Nodulation of white clover (Trifolium repens) in the absence ofRhizobium

Summary Spontaneous nodules developed on the roots of white clover (Trifolium repens cv. Ladino) in the absence ofRhizobium. A small subpopulation of uninoculated clover plants (0.2%) exhibited white, single-to-multilobed elongated structures on their root systems when grown without fixed nitrogen. Clonal propagation using aseptic stolons confirmed the genetic stability of the observation. Few if any viable bacteria of unknown origin were recovered from surfacesterilized structures. Nodule contents were incapable of eliciting nodulation. Histological observations showed that these structures possessed all the characteristic features of indeterminate nodules, such as active meristem, cortex, endodermal layer, vascular strands, and a central zone with parenchyma cells. Infection threads, intercellular or intracellular bacteria were absent. Instead, numerous starch grains were observed in the central zone, a feature absent in normal nitrogen-fixing nodules. Our observation broadens the concept of spontaneous nodulation, believed to be restricted to alfalfa (Medicago sativa), to other legumes, and suggests a degree of generality among indeterminately nodulated legumes displaying natural heterozygosity.