The simple ultrastructure of the maize kinetochore fits a two-domain model

Light microscope observations suggest there are two discrete biochemical domains in the plant kinetochore, an inner domain containing structural proteins, and an outer domain containing proteins involved in motility. We analyzed the ultrastructure of maize meiotic kinetochores following high pressure freezing and freeze substitution, a method that provides excellent sample preservation. Data from meiosis II support previous descriptions of plant kinetochores as diffuse, nearly invisible domains, sometimes nesting in a cup of darkly staining chromatin. The ultrastructure is similar in meiosis I but there are two sister kinetochores that each protrude away from the chromosome and form their own distinct kinetochore fibers. Microtubules terminate within kinetochores where their ends are splayed in a cone-shaped configuration suggestive of microtubule disassembly. We could not detect any significant substructure within the kinetochore proper. We suggest that the diffuse structure classically defined as the kinetochore represents only the outer domain of a two-domain organelle. The inner domain, known to contain chromatin-binding proteins, probably extends into the electron-dense chromatin of the primary constriction.