Most vascular flowering plants are able to form symbiotic associations with arbuscular mycorrhizal (AM) fungi. These associations, named ‘arbuscular mycorrhizas’, develop in the roots, where the fungus colonizes the cortex to access carbon supplied by the plant. The fungal contribution to the symbiosis includes the transfer of mineral nutrients, particularly phosphorus, from the soil to the plant. In many soils, phosphate exists at levels that are limiting for plant growth. Consequently, additional phosphate supplied via AM fungi can have a significant impact on plant development, and this symbiosis influences the structure of plant communities in ecosystems worldwide.
The long-term goals of our research are to understand the mechanisms underlying development of the AM symbiosis and phosphate transfer between the symbionts. A model legume, Medicago truncatula, and arbuscular mycorrhizal fungi, Glomus versiforme, Glomus intraradices and Gigaspora gigantea are used for these analyses. Currently, a combination of molecular, cell biology, genetic and genomics approaches are being used to obtain insights into development of the symbiosis, communication between the plant and fungal symbionts, and symbiotic phosphate transport.
In nature, certain plants and fungi have evolved a complex, symbiotic relationship in which the plants provide the fungi with carbon while the fungi provide the plants with phosphate needed for cell function and growth. Understanding this relationship could result in scientists’ ability to develop plants that require fewer applications of phosphate fertilizers.
Working with soil fungi called arbuscular mycorrhizal fungi and a model legume,Medicago truncatula,Maria Harrison’s laboratory is unraveling the mechanisms underlying mineral transfer from fungus to plant. The fungi, which are ubiquitous in soil, live in close proximity to the plant’s roots. The fungal spores grow on the root surface and, in response to a signal from the plant, grow into the cells of the root. Once there, the plant forms a membrane, called the arbuscular membrane, through which the mineral exchange occurs.
Harrison theorized that a particular transporter protein in the arbuscular membrane mediates the movement of phosphorus from the fungus into the plant cell. In 2007, her team demonstrated that this theory was correct. When the plant gene that produces the transporter protein in question was “knocked out,” or disabled, phosphate in the arbuscule did not cross into the plant cell.
Harrison’s research yielded another, somewhat surprising result. She discovered that in the mutant plant, the arbuscules die very quickly. One interpretation is that the plant, on detecting that the phosphate transfer is not occurring, responds by triggering the death of the arbuscule. Understanding how and why this occurs will be a next step in her research.
Today, growers use fertilizers derived from rock phosphate to enhance plant nutrition, but rock fertilizer reserves are being depleted and, at the current rate of use, they will last only an additional 90 years. Furthermore, excessive application of phosphate fertilizers contributes to the pollution of streams. Harrison’s work may lead to plants that can use naturally occurring phosphate in the soil more completely and efficiently through enhanced symbiotic relationships with fungi – an advance that would lead to more environmentally friendly, sustainable agriculture.
At first glance, plants and fungi look about as amicable as the Montagues and Capulets. Fungal strains decimated the American chestnut tree population, and now threaten the world’s banana supply. But underground, some plants and fungi forge a much friendlier relationship.
In fact, arbuscular mycorrhizal (AM) fungi partner with most plants worldwide, helping them extract phosphate, nitrogen, and other minerals from the soil in exchange for carbon. When plant roots sense AM fungi nearby, they create a welcoming home by ramping up production of proteins from some genes and suppressing others. This change in gene expression keeps the fungus from being destroyed by the plant’s defenses and encourages it to form arbuscules, the specialized structures through which the two species trade minerals and carbon.
To find out more about this sophisticated partnership, Maria Harrison’s lab studies the interaction of alfalfa relative Medicago truncatula with several AM fungal varieties. Using microarrays—glass slides that can detect RNA from numerous genes—they compare the expression levels of Medicago genes before and after the fungi move in. This yields a snapshot of which genes might be needed to forge the symbiosis; the Harrison lab and collaborators have identified close to 900 such genes so far.
The next step is to comb through those genes to find the few that are crucial for symbiosis. To do this, the lab “knocks out” candidate Medicago genes one by one and looks for a disruption in the resulting plants’ interaction with AM fungi. In this way Harrison and her collaborators are piecing together the delicate subterranean choreography that goes into a good AM working relationship. For example, the group recently identified a calcium-dependent enzyme needed for root development and for symbiosis with AM fungi and certain beneficial bacteria. The discovery indicates that calcium may be an important signal for both kinds of symbioses.
Each summer, over 5,000 square miles of the Gulf of Mexico go eerily dead. Agricultural runoff from the Mississippi River creates this so-called Dead Zone off the coast of Louisiana: Excess nutrients feed algal blooms, which leech most of the oxygen from the water. Finding ways to make crops take up phosphorus and other nutrients more efficiently would drastically reduce the fertilizer dumped on fields, saving money as well as watery ecosystems. One promising strategy is to harness a symbiotic relationship as ancient as land-dwelling life.
If you pull up a plant and look at its roots, chances are that part of what you see is actually fungus. Fossil records show plants and fungi teamed up hundreds of millions of years ago, perhaps enabling water-dwelling plants to colonize land. Yet modern farming regimes don?t take full advantage of the fungi’s potential to harvest fungi for the plant. Learning more about the plant-fungus symbiosis could help farmers come up with ways to benefit from this interaction.
Maria Harrison studies the signaling system that allows plants and fungi to work in tandem. Her lab is scanning the genes in model plant Medicago truncatula (a relative of alfalfa) to find which coordinate its relationship with a certain fungus. In the initial phases of a four-year project, they looked for genes whose level of activity changes as fungi move into the root. The lab identified 92 such genes using microarrays. They next fine-tuned a system for knocking out each candidate gene in turn and looking at the resulting plant to see whether symbiosis was affected. Working with collaborators in Minnesota, Harrison?s lab will plough through hundreds of genes in the next few years to discover which are most essential for symbiosis.
