Three-dimensional architecture of the endoplasmic reticulum network in soybean roots reveals extensive remodeling during cyst nematode parasitism

Abstract The soybean cyst nematode Heterodera glycines is a major pathogen of soybean ( Glycine max ) that induces a specialized feeding structure, the syncytium, in infected roots. The syncytium is the nematode’s sole nutrient source throughout development and contains abundant endoplasmic reticulum (ER). Although cisternal ER (cER) has been described in these feeding sites, the presence and distribution of tubular ER (tER) remain unclear. We investigated the three-dimensional (3D) ultrastructure of the ER in mature H. glycines -induced syncytia using two complementary methods: high-voltage electron tomography (HV-ET) of thick sections fixed and stained with zinc iodide–osmium tetroxide (ZIO), and field-emission scanning electron microscopy (FE-SEM) of osmium-macerated specimens. Three-dimensional reconstructions showed dense tER and cER networks within the examined syncytial regions. Notably, some tER strands were closely associated with and appeared to traverse intercellular openings formed by partial cell-wall dissolution, consistent with local ER continuity between adjacent syncytial compartments. These findings reveal extensive ER remodeling in mature syncytia and provide a structural basis for investigating possible transport functions within the feeding site.