By: Dr. Moshe Bar, R&D Manager & Dr. Arthur Schaffer / moshe@zeraim.co.il

As the Company launches hybrid tomato cultivars from its "Taste Tomato" series, we present the history of the fruitful cooperation that has led to the success of the Project. The Company tomato breeding group, originally led by Dr. Moshe Bar and now by Ari Efrati, has been working together with a Volcani Center research group led by Dr. Ari Schaffer on a joint R&D project, with the goal of developing tomato varieties with improved taste, reminiscent of the 'tomatoes of old". The strategy adopted was to utilize the wild species of tomato to modify and improve sugar metabolism and accumulation in the commercial tomato fruit. The research project began in 1986 with the initial interspecific cross at the Volcani Center and, about 10 years later, the joint R&D project began, based on the tomato germplasm that had been developed.

The researchers began with a wild species of tomato found in Peru, Lycopersicon hirsutum, which is an inedible, small green and hairy fruit. But the researchers realized that underneath its hirsute skin lay a unique and genetically-determined sugar biochemistry that could theoretically be advantageous for improving sugar levels in tomatoes. Sugar content is the primary component determining tomato fruit taste, along with the acid and volatile aroma contents, and this became the target of a multifaceted research program to identify the genetic variability for sugar metabolism in the wild species and to introgress it into tomato germplasm.

One of the first traits to be transferred was that of sucrose accumulation. All cultivated tomatoes contain approximately equal amounts of the two hexoses, glucose and fructose, while the disaccharide sucrose is absent. Since practically all high-sugar fruits store sucrose, it was considered a promising strategy to transfer this trait to tomato. Sucrose accumulation is determined by a single recessive gene that encodes for the invertase enzyme that is responsible for the breakdown of sucrose. The wild species have no such invertase activity allowing sucrose to accumulate whereas the cultivated tomato has very high enzyme activity, preventing sucrose accumulation. During the research program, the trait of sucrose accumulation was successfully transferred by classical breeding techniques, molecular markers for the trait were developed and the breeding group was able to create novel varieties of sucrose-accumulating tomatoes. These have shown special promise in the Japanese market, seeking sweet, low-acid tomatoes.

The research and development is continuing as additional genes are being identified that further modulate sucrose accumulation. Major strides have also been made in understanding the molecular control of sucrose accumulation and even shedding light on the evolution of the cultivated tomato from a wild sucrose accumulator to the cultivated hexose accumulator to which we have grown accustomed.

Another strategy taken by the research group was to modify the young tomato fruit metabolism so that it stores a larger reservoir of transient starch that turns to sugar in the ripening fruit. The cultivated tomatoes store very little starch but we can identify tomato plants from the wild species crosses that had a higher sugar content in the ripe fruit due to a higher starch content in the young ovary. This has led to the identification of the gene responsible for the increase, enabling development of a molecular marker for use by the breeding unit and the Company molecular marker unit. The ongoing research has even led to an understanding of how the wild species sub-unit improves the starch synthesis in the tomato fruit and this serves as an example of what is referred to a molecular heterosis.

A third strategy adopted was to modulate the fructose and glucose levels in the tomato fruit, considering that fructose is twice as sweet as glucose. Therefore, a tomato fruit storing primarily fructose would be sweeter with the same amount of total sugar, or just as sweet as a regular tomato even with less sugar, allowing for the development of novel low-sugar tomatoes. At least two genes have been identified which act in consort to increase the ratio of fructose to glucose in the ripe fruit and germplasm with 3-4 fold fructose has been developed.

The two groups, Research and Breeding, have now been cooperating for nearly 10 years in a truly synergistic manner. Plant material, information and test results flow back and forth freely and this has been instrumental in the success of the joint project. All of this is part of Zeraim Gedera’s strategy to join research and breeding efforts to produce improved quality products.