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Vascular plants have an open body plan and continuously generate new axes of growth, such as shoot or root branches. Apical-to-basal transport of the hormone auxin is a hallmark of every axis, and the resulting pattern of auxin distribution affects plant development across scales, from overall architecture to cellular differentiation. How the first axis is initiated in the early embryo is a long-standing question. While our knowledge is still sparse, some of the key players of axialization have emerged, and recent work points to specific models for connecting cellular polarity to the asymmetric division of the zygote and domain specific gene expression to the organization of basipetal auxin flux.

Summary Background The division of plant zygotes is typically asymmetric, generating daughter cells with different developmental fates. In Arabidopsis , the apical daughter cell produces the proembryo, whereas the basal daughter cell forms the mostly extraembryonic suspensor. Establishment of apical and basal fates is known to depend on the YODA ( YDA ) mitogen-associated protein (MAP) kinase cascade and WUSCHEL-LIKE HOMEOBOX ( WOX ) homeodomain transcription factors. Results Mutations in GROUNDED ( GRD ) cause anatomical defects implying a partial loss of developmental asymmetry in the first division. Subsequently, suspensor-specific WOX8 expression disappears while proembryo-specific ZLL expression expands in the mutants, revealing that basal fates are confounded. GRD encodes a small nuclear protein of the RWP-RK family and is broadly transcribed in the early embryo. Loss of GRD eliminates the dominant effects of hyperactive YDA variants, indicating that GRD is required for YDA-dependent signaling in the embryo. However, GRD function is not regulated via direct phosphorylation by MAP kinases, and forced expression of GRD does not suppress the effect of yda mutations. In a strong synthetic interaction, grd;wox8;wox9 triple mutants arrest as zygotes or one-cell embryos lacking apparent polarity. Conclusions The predicted transcription factor GRD acts cooperatively with WOX homeodomain proteins to establish embryonic polarity in the first division. Like YDA , GRD promotes zygote elongation and basal cell fates. GRD function is required for YDA-dependent signaling but apparently not regulated by the YDA MAP kinase cascade. Similarity of GRD to Chlamydomonas MID suggests a conserved role for small RWP-RK proteins in regulating the transcriptional programs of generative cells and the zygote.

SummaryDivision of the Arabidopsis zygote defines two fundamentally different developmental domains, the proembryo and suspensor. The resulting boundary separates domain-specific gene expression, and a signal originating from the proembryo instructs the suspensor to generate the root stem cell niche. While root induction is known to require the phytohormone auxin and the Auxin Response Factor MONOPTEROS, it has remained largely elusive how the two domains involved in this process are initially specified. Here, we show that the GATA factor HANABA TARANU (HAN) is required to position the inductive proembryo boundary. Mutations in HAN cause a coordinated apical shift of gene expression patterns, revealing that HAN regulates transcription in the basal proembryo. Key auxin transporters are affected as early as the 8 cell stage, resulting in apical redistribution of auxin. Remarkably, han embryos eventually organize a root independent of MONOPTEROS and the suspensor around a new boundary marked by the auxin maximum.

The primary axis and main tissue types of Arabidopsis are laid down in the early embryo. Apical-to-basal auxin flux functions as a global organizer of the axis, and recent reports are clarifying our mechanistic understanding of how a graded auxin distribution is generated and interpreted. Polar targeting of PIN transporters in the cells of the embryo is dynamic and linked to their phosphorylation status, suggesting a flexible mechanism for regulating auxin flux in space and time. PLETHORA transcription factors then interpret the graded auxin distribution to provide positional values along the axis in a dose-dependent manner. A comparable framework for tissue patterning in the radial dimension is still lacking, although cell surface signaling probably plays a key role.

The Arabidopsis zygote divides asymmetrically into an embryonic apical cell and a basal cell with mostly extra-embryonic fate. This fundamental asymmetry sets the stage for further embryonic development, but the events mediating it are poorly understood. We have identified a MAPKK kinase gene, named YODA, that promotes extra-embryonic cell fates in the basal lineage. In loss-of-function mutants, the zygote does not elongate properly, and the cells of the basal lineage are eventually incorporated into the embryo instead of differentiating the extra-embryonic suspensor. Gain-of-function alleles cause exaggerated growth of the suspensor and can suppress embryonic development to a degree where no recognizable proembryo is formed. Our results imply that a MAP kinase cascade acts as a molecular switch promoting extra-embryonic fate.

The segmentation gene hunchback (hb) is involved in setting up the anterior-posterior axis of the Drosophila embryo. It is expressed maternally and zygotically and it plays a key role in integrating the effects of the anterior and posterior maternal systems. The hb gene from D. virilis has previously been cloned and was shown to be well conserved in its coding region, but less so in its upstream region which shows a more patchy pattern of conserved and diverged sequences. This work deals with the functional conservation of hb between the two species. We have mapped two additional regulatory elements for the expression of hb in the early embryo, namely the enhancer for the maternal expression and the enhancer region for the late blastoderm expression. Fragments containing these two elements, the previously identified bicoid dependent element for the early blastoderm expression of hb and the coding region were taken from D. virilis and tested in the D. melanogaster background. We find that all enhancer elements as well as the coding region are functionally conserved between the two species. Comparison of the upstream sequences that include the enhancer region for the late blastoderm expression reveal seven highly conserved blocks. Some of these contain consensus binding sites for transregulatory factors that are likely to control the respective expression domains.

Summary Cytokinesis partitions the cytoplasm of dividing eukaryotic cells. In higher plants, a dynamic microtubule array—phragmoplast—mediates the formation of the partitioning membrane—cell plate—in a centrifugal fashion [1, 2]. This phragmoplast dynamic involves microtubule-associated proteins [3–7]. Mutations in a novel Arabidopsis gene RUNKEL ( RUK ) result in cytokinesis defects caused by abnormal phragmoplast organization and arrested cell plate expansion. RUK encodes an essential cell-cycle-regulated 152 kDa protein with a putative serine/threonine kinase domain and a large microtubule-binding domain, both of which are largely conserved in uncharacterized proteins from protozoa, plants, and animals. RUK directly bound to microtubules in vitro and colocalized with mitotic preprophase band, spindle, and phragmoplast in vivo. An engineered RUK fusion protein that was degraded before telophase did not rescue the ruk mutant phenotype, demonstrating RUK action during cytokinesis. Both microtubule-binding domain and putative kinase domain were essential for RUK function. Surprisingly, RUK did not show kinase activity in vitro, and transgenically expressed "kinase-dead" RUK rescued the seedling lethality of ruk mutants. Our results suggest that RUK plays a regulatory, rather than catalytic, role in phragmoplast microtubule organization during cell plate expansion in cytokinesis.