Exposure of Cotyledons to Light Affects Rhizobium Infection and Nodule Development in Soybeans

Abstract

In soybean seedlings the exposure of cotyledons/hypocotyls to light strongly affects the initiation and development of root nodules. Two day-old soybean seedlings in plastic growth pouches were inoculated with R. japonicum in very dim light. At the time of inoculation, the position of the root tip (RT) of each seedling was marked on the face of the pouch. Upper portion of seedlings (cotyledons/hypotocotyl) were exposed to light at various times before and after inoculation. The number and position of nodules on the primary roots relative to RT mark were scored 7–8 days after inoculation or first exposure to light whichever came last. A set of 100 seedlings were used for each treatment. For anatomical studies representative root samples fixed in paraffin were serially sectioned, stained and then examined under microscope to score the location and stage of infection development.