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The root-nodule bacteria Rhizobium, Bradyrhizobium and Azorhizobium (collectively rhizobia) invade and nodulate the roots of their host plants via either wounds or root hairs. The choice is made by the host plant, e.g. the same rhizobial strain infects Vigna roots via root hairs and Arachis roots via wounds (Sen & Weaver, 1984), whereas another strain infects Parasponia via root epidermal cracks and Macroptilium via root hairs (Marvel et al., 1985). Shortly before or during root invasion, rhizobia induce cell divisions in the root cortex, resulting in formation of a nodule primordium. Through infection threads (tip-growing tubular structures containing invading rhizobia) and/or between cortical cells the rhizobia migrate towards the growing primordium, are endocytosed by young nodule cells, and differentiate into dinitrogenfixing bacteroids (see also Brewin et al., this volume).
Rhizobial invasion of most agronomically important legumes such as pea (Pisum sativum), soybean (Glycine max) and bean (Phaseolus vulgaris) occurs through root hairs. Infection of a living plant cell is an unusual phenomenon in plant–bacteria interactions. Plants are open organisms. At many sites, the intercellular space of a plant is in direct contact with the environment, e.g. in stomata, hydathodes or wounds resulting from emergence of lateral roots. A plant is used to regular visits of (plant-associated) bacteria to its interior. Therefore, wound-infection by rhizobia is a normal phenomenon whereas root hair infection is special.
A case of cutaneous T-cell lymphoma is presented in which the diagnosis was obscured by a concomitant granulomatous infiltrate. A working diagnosis of tuberculosis delayed appropriate treatment for several months over which time rapid progression of the disease was seen. Inclusion of overlying skin in the repeat biopsies yielded the histological information to establish the correct diagnosis and there was rapid regression on completion of a course of radiotherapy.