Attachment of soil bacteria to plant cells is supposedly the very early step required in plant-microbe interactions. Attachment also is an initial step for the formation of microbial biofilms on plant roots. For the rhizobia-legume symbiosis, various mechanisms and diverse surface molecules of both partners have been proposed to mediate in this process. The first phase of attachment is a weak, reversible, and unspecific binding in which plant lectins, a Ca(+2)-binding bacterial protein (rhicadhesin), and bacterial surface polysaccharide appear to be involved. The second attachment step requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. Cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment of Agrobacterium to plant cells. Attachment of Azospirillum brasilense to cereals roots also can be divided in two different steps. Bacterial surface proteins, capsular polysaccharide and flagella appear to govern the first binding step while extracellular polysaccharide is involved in the second step. Outer cell surface proteins and pili are implicated in the adherence of Pseudomonas species to plant roots.
Several strains isolated from the legume Pachyrhizus erosus were characterized on the basis of diverse genetic, phenotypic and symbiotic approaches. These novel strains formed two groups closely related to Bradyrhizobium elkanii according to their 16S rRNA gene sequences. Strains PAC48T and PAC68T, designated as the type strains of these two groups, presented 99.8 and 99.1% similarity, respectively, in their 16S rRNA gene sequences with respect to B. elkanii USDA 76T. In spite of these high similarity values, the analysis of additional phylogenetic markers such as atpD and glnII genes and the 16S-23S intergenic spacer (ITS) showed that strains PAC48T and PAC68T represented two separate novel species of the genus Bradyrhizobium with B. elkanii as their closest relative. Phenotypic differences among the novel strains isolated from Pachyrhizus and B. elkanii were found regarding the assimilation of carbon sources and antibiotic resistance. All these differences were congruent with DNA-DNA hybridization analysis which revealed 21% genetic relatedness between strains PAC48T and PAC68T and 46% and 25%, respectively, between these strains and B. elkanii LMG 6134T. The nodD and nifH genes of strains PAC48T and PAC68T were phylogenetically divergent from those of bradyrhizobia species that nodulate soybean. Soybean was not nodulated by the novel Pachyrhizus isolates. Based on the genotypic and phenotypic data obtained in this study, the new strains represent two novel species for which the names Bradyrhizobium pachyrhizi sp. nov. (type strain PAC48T=LMG 24246T=CECT 7396T) and Bradyrhizobium jicamae sp. nov. (type strain PAC68T=LMG 24556T=CECT 7395T) are proposed.
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