Molecular Characterization of the Plant Hypersensitive Response and Maize Lesion Mimic Mutants

Ryan L Benke, Purdue University

Abstract

The rapid localized cell death at and around sites of attempted pathogen infection, termed the hypersensitive response (HR), is an immune response mechanism commonly utilized in plants. This cell death limits pathogens from accessing host nutrients which often leads to resistance. The interaction of pathogen signals and host receptors that are required for the HR are well studied; however, the processes that regulate cell death during the HR remain enigmatic. The plant lesion mimic mutants, which form spontaneous lesions and/or undergo autoactive cell death in the absence of infection or stress, are commonly used as model systems to study the HR. Some lesion mimic mutants are caused by autoactive alleles of the resistance genes that recognize pathogen signals and trigger the HR. These mutants have facilitated studies of the HR as they allow the study of the HR without the need to control for pathogen infection. Currently, the etiologies of most maize lesion mimic mutants are unknown. Lesion mimic mutants contain numerous metabolic perturbations, including the increased accumulation of salicylic acid (SA), phenylalanine, and intermediates in heme and chlorophyll biosynthesis and catabolism. Some of these perturbations are dependent on the cause of lesion formation. As such, the accumulation of any of these metabolites in a lesion mutant may infer the etiology of that mutant. This dissertation contains three projects related to the molecular characterization of HR and maize lesion mimic mutants. In the first project (Chapter 2), I compared the metabolite profile of 23 maize lesion mimic mutants. This work identified two major findings that were further explored in the other projects in this dissertation. The first major finding is that four of the 23 mutants have metabolic perturbations that are like those of the known HR lesion mutant, Rp1-D21. In project two (Chapter 3), I molecularly characterize, Lesion10, which is one of the mutants that has HR-like metabolic perturbations. Using genome-wide association studies, I identified a gene candidate that may modify Lesion10phenotypic severity. The second major finding from project one is that SA accumulates to higher than wild-type levels in most of the lesion mutants analyzed. In the third project (Chapter 4), I characterized how SA is synthesized in maize and if SA is necessary or sufficient for the formation of lesions during the HR in maize. Using untargeted metabolite analysis, stable isotope feedings, and enzyme assays, I provide evidence of both known SA biosynthetic pathways in maize and demonstrate that the two pathways are interdependent. In addition, I show that increased accumulation of SA is not required for the HR in maize.

Degree

Ph.D.

Advisors

Dilkes, Purdue University.

Subject Area

Agronomy|Bioengineering|Cellular biology|Genetics|Immunology

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