ABSTRACT
This chapter discusses the nodulins, nodulin genes, the relationship between nodulin gene expression and nodule development, and the communication between the two partners in the symbiosis correlated with regulation of nodulin gene expression. By definition, nodulins are plant gene-encoded proteins which are found only in root nodules and not in uninfected roots nor in other parts of the host plant. Nodulin genes are, by consequence, plant genes exclusively expressed during the development of the symbiosis. In general, temperate legumes, such as Pisum, Vicia, Trifolium, and Medicago species, develop indeterminate nodules, while determinate nodules occur on the roots of tropical legumes such as Glycine, Phaseolus, Vigna, and Arachis species. Leghemoglobin is supposed to be a true "symbiotic protein" in the sense that the heme moiety is a presumed product of the bacteroid, whereas the globin part is plant-genome encoded. Uricase activity in root or leaf tissue is due to a diamine-oxidase/peroxidase system, requiring a soluble cofactor.
