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

Abstract

Polarity is a fundamental characteristic of most cell types, and is crucial to early development of the brown alga Silvetia compressa. In eukaryotes the cytoskeleton plays an important role in generating cellular asymmetries. While it is known that F-actin is required for polarization and growth in most tip-growing cells, the roles of microtubules are less clear. We examined the distribution and function of microtubules in S. compressa zygotes as they polarized and initiated tip growth. Microtubules formed asymmetric arrays oriented toward the rhizoid hemisphere early in the polarization process. These arrays were spatially coupled with polar adhesive deposition, a marker of the rhizoid pole. Reorientation of the light vector during polarization led to sequential redistribution of polar axis components, with the microtubules and the polar axis reorienting nearly simultaneously, followed by cell wall loosening and then deposition of new polar adhesive. These findings suggested that microtubules may organize and target endomembrane arrays. We therefore examined the distribution of the endoplasmic reticulum during polarization and found it colocalized with microtubules and became targeted toward the rhizoid pole as microtubule asymmetry was generated. Endoplasmic reticulum association with microtubules remained fully intact following pharmacological disruption of F-actin, whereas microtubule disruption led to aggregation of the endoplasmic reticulum around the nucleus. We propose that brown algae utilize microtubules for organization of the endoplasmic reticulum and migration of exocytotic components to the rhizoid cortex, and present a model for polarity establishment to account for these new findings.

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

“…By 4 h after fertilization of the alga Silvetia compressa , a polar axis is formed on the dark side of a light vector and manifests itself by local secretion of adhesive at the prospective rhizoid tip, along with an increased number of microtubules aligned along the polar axis, emanating from the central nucleus and terminating at the prospective rhizoid tip (Peters and Kropf 2010). After a further 2 h (6 h after fertilization), the ER is preferentially co-localized with the increased microtubules along the polar axis.…”

Signaling mechanisms in the establishment of plant and fucoid algal polarity

“…For example, in response to a light gradient, zygotes will form their rhizoids on the shaded hemisphere. An early indication of polarity (beginning 4–5 hr AF) is the accumulation of endoplasmic reticulum (Peters and Kropf, 2010) and other endomembrane organelles (Hadley et al, 2006) within the rhizoid hemisphere, which is coupled to increased secretion of adhesive locally at the rhizoid pole. This thickening of adhesive more stably anchors the zygote and prevents it from being washed out to sea in the next tidal cycle.…”

“…The roles of these earlier Ca 2+ events may be to ensure that the cell cycle is coordinated with cell polarization, as biolistic loading of zygotes with Ca 2+ (BAPTA) buffers blocks both entry into S phase and activation of the polarization pathway in Fucus serratus (Bothwell et al, 2008). Shortly after F‐actin and Ca 2+ localization, microtubule (MT) arrays extending from the nucleus to the cortex become enriched in the rhizoid pole (Peters and Kropf, 2010). The endomembrane system (Hadley et al, 2006) also becomes more prevalent at the rhizoid pole, leading to increased secretion of adhesive.…”

“…These changes are labile, and if a new signal is perceived, the Arp2/3 complex nucleates F‐actin at the new rhizoid pole where Ca 2+ , MTs, the endomembrane system and secretion also become polarized. The F‐actin patch appears to be responsible for targeting polarization of Ca 2+ and MTs as depolymerization of F‐actin prevents polarization of both molecules (Pu et al, 2000; Peters and Kropf, 2010). The Ca 2+ gradient and targeted endomembrane activity may be part of an amplification loop in which new Ca 2+ channels are deposited at the rhizoid pole, enhancing the Ca 2+ gradient, which itself enhances secretion (Kropf et al, 1999).…”

Gene silencing inFucusembryos: developmental consequences ofRNAi‐mediated cytoskeletal disruption

“…Microtubule connections between the growing rhizoid apex and the mitotic apparatus ensure appropriate alignment of the division axis (Bisgrove et al. , Peters and Kropf ). Mitotic divisions are associated with substantial dynamics of the microtubule cytoskeleton (e.g., Hable et al.…”