Doctoral Dissertations

Date of Award

8-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Zoology

Major Professor

L. Evans Roth

Committee Members

David Joy, Peter Gresshoff, Gary Stacey

Abstract

LotusLotus japonicus has been proposed as a model system for the study of plants that form determinate nodules and is now in the early stages of use as a model. Several groups have begun to describe the genetics and gross morphology of the plant. L. japonicus nodules share many characteristics with other determinate-type nodules. The cells of most nodules are arranged into distinct functional zones. This organization differs markedly between determinate and indeterminate nodules. Most authors rely on a set of, what are believed to be, standard characteristics of a given nodule type when studying their experimentally altered plants, to determine what if any effect their experimental conditions had on the plant. This is especially important in the use of model systems. Over the course of time, initially reported characteristics become accepted as rule, and any variation on them is considered to have been induced by the experiment. I will show in this study that many of these so called "typical" features represent only one of several possibilities. I furthermore think that descriptions of these characteristics should reflect this fact. For example, infection threads of L. japonicus are usually broad and resemble those found in alfalfa, which forms indeterminate nodules, while most threads found in soybean and cowpea are thin. However, L. japonicus can form single file infection threads, and broad threads have been observed in soybean. Some characteristics are unique to a given species. There are several such differences observed between L. japonicus and soybean. First, L. japonicus roots contain numerous cells with large deposits of tannins which are not found in the roots of soybean or cowpea. Second, the infected cells of L. japonicus have large centrally located vacuoles that are notably absent from healthy soybean nodules. Third, the prominent scleroid layer found in soybean nodules is absent in L. japonicus. I support L. japonicus as a viable model system but feel that natural inter- as well as intra-species variations must be acknowledged and discussed.

Cadmium Since cadmium is becoming an increasingly important environmental pollutant, I studied the effects of cadmium on soybean nodules by using electron microscopy and X-ray microprobe analysis. I have found that, although plants can accumulate modest amounts of cadmium without significant observable effects, exposures to cadmium at levels in the range of 1.0 to 10mM in laboratory conditions result in a total disarray of infected cells, nodules and plants at 28 days post inoculation. Cellular effects were also seen earlier and at lower concentrations. The earliest and strongest accumulations of cadmium were found in the polyphosphate bodies of bacteroids which, because of their internal location, could only be exposed after the metal had traversed the cell and symbiosome membranes as well as having penetrated the cortical layers of the nodule. Cadmium was also found in nuclei of infected cells in the euchromatin, heterochromatin and nucleoli, results suggesting that cadmium binds to DNA in host cells. Binding to the DNA in bacteroid chromatin could also be expected. The metal was also localized in cell walls of most nodule cells, in amyloplasts, in granules inside tonoplasts, and in the cytosol of almost all cell types in treated plants. Cadmium was also localized in polyphosphate bodies in control cells, though at a lesser frequency and with weak X-ray peaks. Other metals localized were calcium and magnesium, found in both control and treated tissues about equally, and iron which was similarly found, except that it was concentrated more strongly in polyphosphate bodies of treated than untreated cells. I suggest that the primary pathogenicity of cadmium is due to changes in the properties of DNA, the destruction of membranes and the death of bacteroids, though numerous other processes are undoubtedly also affected. The finding of cadmium in numerous locations and causing such widespread devastation in nodule cells helps explain why yields of soybeans in fields treated with industrial sewage sludge and otherwise cadmium-contaminated are so quickly and often permanently reduced. The finding of cadmium in both control and treated nodules and in plants that may or may not show toxic effects suggests that cadmium can be inserted in the food chain for human beings both by ingestion and inhalation at significant levels beyond those presently recognized.

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