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An Orchid by Any Other Name

Posted Oct. 22, 2013

Fungi found in the roots of a particular orchid may hold the key to advancing agriculture
Principal Investigator Kelly Craven, Ph.D., (right) examines orchids in the Noble Research Institute greenhouse with Postdoctoral Fellow Prasun Ray, Ph.D.
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The delicate pastel petals of the orchid have a lot in common with the rugged, rangy switchgrass of the Great Plains - at least in Kelly Craven's laboratory.

Craven, Ph.D., an assistant professor at the Noble Research Institute, is exploring the idea of enhancing the growth of switchgrass with orchids. Or rather, the roots of the orchid and the fungi that dwell there, co-existing in a relationship that benefits both plant and fungus.

Expanding this naturally occurring symbiotic relationship to other plants could enhance their growth, increase crop yield, and improve productivity and crop economics. The impact could be a boon to both food sustainability and the energy sector. In one particular application, a switchgrass-fungi combination might enhance the economics of this crop, making it more productive and less costly to refine into advanced biofuels.

"In the next century, we have to produce more with less: less arable land, less water and less fertilizers," Craven said. "The population is predicted to increase by three billion people by the end of the century. In combination with plant breeding approaches at the Noble Research Institute, we're developing plants that have higher yields or grow better under a greater variety of environmental conditions. Mother Nature has already provided us examples through some of these symbiotic relationships. Now we want to use them in new crops."

The ultimate goal of Craven's research is to produce crops with the best characteristics that will grow on the least amount of land. That's where the delicate orchid comes into play. Orchids, you see, rely on fungi enmeshed in their roots, a symbiotic relationship of a type that Craven said scientists believe stretches back to the time when plants first colonized land.

"Fungi are used commonly by plants in nature. In fact, it's thought that they've been associated with plants since the first plants emerged from the sea," said Craven, explaining that in 460-million-year-old fossilized specimens, scientists have found evidence of fungi in the roots. They theorize that these fungi - called arbuscular mycorrhizae (AM) - played a key role in facilitating the transition from the ocean to the land because they provided critical mineral nutrition. "When you think about what the landscape looked like before vegetation, it was rock. These fungi were able to break down these rocks to produce mineral nutrition."

Fungi and agriculture
The research into fungi and agriculture-based plants isn't new. Researchers have known for decades that mycorrhizal fungi are vital to an orchid's development, so vital that orchid seeds can't germinate without them and some types of orchids remain incapable of photosynthesis for their entire life span. In these instances, the fungus provides not only mineral nutrition, but carbon as well.

Craven is attempting to develop improved switchgrass, using the fungi to bring super-charged benefits to its new host. He said that often, biomass gains to one part of the plant come at the expense of other parts. For example, gains above-ground (the shoot) typically would mean a lower density below ground (roots).

"In this novel symbiosis, we see an expansion of both systems," he said, noting the potential benefit of improving a plant that already has an enviable root system (a yard of roots for every foot of shoot) and towering height (up to 10 feet).

"If we can get more biomass per acre out of switchgrass, you need less land to produce it," Craven said. "Since it's a native grass of the Great Plains, it has adapted to life in the weather-challenged area and already tolerates drought well. We want to use it as a bioenergy crop, but also use it to anchor soil, which could have important range, pasture and conservation applications."

That dual role could be critical, because the plant would have an ecology-based use (not only do the roots anchor soil, they contribute organic carbon to it) and would be a perennial crop whose cellulose could be transformed into ethanol or other advanced biofuels.

Think of corn as a first-generation biofuel, whose starch is easily converted into ethanol. But that crop is relatively expensive to produce; requires good, arable land; and its use as fuel can influence food prices. "It is controversial," Craven said simply. "Finding an option for cellulose-based fuel production, rather than relying on the ethanol fuel produced from corn, would return good farmland to food and feed production."

Cellulosic biofuel, a second-generation biofuel, and next-generation advanced biofuels provide energy via cellulose (what plant cell walls are made of).

Specific orchid fungi
To help improve switchgrass (as part of research to advance the Department of Energy's national biofuels initiative), Craven's laboratory has pinpointed a particular group of orchid fungi (the Sebacinales order). Recent research conducted by outside laboratories suggested these fungi may have broad host ranges. In essence, they have the ability to interact with many plants instead of being host-specific.

Craven wanted to see if they could take the culturable orchid fungi and infect switchgrass. They successfully completed this experiment last year. The match worked, proving just how adaptable these particular orchid fungi can be. Additionally, the results revealed that the fungi-infused switchgrass had three times the biomass. But switchgrass was just the beginning.

"We can take the orchid fungi and infect every plant we've tried so far," Craven said. "That includes Arabidopsis thaliana, a plant species that even the AM fungi, renowned for their broad host range, cannot colonize." A. thaliana belongs to the plant family Brassicaceae, whose members typically lack key genes that are known to regulate symbiosis with both AM fungi and nitrogen-fixing bacteria (rhizobia).

"The fact that we can infect A. thaliana with these orchid fungi suggests that they colonize the roots through an entirely different pathway than either AM fungi or rhizobia" Craven said. "That's a fascinating aspect of the basic biology that may ultimately challenge notions of how plants enter into symbiosis and may also open up new avenues for incorporation of these partnerships into new agricultural crops."

However, there's a hitch. While these particular fungi can be cultured out of their orchid hosts, attempts to culture similar strains out of other plants proved unsuccessful, even though tests showed such fungi were present within the plants. "We know it is there, but if the fungi are not derived from an orchid, it can't be cultured [and grown outside the plant]," Craven said. "This is a problem because it currently limits the strains at our disposal. However, as we learn more about the basic biology of the interaction using the genetic tools available for A. thaliana and Medicago truncatula, we believe we can overcome this challenge and decipher how to entice the non-orchid strains out of their host plants."

There also is the broader potential application for the orchid fungi. Craven has initiated a study of their impact on valuable food crops like wheat, oats and soybeans. So far, the findings have been encouraging. Craven's team has been able to colonize each of the crops. Among its many positive attributes, these fungi serve as an extension of the plant's root system, helping to improve nutrient acquisition from the soil and sustain growth.

"In general, plants depend on microbes. You can't find a plant in nature that lacks symbiotic microbes," Craven said. "The plant depends on them the most in times of stress like weather extremes, including drought. We are simply taking advantage of solutions forged over eons of evolution and maximizing their effects in ways that benefit agricultural production for a growing population in a world of dwindling agronomic resources and extreme climatic variability."

Not bad for a little orchid.