Abstract text: During root nodule symbiosis, the bacterial infection pathway is tightly linked to a nodule organogenesis programme which poses great challenges to the cell wall. Symbiotic rhizobia colonize root hairs via membrane-confined infection threads (ITs) delimiting rhizobial spread within the cell and guiding the microbes along a pre-defined IT trajectory towards the growing nodule. While these cell wall-devoid microbial compartments are initially stabilized by a proteinaceous rather than a cell wall scaffold (1), they are later sealed by an undefined cell wall matrix. The transcellular passage of ITs requires dynamic changes in pectin composition in the apoplast. Using correlative light-electron microscopy and high resolution imaging allowed us to unravel the sequence of events driving this specific cell wall space (2) that we named the transcellular passage cleft. Reaching their final cellular destination, rhizobia differentiate and fix atmospheric nitrogen in membrane-enclosed symbiosomes. While these organelles lack a rigid cell wall, pectins with antimicrobial activity are secreted into them, but constantly cleared by polygalacturonases (3). We will present our newest data and a comprehensive view on the cell wall during this symbiosis.

(1) Su et al (2023); Nat Commun
(2) Su et al (2023); Curr Biol
(3) Gao et al (2025); Nat Commun