Localization of Azospirillum brasilense Cd in inoculated tomato (Lycopersicon esculentum Mill.) roots

Tomato (Lycopersicon esculentum Mill.) seeds inoculated with Azospirillum brasilense Cd were cultured in vitro on solid medium with and without combined nitrogen. Inoculated and control roots from 30-day-old plants were examined by light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to monitor the adhesion, penetration, and localization of bacteria on and within the plant. We also studied the first colonisation steps of three-day-old plants under the microscope and bacteria in culture. Azospirillum brasilense Cd was identified by immunogold or immunofluorescence labelling. In culture, this bacterium has a thick polar flagellum and many thin peritrichous flagella. In aged dark cultures, small cells appeared within cyst-like forms. The bacteria used the polar flagellum to adhere to the epidermis, root hair and root cap areas of tomato roots. They also penetrated the epidermis, root hairs and outer cortex cells. Within the root, A. brasilense formed cyst-like cells that did not divide inside and were rich in poly-β-hydroxybutyrate granules and glycogen. Bacteria within the root had a thick capsule, many granules of different types, and high levels of SOD activity, suggesting that they can fix nitrogen in the intercellular spaces of tomato roots.