One approach for obtaining useful genetic variation is to select for somaclonal variants generated by tissue culture techniques. Increased levels of resistance to bacterial leaf spot (Xanthomonas campestris pv. pruni) have been observed in toxin-selected and unselected peach regenerants in vitro, in the greenhouse and under field conditions. Peach regenerants have also demonstrated increased levels of bacterial canker (Pseudomonas syringae pv. syringae) and root-knot nematode (Meloidogyne incognita) resistance. Random amplified polymorphic DNA (RAPD) primers have been used to study genetic variation at the DNA level among the somaclonal variants. Sixty RAPD primers (10-mers) were screened and 10 proved useful as markers to detect polymorphisms, thus establishing a genetic basis for somaclonal variation. These studies demonstrate the feasibility of using tissue culture techniques to generate fruit trees with increased levels of disease resistance.
Freddi A. Hammerschlag, Ghazala Hashmi, Robin Huettel, Dennis Werner, and David Ritchie
Ghazala P. Hashmi, F.A. Hammerschlag, R.N. Huettel, and L.R. Krusberg
Somaclonal variation has been reported in many plant species, and several phenotypic and genetic changes, including pathogen and pest resistance, have been described. This study was designed to evaluate somaclonal variation in peach [Prunus persica (L.) Batsch] regenerants in response to the root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood. Regenerants SH-156-1, SH-156-7, SH-156-11, and SH-156-12, derived from `Sunhigh' (susceptible) embryo no. 156, and regenerants RH-30-1, RH-30-2, RH-30-4, RH-30-6, RH-30-7, and RH-30-8, derived from `Redhaven' (moderately resistant) embryo no. 30, were screened in vitro for resistance to the root-knot nematode. Under in vitro conditions, fewest nematodes developed on regenerants SH-156-1 and SH-156-11, `Redhaven', and all `Redhaven' embryo no. 30 regenerants. The most nematodes developed on `Sunhigh', `Sunhigh' seedlings (SHS), and regenerant SH-156-7. Nematodes did not develop on `Nemaguard'. In greenhouse tests, fewer nematodes developed and reproduced on the no. 156-series regenerants than on `Sunhigh'. Under in vitro conditions, significant differences among uninfected (control) regenerants, cultivars, and rootstock `Nemaguard' were observed for shoot height and fresh root weights. Significant differences were also observed among infected regenerants, cultivars, and `Nemaguard' for these characteristics, but differences were not observed between control and infected regenerants. Different concentrations of α-naphthaleneacetic acid in half-strength Murashige and Skoog salt medium induced rooting of two peach cultivars, one rootstock, and four regenerants.