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In seed plants, the canonical role of 1-aminocyclopropane-1-carboxylic acid (ACC) is to serve as the precursor in the biosynthesis of the phytohormone ethylene, and indeed, ACC treatment is often used as a proxy for ethylene treatment. Increasing evidence suggests that ACC can also act independently of ethylene to regulate various aspects of plant growth and development. Here, we explore the effects of ACC onArabidopsis thalianaroot growth and the mechanisms by which it acts. ACC inhibits growth of the primary root inArabidopsisseedlings when ethylene signaling is blocked, which becomes evident after 36 h of treatment with ACC. This reduced root growth is in part the result of suppressed cell proliferation in the root meristem resulting from altered expression of a key regulator of stem cell niche activity, WOX5. ACC also promotes lateral root (LR) development, in contrast to ethylene, which inhibits LR formation. Transcriptomic analysis of roots revealed no significant changes in gene expression after 45 min or 4 h of ACC treatment, but longer treatment times revealed a large number of differentially expressed genes, including the downregulation of the expression of a small group of phylogenetically related CLE peptides. Reduced expression of these group 1 CLEs in response to ACC leads to the activation of a transcription factor, LBD18, which promotes LR development. These results suggest that ACC acts to modulate multiple aspects ofArabidopsisroot growth independently of ethylene via distinct transcriptional effects in the root meristem and LR precursor cells. 

The study of plant root growth in real time has been difficult to achieve in an automated, high-throughput, and systematic fashion. Dynamic imaging of plant roots is important in order to discover novel root growth behaviors and to deepen our understanding of how roots interact with their environments. We designed and implemented the Generating Rhizodynamic Observations Over Time (GROOT) robot, an automated, high-throughput imaging system that enables time-lapse imaging of 90 containers of plants and their roots growing in a clear gel medium over the duration of weeks to months. The system uses low-cost, widely available materials. As a proof of concept, we employed GROOT to collect images of root growth ofOryza sativa,Hudsonia montana, and multiple species of orchids includingPlatanthera integrilabiaover six months. Beyond imaging plant roots, our system is highly customizable and can be used to collect time- lapse image data of different container sizes and configurations regardless of what is being imaged, making it applicable to many fields that require longitudinal time-lapse recording. 

Brassinosteroids are plant steroid hormones that regulate diverse processes, such as cell division and cell elongation, through gene regulatory networks that vary in space and time. By using time series single-cell RNA sequencing to profile brassinosteroid-responsive gene expression specific to different cell types and developmental stages of theArabidopsisroot, we identified the elongating cortex as a site where brassinosteroids trigger a shift from proliferation to elongation associated with increased expression of cell wall–related genes. Our analysis revealedHOMEOBOX FROM ARABIDOPSIS THALIANA 7(HAT7) andGT-2-LIKE 1(GTL1) as brassinosteroid-responsive transcription factors that regulate cortex cell elongation. These results establish the cortex as a site of brassinosteroid-mediated growth and unveil a brassinosteroid signaling network regulating the transition from proliferation to elongation, which illuminates aspects of spatiotemporal hormone responses. 

In all multicellular organisms, transcriptional networks orchestrate organ development. The Arabidopsis root, with its simple structure and indeterminate growth, is an ideal model for investigating the spatiotemporal transcriptional signatures underlying developmental trajectories. To map gene expression dynamics across root cell types and developmental time, we built a comprehensive, organ-scale atlas at single-cell resolution. In addition to estimating developmental progressions in pseudotime, we employed the mathematical concept of optimal transport to infer developmental trajectories and identify their underlying regulators. To demonstrate the utility of the atlas to interpret new datasets, we profiled mutants for two key transcriptional regulators at single-cell resolution, shortroot and scarecrow. We report transcriptomic and in vivo evidence for tissue trans-differentiation underlying a mixed cell identity phenotype in scarecrow. Our results support the atlas as a rich community resource for unraveling the transcriptional programs that specify and maintain cell identity to regulate spatiotemporal organ development. 

Early root growth is critical for plant establishment and survival. We have identified a molecular pathway required for helical root tip movement known as circumnutation. Here, we report a multiscale investigation of the regulation and function of this phenomenon. We identify key cell signaling events comprising interaction of the ethylene, cytokinin, and auxin hormone signaling pathways. We identify the geneOryza sativahistidine kinase-1 (HK1) as well as the auxin influx carrier geneOsAUX1as essential regulators of this process in rice. Robophysical modeling and growth challenge experiments indicate circumnutation is critical for seedling establishment in rocky soil, consistent with the long-standing hypothesis that root circumnutation facilitates growth past obstacles. Thus, the integration of robotics, physics, and biology has elucidated the functional importance of root circumnutation and uncovered the molecular mechanisms underlying its regulation.