Valerie M. Williamson

Professor and Nematologist
Office: 560 Hutchison Hall
Office Phone: 530-752-3502
Lab Phone: 530-752-1050
FAX: 530-752-5809
E-Mail: vmwilliamson@ucdavis.edu

Biography

Appointment: 1987
B.A., 1971, Biology. Northeastern University, Boston, MA
Ph.D., 1978, Biochemistry, University of California, Davis

Teaching Disciplines

Molecular Biology Laboratory Techniques (ANG111)
Agricultural Biotechnology (PLP140)

Research

Root-knot nematodes are obligate endoparasites that induce roots to form morphologically distinct galls and "giant" cells. These specialized host cells act as feeding sites to support nematode development. This group of plant parasitic nematodes causes billions of dollars in damage and infects thousands of plant species. Our laboratory is using molecular and genetic tools to study the interactions between host and parasite.

Host resistance to root-knot nematodes
Many commercial tomato lines contain a gene, Mi-1, that confers resistance against root-knot nematodes. Mi-1 has been isolated in our lab and shown to encode a member of the large NBS-LRR family of pathogen resistance proteins in plants. By analysis of transgenic plants, we showed that this same gene confers resistance against the potato aphid. We have been investigating the mechanisms by which Mi-1 recognizes the presence of the nematode and triggers a signal transduction pathway leading to resistance. Using in vitro mutagenesis followed by examination of phenotypes in transformed roots or after transient expression in leaves, we have found that intramolecular interactions are important for regulation of induction of the defense response. We have also used our mutants in Mi-1 to identify components of the signal transduction pathway leading to resistance.

Another line of research has been to identify additional resistance genes that can confer resistance to Mi-1-virulent nematodes. We have mapped one such gene, Mi-3, in the wild tomato species Lycopersicon peruvianum. Using a combination of classical genetics and molecular techniques, we have localized Mi-3 to a DNA clone of <50 kb. Transformation studies to identify the resistance gene are in progress.

Root-knot nematode pathogenicity
Although the Mi-1 gene has been effective for controlling root-knot nematodes on tomato for many years, some nematode isolates have the ability to reproduce on plants with this gene. Using DNA markers and RNAi, we have identified a candidate gene that may be required for the nematode to be recognized by plants with Mi-1.

Our group is interested in understanding host recognition and pathogenicity of root-knot nematodes. We have identified strains of the root-knot nematode species Meloidogyne hapla that differ in attraction to specific plant hosts, and also in ability to reproduce on resistant plants. We have developed attraction assays and identified volatile compounds that may be important for host recognition. We have established a genetic system for M. hapla and have successfully carried out crosses between inbred strains that differ in virulence, host attraction, and molecular markers. Analysis of segregation of phenotypes in progeny should lead to identification of traits involved in host recognition and pathogenicity of this complex and fascinating parasite. Several research groups are now focusing on M. hapla as the canonical species for exploring the interactions of endoparasitic nematodes with their plant hosts. A large number of ESTs are available, BAC libraries have been made, and a project has been initiated to sequence the genome of this organism. These resources will expedite our efforts to understand factors important for host recognition and pathogenicity in root-knot nematodes.

Nematode identification
Nematodes are numerous and ubiquitous in most environments. Identification of plant parasitic nematode species in field samples can often be quite difficult. We have developed molecular assays for identification of several plant parasitic nematodes of agricultural importance in California and Africa.

Lab members:
Kevin Fort       Graduate Student, Genetics Graduate Group
Chin-Feng Hwang       Assistant Specialist
Ann Liu       Undergraduate Assistant
Qingli Liu       Graduate Student, Plant Pathology Graduate Group
Steven Lower       Post Doctoral Scientist
Waclawa Pudlo       Lab Assistant
Adam Telleen       Graduate Student, Plant Biology Graduate Group
Yong Wen       Lab Assistant
Jafar Yaghoobi       Research Scientist