Asymmetric cell division in land plants and algae: the driving force for differentiation.

Asymmetric cell division generates two cells with different fates and has an important role in plant development. It produces distinct cell types and new organs, and maintains stem cell niches. To handle the constraints of having immobile cells, plants possess numerous unique features to obtain asymmetry, such as specific regulators of intrinsic polarity. Although several components have not yet been identified, new findings, together with knowledge from different developmental systems, now allow us to take an important step towards a mechanistic overview of asymmetric cell division in plants and algae. Strikingly, several key regulators are used for different developmental processes, and common mechanisms can be recognized.

Phenotypic variation in sexually and asexually recruited individuals of the Baltic Sea endemic macroalga Fucus radicans: in the field and after growth in a common-garden.

BACKGROUND: Most species of brown macroalgae recruit exclusively sexually. However, Fucus radicans, a dominant species in the northern Baltic Sea, recruits new attached thalli both sexually and asexually. The level of asexual recruitment varies among populations from complete sexual recruitment to almost (> 90%) monoclonal populations. If phenotypic traits have substantial inherited variation, low levels of sexual activity will decrease population variation in these traits, which may affect function and resilience of the species. We assessed the level of inherited variation in nine phenotypic traits by comparing variation within and among three monoclonal groups and one group of unique multilocus genotypes (MLGs) sampled in the wild. RESULTS: Of the nine phenotypic traits, recovery after freezing, recovery after desiccation, and phlorotannin content showed substantial inherited variation, that is, phenotypic variation in these traits were to a large extend genetically determined. In contrast, variation in six other phenotypic traits (growth rate, palatability to isopod grazers, thallus width, distance between dichotomies, water content after desiccation and photochemical yield under ambient conditions) did not show significant signals of genetic variation at the power of analyses used in the study. Averaged over all nine traits, phenotypic variation within monoclonal groups was only 68% of the variation within the group of different MLGs showing that genotype diversity does affect the overall level of phenotypic variation in this species. CONCLUSIONS: Our result indicates that, in general, phenotypic diversity in populations of Fucus radicans increases with increased multilocus genotype (MLG) diversity, but effects are specific for individual traits. In the light of Fucus radicans being a foundation species of the northern Baltic Sea, we propose that increased MLG diversity (leading to increased trait variation) will promote ecosystem function and resilience in areas where F. radicans is common, but this suggestion needs experimental support.

Emergence of symmetry breaking in fucoid zygotes.

Fucoid zygotes are model cells for the study of symmetry breaking in plants. After fertilization, their initial spherical symmetry reduces to an axial symmetry, even in the absence of any external cue. This indicates that zygotes have an intrinsic ability to break symmetry in a way that is solely dependent on their internal biochemical and/or biophysical state. In our opinion, symmetry breaking is a self-organized process. It arises around the fucoid zygotes from the ion dynamics through channels (voltage-dependent calcium channels and a potassium leak) and outside the membrane (electrodiffusion owing to slower calcium diffusion compared with potassium). The robustness of this self-organized process and its lability ensure its relevance in plants where symmetry breaking is correlated with transcellular ion currents.

Diffusion or advection? Mass transfer and complex boundary layer landscapes of the brown alga Fucus vesiculosus.

The role of hyaline hairs on the thallus of brown algae in the genus Fucus is long debated and several functions have been proposed. We used a novel motorized set-up for two-dimensional and three-dimensional mapping with O2 microsensors to investigate the spatial heterogeneity of the diffusive boundary layer (DBL) and O2 flux around single and multiple tufts of hyaline hairs on the thallus of Fucus vesiculosus. Flow was a major determinant of DBL thickness, where higher flow decreased DBL thickness and increased O2 flux between the algal thallus and the surrounding seawater. However, the topography of the DBL varied and did not directly follow the contour of the underlying thallus. Areas around single tufts of hyaline hairs exhibited a more complex mass-transfer boundary layer, showing both increased and decreased thickness when compared with areas over smooth thallus surfaces. Over thallus areas with several hyaline hair tufts, the overall effect was an apparent increase in the boundary layer thickness. We also found indications for advective O2 transport driven by pressure gradients or vortex shedding downstream from dense tufts of hyaline hairs that could alleviate local mass-transfer resistances. Mass-transfer dynamics around hyaline hair tufts are thus more complex than hitherto assumed and may have important implications for algal physiology and plant-microbe interactions.

[Role of auxin in induction of polarity in zygotes of Fucus vesiculosus L]. / Rol' auksina v induktsii poliarnosti u zigot Fucus vesiculosus L.

We studied the effects of auxin (indolyl-3 acetic acid) on formation of the primary polarity axis in zygotes of the brown algae Fucus vesiculosus. Within the first 2.5 h after fertilization, the zygotes release this phytohormone in the environment. The treatment of developing zygotes with the inhibitor of indolyl-3-acetic acid transport from the cell triiodobenzoic acid at 5 mg/l arrests the auxin secretion and leads to its accumulation in the cells. This causes a significant delay in zygote polarization. The treatment of zygotes with the exogenous indolyl-3-acetic acid at 1 mg/l stimulates cell polarization and formation of a rhizoid process. When auxin was added to the medium with triiodobenzoic acid, the inhibitory effect of the latter was fully relieved. It has been proposed that the content of indolyl-3-acetic acid in the environment is a key factor in the induction of polarity of the F. vesiculosus zygotes.

A tip-high, Ca(2+) -interdependent, reactive oxygen species gradient is associated with polarized growth in Fucus serratus zygotes.

We report the existence of a tip-high reactive oxygen species (ROS) gradient in growing Fucus serratus zygotes, using both 5-(and 6-) chloromethyl-2',7'-dichlorodihydrofluorescein and nitroblue tetrazolium staining to report ROS generation. Suppression of the ROS gradient inhibits polarized zygotic growth; conversely, exogenous ROS generation can redirect zygotic polarization following inhibition of endogenous ROS. Confocal imaging of fluo-4 dextran distributions suggests that the ROS gradient is interdependent on the tip-high [Ca(2+)](cyt) gradient which is known to be associated with polarized growth. Our data support a model in which localized production of ROS at the rhizoid tip stimulates formation of a localized tip-high [Ca(2+)](cyt) gradient. Such modulation of intracellular [Ca(2+)](cyt) signals by ROS is a common motif in many plant and algal systems and this study extends this mechanism to embryogenesis.

Spatial re-organisation of cortical microtubules in vivo during polarisation and asymmetric division of Fucus zygotes.

Fucus zygotes polarise and germinate a rhizoid before their first asymmetrical division. The role of microtubules (MTs) in orienting the first division plane has been extensively studied by immunofluorescence approaches. In the present study, the re-organisation of MT arrays during the development of Fucus zygotes and embryos was followed in vivo after microinjection of fluorescent tubulin. A dynamic cortical MT array that shows dramatic reorganization during zygote polarization was detected for the first time. Randomly distributed cortical MTs were redistributed to the presumptive rhizoid site by the time of polarisation and well before rhizoid germination. The cortical MT re-organisation occurs independently of centrosome separation and nucleation. By the time of mitosis the cortical array depolymerised to cortical foci in regions from which it also reformed following mitosis, suggesting that it is nucleated from cortical sites. We confirm previous indications from immunodetection studies that centrosomal alignment and nuclear rotation occur via MT connexions to stabilised cortical sites and that definitive alignment is post-metaphasic. Finally, we show that cortical MTs align parallel to the growth axis during rhizoid tip growth and our results suggest that they may be involved in regulating rhizoid growth by shaping the rhizoid and containing turgor pressure.

Pattern formation of stationary transcellular ionic currents in Fucus.

Stationary and nonstationary spatiotemporal pattern formations emerging from the cellular electric activity are a common feature of biological cells and tissues. The nonstationary ones are well explained in the framework of the cable model. Inversely, the formation of the widespread self-organized stationary patterns of transcellular ionic currents remains elusive, despite their importance in cell polarization, apical growth, and morphogenesis. For example, the nature of the breaking symmetry in the Fucus zygote, a model organism for the experimental investigation of embryonic pattern formation, is still an open question. Using an electrodiffusive model, we report here an unexpected property of the cellular electric activity: a phase-space domain that gives rise to stationary patterns of transcellular ionic currents at finite wavelength. The cable model cannot predict this instability. In agreement with experiments, the characteristic time is an ionic diffusive one (<2 min). The critical radius is of the same order of magnitude as the cell radius (30 microm). The generic salient features are a global positive differential conductance, a negative differential conductance for one ion, and a difference between the diffusive coefficients. Although different, this mechanism is reminiscent of Turing instability.

Localization to the rhizoid tip implicates a Fucus distichus Rho family GTPase in a conserved cell polarity pathway.

Generation and expression of cell polarity in brown algal zygotes of the Fucales involve regulation of the actin cytoskeleton and localized secretion. We used degenerate PCR to isolate cDNAs that encode two small GTPases, FdRac1 and FdRab8, from zygotes of Fucus distichus (L.) Powell. Sequence analysis placed FdRac1 in the Rho family, which regulates actin, and FdRab8 in the Rab family, which regulates vesicle transport. As expected, bacterially expressed forms of both proteins bound GTP in vitro. When expressed in budding yeast, FdRac1 showed some functional overlap with CDC42, the Saccharomyces cerevisiae Rho family gene required for yeast cell polarity. Immunolocalization revealed an asymmetric distribution of FdRac1 in polarized zygotes and embryos, with FdRac1 concentrated at or near the growing tip of the algal rhizoid. Our data support the hypothesis that FdRac1 regulates algal cell polarity, possibly via the actin cytoskeleton. Because brown algae belong to the heterokont group, which diverged from other groups early in eukaryotic evolution, we argue that the Rho family function of regulating cell polarity is ancient and may extend throughout the eukaryotes.