Nutrient storage and release in uninfected cells of soybean nodules support symbiotic nitrogen fixation in infected cells

This study is led by Professor Zhichang Chen (Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China). The authors employed the symplastic movement tracer carboxyfluorescein diacetate (CFDA) to observe and model the transport and storage status of nutrients within nodules. Specifically, the authors introduced CFDA by injecting a CFDA solution into the petiole of the oldest trifoliate leaf, where it is rapidly cleaved by intracellular esterases to produce the fluorescent, membrane-impermeable compound carboxyfluorescein (CF). CF can therefore only diffuse and reach the nodules through the symplastic pathway. Notably, the intensity of the CF signal was significantly higher in the UCs of the N-fixation zone than in the ICs, suggesting that once CF enters the N-fixation zone, it predominantly accumulates in UCs. This result suggest that UCs may serve as a temporary storage site for nutrients transported from the host plant before their final delivery to ICs.

Next the authors examined the ultrastructure of soybean nodule cells using transmission electron microscopy (TEM). The authors observed a near-complete absence of plasmodesmata between ICs, while authors counted abundant plasmodesmata between ICs and UCs, as well as between adjacent UCs. As previous studies have shown that callose deposition occurs at plasmodesmata. Then, the authors used immunostaining with callose-specific antibodies to investigate callose distribution, within the N-fixation zone, observed no callose deposition between adjacent ICs, whereas detected callose deposition between ICs and UCs, as well as between adjacent UCs. These findings suggesting that ICs cannot communicate or exchange nutrients directly with each other. Instead, they likely interact only with adjacent UCs for information and nutrient exchange.

Finally, by artificially inducing callose deposition at plasmodesmata, the authors demonstrate that plasmodesmata permeability between ICs and UCs regulates nutrient import into ICs, thereby influencing nutrient homeostasis and the SNF ability of nodules.

In conclusion, this study reveals a critical role for UCs in the storage and release of nutrients within nodules and elucidates the importance of plasmodesmata between UCs and ICs in nutrient transport. These findings suggest that UCs may serve as a central hub for regulating N fixation, receiving signals and responding accordingly to precisely control the N fixation efficiency and productivity of ICs.

See the article:

Nutrient storage and release in uninfected cells of soybean nodules support symbiotic nitrogen fixation in infected cells

https://link.springer.com/article/10.1007/s42994-025-00247-y