BACTERIAL LEAF NODULE SYMBIOSIS

Leaf nodules represent the most visible aspect of a group of symbiotic relationships, which are arguably the most complex and certainly has the most intimate associations between bacteria and higher plants. This intimacy lies in the fact that the bacteria are constant and obligate companions of the host throughout the plant's entire life cycle and hence these relationships have come to be described as “cyclic symbioses.” Colonies of bacteria are maintained in a protein/carbohydrate based mucilage within every vegetative shoot tip of the host plant, acting as a source of inoculum for the infection of each new developing leaf. At the onset of flowering, the bacteria are transferred into the floral shoot tip and, during floral organogenesis, are placed within the embryo sac of the developing ovule. As the ovule develops into a seed, the bacteria are somehow positioned on the epicotyl of the embryo where, upon germination of the seed, they become enclosed in the shoot tip of the seedling where they infect the first leaves of the next host plant generation. In these relationships, the leaf nodules per se, although highly visible, are but a relatively small part of the story. A more complicated sequence of plant-microbe interactions occurs at a microscopic level within the host plant throughout its entire life cycle. This chapter describes the present state of understanding of the interactions that occur in these complex cyclic symbioses and describes, at the ultrastructural level the interesting, if somewhat bizarre, ecosystem to which these leaf nodule microsymbionts have become adapted. © 1990 Academic Press, Inc.