Tomato Breeding for Organic Agriculture

tomResearch adviser: Dina St. Clair
Team Leader: Jake Uretsky –

Organic growers and seed producers in California have expressed an interest in open pollinated (i.e., inbred) tomato cultivars. At the Organic Breeder Roundtable in February 2015 attended by PI Brummer and co-PI St.Clair, participants indicated a particular need for small-fruited cherry tomatoes with diverse fruit colors, such as yellow, orange, purple, and variegated, to address the needs of their markets and desire of customers. Organic growers and seed producers strongly desire inbred cultivars that can be propagated via self-pollinated seed, which is not possible with F1 hybrid varieties that are produced by large commercial seed companies. Furthermore, commercial F1 hybrid tomato cultivars have not been bred for organic production environments, and some contain transgenes that are not permitted under USDA organic certification rules.

The goal of our project is to develop inbred cherry tomato cultivars with diverse fruit colors for organic production. Cultivated tomato (Solanum lycopersicum) is a naturally inbreeding annual, highly self-pollinated, and easily propagated by seed. Its generation time is approximately 5 months seed-to-seed. We will initiate our inbred cultivar development project by leveraging an existing germplasm collection at UC-Davis that includes heirloom and vintage (pre-1990) inbred cultivars. This tomato germplasm collection was begun in the 1940s by the late tomato breeder Jack Hanna (breeder of the first mechanically harvestable processing tomato varieties in the 1960s). The tomato collection has been added to, maintained, and documented for various traits during the past 27 years by Dina St.Clair, the current UC Davis tomato breeder.

The general plan for this project is as follows, with specifics to be developed by the plant breeding graduate and undergraduate students involved with the project. We will evaluate at least 40 diverse cherry-type and small-fruited inbred cultivars from the St.Clair tomato collection in the field at the UCD Student Farm. Students will evaluate fruit color, fruit size, yield and other key fruit and plant traits determined by the students in prior consultation with organic growers and seed producers. Students will analyze trait data to make parental selections. The students will design and implement one or more crossing schemes with the selected parents (e.g., a series of bi-parental crosses) to generate segregating populations (e.g., F2 or similar). A sample of each segregating population (at least 300 plants/population, or more, if space permits) will be grown and evaluated in the field for fruit color, fruit size and other key traits, and trait data will be used to make individual plant selections. Students will harvest self-pollinated seed from each selected F2 plant to create F3 families using the pedigree breeding method. F3 families will be grown in the field at the Student Farm and potentially on organic producer farms, evaluated for fruit color and other traits and selections will be made within families and between families and selfed seed (F4) obtained. Students will continue, in subsequent self-pollinated generations, cycles of trait evaluation in the field, family selection and propagation by selfing. By the F7 generation, each family (inbred line) is predominantly homozygous. The most promising inbred lines that may be acceptable as inbred cultivars will be evaluated in growers’ fields. Lines chosen as potential inbred cultivars will be re-tested in multi-location trials with growers and seed producers, and seed increased for public distribution.