Why are tomato plants hairy?


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. Even with molecular techniques that enable breeders to locate the position of genes influencing qualities such as fruit form, size, disease resistance, or a variety of additional characteristics such as hair, the job is tedious.

Hair, yes. Plant hairs, known as trichomes, have numerous functions. They reduce transpiration, protect delicate tissue from sun or frost, capture moisture, and interfere with pest movement or feeding.

Therefore, when tomato breeders at the World Vegetable Center (WorldVeg) 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 quite remarkable,” Dr. Hanson stated. “It has the potential to transform the way tomatoes are grown all over the globe.”

(left to right) Peter Hanson, Mohamed Rakha , and David Johnson in the field with a very special tomato plant.

Wild relatives provide a significant contribution to the tomato gene pool.

The two breeders are working together to create enhanced, open pollinated tomato lines for the tropics with features that will assist small-scale farmers in dealing with pests, illnesses, changing climatic conditions, and market preferences. Breeding disease tolerance and pest resistance into vegetables is a cost-effective and efficient technique for supporting a sustainable and safe approach to agriculture, and it is the strategy that WorldVeg has pursued during its 40+ years as a worldwide centre 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. “There is no insect resistance in cultivated tomato varietals,” Dr. Rakha remarked. “If an infestation occurs, tomato producers have no option except to apply pesticides or lose their harvest.”

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. Producers may cut pesticide applications and operating expenses, produce healthier tomatoes for customers, and contribute to a better environment for everybody.

Wild tomato cousins prefer to produce tiny, grape-sized fruits on robust, hairy vines. Rakha and Hanson were interested by the ability of those hairs to repel bugs. The species, cultivar, tissue, ambient circumstances, and plant age all influence the kind, location, size, and density of trichomes. Tomatoes belong to the genus Solanum, which has seven distinct kinds 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. Around 350 USDA accessions were also assessed. 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, the trichome kinds and density of 2500 F2 (second generation) plants resulting from crossings between cultivated tomato and a wild cousin, S. galapagense, were assessed. Rakha and Hanson chose 200 candidates with varying concentrations of type IV trichomes from this group and used marker-assisted selection to choose those with 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 with high whitefly resistance were backcrossed to vulnerable tomato lines.

Plants are crossed, or bred, to create new lines with the best qualities of their parents. The plants generated are then backcrossed, or bred to one of the parent plants, to aid in the correction of a certain characteristic. A breeding aim might require several generations of backcrossing and years of effort to attain.

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 almost ten times the size of a typical S. galapagense fruit.” The new backcross produces fruits around the size of a tennis ball.

That’s huge—really huge. 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. Begomoviruses are to blame. Leaf curl hinders plant development; flowers seldom form and those that do fall off. Fruit output has been drastically decreased.

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’ tomato,” Dr. Rakha said. “Because of the close genetic link with cultivated kinds, an insect-resistant cherry tomato may be developed.” To that goal, he screened 1700 third-generation plants and chose 350 to generate a fourth generation for further evaluation. Cherry tomatoes are becoming more popular as new kinds are produced.

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 prominent research initiatives on wild tomato cousins include investigations on S. habrochaites at the University of Florida (Dr. Rakha’s alma mater) and S. pennellii at Cornell.

“It’s my ambition to discover pest resistance in farmed tomatoes,” stated Dr. Rakha. “It would assist so many farmers, particularly smallholders in Africa and Asia. That would be fantastic for farmers and consumers worldwide.”

Story and photos: Maureen Mecozzi

Related Questions

  • What are the little hairs on tomato plant?

    Tomato plant hairs are actually trichomes.
    Trichomes are covering your tomato plants. Some are lengthy and resemble white fuzz on a tomato stem. The shiny threads of the lengthy trichomes shooting out all around the stem may be seen in the shot below.

  • Why are my tomatoes Wooly?

    A. Often, the first tomatoes of the season or tomatoes grown in containers may have a mealy texture. Inconsistent soil moisture levels are to blame. This is more common in tomatoes in pots because it’s difficult to maintain consistent moisture in a contained area.

  • Do tomato plants usually have hairy stems?

    Tomato plants’ stems are often hairy. Tomato plants that are homozygous recessive for this feature have hairless stems. If the stem characteristics are determined by a single gene, what is the expected outcome of crossing two tomato plants that are heterozygous for hairy stems?

  • Do tomato hairs turn into roots?

    Take a close look at your tomato and you’ll notice lots of tiny silver “hairs” growing from the main stem of the plant. All of the “hairs” buried under the surface will develop into roots from the stem, resulting in a much stronger, healthier plant.

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