How one badass grass thrives at the thermal limits of eukaryotic life

It's not an exaggeration to say grasses as a family have adapted amazingly well to most habitats on earth. But there is at least one species that takes that adaptability to new levels.

Living in the active geothermal areas of Yellowstone National Park takes some major adaptations. It is an extreme and hostile environment with rhizosphere temperatures exceeding 40°C. Many proteins denature above 41°C, and in people a body temperature at 43°C or above normally results in death or at the least serious brain damage and cardio-respiratory collapse.

In such an extreme environment, there are only nine vascular plants that have managed to successfully colonize this habitat. Chief among them are the grasses, which are both the most prevalent and most heat-resistant species in the area. But the award for the most extreme plant goes to Dichanthelium thermale (formerly a subspecies of D. lanuginosum) , which was flourishing in soils with an average of 44° C and a recorded maximum of 57°C! (Stout and Al-Niemi, 2002)

(c) James St. John, Wikipedia

When researchers studied D. thermale, they found that it was even more amazing than they initially thought. This grass species had little problem living at the absolutely mind boggling temperature of 65°C for days on end, and they discovered that this ability was the result of a symbiosis with a new endophytic species of the fungal genus Curvularia (Redman et al, 2002).

Curvularia protuberata was found in the roots, leaves, crowns, and seed coats of D. thermale, and neither the grass nor the endophyte could survive on its own above 38°C. But when endophyte-free grass was inoculated with the fungus, it regained its remarkable thermotolerance.

The researchers surmised that this protection from heat could be the result of several possible mechanisms. One possibility is that the fungal endophyte produces cell wall melanin that may dissipate heat along the hyphae and/or complex with oxygen radicals generated during heat stress. Another possibility is that the endophyte may act as a “biological trigger” that allows the symbiotic D. thermale to activate stress response systems more rapidly and strongly than non-symbiotic plants.

Redman et al, 2002

The symbiosis became even more complex when researchers announced that they had found a third party in the relationship several years later. It turns out that the fungal endophyte was infected with a double stranded RNA virus, and that its ability to confer thermotolerance on D. thermale was dependent on this infection!

Endophyte-free D. thermale that was inoculated with the fungus+virus survived in heated environments, whereas endophyte-free D.thermale  inoculated with the fungus without the virus did not survive in the same environment (Márquez et al, 2007).

Márquez et al, 2007

The unraveling of this complex symbiotic relationship could even have applications in agriculture. When tomato plants were inoculated with the wild-type fungus C. protuberata, their tolerance to heat was improved, albeit only slightly, indicating that the mechanism behind this thermotolerance may be conserved among many different plants.

But this potential novel application in the future would not have been thinkable if it were not for the humble D. thermale in Yellowstone and other geothermal parks. So as we marvel with awe at such an intricate and beautiful example of symbiosis, be sure to also give a respectful nod at this one badass thermotolerant grass and its fungal and viral partners!

Modified from Marin et al (2020), te Velthuis (2014), and J. St, John (Wikipedia)

Literature Cited

Marin, Yasmina & Hernández-Restrepo, Margarita & Crous, Pedro. (2020). Multi-locus phylogeny of the genus Curvularia and description of ten new species. Mycological Progress. 19. 559–588. 10.1007/s11557-020-01576-6.

Márquez LM, Redman RS, Rodriguez RJ, Roossinck MJ. A virus in a fungus in a plant: three-way symbiosis required for thermal tolerance. Science. 2007 Jan 26;315(5811):513-5. doi: 10.1126/science.1136237. Erratum in: Science. 2007 Apr 13;316(5822):201. PMID: 17255511.

Redman RS, Sheehan KB, Stout RG, Rodriguez RJ, Henson JM. Thermotolerance generated by plant/fungal symbiosis. Science. 2002 Nov 22;298(5598):1581. doi: 10.1126/science.1072191. PMID: 12446900.

Stout RG, Al-Niemi TS. Heat-tolerant flowering plants of active geothermal areas in Yellowstone National Park. Ann Bot. 2002 Aug;90(2):259-67. doi: 10.1093/aob/mcf174. PMID: 12197524; PMCID: PMC4240417.

te Velthuis, A.J.W. Common and unique features of viral RNA-dependent polymerases. Cell. Mol. Life Sci. 71, 4403–4420 (2014). https://doi.org/10.1007/s00018-014-1695-z

Sporobolus airoides (alkali sacaton) in White Sands National Monument, New Mexico

Sporobolus airoides specimen next to dune

I visited the White Sands National Monument in New Mexico in September 2019 in order to see some of the tenacious grasses that have managed to gain a foothold in this rugged environment, and I wasn't disappointed.

The dunes themselves were eerily beautiful, an otherworldly world of towering white sand dunes that stretched away for kilometers in all directions. The origin of this natural wonder lies several hundred million years in the past, when the continents still formed the massive landmass called Pangaea. During this time, the area that would become southern New Mexico was covered by a shallow sea called the Permian Sea. The rise and fall of the sea level at this time created layers of gypsum and other minerals on the sea floor. Over the next several million years the bottling up of the Tularosa Basin where the monument lies, the deposition of more minerals from the surrounding mountains, and the final drying of the lake (Lake Otero) that covered the basin slowly resulted in t he creation of the White Sands Monument that people see today.

You can learn more about the origins of the monument here.

The environment in the dunes is a harsh one for any organism trying to live on it. Not only does the sand shift and threaten to bury plants that take root in it, but water is usually at a premium and the very high salt concentrations in the gympsum sand make it toxic to most plants.

Nevertheless, I found many plants living in the relatively flat areas between dunes. Chief among them was Sporobolus airoides, which is commonly called Alkali Sacaton in the local language. This grass is a halophyte, an organism that tolerates and in fact flourishes in areas of high salt concentrations, and I found that most of the grasses I encountered were of this species.

In some of the more established specimens, mounds of sand had accumulated at the bases, and this reminded me of how beach dune grasses also trapped grains of sand on their bases, and thus helped in the stabilization of the beach dunes.

Sporobolus airoides during sunset

I spent some time taking in situ macro photographs of the inflorescence, which had tiny spikelets with unequal glumes and one floret nestling in each of them.

In addition to the widespread S. airoides, I also stumbled on other grasses. One was Oryzopsis hymenoides, or Indian Rice Grass, which looks superficially like S. airoides, and whose seeds are a major food source for various desert animals.

The Native Americans also used the seeds to make bread when their maize crops did not do well.

Spikelets of Indian Rice Grass

The other grass I found while on a hike was Schizachyrium scoparium, commonly known as Little Bluestem. This species is commonly a denizen of the prairies, and in fact was one of the major components of the tallgrass prairies of yore. Its presence in the dunes was at first a mystery, but I found out it was explained by the fact the water table in the lower parts of the dunefield is quite high, which provides enough water for this species even in the absence of enough precipitation.

All in all, the trip to the White Sands National Monument was fantastic, and I would wholeheartedly recommend this for anyone interested in how organisms can survive in extreme environments.

Redfieldia flexuosa (Blowoutgrass) in Great Sand Dunes National Park, Colorado

The huge dunefields of Great Sand Dunes National Park in Colorado, which feature sand dunes up to 200 meters tall, is a tough place for any plant to survive. It is dry, the dessicating winds are harsh, and the threat of being buried by the ever shifting sands is a daily battle for those few organisms that make this area their home.

I visited this National Park in September 2019, and the first plants I noticed as I entered the dunefield was Psoralidium lanceolatum (lemon scurfpea), which formed large colonies along  the sandy perimeter of the actual dunes.

A colony of blowout grass

This legume gave way to Redfieldia flexuosa (blowoutgrass), which was the most common species I encountered as I slogged my way deeper into the sandy depths of the dunefield. As indicated by their common name, blowout grass tends to be found in the sandy depressions between dunes called "blowouts." However, I also found them high up the sides of tall dunes, clinging improbably to the sand in almost vertical colonies.

A colony of blowout grass clinging to the side of a dune

I did not encounter individual specimens of this species in the dunes, but small to large colonies, a possible consequence of the fact the grass has fast growing rhizomes that allow it to quickly spread.

The rhizomes grow at varying depths, which might be an adaptation to the constantly fluctuating height of the sand dunes. Entire plants can be regrown from the rhizomes, even if they are buried deep in the sand.

Habit of blowout grass

Blowout grass is readily identified by the presence of very fine panicles, which can be up to half the height of the plant. I took in situ macro shots of the spikelets, which are awn-less and have equal length glumes and two to six florets. The strong winds and fine sand particles that continually threatened to grit up my camera made the going rough, but I was happy with the results.

In the end, I really enjoyed finding and taking photographs of the blowout grass. It reminded me of the tenacity and adaptability of life, even in the harsh dry environment of  the sand dunes. If I had more time, it would have been interesting to map out the locations of all the R. flexuosa colonies in the various dunes.