Host Status of False Brome Grass to the Leaf Rust Fungus Puccinia brachypodii and the Stripe Rust Fungus P. striiformis

Barbieri, M., Marcel, T. C., and Niks, R. E. 2011. Host status of false brome grass to the leaf rust fungus Puccinia brachypodii and the stripe rust fungus P. striiformis. Plant Dis. 95:1339-1345. Purple false brome grass (Brachypodium distachyon) has recently emerged as a model system for temperate grasses and is also a potential model plant to investigate plant interactions with economically important pathogens such as rust fungi. We determined the host status of five Brachypodium species to three isolates of Puccinia brachypodii, the prevalent rust species on Brachypodium sylvaticum in nature, and to one isolate each of three formae speciales of the stripe rust fungus P. striiformis. Two P. striiformis isolates produced sporulating lesions, both in only one of the tested interactions, suggesting a marginal host status of B. distachyon. P. brachypodii formed sporulating uredinia on the five Brachypodium species tested, and a range of reactions was observed. Surprisingly, the B. sylvaticum–derived rust isolates were more frequently pathogenic to B. distachyon than to their original host species. The B. distachyon diploid inbred lines, developed and distributed as reference material to the Brachypodium research community, include susceptible and resistant genotypes to at least three of the four P. brachypodii isolates tested. This creates the opportunity to use B. distachyon/P. brachypodii as a model pathosystem. In one B. distachyon accession, heavy infection by the loose smut fungus Ustilago bromivora occurred. That pathogen could also serve as a model pathogen of Brachypodium. False brome grass (Brachypodium P. Beauv.) is a genus of temperate wild grasses belonging to the Brachypodieae tribe, which is evolutionarily related to the Triticeae, Avenae, Bromeae, and Poeae (12,25). Within this genus, purple false brome (Brachypodium distachyon (L.) Beauv.) has recently emerged as a model system for temperate grasses, including small grain cereals like wheat and barley and herbaceous energy crops like switchgrass (Panicum virgatum L.) (12). B. distachyon has a suite of characteristics qualifying it to become an important model plant: a mostly diploid nature, small plant size, self-fertility, short life-cycle, and small genome size. Besides its physical and genomic attributes, several molecular studies indicated that B. distachyon is phylogenetically closer to temperate grasses and cereals than rice, which has been considered to be the major resource for cereal genomics research up to now (5,7,8,39). Since B. distachyon was proposed as a model in 2001 (12), genomic tools and resources have been developed and are being made available to the research community (16,37): germplasm resources, inbred lines, and segregating populations (15,16,38,41), transformation protocols (29,34,38,40), a collection of T-DNA lines (33), a virus-induced gene-silencing protocol (11), BAC libraries (20–22), a BAC-based physical map (14,19) integrated with an SSR-based genetic linkage map (17), and the genome sequence of the diploid inbred line Bd21 (23). B. distachyon is a suitable model plant to investigate the molecular basis of plant–pathogen interactions because it has been reported to be a host for a range of commercially relevant fungal pathogens of cereals and grasses (16). An early characterization of a Brachypodium–pathogen interaction involves the fungus Magnaporthe grisea, the causal agent of rice blast disease (12,31). Screening of 21 B. distachyon accessions with M. grisea isolates indicated different degrees of susceptibility and resistance (31). B. distachyon accessions were also challenged with cereal rust species (12). Isolates of Puccinia hordei Otth and P. triticina Erikss. (barley and wheat leaf rust fungi, respectively) elicited brown flecking symptoms on some of those accessions, but no occurrence of uredinia was observed. In contrast, when challenged with isolates of P. striiformis Westend (stripe rust), the plants displayed a range of responses, from brown flecking to uredinial formation in necrotic areas (12). Wilson and Henderson (42) mentioned unconfirmed reports of stripe rust occurring on B. sylvaticum in Britain. They presumed that such reports might be erroneous because of possible confusion with P. brachypodii, which also produces uredinia in lines. A large number of Puccinia species have been reported to produce uredinia on at least one Brachypodium species. In addition to stripe rust, the list includes P. graminis, P. agropyricola, P. coronata, P. recondita, P. corteziana, P. brachypodii, and P. brachypodii-phoenocoides (10,13). Reports on the formation of uredinia by these cereal and grass rusts were predominantly based on greenhouse/climate room experiments (12,13), and have not been supported by reports of the occurrence of those rust species on Brachypodium in nature. In wild populations of Brachypodium, the prevalent rust species is Puccinia brachypodii Otth (10,32,42). In Corresponding author: Rients E. Niks, E-mail: [email protected] *The e-Xtra logo stands for “electronic extra” and indicates that two supplemental tables are available in the online edition. Accepted for publication 9 June 2011. doi:10.1094 / PDIS-11-10-0825 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2011. e-Xtra*