Fungi essential for land plants live with mysterious bacteria
Soil fungi colonize roots and provide essential nutrients for the majority of the world's land plants, but new research sheds light on a class of bacteria found living within these fungi.
A Cornell study, published in May in the Proceedings of the National Academy of Sciences, found these so-called mycoplasma-related endobacteria (MRE) operate with minimal genomes, likely because their fungal hosts provide crucial services for the bacteria's survival. Furthermore, the researchers found that the bacterial genomes had acquired genes transferred from these arbuscular mycorrhizal fungi, which could help the bacteria manipulate their fungal hosts, though more study is needed.
Further understanding of these relationships has important implications for developing tools for sustainable agriculture.
"We know that the bacteria are metabolically dependent on fungus, because their genomes are highly reduced, but they also maintain mechanisms for plasticity and diversity," said Teresa Pawlowska, associate professor of plant pathology and plant-microbe biology and the paper's senior author.
The researchers found the communities of MREs found within fungi are very diverse genetically. In insects, genetically diverse bacteria are usually parasites, while friendly bacteria that share a mutually beneficial interaction with their hosts are usually homogenous, meaning they have no genetic diversity.
"We speculate they (MREs) might be parasites, because they have diverse populations," Pawlowska said. Mizue Naito, Ph.D. '14, now a postdoctoral researcher at University of California, Los Angeles, is the paper's first author. The bacteria "might be giving back something, but we don't know what that is at this point," Pawlowska said.
The association between mycorrhizae and plants is very common and very old, dating back to the early Devonian era (419 million years ago) when plants first emerged onto land. Researchers believe these fungi facilitated plants' transition to land, Pawlowska said. That's because the fungi supply plants with phosphorus, an essential nutrient that is readily available in aquatic environments but not readily available in soil.
Conventional farmers add phosphorus to the soil in fertilizers, but researchers estimate that the world's terrestrial phosphorus supply will be exhausted within the next 100 years. At the same time, modern farming practices such as tilling soil disrupt mycorrhizae, making fertilizer inputs necessary for growing crops, Pawlowska said.
For these reasons, studies such as this one are "important for developing tools for sustainable agriculture. We need to understand how these fungi interact with other organisms and since the association between bacteria and fungi is ubiquitous, we need to understand what this interaction is about," she added.
Thus far, scientists have described nearly 200 fungal species known to associate with the entire diversity of land plants. In the study, Pawlowska and colleagues analyzed three such fungi and examined their bacteria. They sequenced metagenomes that represented entire bacteria populations and found the three were different, but they each retained mechanisms that allow them to be diverse and adaptable to host conditions.
The researchers are conducting experiments to better understand the functional roles of the bacteria in the fungi. Both the host fungi and their endobacteria are very hard to grow in the lab, due to inadequate growth media, so both the fungi and bacteria were grown with plants for study.