In the Pampas, the annual ryegrass is an excellent foraging resource that lives symbiotically with a fungus that, although non-toxic for animals, makes ryegrass more tolerant of plagues, droughts or high temperatures. Today, following the climate change frame, a new stressor would threaten the continuity of this “beneficial” association: the ozone (O3), a gas increasing in the atmosphere. A research by the School of Agriculture of the University of Buenos Aires (FAUBA) found out that, in controlled conditions, a high level of O3 in the air reduced the biomass of the seeds of ryegrass plants with endophyte as well as the fungus’ survival capacity. The researcher discusses the potential impacts on livestock production.

“On my PhD thesis I studied how a high concentration of ozone in the atmosphere could modify the symbiosis between the grass <i>Lolium multiform</i> —or annual ryegrass— and the endophytic fungus <i>Epichlöe occultans</i>. I did it not just because  this was almost scientifically undocumented, but also because this grass is very productive during the cold season of the Pampas, just when other grasses are less so. And that has a lot to do with the relationship between the two ‘associates’ of this symbiosis,” said Andrea Ueno, professor of the Chair of Ecology of the FAUBA.

Andrea explained that the symbiosis between <i>E. occultans</i> and ryegrass is considered ‘beneficial’ because both sides obtain advantages. On the one hand, the fungus creates some chemical compounds —called alkaloids— that allow ryegrass to tolerate various biotic and abiotic stresses. On the other hand, plants give the fungus protection and a spreading method, as by being inside the seeds the fungus uses that way to reach another generation. Besides, she highlighted the importance of such alkaloids not being toxic to livestock, above all because this symbiosis exists in almost 100% of the grasslands in the region.

“Then, to see in detail how ozone affects the symbiosis, we grow <i>L. multiflorum</i> plants, with and without endophyte, inside some transparent plastic boxes 2.5 m high and with open roof, in which we control the concentration of O₃ in the air. Some had the normal levels in the atmosphere and others almost the triple amount, a level that according to the literature can damage the fungus as well as the plants,” pointed out Ueno, who carried out such experiments during her PhD study at the School of Agriculture of the University of Buenos Aires FAUBA.

“The results show that when exposed to O₃, the plants with endophyte decreased 17% its reproductive effort. In other words, in an atmosphere rich in ozone, the plants in symbiosis with <i>E. occultans</i> destine 17% less energy to produce seeds, in comparison with the ones that do not have the fungus in their tissues. In this situation, the symbiosis with the endophyte would not be advantageous for the annual ryegrass,” highlighted the researcher.

And she added: “We also noticed that the seeds with <i>E. occultans</i>, just by having this fungus, were less viable than the ones that did not have it, no matter if the mother plants of those seeds had been exposed to ozone or not. The seedlings that grew from the less-viable seeds died 17 days earlier than the rest, in average.”

“The third important result was that the longevity of the endophyte —which we asses in the seedlings that grew from the seeds that were the ‘children’ of the plants grown with high O₃— was 4 days lower than the ones that came from the normal tenors of ozone,” stated Andrea.

<b>Run-away endophytes</b>

Andrea Ueno commented that her work provided valuable information to the ecology, in general, and to the field of the biological interactions, in particular. “We apply a new approach to study the ryegrass-endophyte symbiosis, as instead of considering just the plant without the fungus —as it is usually done— we study the plant as a whole, together with the microorganisms with which it lives in symbiosis. This group is known as holobiont, and that is what we studied.”

According to the researcher, the results of her thesis suggest that by increasing the O₃ in the atmosphere it is more probable that the endophyte —and therefore, the symbiosis— disappears from the grasslands in the Pampas. This could happen in the context of climate change. The loss would be regrettable, firstly, because of the beneficial effect that the symbiosis gives ryegrass by making it more tolerant to the aforementioned biotic and abiotic stresses. And broadly speaking, ecological, because of the disappearance of biological interactions in the agrosystems.

“Certainly, there are side effects due to ozone for the organisms, but they are quite difficult to detect and vary depending on which morphological or physiological aspect is being studied. If one were to study in the laboratory what is going on inside a plant, one would obtain results than cannot be extended to what happens in grasslands, where at the same time there is a great amount of other organisms, factors and processes that cannot be controlled,” pointed out Ueno.

On this line, Andrea highlighted that the important thing is keeping in mind that grasslands have symbionts, that said symbionts could bring advantages and help improve the productivity of the agricultural systems, and that the O₃ in the atmosphere could alter this relationship.

<b>Where does ozone come from?</b>

“We all know about the ozone layer in the stratosphere, which protects us from the ultraviolet radiation. But the ozone that piles up near the earth surface is, to some degree, product of human activity. This ozone is formed through several chemical reactions in the presence of oxygen and sunlight, and those chemical reactions have much to do with the molecules that come from burning fossil fuels in the industries and in the vehicles,” explained the professor.

Finally, Ueno said that, in the Global Change frame, the general consensus is that O₃ in the atmosphere is increasing. “There is a report of the Royal Society of London for Improving the Natural Knowledge that shows that this gas, from the pre-industrial era until today and all over the planet, bounced between 20 and 40 parts per trillion. This last number is the amount from which ozone starts to have side effects on the biological systems. This is worrisome, it is a global problem and I believe that we make a contribution to the knowledge in this field.”

Source:  Sobre La Tierra (SLT-FAUBA)