Infection of Brachypodium distachyon by formae speciales of Puccinia graminis: early infection events and host-pathogen incompatibility.

Melania Figueroa,David F Garvin,William F Pfender,Stephen Alderman

doi:10.1371/journal.pone.0056857

Abstract

Puccinia graminis causes stem rust, a serious disease of cereals and forage grasses. Important formae speciales of P. graminis and their typical hosts are P. graminis f. sp. tritici (Pg-tr) in wheat and barley, P. graminis f. sp. lolii (Pg-lo) in perennial ryegrass and tall fescue, and P. graminis f. sp. phlei-pratensis (Pg-pp) in timothy grass. Brachypodium distachyon is an emerging genetic model to study fungal disease resistance in cereals and temperate grasses. We characterized the P. graminis-Brachypodium pathosystem to evaluate its potential for investigating incompatibility and non-host resistance to P. graminis. Inoculation of eight Brachypodium inbred lines with Pg-tr, Pg-lo or Pg-pp resulted in sporulating lesions later accompanied by necrosis. Histological analysis of early infection events in one Brachypodium inbred line (Bd1-1) indicated that Pg-lo and Pg-pp were markedly more efficient than Pg-tr at establishing a biotrophic interaction. Formation of appressoria was completed (60–70% of germinated spores) by 12 h post-inoculation (hpi) under dark and wet conditions, and after 4 h of subsequent light exposure fungal penetration structures (penetration peg, substomatal vesicle and primary infection hyphae) had developed. Brachypodium Bd1-1 exhibited pre-haustorial resistance to Pg-tr, i.e. infection usually stopped at appressorial formation. By 68 hpi, only 0.3% and 0.7% of the Pg-tr urediniospores developed haustoria and colonies, respectively. In contrast, development of advanced infection structures by Pg-lo and Pg-pp was significantly more common; however, Brachypodium displayed post-haustorial resistance to these isolates. By 68 hpi the percentage of urediniospores that only develop a haustorium mother cell or haustorium in Pg-lo and Pg-pp reached 8% and 5%, respectively. The formation of colonies reached 14% and 13%, respectively. We conclude that Brachypodium is an apt grass model to study the molecular and genetic components of incompatiblity and non-host resistance to P. graminis.

Highlights

Puccinia graminis Pers.: Pers., causal agent of stem rust, is an obligate biotroph that belongs to the rust fungi (Pucciniales, Basidiomycota) [1], a group that includes some of the most diverse and economically important fungal pathogens of crops [2]
This paper uses the formae speciales designation, but we note that P. graminis f. sp. tritici, the causal agent of wheat stem rust, is included in Puccinia graminis subsp. graminis, whereas P. graminis f. sp. lolii and P. graminis f. sp. phlei-pratensis are included in P. graminis subsp. graminicola [3,7,9,10]
Variation for stem rust resistance in Brachypodium We examined eight Brachypodium inbred lines for their response to P. graminis f. sp. tritici (Pg-tr), P. graminis f. sp. lolii (Pg-lo) or P. graminis f. sp. phlei-pratensis (Pg-pp)

Summary

Introduction

Puccinia graminis Pers.: Pers., causal agent of stem rust, is an obligate biotroph that belongs to the rust fungi (Pucciniales, Basidiomycota) [1], a group that includes some of the most diverse and economically important fungal pathogens of crops [2]. The variability in host range and morphology among members of the species P. graminis has created a challenge in establishing a consistent taxonomic nomenclature for subspecific populations [3,4,5]. One system, based on spore morphology, divides the species into two taxonomic subspecies (subsp.), graminis and graminicola [3,6]. Graminis infects primarily cereal crops and closely related genera, whereas P. graminis subsp. The other system classifies P. graminis according to host range, and separates the species into different formae speciales Tritici, the causal agent of wheat stem rust, is included in Puccinia graminis subsp. This paper uses the formae speciales designation, but we note that P. graminis f. sp. tritici, the causal agent of wheat stem rust, is included in Puccinia graminis subsp. graminis, whereas P. graminis f. sp. lolii and P. graminis f. sp. phlei-pratensis are included in P. graminis subsp. graminicola [3,7,9,10]

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