Location: Forage Seed and Cereal ResearchTitle: Infection of Brachypodium distachyon by formae speciales of Puccinia graminis: early infection events and host-pathogen incompatibility ) Author
Submitted to: PLoS One
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/15/2013
Publication Date: 2/18/2013
Citation: Figueroa, M., Alderman, S.C., Garvin, D.F., Pfender, W.F. 2013. Infection of Brachypodium distachyon by formae speciales of Puccinia graminis: early infection events and host-pathogen incompatibility. PLoS One. 8(2):e56857. Interpretive Summary: Research to understand the genetics of host resistance to stem rust disease of cereal grains and forage grasses is difficult because of the large size and complexity of the host plant's total genetic material. Brachypodium, a grass plant with a much simpler genetic makeup, is a recently-developed research plant whose entire genome has been recorded. We conducted a detailed characterization of the very early events, and subsequent disease development, of three different strains of the stem rust when inoculated onto Brachypodium. We pinpointed the time window during which critical genetic and molecular events occur that results either in virtually complete resistance to the pathogen strain that causes wheat stem rust, or allows disease development by two other pathogen strains. Our results provide a framework for follow-on research, by us and others, about the fundamental biology and genetics of stem rust resistance in grasses such as wheat and forage grasses.
Technical Abstract: Brachypodium distachyon is an emerging model to study fungal disease resistance in cereals and grasses. We characterized the stem rust-Brachypodium pathosystem to evaluate its potential for investigating molecular and genetic aspects of resistance to P. graminis, the pathogen that causes stem rust. Some important formae speciales of P. graminis and their typical hosts are P. graminis f.sp. tritici (Pg-tr) in wheat, P. graminis f.sp. lolii (Pg-lo) in perennial ryegrass, and P. graminis f.sp. phlei-pratensis (Pg-pp) in timothy grass. Eight Brachypodium inbred lines were challenged with Pg-tr, Pg-lo and Pg-pp and the resulting symptom development demonstrated significant variation in stem rust resistance among isolate/Brachypodium combinations. Histological analysis of early infection events in one Brachypodium line (Bd1-1) indicated that Pg-lo and Pg-pp are markedly more efficient than Pg-tr at establishing a biotrophic interaction. Complete formation of appressoria (60-70%) occurs by 12 hours post-inoculation (hpi) under dark and wet conditions; and after 4 h of light exposure, there is differentiation of fungal penetration structures. The infection by Pg-tr usually stops at appressorial formation, and by 68 hpi, only 0.3% and 0.7% of the Pg-tr urediniospores form haustoria and colonies, respectively. In contrast, Pg-lo and Pg-pp develop advanced infection structures. By 68 hpi, the percentage of urediniospores with a haustorium mother cell or a haustorium in Pg-lo and Pg-pp reaches 8% and 5%, respectively; while the formation of colonies reaches 14% and 13%, respectively. We conclude that Brachypodium is an apt grass model to study the molecular and genetic components of resistance to stem rust.