Systemic and local modulation of plant responses by Piriformospora indica and related Sebacinales species.

Piriformospora indica is a fungus of the order Sebacinales (Basidiomycota) infesting roots of mono- and dicotyledonous plants. Endophytic fungal colonization leads to enhanced plant growth while host cell death is required for proliferation in differentiated root tissue to form a mutualistic interaction. Colonization of barley roots by P. indica and related Sebacina vermifera strains also leads to systemic resistance against the leaf pathogenic fungus Blumeria graminis f.sp. hordei due to a yet unknown mechanism of induced resistance. In order to elucidate plant response pathways governed by these root endophytes, we analyzed gene expression in barley plants exhibiting an established symbiosis with P. indica 3 weeks after inoculation. P. indica-colonized roots showed no induction of defence-related genes, while other genes showed a differential regulation pattern indicating a faster P. indica-dependent root development. Gene expression analysis of leaves detected only few systemically induced mRNAs. Among differentially regulated transcripts, we characterized the pathogenesis-related gene HvPr17b and the molecular chaperone HvHsp70 in more detail. HvPr17b shows similarity with TaWCI5, a wheat gene inducible by chemical resistance inducers and salicylate, and was previously proven to exhibit antifungal activity against B. graminis. HvHsp70 is the first gene found to systemically indicate root colonization with endophytic fungi of the order Sebacinales. Both genes are discussed as markers for endophytic colonization and resulting systemic responses.