A new study shows that legume plants regulate their symbiotic relationship with soil bacteria by using cytokinins—signaling molecules— that are transmitted through the plant structure from leaves into the roots to control the number of bacteria-holding nodules in the roots.

The research, conducted by several Japanese institutes working in cooperation, has shed more light on how certain plants interact with nitrogen-fixing bacteria in the soil.

The signalling molecules cytokins are also known to be responsible for regulating plant growth and development. The exact cytokine that governs the interaction between plant roots and soil bacteria still needs to be identified.

Researchers said their work increases understanding of Legumes' inner workings. The legume plant family includes lentils, soybeans, and peanuts; they have the ability to prosper in nitrogen-poor soil environments thanks to their symbiotic relationship with nitrogen-fixing bacteria called rhizobia, allowing the bacteria to infect them within special structures known as nodules, located along their roots. However, it takes energy to produce and maintain these nodules, which can affect the plant's growth (though legumes also have means of reduce the number of nodules when they aren't necessary).

It has been known for some time that molecules in the plants' shoots above ground regulate the interactions in the roots.

Signals are shot upwards from root to stem by two peptides, but the downwards signals remain relatively mysterious. The work in Japan has shed more light on the downwards signalling that tells plant roots whether to develop or get rid of root nodules.

According to Hitoshi Sakakibara, who led the RIKEN group participating in the project, "cytokinin has been implicated in shoot-to-root long distance signaling for many years. However, no convincing results were obtained from studies using Arabidopsis, the best known model plant. Now, our study with Lotus japonicus has given us the first convincing evidence of a shoot-to-root signal function of cytokinin."

The breakthrough, as well as advancing the knowledge of how nitrogen fixing plants function, could eventually lead to a breakthrough in balancing plant growth and nitrogen-fixing capabilities in crop plants.

With the results achieved using a model lotus plant, the scientists hope to be able to identify the exact molecule responsible for downwards signalling