Root meristem controls: The plant meristem, a small cluster of stem cells generates all of the cell types necessary for the plant’s indeterminate growth pattern. Roszak et al . use single-cell analyses to follow development from the stem cell to the enucleated cell of the phloem vasculature. In the root of the small mustard plant Arabidopsis , this process takes just over 3 days, and the developmental trajectory spans more than a dozen different cell states. A transcriptional program initially held under repressive control is released as those initial repressors dissipate. Reciprocal repression by regulators early and late in the developmental trajectory control a rapid switch in the differentiation program. —PJH

The root cap is a multilayered tissue covering the tip of a plant root that directs root growth through its unique functions, such as gravity sensing and rhizosphere interaction. To maintain the structure and function of the root cap, its constituent cells are constantly turned over through balanced cell division and cell detachment in the inner and outer cell layers, respectively. Upon displacement toward the outermost layer, columella cells at the central root cap domain functionally transition from gravity-sensing cells to secretory cells, but the mechanisms underlying this drastic cell fate transition are largely unknown. Here, using live-cell tracking microscopy, we show that organelles in the outermost cell layer undergo dramatic rearrangements. This rearrangement depends, at least partially, on spatiotemporally regulated activation of autophagy. Notably, this root cap autophagy does not lead to immediate cell death, but is instead necessary for organized separation of living root cap cells, highlighting a previously undescribed role of developmentally regulated autophagy in plants. This article has an associated ‘The people behind the papers’ interview.

Lateral root (LR) formation is an important developmental event for the establishment of the root system in most vascular plants. In Arabidopsis thaliana, the fewer roots (fwr) mutation in the GNOM gene, encoding an Arf GTPase guanine nucleotide exchange factor that regulates vesicle trafficking, severely inhibits LR formation. Local accumulation of auxin response for LR initiation is severely affected in fwr. To better understand how local accumulation of auxin response for LR initiation is regulated, we identified a mutation, fewer roots suppressor1 (fsp1), that partially restores LR formation in fwr. The gene responsible for fsp1 was identified as SUPERROOT2 (SUR2), encoding CYP83B1 that positions at the metabolic branch point in the biosynthesis of auxin/indole-3-acetic acid (IAA) and indole glucosinolate. The fsp1 mutation increases both endogenous IAA levels and the number of the sites where auxin response locally accumulates prior to LR formation in fwr. SUR2 is expressed in the pericycle of the differentiation zone and in the apical meristem in roots. Time-lapse imaging of the auxin response revealed that local accumulation of auxin response is more stable in fsp1. These results suggest that SUR2/CYP83B1 affects LR founder cell formation at the xylem pole pericycle cells where auxin accumulates. Analysis of the genetic interaction between SUR2 and GNOM indicates the importance of stabilization of local auxin accumulation sites for LR initiation.

Salt stress disturbs plant growth and photosynthesis due to its toxicity. The ice plant Mesembryanthemum crystallinum is a highly salt‐tolerant facultative crassulacean acid metabolism (CAM) plant. However, the genetic basis of the salt tolerance mechanisms in ice plants remains unclear. In this study, we constructed a chromosome‐level whole‐genome sequence of the ice plant and performed transcriptome analysis of the effects of salt treatment on the leaves. After 24‐hour 500 mM NaCl treatment to the roots, 1100 and 1394 genes, including CAM pathway, glycolysis, and inositol metabolism, were up‐ and down‐regulated in the leaves, respectively. Salt treatment also influenced the abscisic acid (ABA) signaling components, including genes from the PYRABACTIN RESISTANCE1 (PYR1) family and the PROTEIN PHOSPHATASE 2CA (PP2CA) family. We detected the induction of the genes encoding various ion transporters after salt treatment. The expression of most v‐ATPase subunits is induced, leading to vacuolar acidification, which facilitates sodium ion sequestration in the vacuoles. Additionally, some genes encoding metal ion transporters, including the genes from the ZIP family and NRAMP family, were induced by salt treatment, accompanied by the accumulation of iron, zinc, and copper ions in the leaves. Cis ‐motif enrichment analysis revealed that ABRE‐like motifs and MYB‐binding‐like motifs were enriched in the upstream sequences of genes that were either up‐regulated or down‐regulated by salt. In conclusion, this study highlights how salt treatment induces drastic and rapid transcriptomic changes and unveils the ice plant's genomic foundation. Our resources provide further insights into the regulation of salt tolerance in the ice plants.

The root cap is a multi-layered tissue covering the tip of a plant root that directs root growth through its unique functions such as gravity-sensing and rhizosphere interaction. To prevent damages from the soil environment, cells in the root cap continuously turn over through balanced cell division and cell detachment at the inner and the outer cell layers, respectively. Upon displacement toward the outermost layer, columella cells at the central root cap domain functionally transition from gravity-sensing cells to secretory cells, but the mechanisms underlying this drastic cell fate transition are largely unknown. By using live-cell tracking microscopy, we here show that organelles in the outermost cell layer undergo dramatic rearrangements, and at least a part of this rearrangement depends on spatiotemporally regulated activation of autophagy. Notably, this root cap autophagy does not lead to immediate cell death, but rather is necessary for organized separation of living root cap cells, highlighting a previously undescribed role of developmentally regulated autophagy in plants. Summary statement Time-lapse microscope imaging revealed spatiotemporal dynamics of intracellular reorganization associated with functional transition and cell separation in the Arabidopsis root cap and the roles of autophagy in this process.

The cultivated chrysanthemum is the most important ornamental species in the genus Chrysanthemum. However, because it is predominantly hexaploid and additionally exhibits self-incompatibility, it harbors numerous functionally redundant genes and displays extremely high heterozygosity. As a result, its genomic architecture is highly complex, making it challenging to interpret data obtained from omics analyses such as RNA-seq. To provide a genetically tractable model, we previously developed Gojo-0, a self-compatible, pure line of the diploid wild species C. seticuspe. In this study, we established Gojo-1, an improved self-compatible pure line derived from Gojo-0 and its sibling lines, exhibiting enhanced viability and culture performance. Leveraging these traits, we performed CRISPR–Cas9 editing of the AGAMOUS orthologs and successfully isolated mutants with altered floral organ morphology, demonstrating the line’s suitability for functional genomics. Comparative genome analysis showed that, aside from chromosome 1, the Gojo-1 genome is highly similar to that of Gojo-0, whose complete sequence has been determined. Taken together, these features indicate that Gojo-1 will serve as a valuable resource for future omics-based studies and a broad range of additional research applications.