An unusual infection mechanism and nodule morphogenesis in white lupin ( Lupinus albus )

• The infection of white lupin ( Lupinus albus ) roots and the early stages in organogenesis of the lupinoid nodule are characterized in detail in this work. • Immunolabelling of Bradyrhizobium sp. ( Lupinus ) ISLU16 and green fluorescent protein labelling of Mesorhizobium loti NZP2037, two strains that induce nodulation in L. albus , allowed us to monitor the infection and morphogenesis process. Light and transmission electron microscopy, low-temperature scanning electron microscopy, fluorescence and confocal microscopy were employed. • Rhizobia penetrated the root intercellularly at the junction between the root hair base and an adjacent epidermal cell. Bacteria invaded the subepidermal cortical cell immediately beneath the root hair through structurally altered cell wall regions. The newly infected cell divided repeatedly to form the central infected zone of the young nodule. Bacteria seemed to be equally distributed between the daughter cells. • A new mode of direct epidermal infection and an unusual morphogenesis for indeterminate nodules lead to the formation of the lupinoid nodule with unique characteristics.