New Faculty: Heejin Yoo

by Heejin Yoo

I grew up in a house filled with various plant species because my mother loves gardening. Today, my parents have a huge garden on Yeonsan mountain about forty-five minutes away from our home in Daejeon, South Korea. There they grow lots of flowers, vegetables, trees, etc. My mother’s loving care for plants and my father’s research in biology influenced me a lot, and inspired me to major in plant biology in college.

While an undergraduate, the study of genetically modified organisms (GMO) was a hot topic, and I became fascinated with plant molecular biology. I dreamt about studying how plants grow in continuously changing environments and how to make healthy plants with genetic engineering. In graduate school at Purdue University, my interest in plants became more serious when I learned that plants seem to have very distinct regulatory mechanisms to attract friends as well as to defend against foes. As stationary organisms, plants must have various strategies to handle friends and foes very differently than that of animals which are mobile and have specialized immune cells to defend against pathogens. Plants cannot run away from their enemies or approach their friends. So how do plants then defend themselves from harm while continuing to flourish?

One of plants’ key approaches to this is through volatiles, the metabolites they release into the air. The quantities released are not trivial; almost one-fifth of the atmospheric CO2 fixed by land plants is released back into the air each day as volatiles. Some flowers emit volatiles to attract pollinators, thus solving the problem of not being able to travel themselves. In contrast to the effort to attract, some plants repel insect herbivores by emitting volatiles such as menthol, having unpleasant odors.

During my PhD program, I studied plant volatiles in Petunia hybrida which attracts moths at night. Since Petunia hybrida emits phenylalanine-derived volatiles, my main research was focused on identification and characterization of unknown genes in phenylalanine biosynthesis to ultimately understand the regulatory mechanism of phenylalanine biosynthesis and phenylalanine-derived volatile compounds.

Later, during my postdoc research at Duke University, I began to dig into plants’ strategy to defend against their foes. One of these strategies is to regulate a phytohormone called salicylic acid, a well-known plant defense hormone against biotrophic pathogens and also a natural substance of pharmacological compounds. The famous pain killer aspirin is a synthetic compound derived from salicylic acid through acetylation with acetic anhydride. Methyl salicylate is the methyl ester of salicylic acid. It is a volatile, which gives inter- and intra-species airborne signals to give warning against pathogen attack. Methyl salicylate, a major component of wintergreen oil, smells minty and has been used in oils, food production, sports medicine and cosmetics.

From my post-doc study on regulation of salicylic acid biosynthesis in the model plant Arabidopsis thaliana, our research team discovered that the transcription factor CHE controls circadian rhythm and systemic induction of salicylic acid. This controlled rhythm is critical for plant immunity because the level of salicylic acid peaks at dawn to adapt to the environmental challenge of biotrophic pathogens which are elevated in the morning.

Additionally, systemic induction of salicylic acid is a key immune mechanism to establish systemic acquired resistance (SAR) in distal uninfected leaves. When local plant tissue is infected by a pathogen carrying effectors, the infected tissue dies by activating what we call programmed cell death (PCD) to trap the effectors in the infected site. During PCD in local tissue, uninfected neighboring tissue receives signals from local infected tissue and establishes SAR, which confers a broad spectrum of resistance in uninfected tissue to prepare for potential attacks. Especially in field-grown crop plants, timing of these attacks varies in different parts of plants. In this way, SAR establishment is critical for plants to prepare and protect from unpredictable pathogen infection at the whole plant level.

Other research during my post-doc was focused on translational regulation in plant immunity, which controls the levels of protein synthesis from its mRNA. Translational regulation is another important layer of regulatory mechanisms beyond transcriptional regulation. From this study, we discovered distinct translational regulatory mechanisms for local immune activation.

Stemming from my diverse educational background as well as my past research, the Heejin Yoo lab (HYoo Lab) is interested in studying the dynamic regulation of plant metabolism and immunity for balancing plant growth and defense. The HYoo lab is particularly interested in following metabolites for plant immunity: salicylic acid, amino acids and plant volatiles.

A first direction in this research is to study the regulatory mechanism of salicylic acid in crop species especially focused on Brassica napus (Canola) and Camelina sativa (Camelina), which are both important oilseed plant species. A second direction is to study the role of selected amino acids for plant immunity. A third direction is to study plant volatiles for defense activation and communication with other plants.

Through this progression, the HYoo Lab is attempting to understand the action molecular mechanisms of systemic immunity. We also want to approach this research with a focus on translational regulation in systemic tissue. We expect to find novel regulatory mechanisms during systemic immunity through translational control. By elucidating multiple layers of unknown mechanisms for plant immunity, we try to find the ideal genetic engineering strategy to improve immunity of crop species.

Our research team first mainly focuses on basic science research rather than working from an agricultural (or applied) platform. Ultimately, I believe that the knowledge gained from basic science research will provide a better strategy for agricultural application. My dream is to develop “super-plants” with strong disease resistance and a high yield, which, again, can solve the increasing food demand due to increasing world population.

I feel like I’ve traveled quite a distance from my mother’s gardens in South Korea where I first fell in love with plants, an affinity underscored by my father’s biological training. In fact, I have traveled quite a way from my undergraduate studies at Seoul National University in Korea, to graduate school at Purdue and then to complete a postdoctoral position at Duke in the United States. But my subject model of plants has remained the same even if my research has taken many twists and turns.