Researchers have identified a systemic signaling pathway in soybean that promotes the formation of nitrogen-fixing root nodules, potentially providing new strategies to improve legume productivity under nitrogen-limited conditions.

Legumes rely on symbiotic nitrogen fixation to support productivity in agricultural and natural ecosystems. Root nodules form when rhizobia, soil bacteria capable of fixing nitrogen, infect plant root hairs. Plants tightly regulate nodule formation to balance the benefits of nitrogen fixation with the growth and metabolic demands of the plant.

The study found that soybean plants use a long-distance signaling mechanism to promote rhizobial infection and nodule formation. The pathway connects a hormone peptide produced in the roots with a mobile microRNA produced in the shoots and a lectin gene expressed in soybean root hairs.

Researchers found that inoculating soybean roots with rhizobia suppresses the production of microRNA miR4416-5p in shoots. The microRNA normally moves from shoots to roots, where it limits expression of the vegetative lectin gene Lectin 3, or GmLe3.

When miR4416-5p levels decline in the roots, GmLe3 expression increases. This promotes rhizobial infection, resulting in greater nodule formation and improved plant productivity under low-nitrogen conditions.

The study also found that suppression of miR4416-5p production in shoots is triggered by C-TERMINALLY ENCODED PEPTIDE 7, or GmCEP7, a peptide produced in the roots. This establishes a long-distance GmCEP7-miR4416-5p-GmLe3 regulatory pathway that helps regulate the plant's response to rhizobial infection.

Manipulating the pathway by silencing miR4416-5p, increasing expression of its target gene or increasing expression of the GmCEP7 peptide precursor enhanced nodulation and plant productivity under nutrient-limited conditions.

Comparative genomic analysis showed that the miR4416-5p regulatory module is absent in the model legumes Medicago truncatula and Lotus japonicus. However, the pathway appears to be conserved in economically important legume crops including common bean and pigeonpea.

The findings indicate that the regulatory pathway represents an evolutionary innovation in nodulation control and demonstrate the value of studying both model legumes and crop species to understand the mechanisms regulating symbiotic nitrogen fixation. The pathway could provide a basis for strategies aimed at improving legume productivity under nitrogen-limited conditions while supporting more sustainable agricultural production.

Source: Proceedings of the National Academy of Sciences, "A long-distance signaling loop promotes soybean nodulation and productivity"