Tomatoes taste good because we slowed down their internal clocks
Domestication accidentally created a plant that thrives in long summer days.
Tomatoes were probably first domesticated in the Andean region of Peru and Ecuador. From there, cultivation spread to Mexico, where conquistadors found them, returning with them to Europe in the seventeenth century.
In the years since, we have bred them to produce larger fruit, to resist diseases, and to retain their shape and color in neat supermarket pyramids. But the initial domestication event also selected an agriculturally advantageous trait—it slowed down the plant’s circadian clock so it could thrive in the long sunny days of the new latitudes it found itself in.
We’ve bred tomatoes for high productivity with no regard for their taste.
Circadian clocks are present in all kingdoms of life. They function to coordinate organisms’ metabolic and other activities with the external environment, notably the 24-hour light-dark cycle. Circadian clocks are known to regulate plant physiology and development and therefore almost certainly affect the agricultural fitness of crops; shockingly, they have not yet been intentionally manipulated.
Plants’ clocks are often adapted to specific localities, yet crops often grow over broader geographic ranges that encompass more local variation than wild plants see. So changes in circadian clocks may very well have occurred as a consequence of domestication and later breeding.
To test this hypothesis, a very international team of plant researchers examined 34 varieties of cultivated tomato plants, 44 varieties of its wild ancestor, and 24 varieties of another more distantly related wild plant. They measured two aspects of the circadian clock: the phase and the period.
The phase is the time between the peak of a particular behavior, in this case how the plants’ leaves behaved, and an external reference point, like nightfall. The period is the time between peaks of the behavior, so it is a stand-in for the time of one complete circadian cycle. Compared to the wild plants, the cultivated plants had a phase that was three hours later and a period that was two hours longer.
The delayed phase was found in the earliest domesticated types, Ecuadorian cherry tomatoes. The longer period didn’t show up until the plants moved to Mexico.
A comparison of the genomes of two wild species, with a cultivar called Moneymaker (not kidding), revealed that the cultivated tomato plant had a three-base-pair deletion in a gene already known to be important in circadian regulation, called EID1. It turns out that this deletion is found in all cultivated varieties with published genome sequences but not in any wild ones.
Putting the wild type EID1 into a cultivated tomato plant advanced its phase, so the gene seems to be causative. Moreover, the cultivated plants have very low genetic diversity in the region surrounding EID1, suggesting that it initiated a selective sweep—the trait it conferred was selected so strongly that it dragged neighboring genes along with it whether they were useful or not.
But why? As tomatoes were moved from the equatorial regions where they originated to Mesoamerica and then Europe, they faced longer summer days. The researchers suggest that the slower clock is an adaptation to help deal with that.
In addition to being slower than their wild cousins, plants with the deletion in EID1 are also shorter, flower later, and have more chlorophyll. The elevated chlorophyll content was the only one of these traits that was seen specifically when the plants were exposed to longer (simulated) days. This shift in light harvesting proteins might also enhance the plants’ performance as a crop.
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