Tender, juicy, delicious tomatoes were a hallmark of my childhood and are still available from the plants I grow every summer. However, they have largely disappeared from stores. The ripe fruits cannot withstand shipping, so growers have focused on growing varieties where mutations have partially blocked the ripening process. These tomatoes stay firm longer, but at the cost of texture and flavor, as well as a decrease in their nutritional value.
Now researchers appear to have identified an enzyme that specifically helps soften the tomato during the ripening process. By suppressing its activity, they have hindered softening while leaving other aspects of the ripening process intact. The result is a ripe fruit that can stay at room temperature for two weeks and still remain firm.
In some ways, the surprise of these results isn’t that they happened; it’s that it took so long. A high-quality tomato genome sequence was first published in 2012, and it enabled researchers to identify more than 50 genes that likely code for proteins that can modify plant cell walls. Four of these genes were found to be active at high levels in the ripening fruit, so these genes were targeted through genetic engineering.
The results were, well, less than impressive. “Silencing their expression in transgenic tomato lines has led to very small, if any, observable changes in fruit softening,” the researchers wrote.
The team behind the new work instead decided to derive their signals from strawberries, where targeting a specific gene can reduce the fruit’s softening. That gene codes for a protein (pectate lyase) that breaks down pectin, a complex polymer made up of sugars. Pectin is an important part of the plant cell wall and helps form the glue that allows these cells to stick together. (Although you probably know it as part of the gel that makes jellies gelatinous.)
People had looked at the breakdown of pectin in tomatoes, but the first attempts didn’t work, so researchers paid no more attention to it. However, a quick search for transcribed genes showed that tomatoes activate at least five pectin-digesting genes in their fruit, and one is highly expressed during ripening. The team targeted this gene with a technique (called RNA interference) that limits the gene’s ability to be converted into a protein.
It worked. Tomatoes in which the gene was knocked out remained firm during the ripening process and were clearly more robust than their wild-type counterparts. This was most dramatically manifested by simply leaving both tomato varieties at room temperature for two weeks. The non-engineered tomatoes began to rot, while those with the gene knockdown remained intact.
In fact, the other aspects of maturation seemed to proceed normally. The weight and color of the engineered fruit were normal and they still produced all the chemicals we know are involved in the color, smell and taste of tomatoes.
However, the researchers themselves admit that they have not done one rather critical test: the taste test. And an important part of the experience of eating tomatoes is their texture. If all the researchers produced is a piece of cardboard that tastes like tomatoes, then it’s not clear whether their engineered plant represents much progress.
For the most part, the authors (some of whom work at the ag biotech company Syngenta) focus on ways to minimize the genetic engineering involved by simply eliminating the gene completely, rather than just knocking it down. Presumably, by avoiding any DNA introduced, they will avoid any stigma associated with GMO foods. But this could be a case where more engineering produces a better product – a product that remains firm through shipping and storage, but can go through the ripening process, such as by leaving it at room temperature for two days.
However, this is all speculation at the moment. But if Syngenta wants to mail some of them, I’m happy to compare them to the best my plants have to offer.
Nature Biotechnology2016. DOI: 10.1038/nbt.3602 (About DOIs).