Seed transmission of phytoplasmas: evidence, biological constraints, and perspectives

Phytoplasmas are wall-less bacteria living in the phloem tissue, associated with economically significant diseases in a wide range of crop species. They are traditionally thought to be transmitted by vegetative propagation and phloem-feeding insect vectors. Seed transmission in phytoplasmas is a controversial topic unlike many fungi, bacteria, viruses and nematodes that have long been known as seed-transmissible pathogens. In recent years, reports of the presence of phytoplasmas in embryos, seeds and seedlings have sparked new questions on its biological feasibility and epidemiological relevance. Evidence accumulated over the past two decades that phytoplasma DNA could be present in embryos, seeds and seedlings and can be carried over generations. However, the studies have some methodological weaknesses such as lack of standardization in protocols, viability assessment, long-term studies and limited independent validation. Current evidence is very strong that phytoplasma DNA is present in seeds, but the involvement of viable and infectious phytoplasmas in seed transmission is not yet established. It is important to answer this question since the seed transmission of phytoplasmas would impact disease epidemiology, management, seed certification and international phytosanitary regulations.