Systemic resistance induction by Bacillus mycoides isolate Bac J : the mode of action on Beta vulgaris (sugar beet)
Bargabus, Rebecca Lynn
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Bacillus mycoides isolate Bac J, a non-pathogenic, phyllosphere-inhabiting biological control agent, reduced Cercospora leaf spot of sugar beet by 60-80% in glasshouse experiments, even when spatially separated from the causal agent, Cercospora beticola Sacc. Disease control was attributed to the ability of the bacterium to induce systemic resistance in the host, which was demonstrated through classical induced resistance challenge assays. Additionally, in glasshouse and field experiments three pathogenesis-related proteins, chitinase, beta-glucanase and peroxidase, that are accepted molecular markers of systemic induced resistance, were increased by nearly 2-fold in distal, untreated sugar beet leaves following treatment with Bacillus mycoides isolate Bac J and acibenzolar-S-methyl, a chemical inducer of systemic resistance. The increased activity in all cases was a result of the production of unique isoforms of the enzymes not found in the water treated control. The Bacillus mycoides isolate Bac J-induced systemic defense response was preceded by a biphasic oxidative burst. The hydrogen peroxide production pattern was similar in timing, but not intensity to that elicited by avirulent bacterial pathogens of sugar beet, Erwinia carotovora pv. betavasculorum isolates 1 and 6. Although normally coupled with programmed cell death, the oxidative burst elicited by Bacillus mycoides isolate Bac J was independent of the hypersensitive response. Observations made during the oxidative burst experiments provided keys for understanding the signaling in Bacillus mycoides isolate Bac J-sugar beet interactions, including signal delivery not being reliant upon stomatal conductance and sugar beet receptor location being cytosolic or plasma membrane bound. Additionally, the biochemical and oxidative changes observed in sugar beet following Bacillus mycoides isolate Bac J treatment were consistent with changes seen in other Bacilli-sugar beet interactions in which systemic resistance was induced. These chemical consistencies provided a framework with which to establish a host response-based high throughput screen for the systematic identification of novel, putative Bacilli biological control agents, the first such method of its kind.