This InterView with Maria Harrison, professor at Boyce Thompson Institute for Plant Research, was performed by one of the 2016 IS-MPMI student travel awardees, Kevin Cope, University of Wisconsin-Madison.

Editor’s note: Kevin communicated to me that after the interview, he was interested in joining Dr. Harrison’s lab as a postdoc. Although funding was not immediately available, Kevin and Dr. Harrison worked together to submit an application for an NSF Post-Doctoral Research Fellowship in Biology. This is a great example of how communication between students and senior scientists can lead to opportunities for future collaborations and interactions. If you’re interested in completing your own InterView, please let me know.
—Dennis Halterman, Editor-In-Chief

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Maria Harrison
Boyce Thompson Institute

Kevin Cope

University of Wisconsin-Madison

Kevin Cope (KC): How did you become interested in doing research on arbuscular mycorrhizal fungi? What is it about this area of research that is exciting to you?

Maria Harrison (MH): Early in my career, I became interested in arbuscular mycorrhiza because I heard an amazing talk in the early 1990s from Larry Peterson, a cell biologist and professor from the University of Guelph. In his talk, he discussed his work on many different types of mycorrhiza but mostly with orchid and some arbuscular mycorrhiza. It was really at that point that I realized that arbuscular mycorrhiza was exactly what I was interested in. The broad question that interested me was: How does the fungus live inside the root cells of its host plant, and how does the host plant accommodate such a huge cellular invasion by the fungus? In addition, the other aspect that I liked about AM symbiosis was the potential for use in agriculture because of its ability to improve plant mineral nutrition. And so, I had two overarching questions: (1) How does the cell accommodate the symbiont, working from the plant side (because we can use genetics), and then (2) how does symbiotic phosphate transport work? How is phosphate transported from the soil, through the fungus and to the plant, and how does this regulate the symbiosis? Those are the two topics that were (and remain) particularly fascinating to me.

KC: Looking back on your career, what do you feel has been your most important and exciting discovery so far? What impact did it have on your field of research?

MH: There have been many exciting moments in the lab, so it is hard to say which one was the most exciting. Seeing a mutant phenotype for the first time and realizing that it is totally different from wild-type and therefore that the gene has some really important function is always exciting. Also, the results that do not make sense can be exciting—such as the location of plasma membrane phosphate transporters expressed with the MtPT4 promoter (which led to new and unexpected findings). Maybe the discoveries that collectively have had the most impact were a combination of finding the MtPT4 phosphate transporter, knocking it out, and finding that it is important for regulating/maintaining the symbiosis [1–4] and also the accompanying question of how the polarized location of MtPT4 (just into the periarbuscular membrane) was attained—the answer being that the cell reorients secretion during development of the periarbuscular membrane and targeting occurs by default [5]. These findings collectively have had an impact on the field, and the targeting story has had an impact beyond the mycorrhiza field, as it illustrated a novel approach by which cells can target proteins to specific regions of a membrane. So those are probably the findings that have had the most impact so far, but of course, time will tell which discoveries have the most significance.

KC: What direction do you think arbuscular mycorrhizal research should go in the future? What information do you feel is currently missing, and how do you think it could be further elucidated?

MH: I think there are still many things that we do not understand about the symbiosis from the point of view of the plant, but I think even more overwhelming is that we understand very little about the fungus. And so, if I had to pick one area of importance, I would say that in order to really understand the symbiosis, we need to know more about the biology of the fungus. At the molecular level, what is happening as it colonizes the root, what triggers arbuscule development, how does it move phosphate long distances, how is phosphate efflux occurring, and how is this regulated? These are just a few of the unknowns. The fungus is really still a big black box. I think that the genomes and transcriptomes and new tools coming along (e.g., HIGs) will provide the possibility for reverse genetics analyses, and we will be able to get more insights. There are many unknowns and therefore many opportunities.

KC: How does your current research relate to other plant–microbe interactions? For example, do you think there might be any similarities between arbuscular mycorrhizal and ectomycorrhizal associations?

MH: We work only on arbuscular mycorrhiza, not because I am not interested in other symbioses but rather because this is all we can handle. We have broadened our research with regard to plant species but do not have plans to expand to ectomycorrhiza or orchid mycorrhiza, both of which I think would be fascinating. I am sure there are some parallels between the different mycorrhizas, and given your area of research, you may know better than me whether the signaling pathways are overlapping [6]. Our focus is on the mechanisms underlying accommodation of the endosymbiont, and these mechanisms are probably not occurring extensively in the ectomycorrhizal symbiosis, but there may well be parallels in orchid mycorrhizas. However, arbuscular mycorrhizal fungi also colonize the apoplastic spaces of the root, and in this regard, I could imagine commonalities with ectomycorrhizal fungi. Also, in ectomycorrhizal associations, the fungi are more tractable, and so clues from the ectomycorrhizal symbiosis will likely help to inform the arbuscular mycorrhizal symbiosis.

KC: What advice do you have for young scientists entering your field of research?

MH: For molecular biology students, I would say that it is really important to read the literature, particularly the early literature associated with mycorrhizal symbiosis, which forms the groundwork for current research. With a foundational understanding, they can jump in and apply molecular tools to address some of the many remaining questions. Currently, the opportunities for interaction and multidisciplinary research are much better and easier. I think that looking for a team of people with complementary expertise and going after a big question with multidisciplinary research is another good way to go for the future. But basically, the main thing is ask an important question and go for it!

References

Harrison, M. J., Dewbre, G. R., and Liu, J. 2002. A phosphate transporter from Medicago truncatula involved in the acquisition of phosphate released by arbuscular mycorrhizal fungi. Plant Cell 14:2413-2429.
Javot, H., et al. 2007. A Medicago truncatula phosphate transporter indispensable for the arbuscular mycorrhizal symbiosis. Proc. Natl. Acad. Sci. 104:1720-1725.
Javot, H., et al. 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.
Breuillin-Sessoms, F., et al. 2015. Suppression of arbuscule degeneration in Medicago truncatula phosphate transporter4 mutants is dependent on the ammonium transporter 2 family protein AMT2;3. Plant Cell 27:1352-1366.
Pumplin, N., et al. 2012. Polar localization of a symbiosis-specific phosphate transporter is mediated by a transient reorientation of secretion. Proc. Natl. Acad. Sci. 109:4041-4042.
Garcia, K., et al. 2015. Molecular signals required for the establishment and maintenance of ectomycorrhizal symbioses. New Phytol. 208:79-87.

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