Fumonisin B₁ from the fungus Fusarium moniliforme causes contact toxicity in plants: Evidence from studies with biosynthetically labeled toxin

Fumonisin B₁ (FB₁) is the most abundant of a series of sphingosine analog mycotoxins produced by the fungus Fusarium moniliforme, a ubiquitous contaminant of stored corn (maize) worldwide. FB₁ exhibits a variety of biological activities including phytotoxicity, which is of particular interest for its potential role as a virulence factor to facilitate invasion of plant tissues by the fungus. Droplets of FB₁ solution applied to the leaf surface of jimsonweed, black nightshade, and susceptible tomatoes caused necrosis, growth inhibition, and death. With Arabidopsis thaliana grown on agar plates, an IC₅₀ (concentration causing half maximal phytotoxicity) of less than 1 ppm was observed. [³H]FB₁ was prepared by biosynthetic incorporation of commercially-available radiolabeled presumptive precursors-into the toxin in rice medium solid cultures of F. moniliforme JW 1. The labeled toxin produced by incorporation of [9,10-³H]palmitate induced phytotoxic symptoms identical to unlabeled material, indicating it had full biological activity. The area of necrosis on treated leaves was similar in light and dark treated plants. Using liquid scintillation counting to quantify radioactivity in excised plant parts, over 95% of the [³H]FB₁ radioactivity applied to leaves of light or dark-treated plants was recovered from the treated leaf. When [³H]FB₁ was applied to a wound site on target plants, severe damage occurred at the site of FB₁ application and in tissue above the site. These results indicate that FB₁ applied to intact surfaces of target plants exhibits primarily contact activity. Translocation of FB₁ is limited, occurring only when FB₁ is applied to a wound site, and it results in damage to tissue above the point of application, indicating that FB₁ is xylem mobile.