The Endophytic Fungus Piriformospora Indica Reprograms Barley to Salt-Stress Tolerance, Disease Resistance, and Higher Yield
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The endophytic fungus Piriformospora indica reprograms barley to salt-stress tolerance, disease resistance, and higher yield Frank Waller*†, Beate Achatz*†‡, Helmut Baltruschat*†,Jo´ zsef Fodor§, Katja Becker¶, Marina Fischer¶, Tobias Heier*, Ralph Hu¨ ckelhoven*, Christina Neumann*, Diter von Wettsteinʈ, Philipp Franken‡, and Karl-Heinz Kogel*,** *Institute of Phytopathology and Applied Zoology, University of Giessen, D-35392 Giessen, Germany; ‡Institute for Vegetables and Ornamental Crops, D-14979 Grossbeeren, Germany; §Plant Protection Institute, Hungarian Academy of Sciences, H-1525 Budapest, Hungary; ¶Institute of Nutritional Biochemistry, University of Giessen, D-35392 Giessen, Germany; and ʈDepartment of Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6420 Contributed by Diter von Wettstein, May 31, 2005 Disease resistance strategies are powerful approaches to sustain- nuclear DNA sequences from the D1͞D2 region of the large able agriculture because they reduce chemical input into the ribosomal subunit (12). In contrast to arbuscular mycorrhiza environment. Recently, Piriformospora indica, a plant-root-coloniz- fungi, the fungus can be easily cultivated in axenic cultures, ing basidiomycete fungus, has been discovered in the Indian Thar where it asexually forms chlamydospores containing 8–25 nuclei desert and was shown to provide strong growth-promoting activ- (10). The fungus associates with roots of various plant species, ity during its symbiosis with a broad spectrum of plants [Verma, S. where it promotes plant growth. Hosts include the cereal crops et al. (1998) Mycologia 90, 896–903]. Here, we report on the rice, wheat, and barley as well as many Dicotyledoneae, including potential of P. indica to induce resistance to fungal diseases and Arabidopsis (13, 14). Interaction of the endophytic fungus with tolerance to salt stress in the monocotyledonous plant barley. The Arabidopsis roots is accompanied by a considerable requisition beneficial effect on the defense status is detected in distal leaves, of nitrogen from the environment (14). In the interaction with demonstrating a systemic induction of resistance by a root-endo- Arabidopsis and tobacco, the fungus stimulates nitrate reduction phytic fungus. The systemically altered ‘‘defense readiness’’ is (15), in contrast to the activity of arbuscular mycorrhiza fungi. associated with an elevated antioxidative capacity due to an We report here on the enormous agronomical potential of the activation of the glutathione–ascorbate cycle and results in an fungus. First, and most importantly, the growth-promoting ac- overall increase in grain yield. Because P. indica can be easily tivity of the fungus resulted in enhanced barley grain yield. propagated in the absence of a host plant, we conclude that the Second, P. indica amended tolerance to mild salt stress, and fungus could be exploited to increase disease resistance and yield third, P. indica conferred resistance in barley against root and in crop plants. leaf pathogens, including the necrotrophic fungus Fusarium culmorum (root rot) and the biotrophic fungus Blumeria grami- root endophyte ͉ powdery mildew ͉ symbiosis ͉ ascorbate ͉ glutathione nis. Thus, interaction of barley with P. indica constitutes a model system for systemic disease resistance in cereals. espite a worldwide intensification of agriculture and tre- Dmendous progress toward increasing yields in major crops Materials and Methods over the last decades, the goal to reduce the problems associated Plant and Fungal Material, Yield Experiments. Barley was grown in with hunger is far from being reached (1). Major causes for crop a 2:1 mixture of expanded clay (Seramis, Masterfoods, Verden, losses are abiotic and biotic stresses due to unfavorable climate Germany) and Oil-Dri (Damolin, Mettmann, Germany) in a and plant diseases and pests. Increased plant productivity, growth chamber at 22°C͞18°C day͞night cycle, 60% relative therefore, relies on a high chemical input and is achieved at the humidity, and a photoperiod of 16 h (240 mol⅐mϪ2⅐sϪ1 photon expense of detrimental effects on the environment (2, 3). flux density) and fertilized weekly with 20 ml of a 0.1% Wuxal Abiotic-stress tolerance can be evoked in crops by the exploi- top N solution (Schering, N͞P͞K: 12͞4͞6). Hydroponic cultures tation of worldwide abundant endophytic arbuscular mycorrhiza contained expanded clay (Seramis, Masterfoods) as substrate. fungi, which live in reciprocally beneficial relationships with For inoculation with P. indica,2gofmycelium were added to Ϸ80% of land plants (4). However, mycorrhizal plants, albeit 300 g of substrate before sowing. P. indica was propagated in effective against many root diseases (5, 6), often show enhanced liquid Aspergillus minimal medium (14). For yield evaluations, susceptibility to biotrophic leaf pathogens (7, 8). On the other barley was sown in soil containing P. indica mycelium (4 g in hand, ascomycete endophytes have been frequently reported to 300 g of substrate) and grown for 4 weeks in the growth chamber. protect against plant pathogens and pests. Grasses (Poaceae) and Before transplantation to outdoor conditions, root samples were fungi of the family Clavicipitaceae have a long history of asso- checked for P. indica infestation. In the beginning of April 2004, ciations, ranging from mutualism to antagonism (9). These fungi when plants reached growth stage (GS) 30 (16), they were are strictly confined to upper parts of the plant, grow only transplanted into 6-liter Mitscherlich pots (Stoma, Siegburg, intercellularly, and exert a rather narrow host range. A critical Germany) (six plantlets per pot) and filled with a mixture of a review of the literature suggests that the beneficial action of loam (loess) soil and sand (1:2). The preceding crop grown in the these endophytes is based on direct antimicrobial and insecti- soil was potato. Soil nutrient additives were 0.25 g of N, 0.4 g of cidal activity due to alkaloid production. P, 1.6 g of K, and 0.2 g of Mg; N was applied a second time at We used the cereal model plant barley (Hordeum vulgare L.) to test whether growth-promoting activity of the recently dis- covered root-endophytic fungus Piriformospora indica (10) as- Abbreviations: GR, glutathione reductase; GSH, reduced glutathione. sociates with agronomically desirable traits. Discovered in the †F.W., B.A., and H.B. contributed equally to this work. Indian Thar desert in 1997 (11), P. indica has been recently **To whom correspondence should be addressed. E-mail: [email protected] related to the Sebacinales [ordo nov.] (form genus Rhizoctonia; giessen.de. Hymenomycetes, Basidiomycota) on the basis of an alignment of © 2005 by The National Academy of Sciences of the USA 13386–13391 ͉ PNAS ͉ September 20, 2005 ͉ vol. 102 ͉ no. 38 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0504423102 Downloaded by guest on September 26, 2021 a rate of 0.25 g per pot, 2 weeks after planting (GS 32). The fungicide Opus Top (250 g⅐literϪ1 Fenpropimorph and 84 g⅐literϪ1 Epoxiconazole; BASF, Ludwigshafen, Germany) was sprayed at a rate of 1.5 liter⅐hectare (ha)Ϫ1 to control powdery mildew and Rhynchosporium secalis. Both systemic fungicides are transported acropetally via xylem and do not reach roots. Aphids and the cereal beetle Oulema spp. were controlled during anthesis by using the insecticide Karate (100 g⅐literϪ1 lambda- Cyhalothrin, Syngenta Agro, Basel) at a rate of 150 ml⅐haϪ1. The presence of P. indica was monitored microscopically throughout the vegetation period. Analysis of powdery mildew infections was done in a de- tached-leaf-segment assay on agar plates containing 0.4% benz- imidazole to inhibit leaf senescence. Plants were inoculated with 15 conidia⅐mmϪ2 (for macroscopic evaluation) or with 25 conidia⅐mmϪ2 (for microscopy) of B. graminis f.sp. hordei, race A6. For gene expression studies, leaves were inoculated with 80 conidia⅐mmϪ2. Colonies were counted at 7 days after inocula- tion. Microscopic inspection of powdery-mildew-infected leaves was done by determining the frequency of the three different interaction types. Cells showing a hypersensitive response were detected by their whole-cell autofluorescence. Successful pene- tration was ascertained by the detection of haustoria formation or the development of elongated secondary hyphae (17). ‘‘Non- penetrated cells’’ are those in which fungal penetration attempts were unsuccessful. For root inoculation with pathogens, oat kernels colonized by F. culmorum strain KF 350 or Cochliobolus sativus were used. Kernels (1 g) were added to 300 g of substrate before sowing. The control pots were amended with1gofautoclaved inoculum. For inoculum production, kernels were autoclaved (125°C, 25 min), inoculated with conidia of F. culmorum or C. sativus, and incubated for 1 week at room temperature. Biochemical Measurements. Ascorbate was determined by using SCIENCES the bipyridyl method (18). Dehydroascorbate reductase activity AGRICULTURAL was assayed spectrophotometrically at 265 nm as reduced glu- tathione (GSH)-dependent dehydroascorbate oxidation (19). The assay mixture contained 50 mM sodium phosphate buffer (pH 6.5), 0.1 mM Na2EDTA, 20 M dehydroascorbate, 50 M GSH, and 20- to 100-l extracts in a total volume of 2.3 ml. Fig. 1. Colonization pattern of P. indica in barley roots. (a) Fungal hyphae Glutathione concentrations ([GSH] and [oxidized glutathione]) enter roots via root hairs from 10-day-old plants. The fungus forms pear- were measured as described in ref. 20 with slight modifications. shaped chlamydospores within root hairs and proceeds into rhizodermis cells. (b) The fungus grows into the root cortex tissue. (c) Longitudinal section. The Briefly, 0.3 g of plant tissue was mixed with 3 ml of 2% fungus was not detected in the central part of the roots beyond the endoder- sulfosalicylic acid containing 0.15 g of ascorbic acid and 1 mM mis. Fungal structures were visualized by 0.01% acid fuchsin-lactic acid (28) Na-EDTA per 100 ml. The sample was centrifuged (10,000 ϫ g (red in a and c), or they were stained for mitochondrial respiratory activity by at 4°C for 10 min), and the supernatant was either used for the succinate dehydrogenase assay (29) (black in b).