Roles for mannitol and mannitol dehydrogenase in active oxygen-mediated plant defense

Reactive oxygen species (ROS) are both signal molecules and direct particip ants in plant defense against pathogens. Many fungi synthesize mannitol, a potent quencher of ROS, and there is growing evidence that at least some ph ytopathogenic fungi use mannitol to suppress ROS-mediated plant defenses. H ere we show induction of mannitol production and secretion in the phytopath ogenic fungus Alternaria alternata in the presence of host-plant extracts. Conversely, we show that the catabolic enzyme mannitol dehydrogenase is ind uced in a non-mannitol-producing plant in response to both fungal infection and specific inducers of plant defense responses. This provides a mechanis m whereby the plant can counteract fungal suppression of ROS-mediated defen ses by catabolizing mannitol of fungal origin.