Embryogenesis is the beginning of plant development, yet the cell fate decisions and patterning steps that occur during this time are reiterated during development to build the post-embryonic architecture. In Arabidopsis, embryogenesis follows a simple and predictable pattern, making it an ideal model with which to understand how cellular and tissue developmental processes are controlled. Here, we review the early stages of Arabidopsis embryogenesis, focusing on the globular stage, during which time stem cells are first specified and all major tissues obtain their identities. We discuss four different aspects of development: the formation of outer versus inner layers; the specification of vascular and ground tissues; the determination of shoot and root domains; and the establishment of the first stem cells.

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Vascular plants provide most of the biomass, food, and feed on earth, yet the molecular innovations that led to the evolution of their conductive tissues are unknown. Here, we reveal the evolutionary trajectory for the heterodimeric TMO5/LHW transcription factor complex, which is rate-limiting for vascular cell proliferation inArabidopsis thaliana. Both regulators have origins predating vascular tissue emergence, and even terrestrialization. We further show that TMO5 evolved its modern function, including dimerization with LHW, at the origin of land plants. A second innovation in LHW, coinciding with vascular plant emergence, conditioned obligate heterodimerization and generated the critical function in vascular development inArabidopsis. In summary, our results suggest that the division potential of vascular cells may have been an important factor contributing to the evolution of vascular plants.

Intercellular communication coordinates hypophysis establishment in the embryo. Previously, TARGET OF MONOPTEROS 7 (TMO7) was reported to be transported to the hypophysis, the founder cell of the root cap, and RNA suppression experiments implicated its function in embryonic root development. However, the protein properties and mechanisms mediating TMO7 protein transport, and the role the movement plays in development remained unclear. Here, we report that in the post-embryonic root, TMO7 and its close relatives are transported into the root cap through plasmodesmata in a sequence-dependent manner. We also show that nuclear residence is crucial for TMO7 transport, and postulate that modification, potentially phosphorylation, labels TMO7 for transport. Additionally, three novel CRISPR/Cas9-induced alleles confirmed a role in hypophysis division, but suggest complex redundancies with close relatives in root formation. Finally, we demonstrate that TMO7 transport is biologically meaningful, as local expression partially restores hypophysis division in a plasmodesmal protein transport mutant. Our study identifies motifs and amino acids that are pivotal for TMO7 protein transport, and establishes the importance of TMO7 in hypophysis and root development.

Plasmodesmata (PD) are membrane-lined pores that connect neighbouring plant cells and allow molecular exchange via the symplast. Past studies have revealed the basic structure of PD, some of the transport mechanisms for molecules through PD, and a variety of physiological processes in which they function. Recently, with the help of newly developed technologies, several exciting new features of PD have been revealed. New PD structures were observed during early formation of PD and between phloem sieve elements and phloem pole pericycle cells in roots. Both observations challenge our current understanding of PD structure and function. Research into novel physiological responses, which are regulated by PD, indicates that we have not yet fully explored the potential contribution of PD to overall plant function. In this Viewpoint article, we summarize some of the recent advances in understanding the structure and function of PD and propose the challenges ahead for the community.

The phytohormone auxin and its directional transport through tissues play a fundamental role in plant development. This polar auxin transport relies on the PIN auxin exporters, whose polarization at the plasma membrane determines the overall direction of auxin flow. Hence, PIN polarization is crucial for development, but its evolution during the rise of developmental complexity in land plants remains unclear. Here, we investigated the evolution of PIN trafficking and polarization by comparing two model bryophytes, Physcomitrella patens and Marchantia polymorpha, with the angiosperm model Arabidopsis thaliana. All examined PINs show a conserved auxin export function, which is reflected by comparable bryophytic growth defects in overexpression lines. In bryophytes, bryophytic PINs polarize to filamentous apices, while Arabidopsis PINs distribute symmetrically on the membrane with additional intracellular puncta. In the Arabidopsis root epidermis, bryophytic PINs show no defined polarity. Pharmacological interference revealed a strong cytoskeleton dependence of bryophytic but not Arabidopsis PIN polarization, which, in contrast, strongly relies on Brefeldin A-sensitive trafficking. The divergence of PIN polarization and trafficking is also observed within the bryophyte clade and between tissues in individual species. These results collectively reveal a divergence of PIN trafficking and polarity mechanisms throughout land plant evolution and suggest co-evolution of PIN sequence-based and cell-based polarity mechanisms.

Intercellular communication coordinates hypophysis establishment in the Arabidopsis embryo. Previously, TARGET OF MONOPTEROS 7 (TMO7) was reported to be transported to the hypophysis, the founder cell of the root cap, and RNA suppression experiment implicated its function in embryonic root development. However, it remained unclear what protein properties and mechanisms mediate TMO7 protein transport, and what role the movement plays in development. Here, we report that in the post-embryonic root, TMO7 and its close relatives are transported into the root cap through plasmodesmata in a sequence, but not size dependent manner. We also show that nuclear residence is critical for TMO7 transport, and postulate that modification, potentially phosphorylation, labels TMO7 for transport. Additionally, three novel CRISPR/Cas9-induced tmo7 alleles confirmed a role in hypophysis division, but suggest complex redundancies with close relatives in root formation. Finally, we demonstrate that TMO7 transport is biologically meaningful, as local expression partially restores hypophysis division in a plasmodesmatal protein transport mutant. Our study identifies motifs and amino acids critical for TMO7 protein transport and establishes the importance of TMO7 in hypophysis and root development.Summary StatementUnique protein motifs, subcellular localization and post-translational modification, rather than protein size regulate plasmodesmatal transport of TMO7 family proteins during Arabidopsis root development.

16Vascular plants provide most of the biomass, food and feed on earth; yet the molecular 17 innovations that led to the evolution of their conductive tissues are unknown. Here, we reveal the 18 evolutionary trajectory for the heterodimeric TMO5/LHW transcription factor complex, which is 19 rate-limiting for vascular cell proliferation in Arabidopsis thaliana. Both regulators have origins 20 predating vascular tissue emergence, and even terrestrialization. We further show that TMO5 21 evolved its modern function, including dimerization with LHW, at the origin of land plants. A 22 second innovation in LHW, coinciding with vascular plant emergence, conditioned obligate 23 heterodimerization and generated the critical function in vascular development. In summary, our 24 results suggest that division potential of vascular cells may have been a major driver in the 25 evolution of vascular plants. 26 27 28 485 486 Evolution of vascular plants through redeployment of ancient developmental 487 regulators 488