How Beefsteak Tomatoes Became So Supersized

Top row: The wild-type has no branches on the flower-supporting stem; the mutants show branching and extra flower petals. Second row: The wild fruit has 2 compartments for jelly and seeds; the mutants have more. C. Xu et al. / Nature Genetics 2015
Janet Fang 28 May 2015, 14:28

Researchers studying the genetic basis of beefed-up fruits have identified a set of genes that control the production of stem cells. Mutations in these genes may explain the origin of beefsteak tomatoes -- widely considered to be freaks of nature. The findings, published in Nature Genetics this week, suggest that it’s possible to supersize any fruit-bearing crop.

The beefsteak tomatoes that you buy at the market these days can weigh in at over half a kilogram (over a pound). But even when conquistadors brought the first domesticated tomatoes to Europe half a millennium ago, the beefsteaks were already huge compared to the wild, berry-sized fruits of Mexico.

The secret is in the plant’s growing tip (called the meristem), located at the uppermost part of the stem. This is where stem cells do their dividing and differentiating, and their carefully balanced production is controlled by genes with opposite roles: WUSCHEL promotes and CLAVATA inhibits. CLAVATA genes code for receptor proteins on the cell’s surface (these act as locks) and for proteins that dock at these receptors (these serve as keys). When a key is made and subsequently fits into a lock, the cell sends a signal for WUSCHEL to slow down -- preventing WUSCHEL from making too many stem cells. But when CLAVATA genes are mutated, the plant makes more stem cells in the meristem. 

A team led by Zachary Lippman of Cold Spring Harbor Laboratory examined mutant tomato plants called fab and fin, which have extra petals and seed compartments compared to wild-type tomatoes. They also have branches on the stem for supporting flowers. You can see the comparisons in the image above. The mutants carry faulty genes that code for enzymes called arabinosyltransferases, which add sugar molecules to a protein called CLAVATA3, one of the CLAVATA keys. Those sugars are required for the key to fit a CLAVATA lock. 

Changing the number of sugars attached to the CLAVATA3 key, they discovered, changes the number of stem cells: three sugars are needed for normal growth, and when one or more sugar is missing on the CLAVATA3 key, it no longer fits properly into the lock. Then when WUSCHEL sends a signal to make new stem cells, there’s no accompanying stop signal. That’s when you get abnormal growth and plump fruit.

According to a news release, the beefsteak doesn’t have enough of the CLAVATA3 key and the overwhelming number of stem cells in its meristem is what produces its massive size. A naturally occurring mutation in CLAVATA3 likely arose hundreds of years ago. You can see how large the meristems of the mutants are compared to the wild tomato in the image below. 

Furthermore, because the CLAVATA pathway exists in all plants, it’s possible that adjusting the number of sugars on the keys could produce custom-sized fruit. 

Images: C. Xu et al. / Nature Genetics 2015

[h/t Science]

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