Summary: Fungal cell walls, which are essential for environmental adaptation and host colonization by the fungus, have been evolutionarily selected by plants and animals as a source of microbe‐associated molecular patterns (MAMPs) that, upon recognition by host pattern recognition receptors (PRRs), trigger immune responses conferring disease resistance. Chito‐oligosaccharides [β‐1,4‐N‐acetylglucosamine oligomers, (GlcNAc)_n] are the only glycosidic structures from fungal walls that have been well‐demonstrated to function as MAMPs in plants. Perception of (GlcNAc)_4–8 by Arabidopsis involves CERK1, LYK4 and LYK5, three of the eight members of the LysM PRR family. We found that a glucan‐enriched wall fraction from the pathogenic fungus Plectosphaerella cucumerina which was devoid of GlcNAc activated immune responses in Arabidopsis wild‐type plants but not in the cerk1 mutant. Using this differential response, we identified the non‐branched 1,3‐β‐d‐(Glc) hexasaccharide as a major fungal MAMP. Recognition of 1,3‐β‐d‐(Glc)₆ was impaired in cerk1 but not in mutants defective in either each of the LysM PRR family members or in the PRR‐co‐receptor BAK1. Transcriptomic analyses of Arabidopsis plants treated with 1,3‐β‐d‐(Glc)₆ further demonstrated that this fungal MAMP triggers the expression of immunity‐associated genes. In silico docking analyses with molecular mechanics and solvation energy calculations corroborated that CERK1 can bind 1,3‐β‐d‐(Glc)₆ at effective concentrations similar to those of (GlcNAc)₄. These data support that plants, like animals, have selected as MAMPs the linear 1,3‐β‐d‐glucans present in the walls of fungi and oomycetes. Our data also suggest that CERK1 functions as an immune co‐receptor for linear 1,3‐β‐d‐glucans in a similar way to its proposed function in the recognition of fungal chito‐oligosaccharides and bacterial peptidoglycan MAMPs.

Plant non-specific lipid transfer proteins (nsLTPs) are usually defined as small, basic proteins, with a wide distribution in all orders of higher plants. Structurally, nsLTPs contain a conserved motif of eight cysteines, linked by four disulphide bonds, and a hydrophobic cavity in which the ligand is housed. This structure confers stability and enhances the ability to bind and transport a variety of hydrophobic molecules. Their highly conserved structural resemblance but low sequence identity reflects the wide variety of ligands they can carry, as well as the broad biological functions to which they are linked to, such as membrane stabilization, cell wall organization and signal transduction. In addition, they have also been described as essential in resistance to biotic and abiotic stresses, plant growth and development, seed development, and germination. Hence, there is growing interest in this family of proteins for their critical roles in plant development and for the many unresolved questions that need to be clarified, regarding their subcellular localization, transfer capacity, expression profile, biological function, and evolution.

Alternaria alternata is a saprophytic mold whose spores are disseminated in warm dry air, the typical weather of the Mediterranean climate region (from 30° to 45°), with a peak during the late summer and early autumn. Alternaria spores are known to be biological contaminants and a potent source of aeroallergens. One consequence of human exposure to Alternaria is an increased risk of developing asthma, with Alt a 1 as its main elicitor and a marker of primary sensitization. Although the action mechanism needs further investigation, a key role of the epithelium in cytokine production, TLR-activated alveolar macrophages and innate lymphoid cells in the adaptive response was demonstrated. Furthermore, sensitization to A. alternata seems to be a trigger for the development of co-sensitization to other allergen sources and may act as an exacerbator of symptoms and an elicitor of food allergies. The prevalence of A. alternata allergy is increasing and has led to expanding research on the role of this fungal species in the induction of IgE-mediated respiratory diseases. Indeed, recent research has allowed new perspectives to be considered in the assessment of exposure and diagnosis of fungi-induced allergies, although more studies are needed for the standardization of immunotherapy formulations.

Summary: Background: Since intestinal absorption of food protein can trigger an allergic reaction, the effect of plant food allergen on intestinal epithelial cell permeability and its ability to cross the epithelial monolayer was evaluated.Objective: To study the interaction of Pru p 3 with intestinal epithelium, its natural entrance, analyzing transport kinetics and cellular responses that trigger.Methods: This was achieved using Pru p 3, the peach LTP, as a model. Enterocytic monolayers were established by culturing Caco 2 cells, as a model of enterocytes, on permeable supports that separate the apical and basal compartments. Pru p 3 was added to the apical compartment, the transepithelial resistance (TEER) was measured, and the transport was quantified.Results: The peach allergen that crossed the cell monolayer was detected in the cell fraction and in the basal medium by immunodetection with specific antibodies and the quantity was measured by ELISA assay. Pru p 3 was able to cross the monolayer without disturbing the integrity of the tight junctions. This transport was significantly higher than that of a non‐allergenic peach LTP, LTP1, and occurred via lipid raft pathway. The incubation of Caco 2 cells with Pru p 3 and LTP1 produced the expression of epithelial‐specific cytokines TSLP, IL33 and IL25.Conclusion: These results suggest that Pru p 3 was able to cross the cell monolayer by the transcellular route and then induce the production of Th2 cytokines. The results of the present study represent a step towards clarifying the importance of Pru p 3 as a sensitizer.Clinical relevance: The capacity of food allergens to cross the intestinal monolayer could explain their high allergenic capacity and its fast diffusion through the body associating to severe symptoms.

Alt a 1 is a protein found in Alternaria alternata spores related to virulence and pathogenicity and considered to be responsible for chronic asthma in children. We found that spores of Alternaria inoculated on the outer surface of kiwifruits did not develop hyphae. Nevertheless, the expression of Alt a 1 gene was upregulated, and the protein was detected in the pulp where it co‐localized with kiwi PR5. Pull‐down assays demonstrated experimentally that the two proteins interact in such a way that Alt a 1 inhibits the enzymatic activity of PR5. These results are relevant not only for plant defense, but also for human health as patients with chronic asthma could suffer from an allergic reaction when they eat fruit contaminated with Alternaria.