In the plant world, fruit is an ovary. This is not a fanciful figure of speech, but the botanical description of every fruit’s form and function.
Consider the familiar tomato (Lycopersicum esculentum). Its fleshy integument packages rows of ovum-like seeds awaiting germination. New tomatoes growing on the vine are hard and green, a tactile and visual signal to the birds and beasts waiting to eat them that the time is not ripe literally — for consumption.
Once sunshine and internal ripening processes turn the fruit soft and red, its animal consumers gorge on the tempting fruit, and eventually excrete its undigested seeds. This natural function spreads them far, wide and fertile the fruit ovary’s method of propagating the species. While human tomato growers, buyers and consumers don’t take part in this evolutionary exchange, they prefer soft, red, ripe tomatoes to their hard, green precursors.
“In order to be fresh for the supermarket,” observed plant biologist Jim Giovannoni, “these tomatoes are harvested while they are still firm and unripe so they can survive the rigors of shipping, and have an extended shelf life. As a result, succulent tomato flavors typical of homegrown fruit do not get a chance to develop on the vine, and store customers complain about their blandness.”
In days gone by, when the fruit was grown on family farms and distributed to local grocery and produce outlets, the tomato’s life cycle suited everyone. But that was then. Now, with the advent of fast, refrigerated rail, road and air transport, the fruit grown wholesale on factory farms is shipped green, and left to ripen on supermarket shelves. That short shelf life isn’t very profitable.
It turned out that a crucial factor in ripening was a gaseous hormone called ethylene, secreted from the fruit. So produce suppliers store the just-delivered green tomatoes in sealed warehouse chambers suffused with exogenous ethylene, which jump-starts ripening but at a cost.
Biotech Creates Long-Life Flavr Savr’ Fruit
Then, in the 1980s biotechnology came on board. “Plant scientists at the University of California at Davis,” recalled Cornell University plant molecular biologist Jocelyn Rose, “introduced into tomatoes polygalacturonase, a protein that degrades cell walls in the fruits, and causes them to soften. They then licensed to Davis-based Calgene their time-bomb-ripening construct for turning off this cell-degrading enzyme. Ethylene’s role was to regulate when that gene gets switched on. So ethylene production within those fruits is normal. They turn red at the same time. The only difference there was that they didn’t turn on this enzyme that digests cell walls.”
Calgene labeled the genetically engineered product the “Flavr Savr tomato,” and now describes it as “the first biotechnology plant product to be approved for commercial production.” The tomato is a model for analysis of ripening, due originally to its significance as a food source and diverse germplasm. A number of tomato-ripening mutants have been useful for research and breeding.
So far so good but not far enough. Tomatoes are only one of numerous fruits demanding controlled ripening and shelf life. Today’s Science, dated April 12, 2002, reports a multipurpose solution, in a paper titled: “A MADS-box gene necessary for fruit ripening at the tomato ripening-inhibitor (rin) locus.” Its senior author is plant geneticist Jim Giovannoni, at Cornell University in Ithaca, N.Y.
“Our procedure,” said Giovannoni, “also should work with strawberries, bananas, bell peppers and melons, in addition to many other fruits for which shelf life and softening are problems, and which don’t respond to ethylene. This could reduce the use of ethylene to kick-start ripening after fruit has been shipped to the warehouse.”
The “MADS” acronym denotes the first four of a numerous family of genes. More than 100 MADS-box sequences have been found in species of microbes, animals and plants. In flowering plants, they are the molecular architects of growth.
The co-authors report discovering two tandemly located MADS-box genes in the tomato genome. One, called LeMADS-RIN (rin for short), and the other, mc, control development of the tomato’s sepal the collar of pointy green leaves that top the tomato. “Finding these genes,” Giovannoni pointed out, “provides the first molecular insight into a non-hormonal way to ripen fruit.”
His co-authors found that the rin gene-ripening inhibitor induces the ripening process at a point prior to ethylene gas production. Their paper reports that a tomato plant whose fruit cannot ripen carries a mutation in the rin gene encoding a MADS-box transcription factor. It causes a deletion of about 3 kilobases from the tomato genome, which results in fusion and inactivation of two adjacent genes in the MADS-box transcription factor family.
Antisense Upends Mutant Ripening Gene
The team was able to modulate the ripening process, creating tomatoes that become soft and red at controllable fast, medium or slow speeds. They inverted the mutated rin gene by means of antisense, thus shutting off the normal gene and putting brakes on the tomato’s ripening process.
Cornell’s Rose told BioWorld Today “precisely how Giovannoni’s particular gene is different from the strategy that was used for the Flavr Savr tomato. It targeted specifically the softening process, and the cell-wall enzyme. So that was one aspect of fruit ripening. The rin gene, in contrast, is a much more global master switch. It does softening but also other aspects of the fruit-ripening process. In this way one can target many ripening-related processes, not just softening. The identification of this rin gene,” Rose observed, “is likely to have more profound implications in terms of generating commercially valuable products than the approach that was taken with Flavr Savr.”
Giovannoni said, “Discovery of the rin gene provides a way to keep the tomato on the vine a little longer so that it develops more nutrients, color and taste, and yet is firm enough for shipping, and with a good shelf life. This means that tastier, more nutritious supermarket tomatoes are not far behind. As a bonus,” he added, “the longer a tomato remains on the vine, the more lycopene its endogenous antioxidant that inhibits cancer and heart disease is produced.
“For understanding tomato ripening, and eventually taste,” Giovannoni concluded, “this could be the Holy Grail.”