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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Molecular Plant Pathology Laboratory » Research » Publications at this Location » Publication #345558
Title: Resistant and susceptible responses in alfalfa (Medicago sativa) to bacterial stem blight caused by Pseudomonas syringae pv. syringae

Author
item Nemchinov, Lev
item Shao, Jonathan
item LEE, MAYA - Towson University
item POSTNIKOVA, OLGA - Academy Of Science Of Russia
item Samac, Deborah - Debby

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2017
Publication Date: 12/15/2017
Citation: Nemchinov, L.G., Shao, J.Y., Lee, M., Postnikova, O.A., Samac, D.A. 2017. Resistant and susceptible responses in alfalfa (Medicago sativa) to bacterial stem blight caused by Pseudomonas syringae pv. syringae. PLoS One. 12(12):e0189781.

Interpretive Summary: Alfalfa is a key forage crop for dairy producers in the U.S. and in countries around the world. Bacterial stem blight of alfalfa, caused by Pseudomonas syringae pv. syringae, is common in the central and western U.S. and the disease occasionally occurs in eastern states. It has also been reported in Australia, Europe, and recently reported in western Iran. Plants with the disease are stunted, with spindly stems that are easily broken. Yield losses from the disease can be as high as 50% of the first harvest. Disease losses in the first harvest are economically damaging because this harvest is typically the highest yielding with the best forage quality. With a changing climate, the possibility exists of expansion in the geographical range and impact of bacterial stem blight in the U.S. and globally. To date, there are no cultivars selected for resistance to this disease and no known chemical control. Little is known about alfalfa-pathogen interactions. In this study, we identified key genes and processes involved in host resistance. Results of this work will be of interest to the researchers in academia and government organizations that are involved in alfalfa studies as well as to the breeders, alfalfa producers and representatives of the industry.

Technical Abstract: Bacterial stem blight caused by Pseudomonas syringae pv. syringae is a common disease of alfalfa (Medicago sativa L.) in the central and western U.S. and has been reported in Australia and Europe. The disease is not always recognized because symptoms are often associated with frost damage. Two cultivars were found to have a high proportion of plants resistant to the disease but little is known about host-pathogen interactions and host defense mechanisms. Here, individual resistant and susceptible plants were selected from cultivars Maverick and ZG9830 and used for transcript profiling by RNA-seq at 24 and 72 hours after inoculation (hai) with the isolate PssALF3. Strand-specific paired-end-reads were mapped onto the reference genome of cultivated alfalfa at the diploid level and M. truncatula. Comparative bioinformatic analysis revealed a number of differentially expressed genes (DEGs) in resistant and susceptible genotypes. Although resistant plants from each cultivar produced a hypersensitive response after inoculation, transcriptome analyses indicated that they respond differently at the molecular level. The number of DEGs was higher in resistant plants of ZG9830 at 24 hai than in resistant plants from Maverick, suggesting that ZG9830 plants had a more rapid effector triggered immune response. Uniquely up-regulated genes in resistant ZG9830 plants included genes with homology to known plant resistance (R) genes encoding putative nematode resistance HSPRO2-like proteins, putative orthologs for the rice Xa21 gene and soybean resistance gene Rpg1-b, and TIR-containing R genes lacking both NBS and LRR domains (TX proteins). In contrast, the suite of R genes up-regulated in resistant Maverick plants had an over-representation of R genes in the CC-NBS-LRR family including two genes for atypical CCR domains, which alone are sufficient for induction of defense responses, and a putative ortholog of the Arabidopsis RPM1 gene. Resistance in both cultivars appears to be mediated primarily by WRKY family transcription factors and expression of genes involved in protein phosphorylation, regulation of transcription, defense response including synthesis of isoflavonoids, and oxidation-reduction processes. These results will further the identification of mechanisms and genes involved in resistance to facilitate selection of parent populations and development of disease resistant commercial varieties.