Genetic control of in vitro organogenesis in Petunia x hybrida Hort.

Despite the fact that Petunia x hybrida plant has become a model plant in genetic field, there are few reports about genetic model of in vitro organogenesis. The aim of the present project was to formulate a hypothesis on the number of genes involved in the control of in vitro organogenesis in Petunia x hybrida. In previous experiments, we had already characterised 16 pure genotypes for their performances in vitro. Among them, we chose two lines, here reported as “K” and “L”, that showed opposite behaviours in culture. Line K showed the best performance, whilst line L was the worst for each of the following characters: callus formation ability (97.8%; 76.7% respectively), organogenesis ability (80.7%; 36.2% respectively) and average number of differentiated shoots per explant (6.7 ± 4.6; 2.6 ± 1.9 respectively). Reciprocal crosses between these lines were performed; further, leaf explants of F1 progenies (KxL; LxK) were induced to differentiate in vitro on Murashige and Skoog medium added with 30 gl-1 sucrose, 0.2 mgl-1 IAA, and 1.0 mgl-1 BAP, pH 5.8. Data on callus proliferation, explant shoot differentiation and number of differentiated shoots per explant were scored after 7, 14, 21, 28, 34 and 41 days of in vitro culture. The frequencies of explants producing callus in each F1 progeny (100.0% and 96.0%) were similar to the parental K genotype, but callus proliferation happened earlier than the parental K line. These results suggested the involvement of dominant and additive genes. Interestingly, the frequencies of leaf explant differentiation (94.0% and 91.7%) and the average number of shoots per differentiated explant (11.6 ± 4.9 and 9.7 ± 4.1) outperformed the values observed in the line K, hence suggesting a heterosis effect. Moreover, the progenies of the two reciprocal crosses showed significant differences for each of the above mentioned characters, suggesting a maternal effect. F2 seeds, obtained after self cross of F1 plants (KxL or LxK hybrids) were sown in vitro. Twenty one seedlings per each reciprocal cross progeny were collected and their leaves were cultured on the above mentioned differentiation medium. Data score, carried out as for the F1 progeny, was performed per each F2 progeny. The frequency of callus proliferation observed in both F2 progeny was according to a segregation rate of 13:3. This result suggested the involvement of two loci with a dominant and recessive epistatic action. In contrast, a segregation rate of 9:7 was observed in the frequency of explants differentiated shoots. This segregation rate was according to a genetic model of two complementary loci. Finally, segregation rate of F2 progeny which were differentiated a high or a low number of shoots per differentiated explant was doubtful. Result allowed us to hypothesize the presence of two epistatic loci but was not possible to understand what kind of epistatic interaction was present.