In vitro interactions between Mentha piperita L. and a non-mycorrhizal endophyte: root morphogenesis, fungus development and nutritional relationships

Field grown Mentha piperita can be colonized by a fungal endophyte, PGP-HSF, which heavily infects roots and enhances growth of in vitro grown plants. In this work, the time course of fungal development and its morphogenetic effects on plant growth, as well as phosphorus shoot concentration, were analyzed. Fungal infection induced longer shoots and roots and increased adventitious root branching. Microscopic analyses showed that hyphae grew between the walls of rhizodermal cells, produced a progressively thicker sheath and colonized the senescent rhizodermal cells. Plant mechanical defences developed early after the contact between hyphae and rhizodermal cells, and specialized host-fungus interfaces were not observed. Phosphorus concentration was lower in colonized plants than in controls. Our data support a passage of carbon from the plant to the fungus, but no transfer of mineral nutrients from the fungus to the root. The role of PGP-HSF on the enhancement of host growth is discussed.