Identifying somaclonal variants of the apple rootstock Malling 7 resistant to white root rot

A study was conducted with the objective to develop a technique for in vitro screening of cells/shoots of apple rootstock Malling 7 (M7) against Dematophora necatrix culture filtrate. Large amounts of profuse, creamish white and friable calli was obtained from leaf explants when cultured on MS medium supplemented with 5 mg/l naphthalene acetic acid (NAA) and 1 mg/l Benzyladenine (BA). Adventitious shoots were regenerated from leaves cultured on MS medium containing BA (4.0, 4.5 mg/l) or thidiazuran (TDZ, 0.2–1.0 mg/l) and indole-3-acetic acid (IAA), indole butyric acid (IBA) or NAA (0.5–1.5 mg/l) for 6 weeks. Friable callus as well as regenerants were exposed separately to selection medium fortified with different levels (10–90%) of fungal culture filtrate (FCF) of D. necatrix to isolate tolerant cells/regenerants. 5.18% callus lines and 40% shoots survived on 70% FCF which were further subjected to three cycles of treatment using continuous in vitro selection approach. The surviving calli were subcultured on shoot regeneration medium to obtain plantlets but were unable to regenerate. This study also examined total sugars, phenol, protein content and activity of enzyme polyphenol oxidase (PPO) in FCF tolerant calli, and found higher levels of all except sugars. FCF tolerant and unselected regenerants were rooted and hardened. These plants were then screened for disease tolerance by planting in pathogen infested soil in pots. The data revealed that all five regenerants survived over control after 60 days of inoculation, thus showing resistance to white root rot disease.