Differential Messenger RNA Gradients in the Unicellular AlgaAcetabularia acetabulum. Role of the Cytoskeleton

Abstract

The unicellular green alga Acetabularia acetabulum has proven itself to be a superior model for studies of morphogenesis because of its large size and distinctive polar morphology. The giant cell forms an elongated tube (a stalk of up to 60 mm in length), which at its apical pole makes whorls of hairs, followed by one whorl of gametophores in the shape of a cap. At its basal pole, the cell extends into a rhizoid wherein the single nucleus is positioned. In this study, we have determined the level of specific messenger RNAs in the apical, middle, and basal regions using reverse transcriptase-PCR methodology. Four mRNA classes were distinguished: those that were uniformly distributed (small subunit of Rubisco, actin-1, ADPglucose, centrin, and ␣-and ␤-tubulin), those that expressed apical/basal (calmodulin-4) or basal/apical gradients (calmodulin-2 and a Ran-G protein), and those with development-specific patterns of distribution (mitogen-activated protein kinase, actin-2, and UDP-glucose-epimerase). Restoration of the apical/basal calmodulin-4 mRNA gradient after amputation of the apical region of the cell requires the nucleus and was abolished by cytochalasin D. Accumulation of actin-1 mRNA in the vicinity of the wound set by the amputation needs, likewise, the presence of the nucleus and was also inhibited by cytochalasin. This suggests that actin microfilaments of the cytoskeleton are involved in directed transport and/or anchoring of these mRNAs.In the early 1930s, Haemmerling (1931, 1932, 1934a) introduced the unicellular green alga Acetabularia acetabulum (formerly known as Acetabularia mediterranea) as a research object into cell biology primarily because of its large size and distinct polar morphology. The life cycle of A. acetabulum begins with the fusion of two isogametes to form a zygote, which, after attachment to a solid substratum, grows into a long, tube-shaped stalk. The stalk elongates at the tip, periodically forming lateral whorls of branched hairs. Morphogenesis at the apical pole is completed with formation of a whorl of gametophores known as the cap. In contrast, the basal pole ends in a short, convoluted rhizoid resembling the fingers of a gnarled hand wherein the nucleus is located during all vegetative stages of the cell cycle. Thus, both cellular poles of A. acetabulum execute very different morphogenetic programs (for details, see Berger and Kaever, 1992;Mandoli, 1998).Grafting experiments led Haemmerling to the conclusion that "morphogenetic substances" are released from the nucleus into the cytoplasm where they elicit cellular morphogenesis. He considered them to be "gene products of the nucleus," distinguishing between cap-, whorls of hair-, stalk-, and rhizoidforming substances (Haemmerling, 1934b). A major feature of Haemmerling's morphogenetic substances was their extreme stability, because enucleated cells could perform stalk-, hair-, and cap-formation for several weeks after removal of the nucleus (Haemmerling, 1932). When these concepts were formulated, neither the chemical ...

Comparison of ESTs from juvenile and adult phases of the giant unicellular green alga Acetabularia acetabulum

“…Therefore, aspects of post-transcriptional regulation, such as mRNA stability and mRNA localization, are probably very important to the regulation of gene expression in these unicells. Indeed, more than half of the transcripts (9/16) studied to date in A. acetabulum are localized to one end or the other of the unicell, most often to its apex [31-33]. To achieve this localization, each transcript must contain cis-acting elements within its sequence, also called 'zipcodes' [34].…”

Structural features and gene-expression profiles of actin homologs in Porphyra yezoensis (Rhodophyta)

“…In the giant unicellular green alga Acetabularia acetabulum, four mRNA classes were reported: (i) uniformly distributed in the apical, middle and basal regions, (ii) expressed as apical/basal or (iii) basal/apical gradients, or (iv) containing development-specific patterns of distribution. A previous study revealed that actin-1 and actin-2 from A. acetabulum belong to class (i) and (iv), respectively (Vogel et al, 2002). The uniform expression pattern of PyACT3 should make the development of a molecular marker easier since it would be more easily detectable and increase reproducibility between different samples obtained in the lab or nori fields.…”

The Cytoskeleton and Intracellular Motility in Plants

“…Most prominent and wellcharacterized examples are limited zones of expansin mRNA localization adjusting to the zones of active growth in fibers and tracheids [25], differential prolamine and glutelin mRNA attachment to different endoplasmic reticulum (ER) domains in the rice endosperm [26,27], mRNA gradients along the stalk of Acetabularia [28]. However, a detail studying of the mechanisms of such localization is at its start only.…”