Courses currently taught:

• HORT 55100, Cellular and Molecular Plant Physiology
• HORT 59000, Plant Development and Transport
• HORT 60100, Planning and Presenting Plant Science Research
• HORT 60200, Horticulture Research Seminar
• HORT 69500, Horticulture Seminar

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

Manipulation of plant form to enhance crop value and aesthetic quality of ornamentals is an important and costly aspect of horticultural production.  Development of dwarf cultivars has played an important role in cereal and fruit production, structural stabilisation of higher yielding vegetable crops, and adaptation of native shrubs and flowers for ornamental use. Plant architecture is primarily determined by polar growth mechanisms established during embryogenesis and maintained by apical extension of roots and shoots in post-embryonic plants. Polar growth is also modulated by environmental inputs, most notably those that induce tropic bending in response to light, gravity, and surface contact. These processes involve local and long-distance movement and perception of growth regulators, transcriptional and epigenetic effectors, and cellular trafficking mechanisms. The major long distance signalling compound that regulates plastic polar growth is the hormone auxin, which is transduced through plant tissues by polarised plasma membrane transport complexes.

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. Although an analysis of phototropic bending by Charles Darwin and his son Francis described in the 19^th century text Power of Movement in Plants formed the basis for the identification of the hormone auxin, it is still not known how photoreceptors mediate the differential auxin distribution that was subsequently theorised to initiate the bending response. Similarly, although we know a great deal about the developmental processes that control leaf development, we know much less about processes of auxin redistribution that transduce signals from photoreceptors to alter leaf angle and leaf cell expansion to maximise use of available light. This aspect of auxin-dependent development has taken on new importance as fuel and food shortages drive demand for increased crop productivity.

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 (“lipid rafts”) that stabilise other membrane transporters including the PIN1 auxin efflux carrier. We are currently analysing the internal trafficking components that regulate the maturation of this proteins as well as the cell wall components that stabilise sterol-enriched membrane domains. The lab has also characterised the sites of action of both new and established auxin transport inhibitors, including alkoky-IAA derivatives, gravicin, and phthalimide inhibitors of ABCB transporters.

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 assays and transport assays, and are currently collaborating with Dr. Marshall Porterfield to build nanotube-based electrochemical detectors for non-invasive small molecule detection.

Our lab has also had a longstanding in interest in metalloproteins that regulate plant growth and nutrition responses. In collaboration with Dr. Wendy Peer, we have shown that M1 and M24 zinc metalloproteases play an important role in integrating growth with nutrient availability.  APM1 is a plant homolog of mammalian M1 proteases that regulate sterol uptake and glucose homeostasis in humans. APM1 regulates root development and appears to regulate auxin-dependent and auxin-independent processes. An M24 protease, APP1, is activated by NPA and plays a rate-limiting role in auxin hormone signalling.

The practical implications of this work are also starting to be realised. In collaboration with Dr. Guri Johal (Purdue Botany and Plant Pathology), we previously showed that manipulation of auxin transport can be used to enhance stem thickness in maize. More recently, we have shown that the same techniques can be used to prevent lodging of genetically engineered biofuel feedstocks.  In collaboration with Dr. Roberto Gaxiola at the University of Arizona, we have shown that plant mineral nutrition and water stress resistance can be enhanced by altering root architecture. 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.     Herbicides and plant defence responses

Since the mid-1980s, a syndrome known as Mature Watermelon Vine Decline (MWVD) has had a serious effect on watermelon crops in Southern Indiana, accounting for 20% of watermelon losses in 2000. Field and greenhouse studies indicate that the syndrome is the result of biotic and biotic factors. A putative pathogen has been identified, but all evidence indicates that infection is opportunistic and highly dependent on cultural conditions. We have shown that lesions that precede MWVD incidence correlate with increased soil moisture, low pH sandy soils, use of black plastic to impede weed growth, and use of the herbicide Alanap (NPA). We are also investigating the impact of altered auxin levels on the generation of reactive oxygen species and their impact on the accumulation of active defence compounds. 

C.     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.