SUMMARYFungi of the order Sebacinales (Basidiomycota) are involved in a wide spectrum of mutualistic symbioses with various plants, thereby exhibiting unique potential for biocontrol strategies. Piriformospora indica, a model organism of this fungal order, is able to increase the biomass and grain yield of crop plants, and induces local and systemic resistance to fungal diseases and tolerance to abiotic stress. To elucidate the molecular basis for root colonization, we characterized the interaction of P. indica with barley roots by combining global gene expression profiling, metabolic profiling, and genetic studies. At the metabolic level, we show that fungal colonization reduces the availability of free sugars and amino acids to the root tip. At the transcriptional level, consecutive interaction stages covering pre-penetration-associated events and progressing through to root colonization showed differential regulation of signal perception and transduction components, secondary metabolism, and genes associated with membrane transport. Moreover, we observed stage-specific up-regulation of genes involved in phytohormone metabolism, mainly encompassing gibberellin, auxin and abscisic acid, but salicylic acid-associated gene expression was suppressed. The changes in hormone homoeostasis were accompanied with a general suppression of the plant innate immune system. Further genetic studies showed reduced fungal colonization in mutants that are impaired in gibberellin synthesis as well as perception, and implicate gibberellin as a modulator of the root's basal defence. Our data further reveal the complexity of compatibility mechanisms in host-microbe interactions, and identify gibberellin signaling as potential target for successful fungi.

Colonization of barley roots with the basidiomycete fungus Piriformospora indica (Sebacinales) induces systemic resistance against the biotrophic leaf pathogen Blumeria graminis f. sp. hordei (B. graminis). To identify genes involved in this mycorrhiza-induced systemic resistance, we compared the leaf transcriptome of P. indica-colonized and noncolonized barley plants 12, 24, and 96 h after challenge with a virulent race of B. graminis. The leaf pathogen induced specific gene sets (e.g., LRR receptor kinases and WRKY transcription factors) at 12 h postinoculation (hpi) (prepenetration phase) and vesicle-localized gene products 24 hpi (haustorium establishment). Metabolic analysis revealed a progressing shift of steady state contents of the intermediates glucose-1-phosphate, uridinediphosphate-glucose, and phosphoenolpyruvate 24 and 96 hpi, indicating that B. graminis shifts central carbohydrate metabolism in favor of sucrose biosynthesis. Both B. graminis and P. indica increased glutamine and alanine contents, whereas substrates for starch and nitrogen assimilation (adenosinediphosphate- glucose and oxoglutarate) decreased. In plants that were more B. graminis resistant due to P. indica root colonization, 22 transcripts, including those of pathogenesis-related genes and genes encoding heat-shock proteins, were differentially expressed ?twofold in leaves after B. graminis inoculation compared with non-mycorrhized plants. Detailed expression analysis revealed a faster induction after B. graminis inoculation between 8 and 16 hpi, suggesting that priming of these genes is an important mechanism of P. indica-induced systemic disease resistance.

Piriformospora indica is a mutualistic root-colonising basidiomycete that tranfers various benefits to colonized host plants including growth promotion, yield increases as well as abiotic and biotic stress tolerance. The fungus is characterized by a broad host spectrum encompassing various monocots and dicots. Our recent microarray-based studies indicate a general plant defense suppression by P. indica and significant changes in the GA biosynthesis pathway. Furthermore, barley plants impaired in GA synthesis and perception showed a significant reduction in mutualistic colonization, which was associated with an elevated expression of defense-related genes. Here, we discuss the importance of plant hormones for compatibility in plant root-P. indica associations. Our data might provide a first explanation for the colonization success of the fungus in a wide range of higher plants.

Plant profilins are important panallergens, contributing to numerous food and pollen allergies. They fulfil essential functions in all eukaryotic cells, including yeast and mammals, making breeding of profilin-deficient crop plants an impossible task. To obtain hypoallergenic profilin-variants, a novel yeast based screening system to select for fully active but hypoallergenic profilins was developed. Despite similar biological functions, plant profilins share only limited amino acid sequence identity to nonplant profilins. Thus, it should be possible to select for hypoallergenic profilin variants that have kept their biological function. These variants were subsequently tested for IgE-binding and their 3D folding was analyzed. As prerequisites, we developed a conditional profilin-deficient yeast mutant strain and made use of a newly-discovered profilin variant from tomato that enabled preselection for efficient mutagenesis. After random mutagenesis and transformation of tomato profilin (Lyc e 1) into the designed yeast strain, we were able to screen for functional tomato profilin variants. Testing these variants for IgE binding revealed that two variants showed a strong reduction in IgE binding of four Lyc e 1 sensitized patients and one birch pollen allergic patient. This result, for the first time, provides strong evidence that selection of hypoallergenic Lyc e 1 variants is possible. Furthermore, this knowledge provides the basis for the molecular breeding of hypoallergenic profilin alleles in tomato.