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Title: Using transcription of six Puccinia triticina races to identify the secretome during infection of wheat

Author
item Bruce, Myron
item NEUGEBAUER, KERRI - Kansas State University
item JOLY, DAVID - Agriculture And Agri-Food Canada
item BAKKEREN, GUUS - Agriculture And Agri-Food Canada
item MIGEON, PIERRE - Kansas State University
item WANG, SHICHEN - Kansas State University
item AKHUNOV, EDUARD - Kansas State University
item CUOMO, CHRISTINA - Broad Institute Of Mit/harvard
item Fellers, John
item Kolmer, James

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2013
Publication Date: 1/14/2014
Citation: Bruce, M.A., Neugebauer, K.A., Joly, D.L., Bakkeren, G., Migeon, P., Wang, S., Akhunov, E., Cuomo, C.A., Fellers, J.P., Kolmer, J.A. 2014. Using transcription of six Puccinia triticina races to identify the secretome during infection of wheat. Frontiers in Plant Science. 4:520.

Interpretive Summary: Puccinia triticina is the fungus that causes wheat leaf rust, which is one of the most important wheat diseases worldwide. After the fungus enters the plant through the stomates, special fungal structures called haustoria penetrate and absorb nutrients from the plant cells. The fungal haustoria also begin to secrete proteins (called effectors) that are thought to inhibit the defense response of the plant and facilitate parasitism by the fungus. Some of the secreted effector proteins may be recognized by plant resistance genes and may trigger strong race-specific plant defenses. Therefore, secreted proteins are the key to understanding the interaction between the host and the pathogen. This study used RNA sequencing to identify the proteins that are secreted by six races of the fungus during infection. Differences in secreted proteins between the races were correlated with race specificity and 15 candidate effectors for controlling race specificity were identified. Quantitative gene expression profiles confirmed that the 15 candidates were expressed during the early stages of infection. Further work is needed to confirm the role of the candidate effectors in resistance.

Technical Abstract: Wheat leaf rust, caused by the basidiomycete Puccinia triticina, can cause yield losses of up to 20% in wheat-producing regions. During infection, the fungus forms haustoria that secrete proteins into the plant cell and effect changes in plant transcription, metabolism and defense. It is hypothesized that new races emerge as a result of overcoming plant resistance via changes in the secretome. To understand gene expression during infection and find genetic differences associated with races, RNA was extracted from wheat leaves infected with six different rust races at six days post-inoculation and sequenced using Illumina Solexa. As P. triticina is an obligate biotroph, RNA from both the host and fungi were present and separated by alignment to the P. triticina genome and a wheat EST reference database. A total of 222,571 rust contigs were assembled from 165 million reads. An examination of the resulting contigs revealed 532 predicted secreted proteins among the transcriptomes. Of these, 456 were found in all races. Fifteen genes were found with amino acid changes corresponding to putative avirulence effectors potentially recognized by 11 different leaf rust resistance (Lr) genes. Thirteen of the potential avirulence effectors have no homology to known genes. One gene had significant similarity to cerato-platanin, a known fungal elicitor, and another showed similarity to fungal tyrosinase, an enzyme involved in melanin synthesis. Temporal expression profiles were developed for these genes by qPCR and show that the 15 genes share similar expression patterns from infection initiation to just prior to spore eruption.