Jessica Chitwood-Brown, Gary E. Vallad, Tong Geon Lee & Samuel F. Hutton

Theoretical and Applied Genetics July 2021; vol. 134: 2129–2140.

Key message

Reducing the size of the I-3 introgression resulted in eliminating linkage-drag contributing to increased sensitivity to bacterial spot and reduced fruit size. The I-7 gene was determined to have no effect on bacterial spot or fruit size, and germplasm is now available with both the reduced I-3 introgression and I-7.

Abstract

Tomato (Solanum lycopersicum) production is increasingly threatened by Fusarium wilt race 3 (Fol3) caused by the soilborne fungus, Fusarium oxysporum f. sp. lycopersici. Although host resistance based on the I-3 gene is the most effective management strategy, I-3 is associated with detrimental traits including reduced fruit size and increased bacterial spot sensitivity. Previous research demonstrated the association with bacterial spot is not due to the I-3 gene, itself, and we hypothesize that reducing the size of the I-3 introgression will remedy this association. Cultivars with I-7, an additional Fol3 resistance gene, are available but are not widely used commercially, and it is unclear whether I-7 also has negative horticultural associations. To characterize the effect of I-3 on fruit size, segregating populations were developed and evaluated, revealing that the large I-3 introgression decreased fruit size by approximately 21%. We reduced the I-3 introgression from 5 to 140 kb through successive recombinant screening and crossing efforts. The reduced I-3 introgression and I-7 were then separately backcrossed into elite Florida breeding lines and evaluated for effects on bacterial spot sensitivity and fruit size across multiple seasons. The reduced I-3 introgression resulted in significantly less bacterial spot and larger fruit size than the large introgression, and it had no effect on these horticultural characteristics compared with Fol3 susceptibility. I-7 was also found to have no effect on these traits compared to Fol3 susceptibility. Together, these efforts support the development of superior Fol3-resistant cultivars and more durable resistance against this pathogen.

See: https://link.springer.com/article/10.1007/s00122-021-03810-5

Figure 1: Breeding strategy used to obtain a minimal I-3 introgression. The original introgression size exceeded 5.0 Mb. Recombinant inbred lines (RILs; derived from R12 and R18 (Li et al. 2018)) resulting from recombination events on either side of I-3 were intercrossed, and the F₁ was self-pollinated. Flanking markers 7g728 and 7g6362 were used to screen an F₂ population and identify the desirable of two possible products of crossing-over which resulted from further recombination within the overlapping homologous region. A plant containing the minimal I-3 introgression was self-pollinated to obtain a homozygous breeding line, Fla. 8978.