Development of an ineffective pea root nodule: morphogenesis, fine structure, and cytokinin biosynthesis

Abstract

Roots of the garden pea Pisam sativum L. cv. Little Marvel inoculated with Rhizobium leguminosarum strain 1019 produce small white nodules which are ineffective in fixing atmospheric nitrogen. Analyses of cytokinin contents of the nodules at different ages using extraction, purification, and thin-layer chromatographic separation showed that the cytokinins zeatin and zeatin riboside and isopentenyladenine and its riboside were present in greatest amounts early in nodule development and decreased thereafter. A new unidentified cytokinin was present in older nodules. The early stages of the infection process in the ineffective nodules were similar to those observed in effective nodules. However, bacteria released from the bacterial thread via an unwalled droplet were not always surrounded by a host membrane. In later stages of nodule development many infected cells contained rhizobia with no enclosing membranes so that the bacteria were free within the host cytoplasm. Such cells showed very low frequencies of mitochondria, of polyribosomes, and endoplasmic reticulum. Thus, the biosynthetic capacity of the cells appeared to be impaired and membrane synthesis defective. The failure of the nodules to develop nitrogenase activity is probably related to the failure of membrane formation around the bacteria. Abnormalities in amyloplast formation were also noted, as well as structural differences in the nodule, including a higher proportion of uninfected cells and earlier cessation of mitotic activity in the nodule meristem than occurs in effective nodules of pea. Transfer cells were observed in the pericycle in both effective and ineffective nodules.