Backgroud
Fusarium head blight (FHB) is a severe disease that affects cereal crops worldwide. This disease is caused by Fusarium species, such as Fusarium sporotrichioides, F. graminearum, and F. culmorum. The soil-borne pathogen F. sporotrichioides is often observed in cold climates, such as in northern Japan, the northern USA, northern Europe, and Russia. F. sporotrichioides was first isolated from corn in France and was identified as F. tricinctum NRRL 3229; it was later determined that this strain included both F. sporotrichioides and F. tricinctum. Similarly, the F. sporotrichioides IFO 9955 strain was identified from a bean hull, although it had previously been misidentified as F. solani. It has been reported that overwintered cereals colonized by F. sporotrichioides caused the deaths of approximately 1,100 people in the erstwhile USSR during World War II.
Some Fusarium species also produce trichothecene mycotoxins, which are known to inhibit protein synthesis in eukaryotes. Trichothecenes encompass many molecular species, which can be classified into 4 major groups (types A–D). Among these species, the type A (i.e., T-2 toxin) and type B (i.e., deoxynivalenol [DON]) trichothecenes are distinguished by the absence or presence of a ketone group at the C-8 position, respectively. Trichothecenes are often found in cereal grains and cereal-derived commodities. T-2 toxin has been reported to be approximately 10 times more toxic to mammals and plants than the type B trichothecenes, such as DON. The tri5 mutant of F. graminearum, which cannot produce DON, can infect wheat florets and spikes, but it has decreased virulence against host plants. Mitogen-activated protein kinase 3 (MPK3) and mitogen-activated protein kinase 6 (MPK6) in Arabidopsis have also been shown to be activated by T-2 toxin. Nishiuchi et al. suggested that T-2 toxin possesses an elicitor-like activity.
Subject 1; Functional analysis of a novel MAPKKK induced by Fusarium phytotoxin trichothecenes.
Trichothecenes are a closely related family of phytotoxins produced by a phytopathogenic fungi. Type A trichothecenes, such as T-2 toxin, caused rapid and prolonged activation of MAPKs, and triggered the cell death by activation of an elicitor-like signaling pathway in Arabidopsis. However, a MAP kinase cascade in the response to trichothecenes is unknown. Novel MAPKKK (MKD1) was identified as a subunit of an AtNFXL1 protein complex. We had revealed that AtNFXL1 functions as a negative regulator of the trichothecene-induced defense response. mkd1 mutant growing on a medium without trichothecenes showed no phenotype, whereas a resistant phenotype was observed in T-2 toxin-treated mkd1 mutant compared with wild type. In addition, the expression of MKD1 was induced by application of T-2 toxin. In gel kinase assays showed that the activation of MPK3 and MPK6 by T-2 toxin in mkd1 mutant was decreased compared with wild type.
Subject 2; The Secreted Antifungal Protein Thionin
Thionin 2.4 (Thi2.4) protein was localized in cell walls and membranes in both Arabidopsis and Thi2.4-treated F. graminearum by indirect immunofluorescence technique using the anti-Thi2.4 antibody. Thi2.4 protein acted as an antifungal peptide to F. graminearum. On the other hand, we purified Thi2.4 protein and GST-Thi2.4 expressed in E. coli, and these proteins were coupled to NHS-activated column. The total protein from F. graminearum was applied into GST-Thi2.4 or Thi2.4-binding columns. The Fungal Fruit Body Lectin (FFBL) in F. graminearum was identified as a Thi2.4-interacting protein. Interaction between Thi2.4 and FFBL was confirmed by yeast two-hybrid analysis. However, the biological function of FFBL was largely unknown. When FFBL was infiltrated into Arabidopsis leaves, cell deaths were observed in leaves. Application of FFBL significantly enhanced the virulence of necrotrophic pathogen, F. graminearum. On the other hand, the FFBL-induced host cell deaths were effectively suppressed in Thi2.4-overexpressing plants.
