MSA 2012, Yale

MSA 2012 Karling Lecture

Stealth Strategies of a Cereal Killer

Dr. Barbara Valent

Department of Plant Pathology, Kansas State University

Barbara Valent

Magnaporthe oryzae includes host-adapted populations that collectively threaten rice, wheat and other cereal crops worldwide. Past studies have focused on understanding development and function of the pathogen’s appressorium, the dome-shaped cell that builds up and focuses enormous pressures to force a tiny penetration peg through the tough outer plant surface. Now, new understanding of host tissue colonization after penetration has emerged from live cell imaging of the fungus expressing fluorescently-labelled components during invasion of optically-clear rice sheath cells. The blast fungus colonizes its host using specialized intracellular hyphae that successively invade living plant cells. Filamentous hyphae that enter each rice cell invaginate the host plasma membrane, and then differentiate into enlarged, bulbous invasive hyphae that are cloaked in a tight coat of plant membrane. Fungal cells directly involved in the morphological switch are associated with a highly-localized biotrophic interfacial complex (BIC) that appears to function as the staging center for translocation of effectors, pathogen proteins that hijack host processes, into the cytoplasm of the invaded host cell. Indeed, some of the translocated effectors move ahead into neighboring rice cells before the fungus enters them, possibly preparing rice cells before invasion. The thickened invasive hypha again undergoes extreme constriction, forming a tiny penetration peg-like structure for crossing the plant cell wall into the next cell. The fungus searches for locations to cross the wall, suggesting that it recognizes and manipulates the plant’s plasmodesmata, the tiny communication channels between plant cells, for its cell-to-cell movement. Knowledge important for controlling a major disease threat to global food security comes back down to understanding fundamental mechanisms for hyphal polymorphism, for fungal protein secretion and targeting, and for sensing and adapting to harsh environments.

Biography

Barbara Valent earned a B.A. degree with a major in chemistry at the University of Colorado, Boulder. Still working at the University of Colorado, she earned a Ph.D. in biochemistry, identifying and characterizing a structural carbohydrate from Phytophthora sojae cell walls that alerts soybean plants to respond to the invading pathogen. She was awarded a National Research Service Award from the National Institutes of Health for post-doctoral training as a yeast molecular geneticist at Cornell University. While there, she proposed and began developing the rice blast fungus, Magnaporthe oryzae, as a model for fungal plant pathogens.

Dr. Valent accepted a research position in the DuPont Company in 1985 to pursue basic development of the rice blast system, and to develop novel strategies for control of this important pathogen. In 1997, she became one of the first women ever promoted to the position of Research Fellow, the top scientific position within DuPont. She then served as Technical Leader of the Genetic Disease Resistance Program in DuPont Agricultural Products. Dr. Valent became a Professor in the Department of Pathology at Kansas State University in 2001, and a University Distinguished Professor in 2002. She has chaired the KSU Interdepartmental Genetics Program since 2004.

Dr. Valent and her colleagues have been the “first” or contributed significantly to several fundamental advances. Her laboratory was the first to identify and clone both a fungal avirulence gene and corresponding rice resistance gene. They demonstrated that the products of these two genes physically interact to trigger resistance, a finding with important implications for engineering enhanced disease resistance in plants. Other results include new fungicide targets for chemical control of fungal diseases, a patented fungal glue (“Spore Tip Mucilage”) that sticks to hydrophobic surfaces including plant surfaces and DuPont’s Teflon, and strategies for deploying rice resistance genes using molecular markers for analysis of the fungal population structure. Currently, Dr. Valent’s laboratory is applying functional genomics and advanced techniques in cell biology to understand how specialized biotrophic hyphae invade and hijack living rice cells to cause blast disease. She has become a leader in research on wheat blast, a disease that recently emerged in South America and now threatens global wheat production.

Dr. Valent is a Fellow of the American Phytopathological Society (APS) and of the American Association for the Advancement of Science (AAAS). Notable MSA activities include presenting the 1st John S. Karling Lecture in 1996, and serving as councilor for genetics and molecular biology. Additional career highlights include organizing a Fungal Metabolism Gordon Conference, and serving on the selection jury for the prestigious Wolf Prize in Agriculture. She has served on the Board of Trustees for the International Center of Tropical Agriculture in Colombia, and as organizer for International Rice Blast Congresses held in Hunan, China, and in Little Rock, Arkansas. She is currently a Samuel Roberts Noble Foundation Non-Resident Fellow in Plant Biology, and a member of the Scientific Advisory Board for the Max Planck Institute for Terrestrial Microbiology in Germany. Dr. Valent’s fascination with fungi extends to collecting, identifying and eating wild mushrooms. As a long-standing member of the North American Mycological Association, she helped organize the NAMA foray in Granby, Colorado in 1983.