| Pea (Pisum sativum) root nodules are cylindrical growths that arise on the root when the diazotroph Rhizobium leguminosarum invades the plant root cells. The bacteria enter the host cell via a tubular infection thread which extends from the epidermal root hair to the nodule cells derived from inner cortical cells. Cells at the distal portion of the nodule are generally meristematic and do not become infected, but contribute cells to the growth of the nodule. Daughter cells however do become infected and bacteria initially occupy the thin layer of cytoplasm in the cortical region of the cell, and as the bacteria replicate, they densely populate the host cell. Infected cells also grow in size and their organelles are repositioned.; In uninfected meristem cells, actin microfilaments and microtubules are generally restricted to the cortex of the cell, envelope the nucleus, and in the case of actin microfilament bundles, extend through cytoplasmic strands from the nucleus to the cortical cytoplasm.; The cortical cytoskeleton in recently infected cells consists of randomly arranged bundles. The cytoplasmic cytoskeleton is more visible in these cells since the layer of cytoplasm has thickened and is more prominent. The actin microfilaments and the microtubules are fragmented, and mostly have a diffuse appearance. Actin microfilaments are also observed as long bundles near the vacuole, and along cytoplasmic strands. MT fluorescence around the nucleus is diffuse, and actin microfilaments form a mesh-like network around the nucleus. Infection threads are associated With diffuse fluorescence of both cytoskeletal components.; Cytoplasmic and cortical actin microfilaments and microtubules reorganize in older infected cells to form new arrays. In the cortex, the actin microfilaments form thick bundles, but are not as abundant as in recently infected cells. The cortical microtubules are long, less bundled, and form an extensive cortical network. In the cytoplasm, which is occupied by a large population of bacteria, both cytoskeletal components form an extensive network of fine bundles that branch and merge and appear to bend in numerous directions. The pattern of this array is random in actin microfilaments, and somewhat radially organized in microtubules.; Microtubules also associate with the bacteria in the cell, and rapid freezing techniques reveal that the rough endoplasmic reticulum also forms associations with the bacteria, infection thread and plasma membrane. This suggests that in older infected cells, the cytoskeleton and rough endoplasmic reticulum organize to form an interconnecting network possibly to position the bacteria in the cell, position plant organelles in the cell, or to facilitate movement of membrane components and proteins to the symbiosomes. |
