Area of Expertise: Molecular Plant Physiology
Description of research programme
My research programme focuses on the manipulation of plant form to enhance plant productivity and sustainable horticultural production. This programme has made a substantial contribution to the elucidation of the mechanisms of hormone transport and signal transduction in plants. Outputs from the programme include improvements in mineral nutrition, enhanced crop plant growth in marginal soils, and increased harvestability of biofuel feedstock plants. As the research has involved analyses of auxinic herbicide mechanisms, it has also contributed to reduced herbicide use in crop plant cultivation and reductions in consequent cost and environmental damage. The basic research in my lab has also made a contribution to efforts to improve the efficacy of anticancer chemotherapeutics, develop new therapies for protein folding and processing disorders, and identify targets of pesticides and beneficial plant compounds that impact human health.
A. Plant hormone transport and modification of plant architecture
My lab investigates the cellular mechanisms that regulate auxin transport in dicots (Arabidopsis, tomato, cucurbits) and monocots (rice, maize, sorghum). This work has included studies of the relationship between proton extrusion and auxin movement, long distance auxin transport and root, shoot, and leaf architecture, the regulation of auxin transport by natural plant flavonoids (in collaboration with Dr. Wendy Peer), the role of auxin movement in nutrient acquisition responses, and studies of the fate of auxin at the termination of signalling events. In an ongoing collaboration with Dr. John Christie from the University of Glasgow, my lab has focused on understanding the mechanisms underlying phototropic auxin redistribution in photoresponding seedlings, mature shoots, and leaves.
Over the last decade, my lab has played a major role in identifying and characterizing the P-glycoprotein ABCB transport proteins and clarifying their interactions with a second transport protein family, the PINs. Currently, our efforts are focused on the structural features of plant ABCB transporters that confer the relative substrate specificity seen in these proteins when compared to mammalian orthologs. We are also examining the structural basis of the substrate-activated ABC transport, determining which of the other 18 ABCB transporters mobilise auxin, and evaluating which groupings of ABCB proteins may have diverged between dicots and monocots. We also study the cellular mechanisms that regulate ABC transporter function. ABCB19 from Arabidopsis is not only sensitive to membrane sterol composition, but appears to define sterol-enriched membrane microdomains that stabilise other membrane transporters including the PIN1 auxin efflux carrier. Our research makes extensive use of protein biochemistry techniques, mass spectral analyses, microelectrode/ microsensor assays, and laser scanning confocal microscopy using simultaneous fluorescence imaging of small molecules and fluorescent proteins in live cells. Comparative systems biology and computer modelling are fundamental tools used in our research programme. We use these approaches to identify new functional components of membrane transport and regulatory complexes.The lab also places an emphasis on improving auxin determination and transport assay techniques as well as integration of those techniques with cellular imaging technologies. We have developed nanoscale radiotracer transport assays and auxin detectors.Funding for our projects has come from the National Science Foundation, the Department of Energy, the Biotechnology and Biological Sciences Research Council (UK), and US Department of Agriculture.
B. Application of results from plant studies to human health Anticancer treatments rely on aggressive uptake of chemotherapeutics by malignant cells after the drugs have been administered at threshold levels of toxicity. However, increased ABCB activity in cancer cells results in increased multidrug resistance. Development of effective chemotherapeutics that are poorly transported by ABCBs as well as formulations for these drugs that reduce efflux is a priority. However, the structural basis underlying ABCB-mediated efflux of a wide range of amphipathic molecules has only partially been determined. When expressed in mammalian or S. pombe cells, plant ABCBs exhibit a greater degree of substrate specificity than their mammalian homologs. Further, their direction of transport can be changed by drug treatments or co-expression with interacting proteins. Comparative studies in our lab are directed at the discovery of the basis of PGP substrate specificity and direction of transport using multiple heterologous expression systems. We have also formed a partnership with Dr. Debbie Knapp in the Purdue Veterinary Medicine School to study the effects of auxinic herbicides on the bladder tumours in dogs. Funding for these efforts has come from the National Science Foundation, the Indiana Elks Charity, the Purdue Botanicals Center, and Kraft Foods.