In vitro rooting and acclimatization of ‘Marubakaido’ apple rootstock using indole-3-acetic acid from rhizobacteria

Rooting tissue-cultured shoots and acclimatizing the rooted plantlets are key final steps in successful micropropagation protocols. We assessed the effect of indole-3-acetic acid (IAA) from synthetic and biological sources for effects on in vitro rhizogenesis and acclimatization stages in micropropagated ‘Marubakaido’ (Malus prunifolia) apple rootstock. Shoots of 3 cm length were transferred to rooting medium, composed of Murashige and Skoog (MS) medium supplemented with up to 2 mg L AIA and grown for 60 days in a controlled environment before assessment. Root initiation rate, callus formation, dry-root biomass, dry-shoot biomass, root length, root volume and diameter were evaluated. After the acclimatization stage, survival rate was determined. The root initiation rate of micropropagated ‘Marubakaido’ apple rootstock was higher in culture media supplemented with IAA, independent of the source. Callus formation at the base of shoots was higher according to increases of synthetic IAA concentration. In contrast, there was no callus formation from shoots cultured on MS medium supplemented with bacterial. ‘Marubakaido’ shoots rooted in vitro were successfully acclimatized. Survival of the plantlets during acclimatization was affected by both IAA concentration and source. Plant survival rate at the acclimatization stage decreased from 96% to 66% as the concentration of synthetic IAA increased. 'Marubakaido' inoculated in MS medium supplemented with bacterial IAA had thinner roots without callus formation, resulting in higher survival (up to 98 %) during acclimatization. Strain N39 was particularly effective in inducing in vitro rooting and acclimatization of ‘Marubakaido’ apple rootstock, resulting in high-quality rooted plantlets.