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Larsen, Paul B
Research Specialization - Presently, my lab is interested in two areas of plant biology. The first focuses on the elucidation of the mechanism of ethylene signal transduction, which is a plant hormone that regulates many physiological processes throughout a plant's life cycle. Additionally, my lab is interested in examining the mechanism(s) that plants utilize to respond to abiotic stress, particularly aluminum toxicity in acid soil. Ethylene Signal Transduction: Ethylene, one of the five classic plant hormones, is a simple gaseous hydrocarbon that has profound effects on plant growth and development. These effects were first described in the early 1900's with ethylene subsequently being linked to a myriad of physiological events at all stages of plant growth and development. Of greatest interest commercially is ethylene's role in controlling the initiation of fruit ripening, tissue senescence, and pathogen defenses. Using Arabidopsis thaliana as a model system, several of the components involved in ethylene signaling have been identified. This was achieved through the isolation of mutants that either could not perceive ethylene (ethylene insensitive, etr and ein mutants) or manifested an ethylene response even in the absence of ethylene (constitutive ethylene response, ctr and ran mutants). The isolated constituents include a family of five ethylene receptors (ETR1, ETR2, EIN4, ERS1 and ERS2) and a MAP kinase kinase kinase (CTR1). Although tremendous strides have been made in identification of the pathway components, our knowledge of how these factors interact to propagate a signal remains limited. One project that is underway in my lab is to understand how the ethylene receptors regulate CTR1 activity. At least two of the receptors (ETR1 and ERS1) directly interact with CTR1, suggesting that this association is responsible for activation of CTR1. My lab has developed a biochemical assay that we hope will allow us to define a mechanism by which the ethylene receptors control CTR1 activity, both in the presence and absence of ethylene binding. Additionally, using this assay, we hope to identify other factors that are involved in CTR1 regulation, with the goal of fully elucidating the upstream events in ethylene signal transduction. A second project involves using a molecular genetic approach to identify new factors involved in ethylene signaling. We are presently using a map-based cloning approach to isolate genes from two Arabidopsis mutants that have increased sensitivity to ethylene. Mutants with enhanced sensitivity (eer for enhanced ethylene response) were isolated with the goal of identifying factors that are responsible for downregulating the pathway following an ethylene signaling event. One of these mutants, eer1, has been extensively characterized and represents a potential negative regulator of ethylene signaling that acts at or downstream of CTR1. eer1's phenotype is localized to the hypocotyl and stem and it is characterized by an exaggeration of ethylene responses in these tissues. Chromosome walking has placed the eer1 locus in a 1 cM window on chromosome 1. Upon isolation of the gene that represents eer1, work will be initiated to investigate its biochemical role in ethylene signaling. Work is also underway to characterize a second mutant with increased ethylene sensitivity. We are presently initiating map-based cloning for this mutant and examining crosses with other ethylene mutants to identify the gene of interest and determine its role in ethylene signaling. Finally, my lab is also interested in how ethylene-dependent physiological processes are manifested, specifically the mechanisms that are responsible for the progression of fruit ripening and flower senescence. Pollination and fertilization lead to dramatic changes in the flower including senescence of tissues such as the petals and styles and at the same time growth of the ovary and developing ovules. Interestingly, in carnation flowers, each physiological process features a burst in ethylene production, yet in one case, ethylene initiates senescence and in the other it promotes growth. It is unclear as to how ethylene can cause such different phenotypes, but such differential responsiveness to ethylene occurs throughout a plant's life cycle. Using carnation flowers as a model system, we hope to understand ethylene's role in pollination-induced senescence and ovary growth. Aluminum Toxicity in Plants: Aluminum (Al) is the most abundant metal in the earth's crust, and when it exists in an acidic environment, it severely inhibits plant growth. Plants have developed two mechanisms for coping with Al in the environment- either tolerance or exclusion. The first mechanism is common to plants that have evolved in Al toxic soils and requires the plant to tolerate Al that is taken up. The second mechanism is more often found in crop plants that have been bred for growth in acid soil and depends on the exudation of compounds that bind Al and prevent its uptake. Although the physiological responses of plants to Al are well documented, the molecular mechanisms of both Al resistance and Al toxicity are not understood. My lab has generated a collection of Arabidopsis thaliana mutants that have altered responses to Al. One group is represented by a locus (alr-128) that, when mutated, results in increased organic acid release from the roots. These mutants, called alr (for aluminum resistant), are semi-dominant and have an increase in release of malic acid, which is capable of chelating Al at the root surface and effectively preventing Al uptake. Chromosome walking has localized this locus to chromosome 1 and work is being initiated to isolate the gene. Additionally, work is planned to further characterize the metabolic changes that lead to the alterations in organic acid release in these alr mutants. Future plans include isolation of new alr mutants with the hope of identifying more genes responsible for Al resistance. A second group of mutants were have increased sensitivity (als) to aluminum. Three of these mutants have been characterized in depth, and each represents a unique aspect of Al toxicity. One mutant apparently has a defect in citric acid release (als5), resulting in a reduced capability to chelate Al at the root surface. A second mutant shows severe root growth inhibition at low levels of Al (als1), indicating that some mechanism of Al detoxification may be negatively affected. Finally, a third mutant shows an extreme response to very low levels of Al (als3), so much so that Al exposure results in complete arrest of tissue growth at both the root and shoot apical meristems. This is unique since Al responses normally are localized to the root. Work is planned to further characterize the developmental effects of Al in these mutants along with work to examine the metabolic changes that result in the changes in citrate release. Additionally, map-based cloning will be employed to isolate the genes of interest. Finally, we plan to identify new als mutants with the hopes of identifying additional sites of Al toxicity and/or mechanisms of Al resistance.
Postdoctoral Associate, Cornell University, 1994-1997
Research in my lab focuses both on ethylene signal transduction and plant responses to abiotic stress. Ethylene is a plant hormone that, among other things, is responsible for controlling many post-harvest related processes including fruit ripening, tissue sensecence, and pathogen responses. Ethylene-related work is concerned with defining a mechanism for upstream events in the signaling process, primarily by determining how certain factors regulate the activity of downstream components. Additionally, work is ongoing to isolate previously unidentified participants in ethylene signaling using a molecular genetic approach. A second area of interest involves identification of genes that are involved in either aluminum resistance or whose products represent sites of aluminum toxicity. Aluminum toxicity in acid soils is a global problem limiting crop productivity. Several mutants representing alterations in response to aluminum have been identified and the affected genes are being isolated in the hopes that this will result in a molecular and biochemical understanding of how plants cope with an aluminum toxic environment.
Larsen, P. B., Ashworth, E. N., Jones, M. L., and Woodson, W. R. (1995). Pollination-Induced Ethylene in Carnation: Role of pollen tube growth and sexual compatibility. Plant Physiology 108:1405-1412.