Agronomic performance and genetic characterization of sugarcane transformed for resistance to sugarcane yellow leaf virus

Sugarcane yellow leaf virus (SCYLV, a Poleovirus of the Luteoviridae family) is already widespread in Florida, and resistance in the Canal Point (CP) sugarcane population is limited. Genetic transformation of sugarcane for disease resistance holds promise but tissue culture and transformation processes produce undesirable agronomic characteristics necessitating thorough field evaluation. A 3-year sugarcane (a complex hybrid of Saccharum species) field study was conducted in Belle Glade, FL with the following objectives: (1) thoroughly evaluate the agronomic performance of two transgenic lines transformed for SCYLV resistance (6-1, 6-2) compared with parental cultivar control CP 92-1666, (2) determine level of SCYLV resistance in the transgenic lines, and (3) characterize genetic differences in the transgenic lines compared with CP 92-1666 using simple sequence repeat (SSR) genotyping. Sugarcane yields of CP 92-1666 were superior to both transgenic lines, as well as tissue culture (C-1) and nptII marker gene (20-1) controls, in the plant cane, first ratoon and second ratoon crops. CP 92-1666 recorded an average of 6.5–8.7 tons sucrose ha−1 yr−1 more than genotypes subjected to tissue culture and biolistic transformation. However, SCYLV infection rates in transgenic lines were only 0–5%, compared with 98% in CP 92-1666. Kanamycin field assays indicated that selectable marker gene nptII was stably expressed in all co-transformed lines. SSR genotyping showed 35 additional fragments to be present and 25 existing fragments absent among 6-1, 6-2, C-1 and 20-1 compared with CP 92-1666. Although all clones had unique genotypes, the four regenerated clones showed a greater genetic distance from the donor clone CP 92-1666 (mean GD 0.4) than to one another (mean GD 0.03). This study reports the first successful gene transfer of SCYLV resistance in sugarcane and the first report of variations in microsatellite repeat number associated with regeneration from embryogenic callus. Our results highlight the potential of genetic transformation methods to incorporate desirable traits into sugarcane, combined with the necessity of thorough agronomic evaluation of transgenic genotypes. Transgenic lines 6-1 and 6-2 are being used as parents in crosses designed to combine SCYLV resistance from these genotypes with agronomic characteristics of high-yielding materials.