The inoculation of plant species with mycorrhiza fungus Piriformospora indica results in enhancement of growth, increase in yield, and induction of resistance to biotic and abiotic diseases through improvement of the root system. The aim of the present study was to optimize in vitro propagation protocol for three indigenous apples (Malus × domestica) cultivars (ꞌGolbaharꞌ, ꞌSharbatiꞌ, ꞌSoltani Shabestariꞌ) and one commercial cultivar (ꞌGolden Deliciousꞌ). Furthermore, the efficiency of P. indica at rooting stage was investigated on three cultivars (ꞌSharbatiꞌ, ꞌSoltani Shabestariꞌ, ꞌGolden Deliciousꞌ). Establishment and proliferation stages were optimized by collecting explants at different seasons and comparing different culture media respectively. Rooting optimization included six treatments containing different concentrations of auxins in the presence or absence of P. indica. Results showed that at the establishment stage, a maximum percent of survival was observed in explants collected in spring. At the proliferation stage, different media had a divergent effects on distinct cultivars. Although all cultivars reacted favourable to micropropagation in MS (Murashige & Skoog 1962) basal medium, the presence or absence of cytokinin 2ip (N6-(2-Isopentenyl) adenine) in the culture media showed significant and incremental improvements in growth indices. In all cultivars highest rooting percent, root length, root thickness, and the number of roots/explant was observed in MS media containing auxins for three weeks followed by a treatment of MS medium containing P. indica for another three weeks. Plantlets treated with P. indica, grow stronger and healthier at the acclimation stage compared to the ones that excluded P. indica.
