FROM THE ARCHIVES: Tomato Fruits - Choose Your Size

By Carl Price and Ellen Reardon. First published in Let’s Talk Plants! February 2009, No. 173.

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The fruits of wild-type tomatoes are yellow and green, and barely the size of the tip of one’s little finger. The tomato probably originated in Peru, but the first written evidence of their domestication was several thousand years ago by Aztecs in what is now Mexico.


The Aztecs selected larger and red-pigmented fruits, which became a staple of the Aztecs’ food supply.


While Spanish explorers brought tomatoes to Europe at least as early as the 16th century, several hundred years went by before Europeans dared to eat them. Why, because tomatoes were associated with deadly nightshade (Atropa belladonna), a related species of the Solanaceae family. It was not until the 19th century that tomatoes became a fixture of American farms, gardens, and dining-room tables.


Thomas Jefferson, who grew tomatoes in his garden in Monticello, is often credited with having promoted tomatoes as good to eat, but the history is uncertain.


Tomato size and the fasciated (fas) gene


Bilocular tomato. Drawing by Thomas Rost.
Bilocular tomato. Drawing by Thomas Rost.
Five-locule tomato. Drawing by Thomas Rost 2.

In modern times we recognize a wide range in the size of tomatoes, from cherry tomatoes, with a diameter of a few cm (under an inch), to the large tomatoes commonly seen on grocery shelves. Today’s tomatoes can be 1000 times as large as the ancient, wild types.

Recently, a team of scientists at Cornell led by Steve Tanksley1 found that the size of tomatoes is due to mutations in a single gene, fasciated. The product of the gene is a transcription factor, a term that refers to the first step in the expression of the cell’s DNA. Fas is expressed before flowers even begin to form, and it has a negative effect on the other genes responsible for flower and fruit development. In other words, the wild-type fas gene represses the expression of other genes responsible for the formation of flowers and fruits.


As a consequence, plants with the wild type of fas have small meristems and correspondingly smaller fruit.


A specific effect of fas is to control the number of locules (or carpels) in tomato fruit. Mutants of fas have more carpels, and therefore the fruits are larger and heavier.

Longitudinal (top left) and transverse (bottom left & right) slices through a tomato fruit show the number of locules or carpels. Tomatoes with the wild type of the fasciated gene have only two locules, whereas tomatoes with mutant versions of fasciated have many more locules.


Future applications


The Tanksley team expects that their research on the fasciated gene will lead to the understanding of events that led to the domestication of fruits. They also expect to apply their understanding of the mechanisms of fruit development to other members of the Solanacea, including bell pepper, eggplant, and potato.


References


1. Cong, B., L.S. Barrero, S.D. Tanksley, 2008. Regulatory change in YABBY-like transcription factor led to evolution of extreme fruit size during tomato domestication. Nature Genetics, 800-804. 2. Tomato Anatomy: http://www-plb.ucdavis.edu/labs/rost/Tomato/Reproductive/anat.html

Carl Price and Ellen Reardon are retired from Rutgers University, where they conducted research on the molecular biology of plastids and served as editors of journals in their field.