A morphological change in the fungal symbiont Neotyphodium lolii induces dwarfing in its host plant Lolium perenne

The endophytic fungus <i>Neotyphodium lolii</i> forms symbiotic associations with perennial ryegrass (<i>Lolium perenne</i>) and infection is typically described as asymptomatic. Here we describe a naturally occurring New Zealand <i>N. lolii</i> isolate that can induce dwarfing of <i>L. perenne</i> and suppress floral meristem development in the dwarfed plants. Further to this we demonstrate that the observed host dwarfing correlates with a reversible morphological change in the endophyte that appears associated with colony age. Mycelium isolated from normally growing plants had a typical cottony appearance in culture whereas mycelium from dwarfed plants appeared mucoid. Cottony colonies could be induced to turn mucoid after prolonged incubation and seedlings inoculated with this mucoid mycelium formed dwarfed plants. Mucoid colonies on the other hand could be induced to form cottony colonies through additional further incubation and these did not induce dwarfing. The reversibility of colony morphology indicates that the mucoid dwarfing phenotype is not the result of mutation. Ten isolates from other locations in New Zealand could also undergo the reversible morphological changes in culture, induce dwarfing and had the same microsatellite genotype as the original isolate, indicating that a <i>N. lolii</i> genotype with the ability to dwarf host plants is common in New Zealand.