Further understanding about the effect of ryegrass endophytes on pastoral productivity
New Zealand leads the world in research on the endophyte associations in ryegrass, which have positive and negative effects on the plants and on grazing livestock. The most recent scientific publication to arise from that research group is around the iron interactions of siderophore molecules produced by the endophytes and the symbiosis between ryegrass plants and endophytes.
Endophytes are fungal microbes that live in the tissues of varied groups of plants, including many grasses. The endophytic association with pasture grasses is a finely balanced inter-relationship whereby each partner is adapted precisely to the other. In general, these relationships are not well understood but they are responsible for debilitating animal diseases. However, it has also been shown that they are critical for plant protection against insect pests. Today new endophyte strains possibly contribute around $200 million each year to the New Zealand economy. Differences amongst endophytes can actually make a greater difference to grass properties than genetic variation of the grass host itself and therefore they have huge impacts on livestock productivity.
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.
The importance of endophytes was first demonstrated in ryegrass staggers in sheep, and thereafter in liveweight gain, heat stress, dag burden and fly strike, along with reproductive performance. Dairy cattle can also be affected and even other nearby plants, like white clover. The first response of scientists was to eliminate high endophyte grass to deal with the adverse animal effects (nil-endophyte varieties), but these are very vulnerable to insect attack. The endophyte associations provide insect pest resistance, to Argentine stem weevil, African black beetle and mealy bug.
Research then concentrated on endophyte strains (novel endophytes) which are not harmful to livestock but still confer insect pest resistance. These came largely from Europe and after pioneering work done in growing, storing and innoculating seed with endophytes, ryegrass varieties with novel endophytes were trialled, evaluated and brought to market – Endosafe, AR1, AR37 and some others.
Also endophytes associated with tall fescue pasture crop, widely used in the United States, have been developed that don’t have toxins that cause fescue toxosis.
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. The iron-grabbing siderophore gene can be experimentally deleted from an endophytic fungus to observe the results. Elimination caused major problems for both the fungal endophyte and the host grass plant, because 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 shows 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.
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.
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. It continues 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, fundamental research into how grasses and endophytes interact is essential.”