|NELSON, JUSTIN - University Of Minnesota|
|ABOU-KARAM, MOHAMED - University Of Minnesota|
|SHIER, THOMAS - University Of Minnesota|
Submitted to: Toxins
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2019
Publication Date: 1/1/2020
Citation: Abbas, H.K., Bellaloui, N., Butler, A.M., Nelson, J.L., Abou-Karam, M., Shier, T.W. 2020. Phytotoxic responses of soybean (Glycine max L.) to Botryodiplodin, a toxin produced by the charcoal rot disease fungus, Macrophomina phaseolina. Toxins. 12(1):25. https://doi.org/10.3390/toxins12010025.
Interpretive Summary: Botryodiploidin ("Bot") is a previously known fungal toxin that was isolated from the mold Macrophomina phaseolina (MP) for the first time by our research group. MP causes a disease called "Charcoal Rot" in soybeans and close to 500 other commercially-important plant hosts including corn, fruits, fir trees, and many decorative plants. It was reported many years ago that the mold MP produces a toxin called phaseolinone that is involved in its virulence. However, in our research we found that MP isolated from Mississippi soybean plants with charcoal rot disease does not produce phaseolinone, but does produce botryodiploidin. Purified botryodiploidin showed dramatic phytotoxic effects in the bioassays used in this study including soybean leaves and roots and whole duckweed plants. It showed that Bot affected the meristematic tissues and then opened the way for this fungus to penetrate into plant hosts. This is a novel finding that can explain important aspects of the mechanism of infection used by this fungus. This information would be very useful for the scientific community, especially those who work with this disease, and also it will also be useful for conducting a wide range of research aimed at developing disease-resistant cultivars.
Technical Abstract: Toxins have been proposed to facilitate fungal root infection by creating regions of readily-penetrated necrotic tissue when applied externally to intact roots. Isolates of the charcoal rot disease fungus, Macrophomina phaseolina, from soybean plants in Mississippi produced a phytotoxic toxin, (-)-botryodiplodin, but no detectable phaseolinone, a toxin previously proposed to play a role in the root infection mechanism. This study was undertaken to determine if (-)-botryodiplodin induces toxic responses of the types that could facilitate root infection. (±)-Botryodiplodin prepared by chemical synthesis caused phytotoxic effects identical to those observed with (-)-botryodiplodin preparations from M. phaseolina culture filtrates, consistent with fungus-induced phytotoxicity being due to (-)-botryodiplodin, not phaseolinone or other unknown impurities. Soybean leaf disc cultures of Saline cultivar were more susceptible to (±)-botryodiplodin phytotoxicity than were cultures of two charcoal rot-resistant genotypes, DS97-84-1 and DT97-4290. (±)-Botryodiplodin caused similar phytotoxicity in actively growing duckweed (Lemna pausicostata) plantlet cultures, but at much lower concentrations. In soybean seedlings growing in hydroponic culture, (±)-botryodiplodin added to culture medium inhibited lateral and tap root growth, and caused loss of root caps and normal root tip cellular structure. Thus, botryodiplodin applied externally to undisturbed soybean roots induced phytotoxic responses of types expected to facilitate fungal root infection.