Expression of coordinately regulated defense response genes and analysis of their role in disease resistance in Medicago truncatula

Authors: Samac, Deborah - Debby; PENUELA, SILVIA - University Of Minnesota; SCHNURR, JUDY - Former ARS Employee; HUNT, E - Fort Valley State University; FOSTER-HARTNETT, DAWN - University Of Minnesota; VANDENBOSCH, KATHRYN - University Of Minnesota; GANTT, J - University Of Minnesota

Submitted to: Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2011
Publication Date: 9/2/2011
Citation: Samac, D.A., Penuela, S., Schnurr, J.A., Hunt, E.N., Foster-Hartnett, D., Vandenbosch, K.A., Gantt, J.S. 2011. Expression of coordinately regulated defense response genes and analysis of their role in disease resistance in Medicago truncatula. Molecular Plant Pathology. 12(8):786-798.

Interpretive Summary: Barrel medic, a plant closely related to alfalfa, is resistant to diseases that cause important economic damage to crop plants. All plants respond to invasion by pathogenic microorganisms by turning on a battery of genes. However, little is known about how these genes function in defense against disease. The genes expressed in response to a root pathogen and two pathogens attacking leaves were identified using a gene microarray. The expression of genes common to all three pathogens and those unique to an interaction were measured. In general, the genes expressed in response to all pathogens were present at the highest levels. We tracked expression of five highly expressed genes in roots and foliage of healthy plants and in pathogen-challenged plants. All genes were expressed at a relatively low level in healthy plants but their expression increased dramatically when invaded by pathogens. The expression level of each gene paralleled the development of the pathogen; that is, as the pathogen grew and invaded more area, gene expression increased. We knocked-down expression of three genes individually and tested the resulting plants for a change in susceptibility to pathogen attack. Inhibiting expression of one of the genes but not the other two had a dramatic effect on defense against pathogens. These plants were highly susceptible to pathogens that cause plant cell death. This gene is part of a biochemical pathway for production of specific antimicrobial compounds, and our results show that the gene is critical for disease resistance. This information can be used to develop improved crop plants with resistance to highly damaging pathogens and reduce the need for fungicides to control disease.

Technical Abstract: Microarray technology was used to identify genes associated with disease defense responses in the model legume Medicago truncatula. Transcript profiles from leaves inoculated with Colletotrichum trifolii and Erysiphe pisi and roots infected with Phytophthora medicaginis were compared to identify genes expressed in response to all three pathogens and genes unique to an interaction. Among the most strongly up-regulated genes in all three interactions were those encoding a hevein-like protein, thaumatin-like protein (TLP), and members of the pathogenesis response (PR) 10 family. Transcripts of genes for enzymes in the phenylpropanoid pathway leading to production of isoflavonoid phytoalexins increased dramatically in response to inoculation with the foliar pathogens. In P. medicaginis-inoculated roots, transcripts of genes in the phenylpropanoid pathway peaked at 5 days post-inoculation, when symptoms became visible. Transcript accumulation of three PR10 family members, a TLP, and chalcone synthase (CHS) in M. truncatula R108 plants responding to the three pathogens paralleled stages of pathogen development. To evaluate the role of a TLP, a PR10 family member, and CHS in disease resistance, plants containing interfering RNA (RNAi) constructs were produced. Reduced expression of PR10 and TLP had no effect on disease phenotypes whereas reduced expression of CHS resulted in increased susceptibility to necrotrophic pathogens.