Abstract
Alternaria alternata tobacco pathotype, the causal organism of brown spot disease of tobacco, produced a host-selective toxin (named AT-toxin) in culture filtrates and spore-germination fluids. The toxin was purified from culture filtrates of this pathogen by a serial use of ion exchange, silicic acid and gel filtration columns, and thin layer chromatography. Purified toxin inhibited the seedling root growth of both susceptible and moderately resistant cultivars of tobacco at 0.2μg/ml, whereas the toxin induced necrosis on each leaf at 1 and 20μg/ml, respectively. The toxin selectively affected the viability of protoplasts derived from tobacco leaves, and caused an increase in the oxygen uptake by leaf tissues. Among several plants tested, only species belonging to the genus Nicotiana were sensitive to the toxin. All isolates of A. alternata that produced the toxin in spore-germination fluids caused brown spot disease of tobacco. Successful infection by nonpathogenic Alternaria on tobacco leaves was obtained when spores were inoculated with the toxin. These results indicate that AT-toxin is a host recognition factor in the genus Nicotiana-A. alternata pathosystem.
