Email: ewalker@bio.umass.edu
E. Walker Biology Dept Website

Ph.D.: The Rockefeller University
Postdoctoral Training: Yale University

Plant Genetics and Epigenetics

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 TM - Bristol-Myers Squibb). Genes discovered during this project will improve our ability to supply sufficient quantities of paclitaxel to the world.

Iron uptake and homeostasis. We recently identified and cloned a transposon-tagged allele of the maize gene, yellow stripe1 ( ys1 ). The product of ys1 mediates iron uptake in grass species, a process that is of fundamental importance for understanding and manipulating iron homeostasis in the world's major grain crops. Grasses have solved the problem of limited bioavailability of iron using a strategy that is fundamentally different from the mechanisms used by other plant groups. Grasses secrete "phytosiderophores": non-proteinogenic amino acid derivatives that form stable Fe[III] chelates. YS1 is the transporter that is located at the root surface and specifically recognizes and takes up the Fe[III] . phytosiderophore complexes, which are the plant's source of iron. With cloning of ys1 , transfer of more efficient iron uptake into non-grass species becomes possible, allowing better growth in calcareous soils, and better iron nutritional quality in edible parts.

Another surprising but important discovery is that genes with strong sequence similarity to ys1 are found in non-grass species (monocot, dicot, gymnosperm, and moss species) that neither synthesize nor have the ability to use phytosiderophores. All these species do, however, contain a related compound, nicotianamine, an iron chelator that is structurally similar to phytosiderophores. Nicotianamine appears to have several roles in plants, all of which relate to allocation of transition metals in plant cells and organs. Nicotianamine-metal chelates are likely to be transported across cell membranes by YS1-like (YSL) proteins, thus by understanding the locations and timing of expression of these transporters we will gain new insight into the patterns of long range metal ion movement in plants, which is poorly understood at present. Much of our current effort is aimed at the functional genomics of the Yellowstripe-like (YSL) family of genes in Arabidopsis thaliana , a model system in which we can answer these and many other questions. For more information about this project, please visit our 2010 web site.

Paclitaxel biosynthesis. We are using molecular approaches to delineate global metabolic control of paclitaxel accumulation in Taxus cell cultures. Paclitaxel (generic name for Taxol TM - Bristol-Myers Squibb) has been approved by the Food and Drug Administration (FDA) for the treatment of breast, ovarian and lung cancers as well as the AIDS-related Kaposi's sarcoma. The original supply of paclitaxel was obtained through the extraction of yew tissue (the largest proportion of paclitaxel is found in the bark of the yew). However, the yew tree is slow-growing with low paclitaxel yields. 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. 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.

Representative publications:

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.

Walker, E. L. and Panavas, T. (2001) Molecular analysis of r1 geographic haplotypes: structural and epigenetic features associated with activity in paramutation. Genetics 159:1201-1215.

Curie, C., Panaviene, Z., Loulergue, C., Dellaporta,, S. L., Briat, J-F., and Walker, E. L. (2001) Maize yellow stripe 1 encodes a membrane protein directly involved in Fe(III) uptake. Nature 409: 346-349.

Bercury, S. D., Panavas, T., Irenze, K., and Walker, E. L. (2001) Molecular analysis of the Doppia transposable element of maize. Plant Molecular Biology 47:341-351.

Walker, E. L. and T. Panavas (2001). "Structural features and methylation patterns associated with paramutation at the r1 locus of Zea mays." Genetics 159(3): 1201-15.

Panavas, T., A. Pikula, P. D. Reid, B. Rubinstein and E. L. Walker (1999). "Identification of senescence-associated genes from daylily petals." Plant Mol Biol 40(2): 237-48.

Panavas, T., J. Weir and E. L. Walker (1999). "The structure and paramutagenicity of the R-marbled haplotype of Zea mays." Genetics 153(2): 979-91.

Walker, E. L. (1998). "Paramutation of the r1 locus of maize is associated with increased cytosine methylation." Genetics 148(4): 1973-81.

Walker, E.L. , Eggleston, W.B., Demopulos, D., Kermicle, J.L. and Dellaporta, S.L. (1997) Insertions of a novel class of transposable elements with a strong target site preference at the R locus of maize. Genetics 146, 681-693.

Walker, E.L., Robbins, T.P., Bureau, T.E., Kermicle, J. and Dellaporta, S.L. (1995) Transposon mediated chromosomal rearrangements and gene duplications in the formation of the maize R-r complex. EMBO Journal 14, 2350-2363.

Robbins, T.P., Walker, E.L., Kermicle, J.L., Alleman M. and Dellaporta, S.L. (1991) Meiotic instability of the R-r complex arising from displaced intragenic exchange and intrachromosomal rearrangement. Genetics 129, 271-283.