Arabinogalactan proteins are expressed at the symbiotic interface in root nodules of Alnus spp

•  We have characterized the origin and distribution of arabinogalactan proteins (AGPs) at the symbiotic interface of dinitrogen (N ₂ )-fixing root nodules of Alnus spp. The interface between the host plant cell and the microsymbiont is an important zone for signaling and growth regulation during nodulation. Arabinogalactan proteins are glycoproteins that have adhesive properties, and, potentially, participate in cell wall assembly, direction of growth, and signaling cascades. These glycoproteins are expressed in several symbiotic systems in an infection-specific pattern, but their occurrence has not been examined in actinorhizal nodules. •  To characterize AGP epitopes in Alnus root nodules, we have used immunogold localization with anti-AGP antibodies, correlated with other techniques. •  Arabinogalactan proteins are abundant in the nodule-infected tissue. One AGP epitope (JIM4) is localized in pectin-rich cell walls, while another (JIM13) is found at the membrane-wall border along the symbiotic interface at the early infection stage, and in the host cytoplasm/vacuoles in mature, infected cells. •  It is likely that AGPs play a significant role in Alnus root nodules, especially in early nodulation stages.