Iron-grabber important for endophyte survival

Scientists at New Zealand’s largest Crown Research Institute, AgResearch, have made a crucial discovery on what makes endophytes succeed in ryegrass.

In a paper recently published in the prestigious journal PLOS Pathogens, lead author Dr Linda Johnson describes the tactic used by an endophyte to extract a vital nutrient, iron, from its ryegrass host.

​Paddocks of flourishing ryegrass are a typical New Zealand scene. Hidden within the grass, however, lies an invisible helper: an endophytic fungus that protects pastures from being decimated by unwanted pests, ultimately increasing pasture production and persistence.

The endophyte produces a small molecule, known as a siderophore, which grabs and binds strongly to iron, an element essential to both the host and fungal partner. This iron-grabbing trick is widespread among fungi, with common human fungal infections such as thrush and athlete’s foot depending on the iron they quietly filch from us. However, it is the fascinating result of experimentally deleting the iron-grabbing siderophore gene from an endophytic fungus that led to its publication in PLOS* Pathogens.

“Eliminating the siderophore caused major problems for both the fungal endophyte and the host grass plant,” explains lead author Dr Linda Johnson, AgResearch science team leader for Plant-Fungal Interactions. “The usual tightly controlled, synchronous growth of the fungus inside the ryegrass became deformed and unregulated. The host plants were stunted, and in extreme cases both the fungus and host plant died.”

This is the first report that siderophores are essential to the mutually beneficial relationship between ryegrass and endophyte. The relationship is crucial for New Zealand agriculture. The research is part of a large AgResearch programme to find better endophytes for our main pasture plant that produce bioactive compounds that protect pasture from insect attack and are also animal-safe.

Although ryegrass can exist without an endophyte, in New Zealand it is rapidly decimated by insects such as the Argentine stem weevil and black beetle.

AgResearch is the world’s major centre for endophyte research, and was the first to discover stock-friendly, anti-insect endophytes that are now commercially available and widely used nationally and internationally. They continue to search for even better ‘designer’ endophytes.

“We don’t yet have the best possible endophyte in terms of pest protection, animal safety and host compatibility,” says Dr Johnson. “We actively look for new endophytes from around the world, but to identify better performers that can live in pasture grasses we need to know more about the fungal-host interaction. To do that well, this fundamental research into how grasses and endophytes interact is essential.”

Fact sheet on endophytes

  • Endophytes are fungi that live inside grasses.
  • They are found around the world in most grasses, and often cause harm to insects and grazing animals.
  • Grasses benefit from hosting endophytes because they gain protection from insects and grazing animals. The endophyte benefits from the relationship by obtaining nutrients and protection from the grass plant, and by being spread via the plant’s seeds.
  • Some of the compounds made by ryegrass endophytes that cause harm to stock are Lolitrem B (which causes ryegrass staggers) and ergovaline (which causes heat stress). Different compounds give protection against insects.
  • Endophytes were used in warfare when horses were the main mode of transport: fleeing Argentinian Indians would purposely direct their horses towards endophyte-infected grass. Their pursuers, who were unfamiliar with the nature of the pasture, would let their horses graze, and the sleepiness and then death of their animals permitted escape by the Indians. (For more history see http://www.rci.rutgers.edu/~white/history.html)
  • The main endophytes used with ryegrass by New Zealand farmers are AR1 and AR37. They are sold inside ryegrass seed when it is bought from seed companies. AR1 does not produce compounds toxic to stock, but does produce compounds that are toxic to some insects. Ryegrass containing AR37 protects against a wider range of insects compared to AR1 but can cause some ryegrass staggers. AgResearch was a partner in the development of MaxQ®, a stock-friendly endophyte for continental tall fescue, the main US pasture crop.
  • The use of AR1 or AR37 endophytes has increased across the country, with parts of New Zealand that previously used ryegrasses without endophyte or with the naturally-occurring standard endophyte now moving to AR1 or AR37.
  • AgResearch have also developed an endophyte for airport grass. The compounds it produces make birds feel sick (but do not kill them) and repel insects. By reducing the number of birds at airports, it potentially reduces the incidence of bird strike, which is expensive and dangerous.