Masters Theses

Date of Award

12-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Microbiology

Major Professor

Gary Stacey

Committee Members

Raymond Beck, Jeffrey Becker

Abstract

The mechanism of host-symbiont recognition in the soybean-Rhizobium symbiosis was investigated utilizing mutants of R. Japonicum defective in nodulation. Soybeans were grown in clear plastic growth pouches allowing the identification of the area on the root most susceptible to Rhizobiun infection: the area between the root tip (RT) and smallest emerging root hair (SERH). The location of nodules in relation to this developing zone is an indication of the rate of initiation of infection leading to subsequent nodulation. Nodules were scored as to the distance from the (RT) mark made at the time of inoculation. Seventy-eight percent of the plants nodulate above the RT mark when inoculated with the wildtype R. Japonicum strain 3Ilbllo with the average distance of the uppermost nodule being 2 mm above the RT mark. These data indicate that the wildtype strain initiates nodulation rapidly within the RT-SERH zone follow-ing inoculation. However, inoculation with the slow-to-nodulate mutant strain HSlll resulted in 100% of the plants nodulating only below the RT mark with the average distance of the uppermost nodule being approximately 56 mm below the RT mark. Thus, mutant strain HSlll is defective in the ability to rapidly initiate infection leading to subsequent nodulation within the RT-SERH zone. The location of nodules suggests that strain HS1ll must "adapt" to the root environment before nodulation can occur. To test this, strain HS1ll was incubated in soybean root exudate prior to inoculation. In this case, 68% of the plants nodulated above the RT mark with the average distance of the uppermost nodule being approximately 1 mm below the RT mark. Experiments indicated that the change in nodule initiation by strain HS1ll was due to a phenotypic, rather than a genotypic change. The half-time of root exudate incubation for strain HS1ll necessary for optimal nodulation enhancement was less than 6 h. Heat sensitivity and trypsin sensitivity of the nodulation enhancement factor(s) in soybean root exudate indicated a protein was involved in the reversal in the delay in nodulation by mutant strain HS1ll. Experiments indicate that the active component in soybean root exudate, responsible for strain HSlll's nodulation enhancement, is soybean lectin. Removal of lectin from soybean root exudate by affinity chromatography eliminates the ability of soybean root exudate to enhance the nodulation of strain HSIll. In addition, nodulation enhancement of strain HS1ll by soybean lectin is inhibited by D-galactose, a hapten of soybean lectin.

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