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Zusammenfassung:
The mutualistic root endophyte P. indica colonizes the roots of a large spectrum of mono- and dicotyledonous host plants, including the non-mycorrhizal plant A. thaliana and the crop plant H. vulgare (barley). The fungus confers a variety of beneficial effects to its host including improved resistance to biotic and abiotic stresses and increased biomass production and seed yield. P. indica colonization of barley can be divided into two distinct phases: Between 3 and 7 days post inoculation (dpi) the fungus penetrates the cell wall of epidermal and root cortex cells and establishes a biotrophic interaction, growing inter- and intracellulary. Past 10 dpi the fungus colonizes and digests dead root cortex cells using its saprophytic traits. Genome data revealed an expanded family of genes encoding a novel family of plant responsive secreted putative effectors, sharing a regular distribution of histidine and alanine residues and a conserved C-terminal 7 amino acid RSIDELD motif. One member of the DELD family proteins, Dld1, is highly upregulated during biotrophic interaction with barley. The protein shows high affinity to several divalent metal ions and binds to iron(III) in a pH dependent manner. A Yeast-two hybrid screen with a cDNA library from A. thaliana colonized by P. indica, indicated interactions between Dld1 and a P. indica membrane transporter and several plant derived metal associated proteins, including a class III peroxidase. Iron(III) specific staining revealed that plant cell wall appositions (CWAs) formed during cell wall penetration attempts by pathogenic fungi contain a large amount of iron(III) (Llu et al. 2007). We show that the amount of iron(III) in barley CWAs, which formation coincides temporally with the expression pattern of PiDLD1, is lower after penetration by P. indica. We therefore suggest Dld1 is produced and secreted upon penetration in the apoplast where it sequesters iron(III) from CWAs inhibiting iron catalyzed reactions to fortify the plant cells wall.
