Endophytes - the phyte back begins here?
Endophytes are predominantly fungi that inhabit the intercellular space, forming mutualistic relationships with their host plant. The endophyte grows within the stem, sheath and leaf of the developing grass plant and shows no visible signs externally. Ultimately, the endophyte grows into the flower and seed.
Endophytic fungi do not produce spores and, as such, the only way they can naturally propagate is to be transferred within infected seed and develop with the plant as it grows. It's also true that these endophyte-host symbioses are natural and have existed for many millions of years and perhaps as long as grass itself.
The mutualistic benefits conferred to the endophyte are the assimilation of carbohydrates from the host, a host body within which to grow and a vessel within which to propagate future progeny. The benefits to the grass plant are numerous and are the main focus of the article.
The fungi are of the genus Neophytodium and have been shown to infect Perennial Rye and Fescue grasses. Efforts to incorporate into Smooth Stalk Meadow Grass and Bent grasses have been unsuccessful and these grasses remain endophyte free. Of particular interest is Perennial Rye grass as this grass is widely used on sports turf surfaces throughout the UK, with many football, rugby and cricket grounds purporting to be 100% Perennial Rye grass.
Studies into endophyte plant interactions are relatively few when considering sports turf and, as with much research on grasses, it is mostly but not exclusively driven by the agricultural sector. It has been understood for some time that endophytes produce compounds called alkaloids as secondary metabolites within the plant that can act as 'systemic pesticide' to certain pests or pathogens. A number of turf grass pathogenic fungi have been shown to be controlled by certain endophyte-host symbioses. These include but are not exclusive to Microdochium nivale and Pythium spp.
There is also evidence of problems surrounding feeding by cattle due to accumulated alkaloids (Fescue toxicosis) produced by the endophytic fungi. The production of alkaloids are not detrimental to the host plant; they, in fact, produce their own when under attack by pests. It is, however, a boost to the host plant's defences. The agricultural market, particularly in New Zealand and the United States of America, has developed a number of grass cultivars infected with endophytes that are lacking the alkaloids that lead to Fescue toxicosis in cattle, but still contained those alkaloids that inhibit pest attack.
The potential for sports turf grass is immense. We are not concerned about cattle feeding on sports turf, so an endophyte-host symbiosis with the broadest pesticidal action could be sought. Of note is the Avanex™ tall fescue cultivar that has been used on airports to reduce pest incidence and, ultimately, potentially dangerous and costly plane strikes of birds. In these times of reduced chemical inputs and a move towards integrated pest management, grasses resistant or tolerant to certain pests (Chafer grubs or Leatherjacket etc.) would be a real biological breakthrough.
It has also been shown that certain secondary metabolites produced by the endophytic fungi control plant parasitic nematodes. There are reports of effects on the lifecycle of plant parasitic nematodes and inhibition of the nematodes ability to outcompete the more 'friendly' bugs in the soil. It would be a significant development if a perennial ryegrass stand could be grown that had inbuilt defence against certain plant parasitic nematodes.
As we know from some recent high profile cases, this has been a problem on high sand content constructions where the parasitic nematodes have become dominant and decimated root mass, with extensive damage occurring to pitches during play. The research so far seems to suggest that non-parasitic nematodes have been unaffected. This makes sense, as it is in the endophyte's interest for its host to succeed. Therefore, any action that makes its host less successful is mutually limiting.
One area of relatively new research, which is particularly exciting, has looked at how the endophyte-host symbiosis effects root exudates and, therefore, the fauna within the soil. A recent study looking at tall fescue cultivars infected with endophytic fungi showed that this interaction had a significant effect on root exudate composition. Critically, it was shown that the root exudate composition is often specific to the endophyte and grass cultivar being examined. In some instances, the amount of carbon released from plant roots increased.
Carbon released from plant roots has an influence on the microbial community around the root (rhizosphere). The rhizosphere has proportionately greater populations of soil microorganisms than the bulk soil, but the two are inextricably linked.
Additional carbon - released mostly as simple sugars - has the potential to feed and grow microbial communities which, in high sand content rootzones low in organic matter, would be beneficial to plant development in particular rooting.
Evidence also exists of increased phosphorus uptake and altered amino acid composition in root exudates. Exudates found to be altered by the endophyte-host symbiosis include carbohydrates, phenolics, lipids and carboxylic acids. Whilst the research in this area studied tall fescue, the Festuca and Lollium genus are very similar and it is thought there is potential for some interesting work with perennial ryegrass cultivars and endophyte symbiosis and the study of root exudate composition.
To sum up, the endophyte-host symbiosis is an ancient and natural phenomenon that is mutually beneficial. There is obvious scope for development within the sports turf industry, particularly when single species turf stands are being grown in artificially created rootzones of low biological diversity.
Just consider the benefits - pest and pathogen resistance, improved stress tolerance, growth stimulant and the ability to positively influence the microbial community of the rhizosphere. To paraphrase a friend of mine, "if you could bottle it, and spray it, you could make an absolute fortune".
It all sounds great but, of course, if it was that simple, we would all be sowing selected for endophyte infected seeds for the betterment of our surfaces. The process of studying and isolating endophytes within host plants is tricky to say the least, but technology moves apace, so it is becoming ever easier.
Potentially more problematic is that there is sometimes crosstalk between two or more species of endophyte that leads to the production of beneficial compounds. Of course, research and development of products with this level of complexity can be expensive to bring to market, sometimes prohibitively so.
Having said that, this certainly is an interesting area for further research and, where the demands of sport end users are increasingly high and the push for perfection so evident, this ancient technology developed to meet our very modern demands could prove of real benefit in years to come.
If nothing else, as turf managers, this article hopefully highlights that there are potential biological answers to questions that have typically been answered by chemical means.