Robert Turgeon

We conduct interdisciplinary research on the cell biology and eco-physiology of phloem transport. Integral to these projects are studies of leaf development, the structure and function of plasmodesmata, and virus movement. Molecular, physiological, and anatomical techniques are employed in approximately equal measure.

Our primary interest is in phloem loading, the active accumulation of sugars in minor vein sieve elements and companion cells. Loading creates the pressure that drives long-distance flow and therefore motivates the distribution of organic nutrients and many protective compounds. One of our contributions to this area is the ‘polymer trap’ model that explains loading through plasmodesmata, long thought to be thermodynamically impossible. The polymer trap mechanism appears to have evolved at least five times.

Until recently, the adaptive purpose of symplastic loading has been entirely obscure. We now think it likely that the polymer trap mechanism has evolved to permit the loading and long-distance transport of beneficial compounds, such as iridoid glycosides, that protect plants from insect herbivory and other biotic stresses.

We have also found that the primary products of photosynthesis in certain plants (e.g. willow and apple) are not loaded at all, in the thermodynamic sense. Rather they diffuse from mesophyll cells into the phloem. This may prove to be a common transport mechanism, since plants with symplastically connected minor vein phloem constitute almost half of all dicotyledonous species. We feel that this diffusive mode of photoassimilate mobility in leaves is again an adaptation that permits the transport of protective secondary metabolites.

Much of our work on symplastic loading now involves the transformation of Verbascum phoeniceum (Scrophulariaceae) a species that translocates raffinose and stachyose, as well as iridoid glycosides. Until we developed this system, there was no way to apply techniques in molecular biology to the phenomenon of symplastic loading, or to test whether exotic compounds are able to enter the transport stream through plasmodesmata. However, we have found that V. phoeniceum can be transformed as readily as tobacco. We are conducting a number studies with this experimental system to confirm the polymer-trap model, to explore and model the transport of exotic compounds, and to refine our understanding of the cell biology of minor vein companion cells. A primary tool in this work is the galactinol synthase promoter that we cloned from melon and which is active only in the companion cells of minor veins.

Since much of our work revolves around plasmodesmata, we also conduct studies on plasmodesmata ultrastructure and function. In collaboration with other labs, we study the systemic transport of viruses.

We are currently involved in two collaborative programs. With a group at the Univ. of Colorado, we study the physical limitations of phloem loading on long-distance transport from an eco-physiological perspective. We have found that loading can limit phloem transport, and therefore photosynthesis, under high light conditions. These limits can be overcome by increases in membrane surface area in the minor vein phloem, or by increases in vein density. With Shmulik Wolf at the Hebrew University of Jerusalem, we are beginning a program to study carbohydrate signaling in leaves triggered by virus infection. Initial results indicate that virus infection initiates a systemic signaling cascade that modifies the mode of phloem loading and sugar transport, in both infected and uninfected leaves. This may be a strategy the virus uses to suppress defense mechanisms.

Selected Journal Publications

Google Scholar profile and publications.

• Slewinski, T., Zhang, C., & Turgeon, E. G. (2013). Structural and functional heterogeneity in phloem loading and transport. Frontiers in Plant Science. 4:244.
• Slewinski, T., Anderson, A., Zhang, C., & Turgeon, E. G. (2012). Scarecrow regulates kranz anatomy in maize leaves. Plant & Cell Physiology. 53:2030-2037.
• Gil, L., Ben-Ari, J., Turgeon, E. G., & Wolf, S. (2012). Effect of CMV infection and high temperatures on the enzymes involved in raffinose family oligosaccharide biosynthesis in melon plants. Plant Physiology. 169:965-970.
• Liu, D. D., Chao, W. M., & Turgeon, E. G. (2012). Transport of sucrose, not hexose, in the phloem. JXB: Journal of Experimental Botany. 63:4315-4320.
• Zhang, C., Yu, X., Ayre, B. G., & Turgeon, E. G. (2012). The origin and composition of cucurbit ÒphloemÓ exudate. Plant Physiology. 158:1873-1882.
• Gil, L., Yaron, I., Shalitin, D., Sauer, N., Turgeon, E. G., & Wolf, S. (2011). Sucrose transporter plays a role in phloem loading in CMV-infected melon plants that are defined as symplastic loaders. The Plant Journal. 66:364-374.
• Turgeon, E. G., & Medville, R. (2010). Amborella trichopoda, plasmodesmata, and the evolution of phloem loading. Protoplasma. 248:173-180.
• Davidson, A., Keller, F., & Turgeon, E. G. (2010). Phloem loading, plant growth form, and climate. Protoplasma. 248:153-163.
• Majeran, W., Friso, G., Ponnala, L., Connolly, B., Huang, M., Reidel, E., Zhang, C., Asakura, Y., Bhuiyan, N. H., Sun, Q., Turgeon, E. G., & van Wijk, K. (2010). Structural and metabolic transitions of C4 leaf development and differentiation defined by microscopy and quantitative proteomics in maize. The Plant Cell. 22:3509-3542.
• Li, P., Ponnala, L., Gandotra, N., Wang, L., Si, Y., Tausta, S. L., Kebrom, T., Provart, N., Patel, R., Myers, C. R., Reidel, E., Turgeon, E. G., Liu, P., Sun, Q., Nelson, T., & Brutnell, T. (2010). The developmental dynamics of the maize leaf transcriptome as revealed through ultra high throughput sequencing. Nature Genetics. 42:1060-1067.

• Charles Reid Barnes Lifetime Achievement Award 2013 American Society of Plant Biologists

• BIOG 1140: Foundations of Biology