Infection of Brachypodium distachyon by Formae Speciales of Puccinia graminis: Early Infection Events and Host-Pathogen Incompatibility

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

The Use and Role of Predictive Systems in Disease Management

Disease predictive systems are intended to be management aids. With a few exceptions, these systems typically do not have direct sustained use by growers. Rather, their impact is mostly pedagogic and indirect, improving recommendations from farm advisers and shaping management concepts. The degree to which a system is consulted depends on the amount of perceived new, actionable information that is consistent with the objectives of the user. Often this involves avoiding risks associated with costly disease outbreaks. Adoption is sensitive to the correspondence between the information a system delivers and the information needed to manage a particular pathosystem at an acceptable financial risk; details of the approach used to predict disease risk are less important. The continuing challenge for researchers is to construct tools relevant to farmers and their advisers that improve upon their current management skill. This goal requires an appreciation of growers’ decision calculus in managing disease problems and, more broadly, their overall farm enterprise management.Keywords: plant pathogen, IPM adoption, diffusion theory, disease control, disease forecastin

Variation of symptom phenotypes induced by the different stem rust formae speciales.

<p>(<b>A</b>) Close-up image of the sporulating lesions developed in the different <i>Brachypodium</i> inbred lines and natural hosts inoculated with fungal isolates <i>P. graminis</i> f. sp. <i>tritici</i> (<i>Pg-tr</i>), <i>P. graminis</i> f. sp. <i>lolii</i> (<i>Pg-lo</i>), and <i>P. graminis</i> f. sp. <i>phlei-pratensis</i> (<i>Pg-pp</i>). (<b>B</b>) Time of symptom appearance in <i>Brachypodium</i> inbred line Bd1-1 according to fungal isolates. Scale bar indicates 5 cm.</p

Infection of <em>Brachypodium distachyon</em> by Formae Speciales of <em>Puccinia graminis</em>: Early Infection Events and Host-Pathogen Incompatibility

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

Pattern of stem rust severities on <i>Brachypodium</i> inbred lines inoculated with urediniospores of <i>P. graminis</i> f. sp. <i>lolii</i> (<i>Pg-lo</i>), <i>P. graminis</i> f. sp. <i>phlei-pratensis</i> (<i>Pg-pp</i>) or <i>P. graminis</i> f. sp. <i>tritici</i> (<i>Pg-tr</i>).

<p>Data were collected at 12 days post-inoculation. Bar height represents the mean of total number of sporulating lesions per plant across two trials. Error bars indicate standard error of the mean for each treatment, pooled across trials. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056857#s3" target="_blank">Results</a> from statistical analysis are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056857#pone-0056857-t001" target="_blank">Table 1</a>.</p

Early events in the infection of <i>Brachypodium</i> inbred line Bd1-1 by <i>Puccinia graminis</i> formae speciales.

<p>Fluorescence micrographs of longitudinal leaf sections show the fungal penetration sites and formation of infection structures according to time in hours post-inoculation (left side). The isolates <i>P. graminis</i> f. sp. <i>tritici</i> (<i>Pg-tr</i>), <i>P. graminis</i> f. sp. <i>lolii</i> (<i>Pg-lo</i>), and <i>P. graminis</i> f. sp. <i>phlei-pratensis</i> (<i>Pg-pp</i>) used for inoculation are indicated at the top of the figure. Fungal tissue was stained with Uvitex 2B (chitin specific) which exhibits blue fluorescence under UV-light. Structures are labeled as follows: u, urediniospore; g, germ tube; a, appressorium; s, leaf stoma; i, primary infection hyphae; v, substomatal vesicle; h, haustorium. Panels D, E, G and H show the formation of callose papillae below fungal penetration sites. Scale bar in panel L indicates 20 µm.</p

Disease severity of <i>Brachypodium</i> inbred lines challenged with <i>P. graminis</i> f. sp. <i>lolii</i> (<i>Pg-lo</i>), <i>P. graminis</i> f. sp. <i>phlei-pratensis</i> (<i>Pg-pp</i>) and <i>P. graminis</i> f. sp. <i>tritici</i> (<i>Pg-tr</i>)^*.

§<p><i>Brachypodium distachyon</i> (Bd) inbred lines.</p>†<p>Trial number.</p>*<p>Numbers within columns represent means of √ (sporulating lesions per plant). Numbers within a column followed by the same letter do not differ (<i>P</i> = 0.01) according to Tukey's test.</p