R.L. Vellanoweth |
We plan to have a detailed description of all the research projects underway on this page soon.
Until then, here is an excerpt from one of our grant proposals discussing the background to our research.
Our research program is focused on gaining an understanding of the mechanisms of life span determination in the model plant Arabidopsis thaliana. We are attacking the problem from three fronts: biochemical, molecular biological, and genetic. The following biochemical approaches are underway: 1) an analysis of lipid peroxidation state as a function of age in rosette leaves; and 2) measurement of the expression patterns of enzymatic antioxidant defenses as leaf aging proceeds in the cytoplasm, chloroplast and mitochondrion. Using molecular biology, we are: 3) constructing antisense clones of antioxidant defense enzymes (superoxide dismutase and ascorbate peroxidase) to produce transgenic Arabidopsis plants to determine the affect of reducing ROS defenses on leaf lifespan; 4) constructing clones of SOD and AP under the control of a strong promoter to overexpress these enzymes in transgenic Arabidopsis in various compartments (cytosol, mitochondria, and chloroplast), using the appropriate signal sequences, to determine the affect of increasing ROS
defenses on leaf lifespan; and 5) isolating and characterizing genomic clones of known Arabidopsis senescence-associated genes to begin the molecular dissection of their promoters. Using genetics, we are 6) screening for mutants that exhibit delayed senescence. An understanding of the mechanism by which a model organism amenable to experimentation undergoes purely age-related senescence, a likely consequence of our three-pronged approach, can provide some insight into similar mechanisms operative in other species.
Evidence is mounting in gradually senescing species that a primary component of the deteriorative effects of aging is the accumulation of cellular damage from oxygen-mediated toxicity. Upon this background of endogenous damage production is overlaid an apparent genetic program that targets certain genes for activation or repression during aging. It remains unclear, however, what mechanisms are responsible for such age-dependent alterations in gene expression. Certainly, variations in the activities of specific transcription factors are directly responsible for the changes in gene activity, but what time-dependent event(s) regulates these transcription factors? It is quite possible that the epigenetic events themselves are responsible for the perceived genetic program. An analysis of the controlling factors in senescence in the annual plant Arabidopsis thaliana will allow us to investigate this possibility. In both plant and animal cells, similar age-dependent changes resulting from oxygen-mediated toxicity are evident. Thus, an understanding of the mechanisms by which a model organism amenable to experimentation undergoes purely age-related senescence can provide some insight into similar mechanisms operative in human aging. Given the enormous economic consequences of a large, indigent elderly population, the improvement of human health through interventions in degenerative aging processes is warranted, but requires an understanding of those basic processes. Besides the potential relevance of plant senescence to human aging, knowledge of senescence control is of fundamental importance to agriculture. Senescence in field-grown, economically important crops decreases seed yield significantly and genetic interventions to delay the senescence of photosynthetic structures could result in higher yields per plant. Thus, studies of the type proposed here have dual value to human health: identification of the bio-molecular targets of an age-related process designed to rapidly terminate life will give direction to researchers analyzing the gradual, age-dependent changes in similar targets in humans, and a purposeful uncoupling of senescence from grain development in agricultural crops will improve the quality of much of the food consumed by the world's multitudes.
Several models have been put forth to explain the highly ordered and controlled nature of leaf senescence, including gene activation via a flowering-induced diffusible substance or various hormones. The ability of such disparate molecules to affect senescence suggests that its control is multigenic, similar in many respects to animal aging. A common molecular theme in plant senescence is evident in more downstream events, however. In most species examined, ROS and their damage products increase during leaf aging. Thompson has suggested that O2· levels rise in senescing plant tissues due to an increase in lipoxygenase activity, placing ROS production and damage at the same location, cellular membranes. Others have suggested, both in plants and in animals that age-related ROS increases result from leaks in the normal electron transfer reactions of metabolic pathways. Whatever their source, ROS are intimately associated with the degradative aspects of senescence physiology in all plants that have been studied.
Like most organisms, the controlling factors determining longevity in Arabidopsis are unknown. However, we do know that the primary regulator of senescence appears to be something other than seed development or ethylene. These results suggest the existence of an endogenous, age-related signal that initiates the senescence program in Arabidopsis leaves. We hypothesize that such potential regulators that could effect changes in gene expression at late leaf ages may be those same ROS known to increase in cellular concentration during senescence.
back to top
Comments or questions to Robert Vellanoweth.
Revised 1/6/98 Copyright © 1998 Robert Vellanoweth. All rights reserved.