Differential expansion and retention patterns of LRR-RLK genes across plant evolution.

To maximize overall fitness, plants must accurately respond to a host of growth, developmental, and environmental signals throughout their life. Many of these internal and external signals are perceived by the leucine-rich repeat receptor-like kinases, which play roles in regulating growth, development, and immunity. This largest family of receptor kinases in plants can be divided into subfamilies based on the conservation of the kinase domain, which demonstrates that shared evolutionary history often indicates shared molecular function. Here we investigate the evolutionary history of this family across the evolution of 112 plant species. We identify lineage-specific expansions of the malectin-domain containing subfamily LRR subfamily I primarily in the Brassicales and bryophytes. Most other plant lineages instead show a large expansion in LRR subfamily XII, which in Arabidopsis is known to contain key receptors in pathogen perception. This striking asymmetric expansion may reveal a dichotomy in the evolutionary history and adaptation strategies employed by plants. A greater understanding of the evolutionary pressures and adaptation strategies acting on members of this receptor family offers a way to improve functional predictions for orphan receptors and simplify the identification of novel stress-related receptors.

Genomic sequencing of Thinopyrum elongatum chromosome arm 7EL, carrying fusarium head blight resistance, and characterization of its impact on the transcriptome of the introgressed line CS-7EL.

BACKGROUND: The tall wheatgrass species Thinopyrum elongatum carries a strong fusarium head blight (FHB) resistance locus located on the long arm of chromosome 7 (7EL) as well as resistance to leaf and stem rusts, all diseases with a significant impact on wheat production. Towards understanding the contribution of Th. elongatum 7EL to improvement of disease resistance in wheat, the genomic sequence of the 7EL fragment present in the wheat Chinese Spring (CS) telosomic addition line CS-7EL was determined and the contribution and impact of 7EL on the rachis transcriptome during FHB infection was compared between CS and CS-7EL. RESULTS: We assembled the Th. elongatum 7EL chromosome arm using a reference-guided approach. Combining this assembly with the available reference sequence for CS hexaploid wheat provided a reliable reference for interrogating the transcriptomic differences in response to infection conferred by the 7EL fragment. Comparison of the transcriptomes of rachis tissues from CS and CS-7EL showed expression of Th. elongatum transcripts as well as modulation of wheat transcript expression profiles in the CS-7EL line. Expression profiles at 4 days after infection with Fusarium graminearum, the causal agent of FHB, showed an increased in expression of genes associated with an effective defense response, in particular glucan endo-1,3-beta-glucosidases and chitinases, in the FHB-resistant line CS-7EL while there was a larger increase in differential expression for genes associated with the level of fungal infection in the FHB-susceptible line CS. One hundred and seven 7EL transcripts were expressed in the smallest 7EL region defined to carry FHB resistance. CONCLUSION: 7EL contributed to CS-7EL transcriptome by direct expression and through alteration of wheat transcript profiles. FHB resistance in CS-7EL was associated with transcriptome changes suggesting a more effective defense response. A list of candidate genes for the FHB resistance locus on 7EL has been established.

Introgression of Thinopyrum elongatum DNA fragments carrying resistance to fusarium head blight into Triticum aestivum cultivar Chinese Spring is associated with alteration of gene expression.

The tall wheatgrass species Thinopyrum elongatum carries on the long arm of chromosome 7E, a locus that contributes strongly to resistance to fusarium head blight (FHB), a devastating fungal disease affecting wheat crops in all temperate areas of the world. Introgression of Th. elongatum 7E chromatin into chromosome 7D of wheat was induced by the ph1b mutant of CS. Recombinants between chromosome 7E and wheat chromosome 7D, induced by the ph1b mutation, were monitored by a combination of molecular markers and phenotyping for FHB resistance. Progeny of up to five subsequent generations derived from two lineages, 64-8 and 32-5, were phenotyped for FHB symptoms and genotyped using published and novel 7D- and 7E-specific markers. Fragments from the distal end of 7EL, still carrying FHB resistance and estimated to be less than 114 and 66 Mbp, were identified as introgressed into wheat chromosome arm 7DL of progeny derived from 64-8 and 32-5, respectively. Gene expression analysis revealed variation in the expression levels of genes from the distal ends of 7EL and 7DL in the introgressed progeny. The 7EL introgressed material will facilitate the use of the 7EL FHB resistance locus in wheat breeding programs.