In an open-access paper published in the journal PLoS ONE, a team lead by researchers from the US Geological Survey reported adapting two commercial rice varieties to cold, salt and drought stress simply by colonizing them with Class 2 fungal endophytes isolated from plants that are salt and temperature stress tolerant.

Although there are numerous reports on the genetic, molecular and physiological bases of how plants respond to stress, the nature of plant adaptation to high stress habitats remains unresolved. However, most ecological studies fail to consider the fact that all plants in natural ecosystems are thought to be symbiotic with fungal endophytes and these endophytes can have profound effects on plant stress tolerance and fitness. For example, fungal endophytes can confer fitness benefits to plants including increased root and shoot biomass, increased yield, tolerance to abiotic stresses such as heat, salt, and drought, and to biotic stresses such as pathogens and herbivores. One group of fungal endophytes [Class 2] confer habitat-specific stress tolerance to plants through a process defined as Habitat Adapted Symbiosis. Remarkably, Class 2 endophytes, are capable of colonizing and conferring habitat-specific stress tolerance to monocot and eudicot plants which may suggest that the symbiotic communication responsible for stress tolerance may predate the divergence of these lineages (est. 145–230 mya).

—Redman et al.

The endophytes conferred salt, drought and cold tolerance to growth chamber and greenhouse grown plants. Endophytes reduced water consumption by 20–30% and increased growth rate, reproductive yield, and biomass of greenhouse grown plants. In the absence of stress, there was no apparent cost of the endophytes to plants, however, endophyte colonization decreased from 100% at planting to 65% compared to greenhouse plants grown under continual stress (maintained 100% colonization).

These findings, Redman et al. say, indicate that rice plants can exhibit enhanced stress tolerance via symbiosis with Class 2 endophytes, and suggest that symbiotic technology may be useful in mitigating impacts of climate change on other crops and expanding agricultural production onto marginal lands.

Conferring heat tolerance to rice is the next step for the research team since rice production decreases by 10% for every temperature increase of 1-degree centigrade during the rice-growing season.

This is an exciting breakthrough. The ability of these fungi to colonize and confer stress tolerance, as well as increased seed yields and root systems in rice—a genetically unrelated plant species from the native plants from which the fungi were isolated—suggests that the fungi may be useful in adapting plants to drought, salt and temperature stressors predicted to worsen in future years due to climate change.

—Dr. Rusty Rodriguez

Rodriguez said that using these endophytes is one of the only real strategies available for mitigating the effects of climate change on plants in natural and agricultural ecosystems.

We have named this emerging area of research “symbiogenics” for symbiosis-altered gene expression. The DNA of the rice plant itself, however, is not changed. Instead, we are re-creating what normally happens in nature. And with rice yields projected to decrease by 15 percent in developing countries by 2050, such strategies are needed.

—Rusty Rodriguez

All plants seem to have symbiotic endophytes—microscopic fungi or bacteria—living in them that do not cause disease in the plant. The kind of endophytes that Rodriguez and his colleagues examined are all mutualistic, meaning the plant and the fungi have a close and positive relationship that bestows benefits on both partners: stress tolerance for the plant, nutrients and a lack of competition for the fungus.

The scientists took fungal endophytes from dunegrass, a species exposed to seawater and therefore salt-tolerant, and colonized the rice plants and seeds with its fungal spores, which germinated and infiltrated the plant’s tissue. The results, said Rodriguez, were dramatic: the endophytes reduced water consumption of the plant by up to one half, and increased its growth, the number of seeds it produced, and how much it weighed by as much as 50% percent.

Conventional thinking was that the dunegrass is salt tolerant because of genetic adaptations that occurred over time (the process of Darwinian evolution), but we found that when we removed the fungus from dunegrass, the plants were no longer salt tolerant. This means that plants in natural habitats may not be adapting themselves genetically to the stress, but instead are establishing a beneficial partnership with a fungus that makes them more salt tolerant.

—Rust Rodriguez

During the last 40 years of climate change, the authors pointed out, the minimum air temperature in rice-growing season has increased in China and the Philippines, resulting in a substantial decrease in rice yields there, decreases predicted to continue.

The authors emphasized that even though it may be possible to compensate for some of the effects of climate change by incorporating, say, earlier-producing varieties of rice into agricultural practices, the adaptive capabilities of rice will be what ultimately determines how severely climate change affects the annual yield of rice.

Resources

• Redman RS, Kim YO, Woodward CJDA, Greer C, Espino L, et al. (2011) Increased Fitness of Rice Plants to Abiotic Stress Via Habitat Adapted Symbiosis: A Strategy for Mitigating Impacts of Climate Change. PLoS ONE 6(7): e14823. doi: 10.1371/journal.pone.0014823