An Invisible Alliance
Across the vast temperate grasslands of New Zealand, an invisible partnership plays out within the tissues of perennial ryegrass (Lolium perenne) and other grasses. This is a mutualism between the grass and an endophytic fungus from the genus Epichloë. The fungus, an endophyte, lives its entire life cycle within the host plant's leaves and stems, existing as a network of fine filaments called hyphae in the spaces between the plant's cells.
This relationship is obligate for the fungus, meaning it cannot survive in the wild without its grass host. For the grass, the benefits are substantial. The Epichloë endophyte produces a potent cocktail of chemical compounds called alkaloids. These alkaloids provide the grass with increased protection against insect pests and improve its tolerance to environmental stresses like drought. In return, the fungus receives shelter and nutrients from the grass. The success of New Zealand's pastoral economy is significantly influenced by this powerful symbiotic relationship.
The alliance is passed down from one generation to the next with remarkable efficiency. The fungus is transmitted vertically; its hyphae grow into the developing seeds of the parent grass. When the seed germinates, the new seedling is already colonized by the fungus, ensuring the partnership continues. This maternal inheritance means the fungus never has to endure a free-living stage, traveling securely from one plant generation to the next.
A Chemical Defense with Consequences
The protective alkaloids produced by Epichloë are a double-edged sword. While they are highly effective at deterring insects, some are toxic to the livestock that graze on the infected grasses. The main alkaloids of concern are lolitrem B and ergovaline.
Lolitrem B is a potent neurotoxin that causes a condition known as "ryegrass staggers" in sheep, cattle, horses, and deer. Animals suffering from this condition exhibit tremors, head nodding, a stiff gait, and may stumble and fall, especially when stressed. The toxin is concentrated at the base of the leaf sheaths and in the seeds, so animals are at higher risk during summer and autumn when they graze pastures more closely.
Ergovaline, an ergot alkaloid, is a vasoconstrictor that can cause "fescue toxicosis". This condition leads to heat stress, reduced weight gain, and poor reproductive performance in grazing animals. A third major alkaloid, peramine, is beneficial from an agricultural perspective. It deters feeding by major pasture pests like the Argentine stem weevil (Listronotus bonariensis) but has little to no toxicity for livestock. The discovery of this relationship was a major development for New Zealand agriculture, which previously faced annual losses of up to NZ$200 million from this one insect pest.