into the mechanisms of nodule formation showed that the ENOD40 gene, coding for a 12–13 amino acid protein [41], is up-regulated during nodule formation [3]. Connection to root structure Root nodules apparently have evolved three times within the Fabaceae but are rare outside that family. The propensity of these plants to develop root nodules seems to relate to their root structure. In particular, a tendency to develop lateral roots in response to abscisic acid may enable the later evolution of root nodules. Nodule-like structures Some fungi produce nodular structures known as tuberculate ectomycorrhizae on the roots of their plant hosts. Suillus tomentosus, for example, produces these structures with its plant host lodgepole pine (Pinus contorta var. latifolia). These structures have in turn been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites. Gallery See also Root gall nematode Rhizobium Sinorhizobium Bradyrhizobium Neorhizobium Pararhizobium References External links Legume root nodules at the Tree of Life Web project Video and commentary on root nodules of White Clover Plant roots Plant organogenesis Fabaceae Nitrogen cycle Symbiosis Oligotrophs