Date Published: June 7, 2018
Publisher: Public Library of Science
Author(s): Kerri A. Neugebauer, Myron Bruce, Tim Todd, Harold N. Trick, John P. Fellers, Richard A Wilson.
Puccinia triticina, the causal agent of wheat leaf rust, causes significant losses in wheat yield and quality each year worldwide. During leaf rust infection, the host plant recognizes numerous molecules, some of which trigger host defenses. Although P. triticina reproduces clonally, there is still variation within the population due to a high mutation frequency, host specificity, and environmental adaptation. This study explores how wheat responds on a gene expression level to different P. triticina races. Six P. triticina races were inoculated onto a susceptible wheat variety and samples were taken at six days post inoculation, just prior to pustule eruption. RNA sequence data identified 63 wheat genes differentially expressed between the six races. A time course, conducted over the first seven days post inoculation, was used to examine the expression pattern of 63 genes during infection. Forty-seven wheat genes were verified to have differential expression. Three common expression patterns were identified. In addition, two genes were associated with race specific gene expression. Differential expression of an ER molecular chaperone gene was associated with races from two different P. triticina lineages. Also, differential expression in an alanine glyoxylate aminotransferase gene was associated with races with virulence shifts for leaf rust resistance genes.
The struggle between fungi and their host plants is an evolutionary battleground. Pathogenic fungi must have a means of overcoming host defenses in order to obtain nutrients and complete their life cycle. Fungi have specialized effector molecules, which are used to combat plant defenses and reprogram host cells. However, plants have several layers of defenses. The first being pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI; ). Pathogen recognition receptors (PRR) span the host plasma membrane and detect PAMPs triggering an accumulation of reactive oxygen species, increased ethylene production, and eventually an induction of the salicylic acid pathway . The next layer is direct or indirect recognition of specific pathogen effectors by nucleotide binding, leucine rich repeat (NB-LRR) containing proteins starting a cascade of events leading to a more intense form of resistance, called effector triggered immunity (ETI; ). The hallmark of ETI is the localization of infection by a hypersensitive response that results in programmed cell death . The ETI interaction between effectors and NB-LRR resistance genes work in a gene-for-gene manor  and is the basis of most of the “major gene” genetic resistance in crop improvement.
Before characterizing mRNA fragments that were differentially expressed during P. triticina infection, the expression of three PR proteins, PR-1, PR-2, and PR-5, were evaluated in response to P. triticina infection. PR proteins are induced in a PTI response to a wide variety of pathogens and are also involved in plant development. Specifically, PR-1, PR-2, and PR-5 have been shown to inhibit growth of a variety of fungi . PR-2 proteins have a β-1,3- glucanase activity  and are induced in the presence of fungi that contain β-1,3-glucans in their cell walls  while PR-5 functions as a thaumatin-like protein [25–27]. The specific function of PR-1 is still unknown . The initial expression patterns of the PR genes indicated that the PTI pathway has been activated (Fig 2). From 0 DPI through 3 DPI, the appressorium is formed, enters into the host, comes in contact with the cell wall, and forms haustoria (Fig 1, Day 3). Between day 3 and 4, the fungus is transitioning between growth and spore formation and beginning secondary growth. The gene expression of PR-2 had the same general trend as PR-1 and PR-5, but “M” and “T” races induced differential expression at 4–6 DPI. This suggests a common factor between these two groups may be inducing the differing responses.