Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

• Published in: Planta 2009-11, Vol.230 (6), p.1093-1103
• Main Authors: Cheng, Fang-yi, Zamski, Eli, Guo, Wei-wen, Pharr, D. Mason, Williamson, John D.
• Format: Article
• Language: English
• Subjects: Agriculture; Antifungal Agents - pharmacology; Biological and medical sciences; Biomedical and Life Sciences; Brefeldin A - pharmacology; Celery; Cell culture techniques; Cell Wall - drug effects; Cell Wall - metabolism; Cells, Cultured; Cytoplasm - metabolism; Dehydrogenase; Dehydrogenases; Ecology; Enzymes; Forestry; Fundamental and applied biological sciences. Psychology; Fungi - growth & development; Fungi - metabolism; Golgi Apparatus - drug effects; Golgi Apparatus - metabolism; Golgi Apparatus - ultrastructure; Immunity, Innate - genetics; Immunoblotting; Leaves; Life Sciences; Mannitol - metabolism; Mannitol Dehydrogenases - genetics; Mannitol Dehydrogenases - metabolism; Microscopy, Immunoelectron; Nicotiana - cytology; Nicotiana - genetics; Nicotiana - metabolism; Original Article; Pathogens; Phytopathology. Animal pests. Plant and forest protection; Plant cells; Plant Diseases - genetics; Plant Diseases - microbiology; Plant Leaves - genetics; Plant Leaves - metabolism; Plant Leaves - ultrastructure; Plant Proteins - genetics; Plant Proteins - metabolism; Plant Sciences; Plants; Plants, Genetically Modified; Protein Transport - drug effects; Proteins; Salicylic Acid - pharmacology; Secretion; Sugar
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s00425-009-1006-3

The sugar alcohol mannitol is an important carbohydrate with well-documented roles in both metabolism and osmoprotection in many plants and fungi. In addition to these traditionally recognized roles, mannitol is reported to be an antioxidant and as such may play a role in host—pathogen interactions. Current research suggests that pathogenic fungi can secrete mannitol into the apoplast to suppress reactive oxygen-mediated host defenses. Immunoelectron microscopy, immunoblot, and biochemical data reported here show that the normally symplastic plant enzyme, mannitol dehydrogenase (MTD), is secreted into the apoplast after treatment with the endogenous inducer of plant defense responses salicylic acid (SA). In contrast, a cytoplasmic marker protein, hexokinase, remained cytoplasmic after SA-treatment. Secreted MTD retained activity after export to the apoplast. Given that MTD converts mannitol to the sugar mannose, MTD secretion may be an important component of plant defense against mannitolsecreting fungal pathogens such as Alternaria. After SA treatment, MTD was not detected in the Golgi apparatus, and its SA-induced secretion was resistant to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of a known extracellular targeting sequence on the MTD protein, these data suggest that a plant's response to pathogen challenge may include secretion of selected defensive proteins by as yet uncharacterized, non-Golgi mechanisms.