(gamma)-Tubulin in Basal Land Plants: Characterization, Localization, and Implication in the Evolution of Acentriolar Microtubule Organizing Centers

Although seed plants have (gamma)-tubulin, a ubiquitous component of centrosomes associated with microtubule nucleation in algal and animal cells, they do not have discrete microtubule organizing centers (MTOCs) comparable to animal centrosomes, and the organization of microtubule arrays in plants has remained enigmatic. Spindle development in basal land plants has revealed a surprising variety of MTOCs that may represent milestones in the evolution of the typical diffuse acentrosomal plant spindle. We have isolated and characterized the (gamma)-tubulin gene from a liverwort, one of the extant basal land plants. Sequence similarity to the (gamma)-tubulin gene of higher plants suggests that the (gamma)-tubulin gene is highly conserved in land plants. The G9 antibody to fission yeast (gamma)-tubulin recognized a single band of 55 kD in immunoblots from bryophytes. Immunohistochemistry with the G9 antibody clearly documented the association of (gamma)-tubulin with various MTOC sites in basal land plants (e.g., discrete centrosomes with and without centrioles and the plastid surface in monoplastidic meiosis of bryophytes). Changes in the distribution of (gamma)-tubulin occur in a cell cycle–specific manner during monoplastidic meiosis in the liverwort Dumortiera hirsuta. (gamma)-Tubulin changes its localization from the plastid surface in prophase I to the spindle, from the spindle to phragmoplasts and the nuclear envelope in telophase I, and back to the plastid surfaces in prophase II. In vitro experiments show that (gamma)-tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta, and microtubules can be repolymerized from the isolated plastids. (gamma)-Tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that (gamma)-tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has a cell cycle–dependent distribution essential for the orderly succession of microtubule arrays.