Alternaria brassicae Produces a Host-Specific Protein Toxin from Germinating Spores on Host Leaves

Abstract

Spore suspensions of Alternaria brassicae, the causal agent of gray leaf spot in Brassica plants, were incubated on the leaves of cabbage (B. oleracea) and spore germination fluid (SGF) was collected after 48 h. A high molecular weight (HMW) fraction (>10 kDa) was separated from the SGF by ultrafiltration. In a detached leaf assay, the HMW fraction induced visible symptoms only on host leaves and the toxicity was lost by treatment with proteinase K or heat at 60 degrees C for 15 min, indicating the presence of host-specific protein toxin(s). A protein toxin in the HMW fraction was purified by several chromatography steps. The toxin induced water-soaked symptoms followed by chlorosis at concentrations of 0.5 to 1 microg/ml on host leaves, but not on nonhost leaves even at 50 microg/ml. The toxin also had infection-inducing activity when added to spore suspension of a nonpathogenic isolate of A. alternata, causing symptoms similar to the infection of A. brassicae only on host leaves. These results indicate that a new host-specific protein toxin named ABR-toxin is released from germinating spores of A. brassicae on host leaves. ABR-toxin migrated as a protein of 27.5 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of ABR-toxin was estimated to be approximately 7.0 and 21 N-terminal amino acid residues were sequenced.

Proteinaceous necrotrophic effectors in fungal virulence

“…is a pathogen of the Brassicaceae. Evidence that this pathogen produces necrotrophic effectors was reported by Parada et al (2008). Semi-purified protein fractions were shown to contain a 27.5 kDa protein, Abr-toxin, which is able to cause necrosis on cabbage (Brassica oleracea L.) and oilseed (Brassica napus L.).…”

Alternaria Black Spot of Crucifers: Symptoms, Importance of Disease, and Perspectives of Resistance Breeding

“…During plant infection, both Alternaria fungi produce and exude phytotoxins that belong to HST clade (host-specific toxins or host-selective toxins; Parada et al 2008, Wight et al 2009). These compounds play a major role in the pathogenesis by determining the host plant spectrum, as well as the isolates' virulence and pathogenicity levels (Nishimura & Kohmoto 1983).…”

“…These compounds play a major role in the pathogenesis by determining the host plant spectrum, as well as the isolates' virulence and pathogenicity levels (Nishimura & Kohmoto 1983). To date, two HSTs have been characterized in detail: AB toxin of A. brassicicola (Otani et al 1998) and ABR of A. brassicae (Parada et al 2008). Both phytotoxins are proteins with a suggested role in evoking disease symptoms on the infected cruciferous plants.…”

“…Destruxin B was shown to be a major one responsible for inducing necrotic lesions on plant leaves and eliciting the phytoalexins brassilexin and sinalbin A (Sodelade et al 2012). Majority of researchers classify destruxin B as an HST (Sodelade et al 2012), while others have demonstrated its unspecific character and questioned its role in the initial host plant colonization (Buchwaldt & Green 1992, Parada et al 2008. While some isolates of A. brassicae produce destruxin B as their sole toxin, others are capable of production of their derivatives (homodestruxin B, desmethyldestruxin B, and destruxin B 2 ; Parada et al 2008).…”

Infection byAlternaria alternatacauses discolouration ofBackhousia myrtifoliafoliage and flowers

“…on pear caused permeability changes in host cell walls and plasma membranes (Park and Ikeda, 2008). The SGF of A. brassicicola contained a proteinaceous toxin that, when purified and applied at 0.5 -1.0 µg ml -1 , induced water-soaked symptoms followed by chlorosis on cabbage (Otani et al, 1998;Parada et al, 2008). Germinating A. alternata spores produced a toxin which caused necrotic spots surrounded by chlorotic halos in tobacco (Slavov et al, 2004).…”