Dynamic expansion of ER surrounding the symbiosomes in M.truncatula nodules. a–c) Representative SEM images of 3‐week‐old nodules in zone II (a), zone II‐III (b), and zone III (c). The red arrowheads indicate ER; s, symbiosome; m, mitochondrion; g, Golgi apparatus. Scale bars = 1 µm. d) Comparison of the ER width with 2D images in different rhizobial zones (II, n = 150; II‐III, n = 118; III, n = 182). e–g) 3D reconstruction derived from a 50‐µm‐thick section showing that ER (blue) forms a cage‐like structure to enclose symbiosomes (magenta) in zone II (e), zone II‐III (f), and zone III (g). The vertically arranged ones formed a group, showing the interaction between ER and symbiosomes from two different perspectives. Scale bars = 1 µm. At least 10 cells were examined, and 2 cells were reconstructed in each zone with the same results. h), Quantification and comparison of the surface area and the volume of 3D symbiosomes in different nodule zones (II, n = 10; II‐III, n = 6; III, n = 10). i, Quantification and comparison of the surface area and the volume of the 3D ER system surrounding the individual symbiosome in different nodule zones (symbiosomes: II, n = 3; II‐III, n = 3; III, n = 3). j–l) Representative partial enlarged display of the e‐g. Scale bars = 0.3 µm. m), Quantification and comparison of the ER contact frequency and area of each symbiosome in different nodule zones (symbiosomes: II, n = 3; II‐III, n = 3; III, n = 3). All data are shown as mean ± standard deviation (SD). The statistical analysis was performed using the two‐way analysis of variance (ANOVA) and post‐hoc comparisons. The letters indicate values with statistically significant (p < 0.05) and non‐significant (p > 0.05) differences, respectively.