Alt a 1 is a strongly allergenic protein present in the spores ofAlternaria alternata (a fungus widespread in the environment) before germination. Alt a 1 is responsible for infections and respiratory conditions in humans and is also considered as the major allergen associated with chronic asthma. Alternaria species also attack most agricultural crops including vegetables and fruits. The pathogenic action of Alt a 1 is linked to the production of toxic compounds, increase in the production of reactive oxygenated species (ROS) by infected plants as well as interactions with plant defense proteins inhibiting their activity. The spores of Alternaria can remain on the surface of the host plant without inducing symptoms while waiting for the right moment to germinate. When this happens, a cascade of molecular events occurs that had remained largely unexplored until now.
Our results reveal that Alt a 1 utilizes a ligand which is determinant for the infection. Our work has identified this ligand as a methylated derivative of quercetin, a flavonoid well-known for its antioxidant properties. When the spores of Alternaria germinate, the infected plant expresses pathogenesis-related proteins and produces ROS as a defense response. The spores release Alt a 1 in the form of tetramer aggregates carrying the ligand with a stability that depends on the pH of the medium. When the Alt a 1-ligand complex reaches other plant compartments, changes of pH lead to aggregate breakdown and monomers and the ligand are released. Then, the joint action of monomeric Alt a 1 inhibiting defense proteins and the ligand detoxifying ROS block plant defenses and favor fungal entry into the plant.
Tomato after 5 days of infection by Alternaria alternata. Structure of the Alt a 1-ligand complex in the tetrameric state. Ligand of Alt a 1 identified as a methylated derivative of the flavonoid quercetin.