Among the bacterial genes required for nodule invasion are the exo genes. These genes are involved in the production of an extracellular polysaccharide. Mutations in the exoD result in altered exopolysaccharide production and defects in nodule invasion [1].

1: The exoD gene of Rhizobium meliloti encodes a novel function needed for alfalfa nodule invasion. Reed JW, Walker GC; J Bacteriol 1991;173:664-677. PMID:1987158