GRASS ENDOPHYTES AS A MODEL
GRASS ENDOPHYTES AS A MODEL
AgResearch, Private Bag 11008, Palmerston North, New Zealand
Fungi in the genera Epichloe and Neotyphodium are the most commonly studied endophytes of grasses, although there are fungi in the genera Atkinsoneiia, Myriogenospora and Balansia which can be considered endophytic. The relationship between grass host and fungal endophyte is a mutualistic one and is important for the survival of both participants. Researchers are using species of Neotyphodium infecting Festuca and Lolium grasses as models to investigate the many aspects of the relationship.
Neotyphodium endophytes are present in grasses as intercellular mycelium and nutrients are obtained from the host plant. Sporulation does not occur in or on the host and so transmission of the endophyte is only through mycelium in the seed. Hence, survival of the fungus is assured provided that the host plant does not die. The mutualistic benefit of the fungus to the grass depends upon a host of chemicals produced by the fungus which directly or indirectly assist the survival of the grass. Some of these chemicals and their function have been identified, many have not. Endophyte-infected grasses tolerate drought stress better than do than endophyte-free grasses. Although the phenomenon has been studied the mechanisms are still unknown. Theories advanced include stomatal resistance, osmotic adjustment, carbohydrate accumulation, phytohormones and nematode predation of the roots. Endophyte-infected grasses are protected from predation by some species of insects and nematodes and from infection by some plant disease organisms.
The recognition that endophytes are beneficial to grass survival and persistence has been recognized by turfgrass breeders and in the USA most turf cultivars released in recent years are infected with endophytes. However, farmers are more reluctant to sow endophyte-infected grasses because the endophytes present in them produce a range of toxic alkaloids which affect animal health. These alkaloids are beneficial to plant survival because infected grass is sometimes not as palatable to animals as endophyte-free grass and can cause intoxication, weight loss, reproductive problems or gangrene of extremities, depending upon the host grass and endophyte. Management practices can alleviate the effect of endophyte toxins but it would be desirable to have grasses which are either endophyte-free or infected with endophytes that cannot produce the animal toxins.
Because endophyte-free tall fescue and perennial ryegrass is generally more persistent than endophyte-free grasses it is difficult in some countries to retain endophyte-free grass in pastures. The alkaloids responsible for some of these endophyte-induced animal disorders are known. We have found a few strains of Neotyphodium endophytes which do not produce the alkaloid lolitrem B, which is the toxin responsible for Ryegrass Staggers, a neurological disorder of animals grazing endophyte-infected perennial ryegrass. A ryegrass cultivar infected with a nontoxic endophyte has been available to farmers in New Zealand for several years and soon a superior strain of nontoxic endophyte will be available in many cultivars of ryegrass. Strains of Neotyphodium from tall fescue grass are also available which do not contain the alkaloid ergovaline, thought to be the cause of Fescue Toxicosis. Hopefully, these strains will be available in tall fescue cultivars in 2-3 ;years provided that grazing trials continue to be satisfactory.
The interaction between different strains of endophytes and host plant genotypes is now being studied. We find that different strains of endophytes can affect plant growth and seasonal production and that alkaloid production is influenced greatly by plant genotype. These findings could account for some of the discrepancies in the results that researchers have reported when investigating the effects that endophytes have on their grass hosts. Stable mutualism has developed between host plant and endophyte over many years. One should therefore be careful when making new combinations of grass and endophyte that the relationship is a stable one. Although only a few grass-endophyte combinations have been studied, the use of endophytes, nontoxic to animals, for biological control of pests and diseases and assisting the growth and persistence of grasses is now a reality for these models.