Background

Motile gametes of land plants are strikingly diverse and develop through transformations that involve repositioning, and reshaping of cellular components, and the assembly of a complex locomotory apparatus (Renzaglia and Garbary, 2001; Lopez and Renzaglia, 2008). Because of constraints imposed by cell walls, elongation of the cell and flagella is around the periphery of a nearly spherical space, resulting in a coiled configuration of the mature gamete (Renzaglia and Garbary, 2001; Lopez and Renzaglia, 2014). The degree of coiling varies from just over one to as many as 10 revolutions per cell. The number of flagella per gamete is even more variable, ranging from two in bryophytes (mosses, hornworts, liverworts and most lycophytes) to an estimated 1,000-40,000 in Ginkgo and cycads, the earliest divergent seed plant lineages. Following the diversification of Ginkgo and cycads, all vestiges of basal bodies and flagella were lost in the remaining seed plants that utilize pollen tubes to deliver non-motile sperm to egg cells (Southworth and Cresti, 1997).

It is widely known that vegetative plant cells lack centrioles and the centrosome is elusive. A lesser-known fact is that in plants with motile sperm cells, centrioles arise de novo during the penultimate or ultimate mitotic divisions that produce the nascent spermatid in antheridia (Renzaglia and Carothers, 1986; Vaughn and Renzaglia, 1998; Vaughn and Harper, 1998; Renzaglia and Maden, 2000; Vaughn and Renzaglia, 2006). In these cell lineages, centriolar centrosomes serve as the nucleation site for spindle microtubules and thus bear striking parallels with centrioles of animal and protist cells. In the developing sperm cells, the centrioles reposition, anchor to form the distinctive basal bodies, and elongate to produce the 2-40,000 flagella in each gamete. These changes occur in synchrony with cell elongation, and the entire process of cytomorphogenesis is guided by the production of unique arrays of microtubules, and fibrillar and lamellar bands or strips. Because of the exclusive occurrence of basal bodies, flagella and associated complexes in developing male gametes, studies of spermatogenesis have revealed important information on the structure, composition, and developmental changes in microtubule arrays as they relate to the cell cycle, microtubule organizing centers (MTOCs), and cellular differentiation in plants. The purpose of this review is to describe the method used in transmission electron microscopic examination and to demonstrate how this approach has advanced understanding of basal bodies, flagella/cilia, and associated structures in land plants.