Written by: Sarah Sharman, PhD
Illustrated by: Cathleen Shaw
Imagine inheriting a rare, heirloom tomato plant from your grandmother. The tomatoes are so juicy, flavorful, and perfectly round that you’ve always dreamed of growing them yourself. But when you try to save the seeds and plant them the next year, the offspring don’t quite match the tomatoes you remember. This is because most plants reproduce sexually, combining genetic material from both parents and producing offspring with varying traits. In some cases, a plant may be able to pollinate itself to produce seed (called ‘selfing’); but unfortunately, the genetic material usually gets shuffled in a way where the offspring still look different than the plant it came from.
But what if there was a way to create exact clones of your grandmother’s prized tomato plant, preserving its unique characteristics for generations to come? Apomixis is a type of asexual reproduction that does just that, creating exact copies of the maternal plant. Let’s learn about the process of apomixis and how scientists are trying to leverage it to help the agriculture industry.
What is apomixis?
Plants are the foundation of life on Earth, providing us with food, oxygen, and countless other benefits. Most of our food comes directly from plants or the animals that eat them. Plant reproduction is an extremely important process that ensures plants continue to grow on our planet and provide us with delicious food to eat.
There are two main types of plant reproduction: sexual and asexual. Sexual reproduction occurs when a male reproductive cell (sperm within pollen) fertilizes the female reproductive cell (the egg within the embryo sac). The fertilized embryo sac develops into a seed, which contains the embryo of the new plant and a food supply for its development. The ovary of the flower often ripens into a fruit that can be dispersed by various means, such as wind, water, or animals. When the seed lands in a suitable environment, it can germinate and grow into a new plant that is genetically diverse from the parents.
Asexual reproduction, on the other hand, produces offspring that are genetically identical to the mother plant. Gardeners are probably familiar with a common form of asexual reproduction called vegetative propagation, where new plants grow from parts of a plant, such as runners or bulbs.
Apomixis is a type of asexual reproduction in plants where the plant makes an exact copy of itself through its seed. A special type of tissue within the plant starts to develop into a seed-like structure where the embryo forms without fertilization. This embryo is genetically identical to the mother plant. The endosperm, which is the food supply for the developing embryo, also forms within the seed-like structure. The seed continues to mature, developing a tough outer coat to protect the embryo and endosperm. Once mature, the seed is dispersed, germinates, and grows into a new plant that is an exact clone of its mother.
How can understanding apomixis benefit the agriculture industry?
Traditional plant breeding has been a cornerstone of agriculture for millennia, allowing humans to select and cultivate plants with desirable traits. Much of plant breeding is structured around hybrid breeding: taking two very different parent plants with some desirable characteristics to make a child plant with many desirable characteristics. But to keep producing that child plant, you have to either maintain the parents forever (so you can cross them) or propagate the child vegetatively somehow. And these methods can take up a lot of time, money, and labor.
Apomixis offers a potential solution to this challenge. By creating exact clones of a plant through its seed, apomixis can “fix” desirable traits and preserve them indefinitely much more efficiently. This eliminates the need for continuous breeding to recreate our favorite plants. Remember that heirloom tomato? By understanding apomixis, this variety could be maintained exactly as you and grandma remember it.
Saving seeds from apomictic plants allows breeders and farmers to maintain their top plants for years without the need to save and maintain the parent plants. This would allow breeders to maintain specific genetic combinations in crop varieties, ensuring that they retain desirable traits like high yield, disease resistance, or specific flavor profiles. Additionally, apomixis can simplify seed storage and distribution, as farmers can easily save and share the seeds without the need for complex breeding protocols.
Many important crop plants have very complicated genomes, containing multiple copies of each chromosome (called polyploidy). Because of their large and complex genomes, breeding some polyploids is difficult, and not all crosses yield fruits and offspring. For example, wheat, a polyploid crop, is essential for global food security. However, breeding new wheat varieties can be challenging due to its complex genome. Apomixis could enable breeders to create and maintain clones of high-yielding, disease-resistant wheat varieties more efficiently.
While apomixis offers significant potential benefits, there are also potential challenges to consider, including a lack of genetic diversity and technical difficulties. However, researchers are making significant progress in understanding the genetics of apomixis and developing strategies to overcome these obstacles. Recent advancements in genetic engineering and molecular biology offer hope for the creation of apomictic crop varieties in the near future.
Uncovering the genetics of apomixis
In plant biology, it has long been a mystery how apomictic plants bypass pollination and fertilization but still form an embryo. One group working to understand the genetics behind this phenomenon is the Harkess lab at HudsonAlpha Institute for Biotechnology. They are experts at identifying and understanding the genetic contributors to genetic sex and plant reproduction in many different plant species.
Apomixis has evolved in hundreds of unrelated plants over millions of years, a phenomenon known as a convergent trait. This means that it is unlikely that unrelated apomictic plants have the same genes controlling the process. For example, researchers have found genes involved in apomixis in daisies and grasses, but these genes were different in both groups. The hunt for genetic contributors to apomixis will likely be on a plant-by-plant basis.
Charity Goeckeritz, PhD, is a Postdoctoral Fellow leading a National Science Foundation (NSF)- funded research project to understand the genetic basis of apomixis in wild apple and blackberry species. Charity is growing blackberry plants in the Kathy L. Chan Greenhouse on HudsonAlpha’s campus that were collected from all over the world and preserved in the USDA’s germplasm database. She also travels to and receives materials from the apple collection maintained by the USDA in Geneva, NY. She is identifying plants that are apomicts and will compare their genomes to those of related sexually-reproducing plants. Major differences between the two groups will be studied more in-depth to see if they contribute to any of the stages of apomixis. The ultimate goal of Charity’s project is to find sections of DNA (genes) that control apomixis, information that can be leveraged by the larger plant breeding community in their quest to integrate apomixis into economically valuable crop plants.
