Asymmetric microtubule arrays organize the endoplasmic reticulum during polarity establishment in the brown alga Silvetia compressa

Plant ER geometry and dynamics: biophysical and cytoskeletal control during growth and biotic response

The endoplasmic reticulum (ER) is an intricate and dynamic network of membrane tubules and cisternae. In plant cells, the ER 'web' pervades the cortex and endoplasm and is continuous with adjacent cells as it passes through plasmodesmata. It is therefore the largest membranous organelle in plant cells. It performs essential functions including protein and lipid synthesis, and its morphology and movement are linked to cellular function. An emerging trend is that organelles can no longer be seen as discrete membrane-bound compartments, since they can physically interact and 'communicate' with one another. The ER may form a connecting central role in this process. This review tackles our current understanding and quantification of ER dynamics and how these change under a variety of biotic and developmental cues.

Photopolarization of Fucus zygotes is determined by time sensitive vectorial addition of environmental cues during axis amplification

Fucoid zygotes have been extensively used to study cell polarization and asymmetrical cell division. Fertilized eggs are responsive to different environmental cues (e.g., light, gravity) for a long period before the polarity is fixed and the cells germinate accordingly. First, it is commonly believed that the direction and sense of the polarization vector are established simultaneously as indicated by the formation of an F-actin patch. Secondly, upon reorientation of the zygote, a new polar gradient is formed and it is assumed that the position of the future rhizoid pole is only influenced by the latter. Here we tested these two hypotheses investigating photopolarization in Fucus zygotes by reorienting zygotes 90° relative to a unilateral light source at different time points during the first cell cycle. We conclude that fixation of direction and sense of the polarization vector is indeed established simultaneously. However, the experiments yielded a distribution of polarization axes that cannot be explained if only the last environmental cue is supposed to determine the polarization axis. We conclude that our observations, together with published findings, can only be explained by assuming imprinting of the different polarization vectors and their integration as a vectorial sum at the moment of axis fixation. This way cells will average different serially perceived cues resulting in a polarization vector representative of the dynamic intertidal environment, instead of betting exclusively on the perceived vector at the moment of axis fixation.

Rac1 signaling in the establishment of the fucoid algal body plan

Fucoid zygotes use environmental vectors, including sunlight, to initiate a growth axis a few hours after fertilization. The first division is then transversely oriented by the growth axis, producing daughter cells of distinct fates. The tip growing rhizoid cell gives rise to the holdfast, anchoring the alga to the intertidal substratum, while the opposite thallus cell mainly generates the photosynthetic and reproductive stipe and fronds. Elaboration of this simple growth axis thus establishes the basic body plan of the adult; and elucidating the mechanisms responsible for formation of the growth axis is paramount to understanding fucoid morphogenesis. Recent studies have culminated in a model whereby sunlight, and perhaps other environmental cues, activate the signaling protein Rac1 at the rhizoid pole. Here it sets in motion nucleation of a patch of actin filaments that in turn, targets ions, proteins, and cellular processes to the future growth site. At germination, Rac1 initiates morphogenesis by inducing transformation of the patch of actin filaments to a structure that delivers vesicles to the growing tip, and a few hours later orients the spindle and cytokinetic plate.

Asymmetric properties of the Chlamydomonas reinhardtii cytoskeleton direct rhodopsin photoreceptor localization

The eyespot of the unicellular green alga Chlamydomonas reinhardtii is a photoreceptive organelle required for phototaxis. Relative to the anterior flagella, the eyespot is asymmetrically positioned adjacent to the daughter four-membered rootlet (D4), a unique bundle of acetylated microtubules extending from the daughter basal body toward the posterior of the cell. Here, we detail the relationship between the rhodopsin eyespot photoreceptor Channelrhodopsin 1 (ChR1) and acetylated microtubules. In wild-type cells, ChR1 was observed in an equatorial patch adjacent to D4 near the end of the acetylated microtubules and along the D4 rootlet. In cells with cytoskeletal protein mutations, supernumerary ChR1 patches remained adjacent to acetylated microtubules. In mlt1 (multieyed) mutant cells, supernumerary photoreceptor patches were not restricted to the D4 rootlet, and more anterior eyespots correlated with shorter acetylated microtubule rootlets. The data suggest a model in which photoreceptor localization is dependent on microtubule-based trafficking selective for the D4 rootlet, which is perturbed in mlt1 mutant cells.