Office: 127 Morrill III South
A.B., Mount Holyoke College, 1984
Ph.D., Rockefeller University, 1989
1990-1993 Yale University
Plant Development and Molecular Genetics
My lab is conducting two distinct research projects. In the first project, we are investigating the mechanisms that control the uptake and homeostasis of iron. This project has important implications both for our basic understanding of these mechanisms, and also for our ability to manipulate the iron content of particular plant parts as a means of improving the iron nutritional quality of food. In the second project, we are using molecular approaches to discover genes involved in production of paclitaxel (generic name for Taxol™ - Bristol-Myers Squibb). Genes discovered during this project will improve our ability to supply sufficient quantities of paclitaxel to the world.
Iron Homeostasis in Plants
Iron is one of the most important and most problematic of all the micronutrients used by living organisms. Iron is an essential cofactor for many cellular redox reactions, yet the same high reactivity that makes it so useful can cause cellular damage if iron is not carefully controlled. Add to this problem that iron is also only sparingly soluble in aqueous solution, and it is easy to see why plants have evolved multifaceted iron homeostatic mechanisms. These mechanisms include control of uptake, translocation from organ to organ and cell to cell, re-mobilization of stored iron, as well as poorly understood sensing and signaling systems by which the plant communicates its iron status between tissues. Many of the mechanisms involved in plant iron homeostasis are not well understood, and this is a major obstacle to devising approaches for biofortification of staple foods with iron. Biofortification refers to the genetic engineering of staple crops to accumulate additional bioavailable iron in edible parts; it is widely regarded as a sustainable means of improving the iron nutrition of the 2-3 billion people worldwide whose inadequate diet causes iron deficiency anemia.
My group has a strong interest in the processes by which plants move iron and other transition metals within their above ground parts. We have recently shown that members of the Yellow Stripe Like (YSL) family of transporters are required for normal iron, zinc, and copper loading into both vegetative and reproductive tissues. In future, my group will continue to perform experiments that will elucidate the mechanisms that plants use to achieve correct distribution of iron and other metals into above ground organs and seeds. We will also pursue experiments that will elucidate the function(s) of additional YSL family members both at the biochemical and whole plant physiological levels. For more information about the YSL family in Arabidopsis thaliana, please visit our 2010 web site.
We are using molecular approaches to delineate global metabolic control of taxol (paclitaxel) in Taxus cell cultures. Paclitaxel is a valuable pharmaceutical currently used primarily for treatment of cancer, but which has also been found to reduce major adverse cardiac events when coated onto coronary stents, and which is being tested in the treatment of Alzheimer’s disease and other neurodegenerative disorders characterized by altered microtubule networks. Because two to four mature trees are needed to supply enough paclitaxel for the treatment of one patient, supply of paclitaxel from natural sources was limiting, and alternative methods of production such as cell culture systems for production are actively sought. The biosynthesis of paclitaxel is complex, and molecular analysis of the system has been limited to work on the biosynthetic genes themselves. We are using transcription profiling to identify genes involved in global metabolic control. Such genes will be involved not just in paclitaxel biosynthesis (where all current efforts are focused), but also in transcriptional regulation, transport, secretion and degradation.
Most recently, we have identified a transcription factor from yew (Taxus cuspidata) that is capable of activating transcription from the promoters of several of the genes for paclitaxel biosynthesis. Our identification of a protein that may regulate the biosynthesis of this compound is an important step forward in efforts to engineer cultured yew cells for enhanced production of paclitaxel.
Chu, H-H., Conte, S.S., Chan-Rodriguez, D., Vasques, K., Punshon, T., Salt, D.E., Walker, E.L. 2013. Arabidopsis thaliana Yellow Stripe1-Like4 and Yellow Stripe1-Like6 localize to internal cellular membranes and are involved in metal ion homeostasis. Frontiers in Plant Science, 4: 283.
Lenka, S. K., Boutaoui, N., Paulose, B., Vongpaseuth, K., Normanly, J., Roberts, S. C. and E. L. Walker. 2012. Identification and expression analysis of methyl jasmonate responsive ESTs in paclitaxel producing Taxus cuspidate suspension culture cells. BMC Genomics 2012, 13: 148.
Roha, A. Patil, Martin E. Kolewe, Jennifer Normanly, Elsbeth L. Walker, Susan C. Roberts. 2012. Taxane biosynthetic pathway gene expression in Taxus suspension cultures with different bulk paclitaxel accumulation patterns – a molecular approach to understand variability in paclitaxel accumulation. Biotechnology Journal, 7: 418–427.
Yordem, B. K., Conte, S. S., Ma, J. F., Yokosho, K., Vasques, K. A., Gopalsamy, S. N. and Walker, E. L. 2011. Brachypodium distachyon as a new model system for understanding iron homeostasis in grasses: phylogenetic and expression analysis of Yellow Stripe-Like (YSL) transporters. Annals of Botany, 108(5): 821-835.
Walker E. L., Waters B. M. 2011. The role of transition metal homeostasis in plant seed development. Current Opinion in Plant Biology, 14(3): 318-24.
Conte, S. S. and Walker. E. L. 2011. Transporters Contributing to Iron Trafficking in Plants. Molecular Plant, 4(3): 464-76.
Chu, H. H., Chiecko, J., Punshon, T., Lanzirotti, A., Lahner, B., Salt, D. E., Walker, E. L. 2010. Successful Reproduction Requires the Function of Arabidopsis YELLOW STRIPE-LIKE1 and YELLOW STRIPE-LIKE3 Metal-Nicotianamine Transporters in Both Vegetative and Reproductive Structures. Plant Physiology, 154: 197-210.
Lee, S., Chiecko, J. C., Walker, E. L., Lee, Y., Guerinot, M. L. and G. An. 2009. Disruption of OsYSL15 leads to iron inefficiency in rice plants. Plant Physiology, 150(2): 786-80.
Tharayil. N., Bhowmik, P. C., Alpert, P., Walker, E., Amarasiriwardena, D., and B. Xing. 2008. Dual purpose secondary compounds: Phytotoxin of Centaurea diffusa also facilitates nutrient uptake. New Phytologist, 181: 424-434.
Walker, E. L. and E. L. Connolly. 2008. Time to pump iron: Iron-deficiency signaling mechanisms of higher plants. Current Opinion in Plant Biology, 11(5): 530-535.
Vongpaseuth, K., Nims, E., Amand, M., Walker, E. L., and S. C. Roberts 2007. Development of a particle bombardment-mediated transient transformation system for Taxus spp. cells in culture. Biotechnology Progress, 23(5): 1180 -1185.
Waters, B. M., H. H. Chu, R. J. Didonato, L. A. Roberts, R. B. Eisley, B. Lahner, D. E. Salt, and Walker, E. L. 2006. Mutations in Arabidopsis yellow stripe-like1 and yellow stripe-like3 reveal their roles in metal ion homeostasis and loading of metal ions in seeds. Plant Physiology, 141: 1446-58.
Nims, E., C. P. Dubois, S. C. Roberts, and Walker, E. L. 2006. Expression profiling of genes involved in paclitaxel biosynthesis for targeted metabolic engineering. Metabolic Engineering, 8: 385-94.
DiDonato, R. J., Roberts, L. A., Sanderson, T., Eisley, R. B. and Walker, E. L. 2004. Arabidopsis Yellow Stripe-Like2 (YSL2): a metal-regulated gene encoding a plasma membrane transporter of nicotianamine-metal complexes. The Plant Journal, 39: 404-413.
Roberts, L. A., Pierson, A. J., Panaviene, Z., and Walker, E. L. 2004. Yellow Stripe1. Expanded Roles for the Maize Iron-Phytosiderophore Transporter," Plant Physiology, 135: 112-120.
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