Yiqun Wang

Mapping the vertebrate developmental landscape As embryos develop, numerous cell types with distinct functions and morphologies arise from pluripotent cells. Three research groups have used single-cell RNA sequencing to analyze the transcriptional changes accompanying development of vertebrate embryos (see the Perspective by Harland). Wagner et al. sequenced the transcriptomes of more than 90,000 cells throughout zebrafish development to reveal how cells differentiate during axis patterning, germ layer formation, and early organogenesis. Farrell et al. profiled the transcriptomes of tens of thousands of embryonic cells and applied a computational approach to construct a branching tree describing the transcriptional trajectories that lead to 25 distinct zebrafish cell types. The branching tree revealed how cells change their gene expression as they become more and more specialized. Briggs et al. examined whole frog embryos, spanning zygotic genome activation through early organogenesis, to map cell states and differentiation across all cell lineages over time. These data and approaches pave the way for the comprehensive reconstruction of transcriptional trajectories during development. Science , this issue p. 981 , p. eaar3131 , p. eaar5780 ; see also p. 967

Abstract During development, animals generate distinct cell populations with specific identities, functions, and morphologies. We mapped transcriptionally distinct populations across 489,686 cells from 62 stages during wild-type zebrafish embryogenesis and early larval development (3–120 hours post-fertilization). Using these data, we identified the limited catalog of gene expression programs reused across multiple tissues and their cell-type-specific adaptations. We also determined the duration each transcriptional state is present during development and suggest new long-term cycling populations. Focused analyses of non-skeletal muscle and the endoderm identified transcriptional profiles of understudied cell types and subpopulations, including the pneumatic duct, individual intestinal smooth muscle layers, spatially distinct pericyte subpopulations, and homologs of recently discovered human best4 + enterocytes. The transcriptional regulators of these populations remain unknown, so we reconstructed gene expression trajectories to suggest candidates. To enable additional discoveries, we make this comprehensive transcriptional atlas of early zebrafish development available through our website, Daniocell.

Abstract Anthocyanins provide ideal visual markers for the identification of mutations that disrupt molecular responses to abiotic stress. We screened Arabidopsis mutants of ABC ( ATP ‐Binding Cassette) and MATE (Multidrug And Toxic compound Extrusion) transporter genes under nutritional stress and identified four genes ( ABCG 25 , ABCG 9 , ABCG 5, and MATE 45 ) required for normal anthocyanin pigmentation. ABCG 25 was previously demonstrated to encode a vascular‐localized cellular exporter of abscisic acid ( ABA ). Our results show that MATE 45 encodes an aerial meristem‐ and a vascular‐localized transporter associated with the trans ‐Golgi, and that it plays an important role in controlling the levels and distribution of ABA in growing aerial meristems and non‐meristematic tissues. MATE 45 promoter‐ GUS reporter fusions revealed the activity localized to the leaf and influorescence meristems and the vasculature. Loss‐of‐function mate45 mutants exhibited accelerated rates of aerial organ initiation suggesting at least partial functional conservation with the maize ortholog bige1 . The aba2‐1 mutant, which is deficient in ABA biosynthesis, exhibited a number of phenotypes that were rescued in the mate45‐1 aba2‐1 double mutant. mate45 exhibited enhanced the seed dormancy, and germination was hypersensitive to ABA . Enhanced frequency of leaf primordia growth in mate45 seedlings grown in nutrient imbalance stress was ABA ‐dependent. The ABA signaling reporter construct pRD 29B:: GUS revealed elevated levels of ABA signaling in the true leaf primordia of mate45 seedlings grown under nutritional stress, and gradually reduced signaling in surrounding cotyledon and hypocotyl tissues concomitant with reduced expressions of ABCG 25 . Our results suggest a role of MATE 45 in reducing meristematic ABA and in maintaining ABA distribution in adjacent non‐meristematic tissues.