistance in wheat and barley, it is critical to understand the molecular mechanisms involved during initial infection and improve wheat and barley type I resistance
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