2013 Annual Report
1a.Objectives (from AD-416):
1) Identify candidate avirulence genes in the wheat leaf rust pathogen, Puccinia triticina, that may condition resistance to wheat leaf rust,.
2)Assemble association mapping population in hard winter wheat and analyze for resistance to leaf rust, stripe rust, and stem rust, and.
3)Construct and analyze mapping populations for resistance to wheat leaf rust, stripe rust, and stem rust.
1b.Approach (from AD-416):
Candidate avirulence genes will be identified from gene expression studies or by analysis of genomic sequences of Puccinia triticina. Candidates will be tested for ability to induce hypersensitive necrotic responses on resistant wheat varieties. An association mapping population representing diverse hard winter wheat germplasm will be tested in the field and in the greenhouse for resistance to the three rusts. Lines will be genotyped using a large set of SNP markers. Genomic regions associated with lower mean disease scores will be further investigated. Mapping populations will be developed using the doubled haploid (DH) or the recombinant inbred line (RIL) approach for native and exotic sources of resistance to leaf rust, stripe rust, and stem rust.
Puccinia triticina is the fungal pathogen of wheat that causes leaf rust. As the fungus penetrates the wheat plant, it produces structures called haustoria which penetrate the host cells. Haustoria absorb nutrients from the invaded host cells. Haustoria also secrete effector molecules that are thought to reprogram the wheat cells to facilitate infection by the fungus. In some cases, particular fungal effectors are detected by host resistance genes and trigger a resistance reaction. Such resistance-triggering effectors are called avirulence factors. Knowledge about effectors is key to understanding the success or failure of infection.
A cDNA library of expressed genes from fungal haustoria was created and sequenced. Ten haustorial genes were found to have secretion signals and three were tested as candidate avirulence effectors. A method was developed to test the candidates in wheat leaf tissues. One fungal gene, designated PtAVRLr26, triggered a resistance reaction by the Lr26 resistance gene in wheat. Work is continuing to verify that PtAVRLr26 is indeed an avirulence factor.
A different set of experiments was performed to identify wheat gene expression differences associated with specific races of P. triticina. We have identified over 25 genes in wheat that show changes in expression during the stages of infection. Three of the genes appear to have race-specific expression. Future work will use reverse genetics approaches to further characterize these genes and determine their relationship to fungal effectors.
Further experiments were directed at identifying rust resistance genes in wheat cultivars and breeding lines. Genome-wide association analysis is a new method for identifying genes that are associated with specific traits in structured or unstructured populations. Two special panels totaling more than 400 wheat cultivars and elite germplasm lines were tested for resistance to leaf rust in the field in Texas and Kansas. These wheat lines were genotyped to obtain a set of genome-wide markers. Association mapping analyses will be performed in the next year to identify resistance genes in these wheat lines.