|SHIER, W. THOMAS - University Of Minnesota|
|NELSON, JUSTIN - University Of Minnesota|
|BAIRD, RICHARD - Mississippi State University|
Submitted to: Toxicon
Publication Type: Proceedings
Publication Acceptance Date: 3/14/2012
Publication Date: 8/1/2012
Citation: Shier, W., Nelson, J., Abbas, H.K., Baird, R.E. 2012. Root toxicity of the mycotoxin botryodiplodin in soybean seedlings. Toxicon. 60:163.
Technical Abstract: The fungus Macrophomina phaseolina causes disease in more than 500 plant species. In soybean it causes charcoal rot disease, which is a major cause of soybean yield losses in Mississippi and a significant problem around the world, particularly under dry conditions. M. phaseolina infects plants from soil-borne inoculum by entering plants through the roots and spreading to other parts through the vascular system. M. phaseolina isolated from infected plants and soil in Mississippi produces the mycotoxin (-)-botryodiplodin, but not phaseolinone. (-)-Botryodiplodin is believed to be a virulence factor that M. phaseolina uses to penetrate roots during the initial infection process. The mycotoxin kills localized plant tissue, and the fungus is then able to readily enter the plant through the necrotic tissue produced by the toxin. The effects of (±)-botryodiplodin on soybean root were investigated in soybean seedlings germinated in soil and transplanted at the cotyledon stage (VC, 4-7 cm growth) to hydroponic growth in individual glass tubes in 10% Villagarcia medium. During 4 days growth at room temperature in continuous light, submergence induced abundant lateral root growth. The seedlings were transplanted to fresh medium containing a range of concentrations (0-80 µg/ml) of (±)-botryodiplodin prepared by chemical synthesis, and cultured an additional 4 days. Lateral root growth was inhibited at =4 µg/ml (±)-botryodiplodin. (±)-Botryodiplodin at 80 µg/ml caused severely stunted roots that were stained pink, the color of the pigment produced when botryodiplodin reacts with protein. Inhibition of root growth was quantified by excising roots, drying lateral and tap roots separately and weighing. (±)-Botryodiplodin inhibited tap root growth at IC50 = 23.5 µg/ml and lateral root growth at IC50 = 4.2 µg/ml. There were no apparent toxic effects to aerial parts of the plants, except secondary to root loss. The results are consistent with botryodiplodin selectively killing actively growing meristematic tissue at root tips. Production of necrotic tissue at root tips would be expected to provide the fungus with ready access to the plant vasculature system.