Genetic stability analysis of in vitro micropropagated fire blight-resistant apple rootstocks

Apple (Malus domestica) is a major fruit crop in Korea, but its production is threatened by viral infections and bacterial diseases such as fire blight. To increase production sustainability, fire blight-resistant apple rootstocks from the Cornell-Geneva (Geneva® series), particularly G11 and G30, have been studied for their disease resistance and cold tolerance. This study aimed to establish an optimized in vitro propagation protocol using apical meristem culture, as no established medium composition was available for the efficient micropropagation of fire blight-resistant apple rootstocks. Meristems (0.2-0.3 mm) were excised from the axillary buds of G11 and G30 and cultured on Murashige and Skoog (MS) medium supplemented with various plant growth regulators (PGRs; 0.5, 1.0, 1.5 mg/L BAP, 0.5 mg/L zeatin, 0.2 mg/L kinetin, 0.5, 1.0 mg/L GA3, 0.2 mg/L 2ip, and 0.1, 0.2, 0.5 mg/L IBA) MS medium containing 2.0 mg/L BAP and 0.05 mg/L NAA showed high shoot induction rates of 95.8% (G11) and 100% (G30), with an average of 3.8 leaves per explant. The highest shoot proliferation rate (100%) and number of shoots (6.0 per explant) were obtained on MS medium supplemented with 0.5 mg/L BAP, 0.5 mg/L zeatin, 0.2 mg/L IBA, and 0.1 mg/L GA3. Rooting rates of 100% (G11) and 75% (G30) were achieved on 1.1 mg/L MS medium containing 0.2 mg/L IBA and 2.0 mg/L IAA. Genetic stability was assessed using 15 simple sequence repeat (SSR) markers, which detected 30 alleles across 20 micropropagated plantlets. Genetic stability was confirmed by the fact that all SSR profiles were monomorphic and matched those of the donor plants. These results support the effectiveness of employing meristem culture and SSR markers for the virus-free propagation and genetic authentication of apple rootstocks.