When legumes and rhizobial bacteria met

Legumes have special nodules in their roots that are home to nitrogen-fixing rhizobial bacteria, the two having formed a symbiotic relationship. The bacteria provide the plant with a form of nitrogen it can use while the plant provides it with sugars and proteins.

Researchers have constructed a draft genome of Medicago truncatula, a model species for the study of legumes, which covers around 94% of its genes. The data are revealing how the legume's nodules evolved over millions of years. They published their results in Nature.

The research team, including Ton Bisseling from the King Saud University, Riyadh, found nodules can be traced back to a an event around 58 million years ago when the complete genome duplicated itself. They suggest that many of the duplicated genes then further developed novel functions that led to the development of nodules. These genes, however, built upon even older nodule-related genes that were formed even earlier, at between 140–150 Myr ago.

The researchers propose a way in which M. truncatula developed the ability to interact with the bacteria after the older genes were modified following the full genome duplication event. This budding relationship led to the complex interdependency between legumes and bacteria that still exists to this day, which makes these plants especially useful to humans.

The gene sequence also revealed that M. truncatula has a 764 nucleotide long binding site and leucine-rich repeat (NBS-LRR) genes, which are genes that play a role in disease detection – more than any other plant sequenced.

"This is primarily interesting from a basic perspective. One potential benefit may be that an inventory of NBS-LRRs gives molecular breeders a parts-list to utilize in dealing with novel disease problems," says Nevin Young, the lead author of the study.

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