Big, hairy tomatoes
With strong natural defenses against several common pests, this tomato bred from wild relatives could provide safer production of one of the world’s most popular crops.
A tomato breeder’s life is one of long quiet hours spent in the lab and in the field, evaluating thousands upon thousands of seedlings in the hope of finding a few—or even just one—with traits of interest than can passed on to future generations. It’s painstaking work, even with the molecular tools that allow breeders to pinpoint the location of genes governing traits for fruit shape, size, tolerance to diseases, or a myriad of other characteristics, such as hair.
Yes, hair. Plant hairs, called trichomes, serve several purposes. They reduce transpiration, protect delicate tissue from sun or frost, capture moisture, and interfere with pest movement or feeding.
So, when World Vegetable Center (WorldVeg) tomato breeders Mohamed Rakha and Peter Hanson found one of their crosses resulted in a hairy vined, pest-resistant tomato plant that produced big, good-tasting fruit in the first generation—a very early stage in the long, meticulous process of plant breeding—they could barely believe their good fortune. “This is pretty special,” said Dr. Hanson. “It has the potential to change how tomatoes are produced worldwide.”
Wild relatives make a big splash in the tomato gene pool
The two breeders work to develop improved, open pollinated tomato lines for the tropics with the traits to help small-scale farmers cope with pests, diseases, changing climate conditions, and market preference. Breeding disease tolerance and pest resistance into vegetables is a cost-effective and efficient strategy to support a sustainable and safe approach to agriculture, and it is the strategy WorldVeg has followed over its 40+ years of existence as a global hub for vegetable research.
Although the cultivated tomato (Solanum lycopersicum) varieties farmers rely on have many good and desirable characteristics, they do lack one very important trait. “Cultivated tomato varieties have no insect resistance,” said Dr. Rakha. “If there is an infestation, tomato farmers have little choice but to use pesticides or lose their crop.”
A commercial cultivated tomato variety able to resist the predations of whitefly, spider mites, Tuta absoluta and other troublesome pests would be of great benefit to everyone. Farmers could reduce pesticide applications and their costs for operation, produce safer tomatoes for consumers, and contribute to a healthier environment for all.
Wild relatives of cultivated tomato tend to produce small, grape-sized fruits on vigorous, hairy vines. The potential of those hairs to repel pests intrigued Rakha and Hanson. The type, location, size, and density of trichomes vary according to species, cultivar, tissue, environmental conditions and plant age. Plants in the genus Solanum, in which tomato is classified, have seven different types of trichomes. Types I, IV, VI and VII are attached to glands that secrete compounds including acylsugars, methylketones, sesquiterpenes and other compounds that can repel insects; type IV trichomes are particularly noteworthy for convincing pests to munch elsewhere.
The researchers plumbed the depths of the WorldVeg Genebank tomato collection for S. galapagense, S. pimpinellifolium, S. cheesmanii, and S. lycopersicum var. cerasiforme accessions. More than 350 accessions from the United States Department of Agriculture (USDA) collection were also evaluated. The breeders were able to identify seven accessions of S. galapagense, and one accession each of S. pimpinellifolium, S. cheesmanii, and S. lycopersicum var. cerasiforme with high levels of insect resistance. In 2016, 2500 F2 (second generation) plants derived from crosses between cultivated tomato and a wild relative, S. galapagense were evaluated for trichomes types and density. From this group, Rakha and Hanson selected 200 candidates with different densities of type IV trichomes and used marker-assisted selection to choose those that also had known disease resistance genes (Ty-3, Ty- 2 [Tomato leaf curl virus disease]; Bwr-12 [Bacterial wilt]; I2 [Fusarium wilt]; and Mi-1 [nematode resistance]).
The 30 F2 plants that met the trichome/disease resistance gene requirement were put to the test: all were exposed to whiteflies in a “no-choice” assay (whiteflies are forced to feed from only one type of plant, which they either attack or ignore). Five plants showed strong resistance to whitefly, which were backcrossed to susceptible tomato lines.
Plants are crossed, or bred, to develop new lines with the good characteristics of the parents. The plants produced are then backcrossed, or bred to one of the parent plants, to help fix a particular trait. It can take many generations of backcrossing and years of work to achieve a breeding goal.
One of the five tomato specimens planted out in the field as a first generation backcross produced a remarkable result. “It had especially large fruit size,” said Dr. Rakha. “This is more than 10 times the size of normal S. galapagense fruit.” The fruits from the new backcross are about the size of a tennis ball.
That’s big news—really big. It means this good-sized tomato with pest and disease resistance can be developed faster, and its traits bred into other tomato lines, so that tomato growers can produce a globally popular vegetable crop without intensive pesticide use.
“Our next step is to backcross the line with other lines that have demonstrated resistance to leaf curl, bacterial wilt, and nematodes,” said Dr. Hanson. Leaf curl—the yellowing, crumpling and upward curling of leaves—is the most destructive disease of tomato, and it can be found in tropical and subtropical regions. It is caused by Begomoviruses. Leaf curl stunts plant growth; flowers usually will not develop and those that do simply drop off. Fruit production is significantly reduced.
Hanson and Rakha are also crossing cultivated tomato to S. pimpinellifolium (closest wild tomato relative) and S. lycopersicum var. cerasiforme (cherry tomato). “S. lycopersicum var. cerasiforme is the ‘gateway’ to cultivated tomato,” said Dr. Rakha. “The close genetic relationship with cultivated types means it may be possible to develop an insect-resistant cherry tomato.” To that end, he has screened 1700 third-generation plants and selected 350 to develop a fourth generation for further review. Cherry tomatoes are growing in popularity as new varieties are introduced.
The World Vegetable Center’s tomato breeding program is one of the few programs worldwide working with wild relatives S. galapagense, S. pimpinellifolium, and S. lycopersicum var. cerasiforme. Other notable research programs investigating wild tomato relatives include studies at the University of Florida (Dr. Rakha’s alma mater) on S. habrochaites and Cornell’s research on S. pennellii.
It’s my dream to find insect resistance in cultivated tomato,” Dr. Rakha said. “It would help so many farmers, especially smallholders in Africa and Asia. Really, it would be a great thing for farmers and consumers everywhere.”
Story and photos: Maureen Mecozzi