Masters Theses

Date of Award

12-1989

Degree Type

Thesis

Degree Name

Master of Science

Major

Life Sciences

Major Professor

Peter M. Gresshoff

Committee Members

G. Douglas Carter, Bob W. Auge

Abstract

Factors affecting Bradyrhizobium nod gene expression derived from extracts and root exudates of soybean cv. Bragg were compared using a bioassay which monitors expression of the plant-inducible nodYABC operon from Bradyrhizobium japonicum strain USDAl 10. Wild-type soybean plants (uninoculated and inoculated with B. japonicum) were compared with an autoregulatory ("supemodulating") and two non-nodulating mutants. Analyses of extracts from seeds, cotyledons, roots, stems, and leaves resulted in levels of of nod gene induction characteristic for each tissue type; however, no significant variation was observed between each plant genotype tested. Extracts from root tip regions of inoculated and uninoculated plants showed similar abilities to induce the nodYABC operon in a time course experiment. Root exudates from wild-type and mutant plants also had similar nod gene inducing ability. The validity of the nod gene induction bioassay was confirmed using HPLC analysis which showed that the peak size of daidzein in fractionated root extracts correlated with nod gene inducibility. The results indicate that the autoregulatory and nonnodulating plant mutants do not have significantly different levels of bacterial nod gene signalling molecules compared to wild-type plants. It appears unlikely that alterations in levels of plant signals are responsible for the mutant symbiotic plant phenotypes examined.

Analyses of the response of B. japonicum to daidzein, a potent nod gene inducing agent and chemoattractant, revealed the compound was not utilized as a metabolite either in the presence or absence of an alternative carbon source.

Radioactively labelled daidzein was isolated by a method that utilizes daidzein accumulation as a result of a plant defense responses in soybean. This method, although not perfected, is an alternative to the chemical synthesis of the compound, which is a costly and complicated procedure.

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