Why don’t we see square vegetables?

If you stroll through your local grocery store’s produce section, you’ll encounter a vast array of tomatoes—ranging from tiny cherries and grapes to pear-shaped, massive beefsteaks, and unique heirlooms. The same variety exists in squash, potatoes, cucumbers, and leafy greens. This incredible diversity in color, shape, and size isn't due to natural selection, but instead, it’s the result of human influence.

Throughout history, farmers and plant breeders have recognized beneficial mutations in fruits and vegetables—whether it’s a tastier fruit, higher yields, or novel shapes—and have carefully preserved these traits through traditional breeding methods. Though the process takes time, repeated cross-breeding can eventually yield a new, distinct variety that’s marketable and recognizable as its own.

The traditional, slow process of plant breeding is set to be revolutionized by breakthroughs in genetic mapping. With the genomes of crops like tomatoes and cucumbers available, breeders no longer have to wait months to see what a plant's fruit will look like. Instead, by examining the DNA of seedlings for specific markers, they can predict the shape, size, and color of the fruit. This method, known as 'marker-assisted selection,' promises to drastically speed up the plant breeding timeline.

Esther van der Knaap is leading cutting-edge research to uncover how a plant’s genetic makeup determines whether its fruit grows long and slender like a cucumber or round and plump like a beefsteak tomato. In her lab at the University of Georgia, postdoctoral researchers and students examine the shapes and sizes of tomatoes by slicing them and scanning them on a flatbed scanner, studying the genetic variations that lead to different fruit characteristics.

In a paper published on November 9, 2018, in the journal Nature Communications, van der Knaap revealed the identification of two gene families that seem to be crucial in determining whether fruits and vegetables take on a round or elongated form. Fruits and vegetables, as edible parts of a plant, develop through cell division, which is central to their growth.

To achieve specific shapes like round or long fruits, cell division patterns must be precise, explains van der Knaap. 'The direction of cell division, whether horizontal or vertical, determines the shape.' Horizontal cell division builds up tissue in a way that results in a rounder fruit, while vertical division creates a different structural form.

This concept makes sense: the more cells in an organ divide horizontally by splitting in the middle, the more tissue accumulates in the horizontal direction, leading to a rounder, fuller fruit.

Van der Knaap and her team have pinpointed a gene in the tomato genome called OVATE, which is thought to produce proteins that instruct cells to divide in a vertical manner. This pattern of cell division results in an elongated fruit. The presence of OVATE is what distinguishes a perfectly round cherry tomato from one that is pear-shaped.

Wild tomato varieties, like those native to Peru, Ecuador, and Mexico, are generally small and round, according to van der Knaap. This means that the pear-shaped and other elongated tomatoes are mutations that appeared later on. As far back as the 1930s, scientists had named the elongation mutation 'OVATE,' but the genetic mechanism behind it was unknown.

Now that van der Knaap has discovered the OVATE protein, along with another protein family known as TRMs that interact with OVATE, this offers plant breeders a new tool in marker-assisted selection. When the OVATE and TRM markers are present, elongated fruits are guaranteed. If either marker is absent, round fruits will result. Van der Knaap believes this will accelerate the breeding process, allowing growers to focus on more complex traits like yield and pest resistance that are not as easily tied to specific genes.

The next question is whether these advancements in plant genetics will result in square tomatoes or pyramid-shaped pumpkins on store shelves. According to van der Knaap, this is unlikely—though not because it's scientifically unfeasible. She notes that the tomato genome contains many unusual mutations that lead to strange-looking fruits, and these mutations could be isolated and replicated in the lab.

However, the issue with square tomatoes and other odd-shaped fruits is twofold, van der Knaap explains. First, there's the GMO concern. If breeders use gene editing to modify or replace genes in food crops, the result is classified as genetically modified (GMO), and people often have reservations about GMOs in their food.

Secondly, newly developed shapes for fruits and vegetables may end up having a terrible taste.

"Some mutations are so strange that no farmer would consider growing them, as they come with a host of other issues," says van der Knaap. "These fruits may only produce a few per plant or may taste awful because growing a fruit in such an unusual shape disrupts its hormone balance. As a result, they might not be juicy or flavorful at all."

If you're really set on growing a square tomato, van der Knaap suggests following the Japanese method for growing square watermelons—just put a box around it. "That would certainly be a luxury tomato," says the researcher, "but I'm not sure I'd want to pay for it."