Two indispensable attributes of all organisms are the abilities to absorb nutrients and adapt their metabolism to changes in the environment. The Judelson laboratory will study these processes in the microbe Phytophthora infestans, which causes the economically important late blight diseases of potato and tomato. P. infestans is believed to absorb nutrients during plant infection through the use of specialized structures that it inserts into host cells, and to adjust its metabolism to adapt to the nutrients encountered in its different hosts, but neither phenomenon is well-understood. A combination of molecular and cellular tools will be used to explore these processes, such as methods to identify genes that are turned on and off in response to different feeding conditions and techniques to localize within P. infestans the proteins that participate in nutrient sensing and uptake. This work is significant since organisms like P. infestans cause disease on hundreds of different plant species, and those diseases are difficult to control with existing tools. The research may lead to practical applications, such as inhibitors of the pathways used to assimilate nutrients which might be used to reduce the incidence of disease. The project also integrates research, education, and outreach at several levels as it will support junior scientists and graduate students, as well as undergraduates that might be interested in careers in science.
Our work has addressed the most fundamental aspect of microbe-plant interactions: the acquisition and use of nutrients by pathogens. Pathogens feed on their hosts in order to reproduce. Colonization thus depends as much on a pathogen's ability to exploit host nutrients as on its capacity to overcome host defenses. Host nutrients also influence progression through the pathogen's life cycle. We studied these topics using a plant pathogen called Phytophthora infestans, which causes an important disease of potato and tomato named Late Blight. We started by identifying all of the proteins encoded by this organism that are involved in absorbing nutrients from the plant host, and the enzymes involved in metabolizing those nutrients to generate energy and the molecular building blocks for growth. One interesting discovery was that the metabolic pathways in P. infestans appear to be an amalgam of proteins obtained by gene transfer from other species, including fungi, plants, and bacteria. This sheds light on gene transfer processes that occur in a nature and the evolutionary origins of cells. A second interesting finding is that P. infestans adapts its metabolism to the nutrients available in its plant hosts, and the conditions under which the plants were grown. Many metabolic pathways are, for example, expressed at different levels in potato tubers than leaves. These include enzymes that digest amino acids which were often expressed higher in tubers, which are amino acid-rich, and enzymes used to convert the carbon in amino acids into sugars, which are used to form the pathogen's cell wall. We also characterized the expression of transporter proteins, which take up nutrients from the host. Many are expressed at higher levels during infection than growth on artificial media, which indicates that P. infestans is able to sense the plant's environment and adjust its pattern of gene expression accordingly. Also, proteins that participate in sugar uptake were found to localize specifically to special cell structures that the pathogen makes during infection. We also discovered a system that the pathogen uses to store and distribute energy to different cell types, including swimming spores which have high energy requirements. The broad impacts of this research include its generation of new knowledge related to evolution, metabolism, and plant pathogenesis. Out work also has the potential of a practical return, since crop protection solutions might be developed against P. infestans and its relatives based on understanding its metabolism, against which chemical inhibitors could be developed. Our activities also contributed to the development of human resources by supporting the training of undergraduate and graduate student researchers, and by sponsoring the participation of the principal investigator in regions science fairs and other outreach activities.
