The Role of Mannitol and Mannitol Dehydrogenase in Plant-Pathogen Interactions
Reactive oxygen species (ROS) are known to act as both signaling molecules and direct antimicrobial agents in plant defense against pathogens. Many plant pathogens have the ability to synthesize mannitol, a potent ROS quencher, and there is growing evidence that at least some phytopathogenic fungi use mannitol to suppress ROS mediated defenses. Here we show that mannitol production and secretion in the phytopathogenic fungus, Alternaria alternata is induced in the presence of host plant extracts. Additionally we demonstrate that the catabolic enzyme mannitol dehydrogenase (MTD) is induced in a non-mannitol-producing plant in response to both fungal elicitor and specific inducers of plant defense responses. This indicates a mechanism whereby the plant can counteract fungal suppression of ROS-mediated defenses by catabolizing mannitol of fungal origin. To further clarify this interaction, tobacco plants were transformed with celery mannitol dehydrogenase cDNA under a constitutive promoter, resulting in an enzymatically active MTD protein. This constitutive MTD expression conferred enhanced resistance to A. alternata, and this resistance did not correlate with expression of PR1a, a protein often used as an indicator of systemic acquired resistance. Constitutive Mtd expression did not enhance tolerance to two non-mannitol secreting pathogens, the fungal pathogen Cercospora nicotianae and the bacterial pathogen Pseudomonas syringae pv. tabaci. These results are consistent with the hypothesis that MTD plays a role in plant resistance to mannitol secreting fungal pathogens by catabolizing mannitol of fungal origin.
Advisor:John Williamson; D. Mason Pharr; Margaret Daub; Steve Clouse
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:02/05/2001