Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

In many land plants, asymmetric cell divisions (ACDs) create, and pattern differentiated cell types on the leaf surface. In the Arabidopsis stomatal lineage, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) regulates ACD division plane placement and cell fate enforcement. Polarized subcellular localization of BASL is initiated before ACD and persists for many hours after the division in one of the two daughters. Untangling the respective contributions of polarized BASL before and after division is essential to gain a better understanding of its roles in regulating stomatal lineage ACDs. Here we combine quantitative imaging and lineage tracking with genetic tools that provide temporally restricted BASL expression. We find that pre-division BASL is required for division orientation, whereas BASL polarity post-division ensures proper cell fate commitment. These genetic manipulations allowed us to uncouple daughter-cell size asymmetry from polarity crescent inheritance, revealing independent effects of these two asymmetries on subsequent cell behavior. Finally, we show that there is coordination between the division frequencies of sister cells produced by ACDs, and this coupling requires BASL as an effector of peptide signaling.

On the edge: Evolution of polarity protein BASL and the capacity for stomatal lineage asymmetric divisions

Asymmetric and oriented stem cell divisions enable the continued production of patterned tissues. The molecules that guide these divisions include several polarity proteins that are localized to discrete plasma membrane domains, are differentially inherited during asymmetric divisions, and whose scaffolding activities can guide division plane orientation and subsequent cell fates. In the stomatal lineages on the surfaces of plant leaves, asymmetric and oriented divisions create distinct cell types in physiologically optimized patterns. The polarity protein BASL is a major regulator of stomatal lineage division and cell fate asymmetries in Arabidopsis, but its role in the stomatal lineages of other plants was unclear. Here, using phylogenetic and functional assays, we demonstrate that BASL is a dicot specific polarity protein. Among dicots, divergence in BASLs roles may reflect some intrinsic protein differences, but more likely reflects previously unappreciated differences in how asymmetric cell divisions are employed for pattern formation in different species. This multi-species analysis therefore provides insight into the evolution of a unique polarity regulator and into the developmental choices available to cells as they build and pattern tissues.

Live imaging of remyelination in the adult mouse corpus callosum

Oligodendrocyte precursor cells (OPCs) retain the capacity to remyelinate axons in the corpus callosum (CC) upon demyelination. However, the dynamics of OPC activation, mode of cell division, migration, and differentiation on a single-cell level remain poorly understood due to the lack of longitudinal observations of individual cells within the injured brain. After inducing focal demyelination with lysophosphatidylcholin in the CC of adult mice, we used two-photon microscopy to follow for up to 2 mo OPCs and their differentiating progeny, genetically labeled through conditional recombination driven by the regulatory elements of the gene Achaete-scute homolog 1. OPCs underwent several rounds of symmetric and asymmetric cell divisions, producing a subset of daughter cells that differentiates into myelinating oligodendrocytes. While OPCs continue to proliferate, differentiation into myelinating oligodendrocytes declines with time, and death of OPC-derived daughter cells increases. Thus, chronic in vivo imaging delineates the cellular principles leading to remyelination in the adult brain, providing a framework for the development of strategies to enhance endogenous brain repair in acute and chronic demyelinating disease.

Arabidopsis stomatal polarity protein BASL mediates distinct processes before and after cell division to coordinate cell size and fate asymmetries

In many land plants, asymmetric cell divisions (ACDs) create and pattern differentiated cell types on the leaf surface. In the Arabidopsis stomatal lineage, BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) regulates multiple aspects of ACD including division plane placement and cell fate enforcement. Polarized subcellular localization of BASL is initiated before the ACD and persists for many hours after the division in one of the two daughters. Untangling the respective contributions of polarized BASL before and after division is essential to gain a better understanding of its roles in regulating stomatal lineage ACDs and to uncover the rules that guide leaf pattern. Here we combine quantitative imaging and lineage tracking with genetic tools that provide temporally-restricted BASL expression. We find that pre-division BASL is required for division orientation, whereas BASL polarity post-division ensures proper cell fate commitment. These genetic manipulations allowed us to uncouple daughter-cell size asymmetry from polarity crescent inheritance, revealing independent effects of these two asymmetries on subsequent cell behavior. Finally, we show that there is coordination between the division frequencies of sister cells produced by ACDs, and this coupling requires BASL as an effector of peptide signaling.

The neurogenic fate of the hindbrain boundaries: a Notch-dependent behavioral switch triggers asymmetric division of boundary stem cells

The generation of cell diversity in the central nervous system occurs during embryogenesis and requires a precise balance between stem cell proliferation, neuronal commitment to specific fates, and further differentiation. Understanding the cellular and molecular mechanisms regulating this balance in the embryonic brain is challenging. Here we reveal how the neurogenic capacity in the hindbrain is differently allocated to distinct domains over time, and how the boundary cells undergo a functional transition to become neurogenic during zebrafish hindbrain segmentation. By generating a CRISPR-based knock-in transgenic line to specifically label the boundary cell population, we tracked their derivatives over time and followed their behavior, allowing us to identify how asymmetric cell divisions arise and to reconstruct the trajectories of the boundary derivatives through the progenitor and differentiated domains. The behavioral switch in boundary cells is triggered by the onset of Notch signaling, based on lateral inhibition at the dorsoventral level. Our findings reveal that distinct neurogenic phases take place during hindbrain growth and suggest that boundary cells contribute to refine the final number, identity, and proportion of neurons in the brain.

Spatiotemporal patterns of neuronal subtype genesis suggest hierarchical development of retinal diversity

How do neuronal subtypes emerge during development? Recent molecular studies have expanded our knowledge of existing neuronal diversity. However, the genesis of neuronal subtypes remains elusive and previous studies have been limited by a lack of quantitative methods for simultaneous detection of subtype diversity in situ. The bipolar interneurons of the mammalian retina represent a diverse neuronal class, characterized by distinct functions, morphologies, and recently discovered transcriptional profiles. Here, we developed a comprehensive spatiotemporal map of bipolar subtype genesis in the retina. Combining multiplexed detection of 16 RNA markers with timed delivery of EdU and BrdU, we analyzed more than 30,000 single cells in full retinal sections to classify all bipolar subtypes and their birthdates. We found that bipolar subtype birthdates are ordered and follow a centrifugal developmental axis. Spatial analysis revealed a striking oscillatory wave pattern of bipolar subtype birthdates, and lineage analyses suggest clonal restriction on homotypic subtype production. These results inspired a hierarchical model of neuronal subtype genesis in the mammalian retina, with the wave pattern of subtype birthdates arising from early asymmetric cell divisions among founding retinal progenitor cells. Our results provide an outline of the developmental logic that generates diverse neuronal subtypes, and establishes a framework for studying subtype diversification.

Kar9 symmetry breaking alone is insufficient to ensure spindle alignment

Spindle positioning must be tightly regulated to ensure asymmetric cell divisions are successful. In budding yeast, spindle positioning is mediated by the asymmetric localization of microtubule + end tracking protein Kar9. Kar9 asymmetry is believed to be essential for spindle alignment. However, the temporal correlation between symmetry breaking and spindle alignment has not been measured. Here, we establish a method of quantifying Kar9 symmetry breaking and find that Kar9 asymmetry is not well coupled with stable spindle alignment. We report the spindles are not aligned in the majority of asymmetric cells. Rather, stable alignment is correlated with Kar9 residence in the bud, regardless of symmetry state. Our findings suggest that Kar9 asymmetry alone is insufficient for stable alignment and reveal a possible role for Swe1 in regulating Kar9 residence in the bud.

Jasmonates, Ethylene and Brassinosteroids Control Adventitious and Lateral Rooting as Stress Avoidance Responses to Heavy Metals and Metalloids

Developmental and environmental signaling networks often converge during plant growth in response to changing conditions. Stress-induced hormones, such as jasmonates (JAs), can influence growth by crosstalk with other signals like brassinosteroids (BRs) and ethylene (ET). Nevertheless, it is unclear how avoidance of an abiotic stress triggers local changes in development as a response. It is known that stress hormones like JAs/ET and BRs can regulate the division rate of cells from the first asymmetric cell divisions (ACDs) in meristems, suggesting that stem cell activation may take part in developmental changes as a stress-avoidance-induced response. The root system is a prime responder to stress conditions in soil. Together with the primary root and lateral roots (LRs), adventitious roots (ARs) are necessary for survival in numerous plant species. AR and LR formation is affected by soil pollution, causing substantial root architecture changes by either depressing or enhancing rooting as a stress avoidance/survival response. Here, a detailed overview of the crosstalk between JAs, ET, BRs, and the stress mediator nitric oxide (NO) in auxin-induced AR and LR formation, with/without cadmium and arsenic, is presented. Interactions essential in achieving a balance between growth and adaptation to Cd and As soil pollution to ensure survival are reviewed here in the model species Arabidopsis and rice.