Deep in the Weeds
Close study of an ordinary plant's cellular mechanisms could lead to big advances in agriculture and medicine
If you were to come across a patch of thale cress poking through a crack in a parking lot, you might not think much of it. Its miniscule yellow flowers are little more impressive than its spindly stalk and rosette of leaves. You could eat those leaves in a salad, it just wouldn’t be much of an epicurean experience. Yet thale cress just might be one of the most important weeds in the world.
This mustard relative, also known as Arabidopsis thaliana, has spread to habitats across the globe. And since the advent of genetics, it has taken root in science labs the world over. One such laboratory belongs to CALS genetics professor Xuehua Zhong, who discovered Arabidopsis’s utility early in her graduate career. Now she uses the plant to study epigenetics, a rapidly growing field concerned with how genes are turned on and off without changes to DNA sequences — in other words, how cells and organisms with the same DNA sequence can exhibit different traits.
Scientists are turning to epigenetics to understand everything from the development and treatment of cancer and other diseases to how crops become resilient to environmental stressors brought on by climate change. Sarah Leichter, a third-year genetics graduate student in Zhong’s lab, sees a lot of possibilities through the lens of tiny molecules.
“If we can really understand how the mechanisms work, then perhaps we can manipulate them in a way that’s favorable for precision agriculture,” she says, referring to efforts to use science to tailor farming practices to specific conditions. And the innovations won’t be limited to the plant kingdom: Some of these processes also occur in mammals. “So discoveries first found in plants can help humans, too.”
If precision agriculture and precision medicine are the future, then the field of epigenetics is helping make the map that leads there.
Xuehua Zhong wasn’t always a geneticist, nor was she always destined to become a scientist at all. But a deep-seated curiosity about the world, paired with a desire to find answers, laid her path. “I grew up in a small town in the southeast of China,” she says, “in a family without a science background.” Her father, a businessman with a burdensome travel schedule, cultivated her inquisitive young mind. During long road trips, he indulged endless questions about trees, animals, and everything else.
When Zhong began middle school, her questions became more focused. On a day when she was home sick from school, she wondered why she, but not some of her classmates, got sick during the flu season. Her sixth-grade biology teacher was the first person to help her understand that science could hold the answers to her myriad questions. “Those early science classes really opened my eyes,” she says. “I saw that some questions were already being answered. But the more that I learned, the more I realized that there is so much we still do not know and that maybe I could be one of the people to figure out the answers.”
By the time Zhong arrived at Wuhan University, understanding human health and individual differences had become a passion. She majored in microbiology and immunology and found herself inside a laboratory for the first time during her sophomore year, barreling toward a career in medical research. But one thing was holding her back: her model organisms.
As a master’s student, Zhong wondered if she could continue to study human diseases without the restraints— and emotional turbulence — of working with mammal models. Enter Arabidopsis thaliana. Thale cress is an excellent model for studying all kinds of biological problems because of a few key traits.
First, its life cycle is very short relative to animals and many other plants: It takes only about six weeks for the plant to go from a seed to a self-pollinating adult to the next generation. That’s important when you want to study transgenerational changes — modifications that are preserved from one generation to the next.
Second, the Arabidopsis genome, or full DNA sequence, is comparatively tiny. At 135 million base pairs (the individual C-G and A-T pairs that make up DNA’s double helix), its genetic catalog is among the shortest in the plant kingdom. That means that scientists can get a handle on the plant’s genes and their functions with greater ease. In fact, in 2000, the Arabidopsis genome was the first to be entirely sequenced.
Finally, the plant is small enough to grow in large quantities inside a laboratory and produces plenty of seeds to support the next research study. Finding Arabidopsis thaliana was Zhong’s ticket to an entirely new world of possibilities in biological research.
While Zhong was beginning her Ph.D. studies in plant virology at The Ohio State University in the early 2000s, ideas that had been around for the better part of the 20th century about how the same DNA can give rise to different traits were undergoing a renaissance. The term “epigenetics,” which has meant many things to various generations of scientists, was gaining ever-narrower definitions.
A few years earlier, in 1996, a group of researchers at the University of Rochester in New York made a pivotal breakthrough. They discovered that gene expression could be regulated by modifications to a substance called chromatin, a material — found inside the nucleus of a cell — that is made up of proteins, DNA, and other chemicals.
Scientists had been aware since the early days of DNA research that mechanisms must exist that determine which sections of an organism’s genome are expressed, or activated. After all, every cell in a given organism’s body has the exact same genome hidden away in its nucleus, and yet cells end up doing vastly different things. Cells must, therefore, have some way of reading only certain parts of the DNA code. A skin cell reads the sections that tell it how to be a skin cell, and a neuron reads the parts that prescribe neuronal functions.
The ability to sequence entire genomes and to “knock out” particular genes or regions of the DNA code presents all kinds of new possibilities and allows scientists to identify many genes that play a role in regulating the expression of other genes. What the team at Rochester discovered was that one of those genes codes for a type of enzyme — a histone acetyltransferase — that modifies chromatin in a particular way to change gene expression without changing the DNA sequence. The mechanism was one of the first thoroughly characterized examples of what today’s experts call epigenetics.
“These things that we knew controlled gene expression are actually writing information on top of the genome,” explains John Denu, a professor of biomolecular chemistry at UW–Madison and one of Zhong’s collaborators, “and that information is what is generally referred to as the epigenome.” Denu was starting his laboratory in 1996, characterizing enzymes and studying how information is transmitted into the nuclei of cells. He became fascinated by epigenetic factors and began trying to understand how they work at the molecular level.
A decade later, Denu was an expert in chromatin in the blossoming field of epigenetics and was on the lookout for colleagues who shared his passion about understanding the machinery of epigenetic changes. When UW–Madison announced the creation of a new collaborative research center, the Wisconsin Institute for Discovery, Denu submitted epigenetics as one of its potential research themes. Part of the proposal was to recruit new experts in the field.
Meanwhile, as a graduate student at Ohio State, Zhong was interested in how plants defend themselves against viruses. Viral invaders commonly carry their genetic information in single-stranded RNA rather than in the double-helical DNA that resides inside every cell of plants and animals. Viral RNA can be similar to the messenger RNA that plant cells use to transfer information from strands of DNA to ribosomes, where the information is read and proteins are made. Viruses can use that structural similarity to sneak from one cell to another and spread infection. Once inside a cell, viruses let the cell do the work of replication: The cell’s own transcriptional machinery creates copies of the viral RNA.
The plants, however, are not defenseless. Zhong and her Ph.D. adviser, Biao Ding, discovered that one way the plants fight back is by silencing parts of the virus’s RNA, turning off genes related to replication. In response, viruses have developed measures to silence plant genes that code for the silencers. So goes the biological arms race that drives evolution. That arms race also seems to be conserved across species, meaning similar defenses and counter defenses exist in the animal kingdom, too. The discovery was Zhong’s introduction to epigenetic mechanisms.
Zhong now studies the machinery of suppressing gene expression by way of methylation. Enzymes called DNA methyltransferases attach methyl groups — bunches of CH3, one car- bon atom bonded to three hydrogen atoms — to specific sites on DNA, changing how that section is expressed.
“Methyltransferases are really ancient proteins,” says Zhong, “and because their structure and function are conserved between plants and mammals, we can gain important information about DNA methylation in humans by studying flowering plants, such as Arabidopsis.”
In 2013, Denu partnered with the Department of Genetics to bring Zhong’s research program — and her model organism — to UW–Madison. “She brought with her a very strong genetic background and a good knowledge of the model organism Arabidopsis to study fundamental processes of the epigenome,” says Denu, who has been impressed with Zhong’s success since her arrival in Wisconsin. Zhong has been honored with an Early Career Development Award from the National Science Foundation, the Alfred Toepfer Faculty Fellow Award from CALS for research benefiting agricultural activities, a Maximizing Investigators’ Research Award from the National Institutes of Health, and, most recently, a Vilas Early Career Investigator Award from UW–Madison recognizing research and teaching excellence. She has also published her research in prestigious journals and cultivated collaborations throughout Wisconsin and beyond.
Below the sunlight-drenched glass windows, bustling labs, and Mesozoic gardens of the Discovery Building on the UW–Madison campus sits a portion of Zhong’s crop of Arabidopsis thaliana, a ragtag collection of plants at various stages of the life cycle growing in a windowless basement corner. Freshly germinated green sprouts can be found on a shelf next to brittle, brown bunches of seeds.
Across University Avenue beneath the Biotechnology Center, thousands more specimens of the plant fill carefully climate-controlled walk-in rooms and stand-up growth chambers. At a given time, Zhong’s lab is cultivating around 30,000 individual plants that may be part of dozens of different experiments.
Leichter, backlit by meticulously calibrated lights, uses a small forceps to pollinate the delicate yellow flower of an Arabidopsis plant. She’s a details person. The processes she tries to understand occur at the molecular level. She studies one particular enzyme, a DNA methyltransferase called DRM2, that is responsible for depositing epigenetic modifications in Arabidopsis’s genome. DRM2 is choosy. It lays down methyl groups at very specific sites on DNA, usually silencing those regions. But Leichter is not satisfied only knowing what DRM2 does — she wants to know exactly how it does it.
“Getting down to the precise, nitty-gritty mechanism is what makes me so excited about it,” she says. “What are the properties of the amino acids within the enzyme that targets DNA? What’s the biochemistry of the DNA that allows this interaction to occur?” Answering those questions means building collaborations with structural biologists, biophysicists, computational modelers, and other experts. “We’re trying to study this in a way that’s very multidisciplinary and that pushes me outside of my comfort zone as a scientist,” she says.
Delving into the nitty-gritty is a common theme for Zhong’s research group. One project, for example, is focused on understanding how plants sense environmental threats, such as ultraviolet (UV) radiation from sunlight, which can alter the epigenetic landscape. That means teasing apart every aspect of UV radiation’s effect on a single plant enzyme, from the photoreceptors that sense the UV light to the plant’s ability to develop and survive.
Zhong and postdoctoral researcher Jianjun Jiang have discovered that extra doses of UV light cause the DNA methyltransferase DRM2 to malfunction, leaving typically silenced regions of Arabidopsis’s genome unmethylated and thus activating genes associated with stress and defense responses. Zhong’s enthusiasm about the discovery is palpable.
“This is big because it’s the first time we are able to link a signaling pathway directly with epigenetic components in a way that we understand the precise mechanism,” she says.
Fully appreciating the mechanisms underlying epigenetic changes is critical, according to Zhong, because such understanding might be the key to turning epigenetics into a powerful tool. If UV light is a potent inhibitor of DNA methylation, it might be an effective means of changing gene expression in a less invasive, less enduring way.
“If we want to change a phenotype,” she says, “perhaps we can just use UV light rather than chemicals or other permanent methods like genetic transformation.”
The epigenome, it turns out, is very responsive to environmental cues not only in plants but also in mammals. Zhong and Denu are part of a campus-wide Epigenetics Hub with researchers who are studying how multitudes of external factors, from lifestyle and diet to trauma and early life experiences, can influence chromatin to cause changes in which genes are turned on or off. Denu, for example, has linked diet to gene expression and health, a relationship that is moderated by gut microbes, metabolism, and epigenetic mechanisms, such as methylation.
The more the mechanics of epigenetics are understood, the easier it is to see the big picture. What might be possible if we could harness the biochemistry of chromatin and bend it to our will? Modifying the genomes of organisms has become a common practice in science and industry. Humans have been making tweaks to the genetic codes of plants and animals using selective breeding since long before the discovery of DNA. Today, nearly all of the soy, sugar beets, canola, and corn grown in the United States has been genetically engineered through various forms of biotechnology to be resistant to disease, drought, and other environmental threats.
That resistance though, is not necessarily responsive to present and ever-changing conditions, and creating transgenic crops can take several generations. What if crops could be made more resilient more quickly but without permanent edits to their DNA?
Zhong thinks epigenetics has a big role to play in the future of agriculture, and she has one big environmental factor on her mind: climate change. As the planet warms up, plants are facing new and intense challenges, such as less predictable growing seasons and skyrocketing temperatures. Jaini Chen, another of Zhong’s postdoctoral collaborators, is investigating how Arabidopsis responds to increasing temperatures, the kind that crops around the world are experiencing as a result of climate change.
Since setting up shop in Madison, Zhong and her collaborators have built an extensive library of Arabidopsis mutants with various genes deleted from their genomes. Chen noticed that when the genes that code for the DNA methyltransferase are mutated, the plants start to struggle to beat the heat: Their stalks grow shorter and their leaves curl, providing less surface area for photosynthesis.
“We started to think that maybe DNA methylation is involved in heat stress,” Chen says.
Chen, who cranks the growing chamber temperatures from 23 degrees Celsius (a comfortable mid-70s Fahrenheit) to a scorching 44 degrees (over 110 degrees Fahrenheit), has spent the last year teasing apart the pathways and processes between the temperature of the environment, the epigenetic response, and the ultimate effects on the plants’ health and fortitude. She’s found, for example, that a gene related to curling leaves is typically methylated and silenced, but without the DNA methyltransferase, it is expressed. Now she’s trying to determine whether the methylation-related consequences of heat stress are passed from one generation to the next. After a few generations of high temperatures, she’ll compare the methylation profile and health of the plants in the experimental lineage to the wild-type controls.
“When we see the difference, we’ll go deeper to study the precise mechanism behind this transgenerational inheritance,” she says.
When it comes to the effects of heat, it’s another case of uncovering the nitty-gritty with an eye toward manipulating the epigenome. Zhong and her group believe that if they can understand how plants use epigenetic mechanisms such as methylation to adapt to heat, they might be able to activate those same mechanisms to help them adapt even better as conditions change.
“If we can understand how it works in a model plant system and then transfer that knowledge out to agricultural plants that need to adapt to the changing environment, we can develop strategies to maintain a sustainable yield in heat-stressed environments,” says Leichter.
As new ways of modifying chromatin — such as UV light and heat — are carefully characterized, they could become powerful tools for fine-tuning the epigenomes of crops and their responses to environmental stresses. And those alterations could be passed on to future generations of more resilient plants.
Sidebar | Science Outreach: A Versatile Training ToolThis article was posted in Cover Story, Fall 2019, Features, Health and Wellness and tagged Agriculture, Biao Ding, changing climate, chromatin, DNA, epigenetics, Epigenetics Hub, epigenome, Genetics, genome, Jaini Chen, John Denu, Medicine, Sarah Leichter, thale cress, Xuehua Zhong.