Exogenous application of melatonin improves eradication of apple stem grooving virus from the infected in vitro shoots by shoot tip culture

Production and maintenance of virus-free planting materials is pivotal for the control of viral diseases. The present study attempted to test exogenous application of melatonin for eradication of apple stem grooving virus (ASGV) from virus-infected in vitro shoots of apple cultivar Gala. Exogenous application of 15 mu m melatonin to the shoot proliferation medium significantly increased the number of shoots and shoot length. The level of endogenous indole-3-acetic acid (IAA) was the highest in the shoots proliferating on the shoot proliferation medium containing 15 mu m melatonin. Shoot regrowth levels were significantly higher in shoot tips of the virus-infected shoots cultured for 4 weeks on this medium than the control. In addition, culture of shoot tips of the virus-infected in vitro shoots proliferated for 4 weeks on this medium resulted in 95% of shoots being virus-free, while no virus-free shoots were obtained in shoot tips of the virus-infected shoots cultured without melatonin. Analyses by microtissue direct RT-PCR and RT-qPCR showed that ASGV concentration decreased in shoot tips of the virus-infected shoots proliferating on the medium containing 15 mu m melatonin for 4 weeks. Virus localization showed that exogenous application of melatonin enlarged the virus-free area in the virus-infected shoot tips. These data provide explanations as to why exogenous application of melatonin can efficiently eradicate ASGV. Exogenous application of melatonin provides an alternative means for plant virus eradication and has the potential to produce virus-free plants.