Masters Theses

Noel Gerahty

Date of Award

12-1990

Degree Type

Thesis

Degree Name

Master of Science

Major

Life Sciences

Major Professor

Peter M. Gresshoff

Abstract

Soybean nodules can only be initiated in a susceptible zone between the emerging root hairs and the root tip, defining a nodulation window that moves acropetally with root development. It is within these windows that cell divisions in the root cortex produce a meristem and ultimately differentiate into nodules. Nodulation is feedback regulated-early nodules inhibit later nodule formation. This control is seen in double inoculation studies and begins within 3 days post-inoculation and is maximal after 7 days. Since nodule maturation takes about 6 days from infection, feedback regulation appears to inhibit the development of pre-nodule cell divisions. A profile of the frequency of cell division stages from four time points spanning soon after inoculation to well beyond nodule maturation showed that feedback regulation was maximal by 5 days post-inoculation, with a major control point at about the time of meristem formation (stage III to stage IV). There were also indications of a second control point at nodule initiation (stage I). An analysis of the distribution of early stages (stages I through VI) in successive nodulation windows along the root was also performed. Observation were taken at two time points, 3 days and 13 days after each window had opened, and showed that inhibition of nodule initiation was significant 48 hours after inoculation, and resulted from a restriction in the transition from stage II to stage III. Again there were indications of a second control point at stage I. Both control points were seen early and late in feedback regulation indicating that control is continuous. The control points have one characteristic in common-each represents a stage where the plant makes an important commitment to nodule organogenesis. The initial hypodermal cell divisions (stage I), the increase in cytoplasm in outer cortical cells prior to meristem formation (stage III) and the first round of periclinal cell divisions (stage V), each represent major steps in nodule development. From these results it appears that feedback regulation has two components; a spatial (acropetal) component that controls nodulation in new nodulation windows as they open, and a temporal component that controls nodule development as the windows mature.

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