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.
Harrison, M.J. 2012. Cellular programs for arbuscular mycorrhizal symbiosis. Curr Opin Plant Biol 15: 691-698
Hong , J.J., Park, Y.S., Bravo, A., Bhattarai, K.K., Daniels, D.A. and Harrison, M.J. 2012. Diversity of morphology and function in arbuscular mycorrhizal symbioses in Brachypodium distachyon. Planta 236: 851-865
Ma, X.F., Tudor, S., Butler, T., Ge, Y.X., Xi, Y.J., Bouton, J., Harrison, M. and Wang, Z.Y. 2012. Transgenic expression of phytase and acid phosphatase genes in alfalfa (Medicago sativa) leads to improved phosphate uptake in natural soils. Molecular Breeding 30: 377-391
Pumplin, N., Zhang, X., Noar, R.D. and Harrison, M.J. 2012. Polar localization of a symbiosis-specific phosphate transporter is mediated by a transient reorientation of secretion. Proc Natl Acad Sci U S A 109: E665-672
Silverberg, J.L., Noar, R.D., Packer, M.S., Harrison, M.J., Henley, C.L., Cohen, I. and Gerbode, S.J. 2012. 3D imaging and mechanical modeling of helical buckling in Medicago truncatula plant roots. A. Proc Natl Acad Sci U S 109: 16794-16799
Javot, H., Penmetsa, R.V., Breuillin, F., Bhattarai, K.K., Noar, R.D., Gomez, S.K., Zhang, Q., Cook, D.R. and Harrison, M.J. 2011. Medicago truncatula mtpt4 mutants reveal a role for nitrogen in the regulation of arbuscule degeneration in arbuscular mycorrhizal symbiosis. Plant J 68: 954-965
Gonzalez-Chavez, M.C., P., O.-L.M., Carrillo-Gonzalez, R., Lopez-Meyer, M., Xoconostle-Cazares, B., Gomez, S.K., Harrison, M.J., Figueroa-Lopez, A.M. and Maldonado-Mendoza, I.E. 2011. Arsenate induces the expression of fungal genes involved in As transport in arbuscular mycorrhiza. Fungal Biol 115: 1197-1209
Harrison, M.J., Pumplin, N., Breuillin, F.J., Noar, R.D. and Park, H.-J. 2010. Phosphate transporters in arbuscular mycorrhizal symbiosis. In Arbuscular Mycorrhizas: Physiology and Function (Koltai, H. and Kapulnik, Y. eds). New York. Springer 0:
Benedito, V.A., Li, H.Q., Dai, X.B., Wandrey, M., He, J., Kaundal, R., Torres-Jerez, I., Gomez, S.K., Harrison, M.J., Tang, Y.H., Zhao, P.X. and Udvardi, M.K.. 2010. Genomic inventory and transcriptional analysis of Medicago truncatula transporters. Plant Physiology 152: 1716-1730
Pumplin, N., Mondo, S.J., Topp, S., Starker, C.G., Gantt, J.S. and Harrison, M.J. 2010. Medicago truncatula Vapyrin is a novel protein required for arbuscular mycorrhizal symbiosis. Plant Journal 61: 482-494
Schultz, C.J., Kochian, L.V. and Harrison, M.J. 2010. Genetic variation for root architecture, nutrient uptake and mycorrhizal colonisation in Medicago truncatula accessions. Plant and Soil 336: 113-128
Zhang, Q., Blaylock, L.A. and Harrison, M.J. 2010. Two Medicago truncatula half-ABC transporters are essential for arbuscule development in arbuscular mycorrhizal symbiosis. Plant Cell 22: 1483-1497
Pumplin, N. and Harrison, M.J. 2009. Live-cell imaging reveals periarbuscular membrane domains and organelle location in Medicago truncatula roots during arbuscular mycorrhizal symbiosis. Plant Physiology 151: 809-819
Oldroyd, G.E.D., Harrison, M.J. and Paszkowski, U. 2009. Reprogramming plant cells for endosymbiosis. Science 324: 753-754
Gomez, S.K., Javot, H., Deewatthanawong, P., Torres-Jerez, I., Tang, Y., Blancaflor, E.B., Udvardi , M.K. and Harrison, M.J. 2009. Medicago truncatula and Glomus intraradices gene expression in cortical cells harboring arbuscules in the arbuscular mycorrhizal symbiosis. BMC Plant Biol 9: 10
Gomez, S.K. and Harrison, M.J. 2009. Laser microdissection and its application to analyze gene expression in arbuscular mycorrhizal symbiosis. Pest Management Science 65: 504-511
Schultz, C.J. and Harrison, M.J. 2008. Novel plant and fungal AGP-like proteins in the Medicago truncatula-Glomus intraradices arbuscular mycorrhizal symbiosis. Mycorrhiza 18: 403-412
Penmetsa, R.V., Uribe, P., Anderson, J., Lichtenzveig, J., Gish, J.C., Nam, Y.W., Engstrom, E., Xu, K., Sckisel, G., Pereira, M., Baek, J.M., Lopez-Meyer, M., Long, S.R., Harrison, M.J., Singh, K.B., Kiss, G.B. and Cook, D.R. 2008. The Medicago truncatula ortholog of Arabidopsis EIN2, sickle, is a negative regulator of symbiotic and pathogenic microbial associations. Plant Journal 55: 580-595
Liu, J.Y., Versaw, W.K., Pumplin, N., Gomez, S.K., Blaylock, L.A. and Harrison, M.J. 2008. Closely related members of the Medicago truncatula PHT1 phosphate transporter gene family encode phosphate transporters with distinct biochemical activities. Journal of Biological Chemistry 283: 24673-24681
Javot, H., R. V. Penmetsa, N. Terzaghi, D. R. Cook, M. J. Harrison. 2007. A Medicago truncatula phosphate transporter indispensable for the arbuscular mycorrhizal symbiosis. Proc Natl Acad Sci USA 104: 1720-1725
Harrison, M. J., . 2005. Signaling in the Arbuscular Mycorrhizal Symbiosis. Annual Review of Microbiology 59: 19-42
Liu, J. Y., L. A. Blaylock, G. Endre, J. Cho, C. D. Town, K. A. VandenBosch, M. J. Harrison. 2003. Transcript Profiling Coupled with Spatial Expression Analyses Reveals Genes Involved in Distinct Developmental Stages of an Arbuscular Mycorrhizal Symbiosis. Plant Cell 15: 2106-2123
Harrison, M. J., G. R. Dewbre, J. Liu. 2002. A phosphate transporter from Medicago truncatula involved in the acquisition of phosphate released by arbuscular mycorrhizal fungi. Plant Cell 14: 2413-2429
Versaw, W.K. and M.J. Harrison, . 2002. A chloroplast phosphate transporter, PHT2;1, influences allocation of phosphate within the plant and phosphate-starvation responses. Plant Cell 14: 1751-1766
Identification Of M. Truncatula Genes Required For Arbuscular Mycorrhizal Symbiosis
Funding for this project was provided by the National Science Foundation Plant Genome Research Program (NSF DBI-0421676: “Use of interfering RNAs to identify gene function in Medicago truncatula“). The main project website and the M. truncatula RNAi database can be found at
Participants at partner institutions include: J. Stephen Gantt, PI, University of Minnesota, Maria J. Harrison, co-PI, Boyce Thompson Institute, Carroll Vance, co-PI, University of Minnesota/USDA ARS, Kathryn A VandenBosch, co-PI, University of Minnesota, Deborah Samac, key-collaborator, University of Minnesota/USDA ARS
Members of the Harrison lab who have participated in the M. truncatula RNAi screen to identify genes required for AM symbiosis:
- Julien Levy (Postdoctoral researcher)
- Jeon Hong (Research assistant)
- Janelle K. Jung (Research assistant)
- Stephen J. Mondo (Research assistant)
- Stephanie Topp (Research assistant)
- Sayan Das (Postdoctoral researcher)
- Jose Aravalo (Summer undergraduate Intern)
- Zach King (Summer undergraduate Intern)
- Aynur Cakmak (Laboratory Technician)
- Jon Zhang (Undergraduate student)
- Tamara Wynne (Undergraduate student)
- Maria J. Harrison (PI) ( email@example.com)
The majority of the vascular flowering plants, including most crop species of agronomic significance, are able to develop symbiotic associations with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the roots where the AM fungi deliver phosphate (Pi) and nitrogen to the root cortex and in return obtain carbon from the plant. Despite the importance of this symbiosis we know relatively little about plant genes that control its development (Harrison, 2005).
In collaboration with colleagues at the University of Minnesota, we undertook an RNAi-based reverse genetic screen to identify M. truncatula genes required for AM symbiosis and nodulation. Previously, we had generated transcript profiles of M. truncatula roots during AM symbiosis and nodulation. These profiles were used to guide the selection of candidate genes and over 1400 RNAi constructs were generated (https://mtrnai.msi.umn.edu/) in the Gantt laboratory at the University of Minnesota. The major activities in the Harrison lab (Boyce Thompson Institute for Plant Research) included generating M. truncatula plants with transgenic roots expressing the RNAi constructs and analysis of their AM symbiosis phenotypes (Figure 1.)
Currently, M. truncatula composite plants with transgenic roots expressing 1392 RNAi constructs have been generated and the majority of these have been analyzed. We have identified 56 RNAi constructs that result in aberrant AM symbiosis phenotypes (Table 1). Further characterization of a selection of these genes is in progress.
This screen enabled the identification of several novel genes required for AM symbiosis, including a new plant-specific protein that we named Vapyrin (Pumplin et al, 2010). Vapyrin is composed of two domains that mediate protein-protein interactions: an N-terminal VAP/MSP domain and a C-terminal ankyrin repeat domain. Putative Vapyrin orthologs exist widely in the plant kingdom, but not in Arabidopsis. In Vapyrin RNAi roots, the fungus is unable to form arbuscules and is blocked at the point of entry into the cell (Figure 2).
Fig. 2. AM symbiosis phenotype of M. truncatula roots expressing a vapyrin RNAi construct. Roots are colonized with the AM fungus Gigaspora gigantea and stained with WGA-alexafluor488 to reveal the fungal hyphae (green).
Vapryin is a cytoplasmic protein but in the cells with arbuscules it is present on ‘mobile bodies’ that are likely vesicles. Based on these data we propose that Vapryin might serve as a scaffold protein involved in secretion of the periarbuscular membrane (Pumplin et al., 2010).
- Harrison, M.J. (2005) Signaling in the arbuscular mycorrhizal symbiosis. Annual Reviews of Microbiology 59: 19-42.
- Pumplin, N., Mondo, S., Topp, S., Starker, C., Gantt, J.S., Harrison, M.J. (2010) Medicago truncatula Vapyrin is a novel protein required for arbuscular mycorrhizal symbiosis. Plant Journal, 61:482-494).
How do soil fungi supply plants with mineral nutrients?
feature released -2008
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.
feature released -2007
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.
Of Phosphates and Fungi.
feature released -2007
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.
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