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Melkamu Woldemariam
 &emdash;  Postdoctoral Scientist

Melkamu Woldemariam

Research Overview

Physiological cost and metabolic fates of plant defense metabolites

In response to insect herbivory, plants produce toxic chemicals that reduce the overall fitness or survival of the attacking herbivores. Beneficial as they are in protecting plants, these toxic metabolites are often costly to produce or they may have undesired negative effects on the plants themselves. Plants solve these issues by (1) having induced defensive metabolites that are only produced in response to herbivory and (2) detoxifying defensive metabolites when they are no longer needed to protect against herbivory. Even though predictions are made regarding tradeoffs in plant defense responses, detailed studies on the effects of defensive metabolites on plant growth and development, as well as the mechanisms by which plants regulate these effects, are lacking.

Regulation of herbivore-induced defense responses in maize

The jasmonate signaling pathway plays a central role in orchestrating herbivore-induced plant defense responses. During herbivore attack, most plants produce a large pool of jasmonic acid (JA) that is conjugated with isoleucine to produce the bioactive jasmonate, jasmonoyl-L-isoleucine (JA-Ile). JA-Ile is perceived by the SCFCOI receptor complex leading to the degradation of the JAZ repressors of the MYC2 transcription factors, thereby inducing MYC2 expression and subsequently the synthesis of defensive metabolites. Perturbation of the jasmonic acid (JA) signaling pathway in several dicots has been shown to alter defense responses.

Despite the presumed overall conservation of the JA signaling pathway in higher plants, there have been relatively few studies of JA signaling in monocots. In my research, I am assessing natural variation in the jasmonate signaling pathway of maize (Zea mays) to identify quantitative trait loci (QTL) that affect insect resistance (Figures 1 and 2). To identify the underlying genetic basis of these QTL, I am testing candidate genes by measuring herbivore-induced metabolic responses in transposon insertion lines and near isogenic lines.

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Figure 1. Transposon insertion mutants grown in the field to generate homozygous lines.

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Figure 2. Seedlings of maize inbred lines grown in greenhouse and subjected to fall armyworm (Spodoptera exigua) caterpillar feeding.

Detoxification of glucosinolate breakdown products in Arabidopsis

Glucosinolates are defensive metabolites produced by cruciferous plants, including Arabidopsis thaliana(Figure 3). When A. thaliana plants are wounded by mechanical damage or insect attack, glucosinolates come in contact with the enzymes that degrade them, called myrosinases. This leads to the rapid production of toxic breakdown products, a process also known as the mustard oil bomb. Indole-3-carbinol (I3C) is one of the degradative products of indole glucosinolates. We recently demonstrated that I3C has a negative effect on the growth of A. thaliana seedlings (Figure 4), but we do not know how A. thaliana plants cope with this challenge. Using the A. thaliana glucosinolate-myrosinase model system and a combination of HPLC, mass spectrometry, and NMR techniques, I am studying how A. thaliana plants detoxify I3C and thereby avert the negative growth consequences.

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Figure 3. Rosette stage (left) and flowering (right) Arabidopsis thaliana plants.

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Figure 4. Effect of I3C on growth of A. thaliana plants.