ASSOCIATION OF ACTIN WITH CORTICAL MICROTUBULES REVEALED BY IMMUNOGOLD LOCALIZATION IN NICOTIANA POLLEN TUBES

It is well known that an extensive array of actin microfilament (MF) bundles exists in the cytoplasm of pollen tubes and that it plays an important role in cytoplasmic streaming in these cells. Less well documented or understood is a cortical MF system, which occurs in two forms: single fine filaments running the length of the cortical microtubules (MTs), and MF bundles. In the present study we have utilized double immunogold labeling of tubulin and actin in rapidly frozen and freeze-substituted pollen tubes of Nicotiana alata in an attempt to clarify the distribution and association of these cytoskeletal proteins. We find that both antibodies bind to antigens associated with cortical MTs, while generative cell MTs label only with the tubulin antibody. Bundles of MFs that show a clear reaction with anti-actin are often seen associated with the cortical MTs, but it remains unclear if the single MT-associated MFs are labeled, and thus, if they are composed of actin. Nevertheless, a majority of cortical MTs show a close association with actin and it is possible that these MTs act as guide elements for MF bundles.