GmYSL7 controls iron uptake, allocation, and cellular response of nodules in soybean

Iron (Fe) is essential for DNA synthesis, photosynthesis and respiration of plants. The demand for Fe substantially increases during legumes-rhizobia symbiotic nitrogen fixation because of the synthesis of leghemoglobin in the host and Fe-containing proteins in bacteroids. However, the mechanism by which plant controls iron transport to nodules remains largely unknown. Here we demonstrate that GmYSL7 serves as a key regulator controlling Fe uptake from root to nodule and distribution in soybean nodules. GmYSL7 is Fe responsive and GmYSL7 transports iron across the membrane and into the infected cells of nodules. Alterations of GmYSL7 substantially affect iron distribution between root and nodule, resulting in defective growth of nodules and reduced nitrogenase activity. GmYSL7 knockout increases the expression of GmbHLH300, a transcription factor required for Fe response of nodules. Overexpression of GmbHLH300 decreases nodule number, nitrogenase activity and Fe content in nodules. Remarkably, GmbHLH300 directly binds to the promoters of ENOD93 and GmLbs, which regulate nodule number and nitrogenase activity, and represses their transcription. Our data reveal a new role of GmYSL7 in controlling Fe transport from host root to nodule and Fe distribution in nodule cells, and uncover a molecular mechanism by which Fe affects nodule number and nitrogenase activity.

Automated summary

Iron (Fe) is essential for DNA synthesis, photosynthesis, and respiration in plants. However, the mechanism by which plants control iron transport from roots to nodules remains unknown. This study demonstrates that GmYSL7 plays a key role in regulating iron uptake and distribution in soybean nodules. GmYSL7 transports iron across membranes and into infected cells of nodules, and alterations of GmYSL7 affect iron distribution between root and nodule, leading to defective nodule growth and reduced nitrogenase activity. Additionally, GmYSL7 knockout increases the expression of GmbHLH300, a transcription factor involved in iron response of nodules, while overexpression of GmbHLH300 decreases nodule number, nitrogenase activity, and iron content in nodules. The study also reveals that GmbHLH300 directly binds to the promoters of ENOD93 and GmLbs, which regulate nodule number and nitrogenase activity, and represses their transcription.