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Characterization of defense pathways and genes involved in host-pathovar level resistance using Arabidopsis-Pseudomonas system

Gangadharan, Anju

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2014, Doctor of Philosophy, Ohio State University, Plant Cellular and Molecular Biology.
Interaction of the model plant, Arabidopsis thaliana, and bacterial pathogen, Pseudomonas syringae, is widely used for studying plant-pathogen interactions at the molecular level. Immune responses are usually triggered after plants recognize the presence of integral pathogen molecules called Pathogen Associated Molecular Patterns (PAMPs) or proteins secreted into the host cell called effectors. Recognition of PAMPs by pattern recognition receptors (PRRs) trigger defense responses including extensive transcriptional reprogramming, modulation of hormone signaling pathways, secretion of antimicrobial elements into the apoplast, and induction of cell wall fortifications. Effector recognition by resistance (R) proteins trigger similar but more robust responses that often include the hypersensitive response (HR). Host-pathovar level resistance or non-host resistance occurs when all cultivars or landraces of a plant are resistant to a particular pathogen. Non-host resistance is governed by multiple genes and overlapping defense pathways. Though the mechanisms are poorly understood, these features are thought to underlie the durability of non-host resistance. In this study I have taken three approaches to decipher resistance mechanisms that govern non-host resistance. Pseudomonas syringae pv phaseolicola (Pph) causes halo blight on beans but is non-pathogenic on Arabidopsis. Pph induces two distinct defense readouts, namely induction of callose containing cell wall fortifications spanning mesophyll cells (big callose) and accumulation of high levels of PATHOGENESIS RELATED-1 (PR-1) protein, an important marker for phytohormone salicylic acid (SA)-signaling. To study the mechanisms associated with the induction of these defense responses, we heterologously expressed a virulence effector from a strain of P. syringae pathogenic on Arabidopsis, called HopM1, that is lacking in Pph. Pph induces the induction of big callose deposition and PR-1 accumulation in manner independent of SA-signaling and HopM1 suppresses both readouts. AtMIN7, a known target of HopM1, and TGA3, a transcriptional activator of PR-1, positively regulate the SA-independent pathway leading to PR-1 expression. HopM1 fails to target and degrade AtMIN7 in the context of a Pph infection showing that it suppresses SA-independent defense by targeting proteins other than AtMIN7. The second approach used RNA-seq to examine genome-wide gene expression changes when plants were challenged with Pph. Wild-type and plants having mutation in a resistance gene, RPS4, contributing to big callose deposition, were infected with Pph. Comparison of gene expression changes between wild-type and rps4 plants revealed differential expression patterns of several genes known to be involved in defense induction and novel gene candidates. The pattern in gene expression changes were validated using real time quantitative PCR and potential candidates for further evaluation identified. The third approach used natural variation to examine genes required for the induction of big callose deposition and PR-1 protein accumulation. We screened a collection of Arabidopsis accessions for changes in defense read outs after challenges with Pph. Accessions partially or completely compromised in defense induction were identified. Accessions compromised in PR-1 protein accumulation included KZ1, which was highlighted as a candidate for further evaluation. Thus my study has contributed substantially towards understanding non-host resistance mechanisms by identifying molecular players involved and have opened up several avenues for future studies on the same.
David Mackey, PhD (Advisor)
David Bisaro, PhD (Committee Member)
Terrance Graham, PhD (Committee Member)
Keith Slotkin, PhD (Committee Member)
165 p.

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Citations

  • Gangadharan, A. (2014). Characterization of defense pathways and genes involved in host-pathovar level resistance using Arabidopsis-Pseudomonas system [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1387815106

    APA Style (7th edition)

  • Gangadharan, Anju. Characterization of defense pathways and genes involved in host-pathovar level resistance using Arabidopsis-Pseudomonas system. 2014. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1387815106.

    MLA Style (8th edition)

  • Gangadharan, Anju. "Characterization of defense pathways and genes involved in host-pathovar level resistance using Arabidopsis-Pseudomonas system." Doctoral dissertation, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1387815106

    Chicago Manual of Style (17th edition)