In this study we have examined the immunocytochemical distribution of calmodulin during mitosis of higher plant endosperm cells. Spindle development in these cells occurs without centrioles. Instead, asterlike microtubule converging centers appear to be involved in establishing spindle polarity. By indirect immunofluorescence and immunogold staining methods with anti-calmodulin antibodies, we found endosperm calmodulin to be associated with the mitotic apparatus, particularly with asterlike and/or polar microtubule converging centers and kinetochore microtubules, in an immunocytochemical pattern distinct from that of tubulin. In addition, endosperm calmodulin and calcium showed analogous distribution profiles during mitosis. Previous reports have demonstrated that calmodulin is associated with the mitotic apparatus in animal cells. The present observation that calmodulin is also associated with the mitotic apparatus in acentriolar, higher plant endosperm cells suggests that some of the regulatory mechanisms involved in spindle formation, microtubule disassembly, and chromosome movement in plant cells may be similar to those in animal cells. However, unlike animal cell calmodulin, endosperm calmodulin appears to associate with kinetochore microtubules throughout mitosis, which suggests a specialized role for higher plant calmodulin in the regulation of kinetochore microtubule dynamics.

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