In-vitro selection for Fusarium-resistance in Gladiolus

The idea of using a toxin produced by a pathogen as a tool for resistance breeding dates from over 20 years ago. Several examples of germplasm screened for disease resistance using a toxin as discriminating agent are published. Such toxins may also successfully be applied in in vitro selection experiments. Before even attempting to exploit such compounds for breeding purposes, their role in plant pathogenesis has to be assessed. The current research is dealing with one of those toxins, fusaric acid. We searched for evidence that fusaric acid, a toxin produced by the pathogen Fusarium oxysporum f.sp. gladioli, the causal agent of gladiolus corm-rot, is involved in the rotting process of the corm. Although the fungus produces fusaric acid abundantly in vitro, a strong interaction between isolate and medium was found. Therefore the fusaric acid production was studied in planta. The toxin was detected in diseased corm tissue in amounts high enough to exert toxic effects on corm tissue. Subsequently, fusaric acid was applied in several bioassays. Fusarium-resistant cultivars were able to tolerate higher concentrations of the toxin than more susceptible ones. Fusaric acid induced part of the disease symptoms on shoots cultured in vitro. Callus tissue, however, was less suitable for such assays. Thirteen isolates of the fungus were tested for aggressiveness in a greenhouse and an in vitro assay. Results from both tests correlated strongly. Also the amount of ergosterol and fusaric acid were determined. Ergosterol reflects fungal growth and was strongly correlated with the visually observed symptoms. The amount of fusaric acid produced per unit fungus was not correlated with the visual symptoms. Therefore the hypothesis that the aggressiveness of isolates is due to their potency to produce fusaric acid was not affirmed. Since, however, the production and degradation of fusaric acid is a dynamic process, the determination of the amount of fusaric acid at a specific time location may be erratic. The second part of the research concerned an attempt to select toxin-tolerant plants from an in vitro experiment. A cell suspension was challenged stepwise with increasing concentrations of fusaric acid (0.12 mM and 0.4 mM). Nine cell lines selected on 0.12 mM fusaric acid showed variable reactions when inoculated directly with conidia of the fungus. The growth of the fungus was reduced at least by 50 percent compared to that on non-selected callus. About 50 percent of the plantlets regenerated from selected calli showed increased tolerance for the toxin.