Abstract
Spores of pathogenic fungi are virtually ubiquitous and cause human disease and severe losses in crops. The endophytic fungi Alternaria species produce host-selective phytotoxins. Alt a 1 is a strongly allergenic protein found in A. alternata that causes severe asthma. Despite the well-established pathogenicity of Alt a 1, the molecular mechanisms underlying its action and physiological function remain largely unknown. To gain insight into the role played by this protein in the pathogenicity of the fungus, we studied production of Alt a 1 and its activity in spores. We found that Alt a 1 accumulates inside spores and that its release with a ligand is pH-dependent, with optimum production in the 5.0–6.5 interval. The Alt a 1 ligand was identified as a methylated flavonoid that inhibits plant root growth and detoxifies reactive oxygen species. We also found that Alt a 1 changes its oligomerization state depending on the pH of the surrounding medium and that these changes facilitate the release of the ligand. Based on these results, we propose that release of Alt a 1 should be a pathogenic target in approaches used to block plant defenses and consequently to favor fungal entry into the plant.
Highlights
Spores of pathogenic fungi are virtually ubiquitous and cause human disease and severe losses in crops
Alt a 1 was located inside spores of Alternaria by immunofluorescence with specific monoclonal antibody; the most intense signal was observed in the spore cytoplasm at time 0
Alternaria species take several forms, ranging from saprophytes to endophytes to pathogens[6]. They are highly successful as fungal pathogens that cause severe losses in a wide variety of economically important crops and in Tosylactivated were coated with Alt a 1 and incubated with its ligand
Summary
Spores of pathogenic fungi are virtually ubiquitous and cause human disease and severe losses in crops. Flavonoids are well-known secondary plant metabolites involved in a variety of processes such as cell signaling, plant growth, and reproduction[10,11,12,13,14] Among this heterogeneous group of molecules, flavone derivatives with two ortho-hydroxyl groups on the B-ring (catechol) are interesting owing to their capacity to detoxify ROS12,15–17 and activate siderophores[18,19]; both features are relevant in plant defense strategies. The allergen Alt a 1 (AAM90320.1, NCBI Protein Database) is detected in the spores of A. alternata before germination[20,21] It has been described as the main allergen associated with chronic asthma[22] and was recently shown to interact with the pathogenesis-related plant defense protein PR523, the molecular mechanism of Alt a 1 in fungal pathogenesis remains unknown[24]. The monomeric crystal structure comprises a β-barrel composed of 11 β-strands that is a unique type of fold with no equivalent in the Protein Data Bank[7]
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