ABSTRACT
This chapter reviews progress in understanding of Parasponia nodulation and nitrogen fixation with particular emphasis on a comparative analysis of this knowledge relative to the rapidly advancing legume nodulation field. The structure of mature Parasponia nodules appears quite different to that of tropical legume nodules even though both are elicited by the same bacterial strain. Studies using flood-inoculated Parasponia plants have shown that, as in the legume infection process, there is a zone of elongating cells directly behind the root tip which is highly susceptible to infection. In legumes such as soybean, the spot inoculation technique has been used to study early infection events. Analysis of the genetics of nodulation in Rhizobium strains has resulted in the identification of some 14 genes implicated in nodule induction. Studies on host-specific nodulation genes in the Rhizobium-legume symbiosis have shown that there are at least two classes of Rhizobium genes that mediate the host-specific response.
