There is a dominant view that plants are shaped by the ability of their outermost cell layers to resist or yield to pressure from internal tissues. During stem development, outer tissues originate from the tunica layers of the shoot meristem, while inner tissues are produced by the rib zone (RZ), named after its distinctive pattern of transverse cell divisions. Despite over a century of studies, the interplay between inner and outer tissues in shaping the stem remains unclear. Here, we show that mutations in a subfamily of IQ domain (IQD) genes disrupt the orientation of cell divisions in the RZ and increase stem diameter in Arabidopsis. Measurements of cell geometry, growth of marked cell clones, tissue-specific expression, and subcellular localization all support the idea that these IQD proteins reduce longitudinal cell divisions in the RZ, limiting the build-up of vertical cell files in inner tissues and the concomitant radial growth of the stem. Thus, the characteristic orientation of cell divisions in the RZ is important for shaping the stem, and the genetic control of organ shape can be exerted in inner plant tissues.

Adult mammalian stem cells typically maintain stem cell identity through proliferative quiescence. In contrast, we demonstrate that stem cell maintenance in Arabidopsis bud precursor cells requires active cell-cycle progression. Inhibiting division silences the shoot meristem marker gene SHOOT MERISTEMLESS ( STM ) and promotes differentiation. Whereas proliferation dilutes H3K27me3 levels to counteract silencing. Meanwhile, we identified two classes of transcription factors recruiting polycomb repressive complex 2 (PRC2) to epigenetically silence STM . The balance between these forces establishes a cell cycle-coupled epigenetic “Sisyphus” mechanism that maintains pluripotency. This cell fate switch is bistable; modeling and experimental data confirm that prolonged quiescence triggers irreversible differentiation. We propose that sequence-dependent PRC2 recruitment in plants enables precise silencing of fate-determining genes, while cell proliferation sustains pluripotency by resetting epigenetic marks. ### Competing Interest Statement The authors have declared no competing interest.

Organogenesis relies on the coordination of cell proliferation with developmental programs. In meristems, where new plant organs initiate, the cell proliferation potential depends on stem cell regulators, but the mechanisms linking their local activity with the cell cycle machinery remain unknown. Here we show a positional gradient of G1 duration in the Arabidopsis root meristem spanning from ~2 h near the meristem boundary to more than 20 h in the early stem cell derivatives. Mutations in the stem cell regulatory PLETHORA (PLT) genes and the cell cycle RETINOBLASTOMA-RELATED 1 gene shortened G1 duration, abolishing the G1 duration gradient, whereas PLT2 overexpression increased G1 duration. Data-driven computer modelling supported the presence of an incoherent feed-forward loop. We found that PLT genes are drivers with simultaneous and opposing roles in maintaining stem cell activity and inhibiting G1 progression through a cascade involving the CDK inhibitor KRP5, a PLT target and RETINOBLASTOMA-RELATED 1. The G1 duration gradient is developmentally regulated and established after the emergence of lateral roots from the primary root and correlates with increased tolerance to genome damage. Our study establishes a previously undescribed proximal–distal G1 duration gradient during root development that is shaped by the balanced activity of stem cell maintenance and cell cycle regulators. A positional and developmentally regulated cell cycle duration gradient exists in the root meristem whereby the G1 phase is very long close to the stem cell niche. This relies on the interplay of PLETHORA with the RETINOBLASTOMA-RELATED pathway.

Innovative genetic improvements in food crops are needed to maintain global food security. Here, we report the map-based cloning of TaWUSCHEL-D1 (WUS-D1) as the gene responsible for the multiovary phenotype in wheat, which produces three fertile ovaries and grains per floret. We generated a 14.5 Gbp chromosome-level assembly of multiovary wheat line "MOV" that shows unique structural variation in the Mov-1 physical region, resulting in widespread gene upregulation. High-resolution genetic mapping refined the locus to a 135 kbp region that contains two genes. We used nine independent deletion mutants, eight TILLING mutants, and genetic complementation of these genotypes to show that a WUSCHEL ortholog, WUS-D1, is the causal gene of the Mov-1 locus. Expression studies showed that WUS-D1 is highly expressed during early inflorescence development in MOV, whereas the gene is inactive in wild-type wheat. The higher WUS-D1 expression is associated with the formation of larger meristems and floret primordia that are competent to produce multiple ovaries. These insights provide a foundation to manipulate floral organ numbers to enhance breeding capabilities of bread wheat.

When exposed to stress, plants reduce growth while activating defense mechanisms-a behaviour proposed to help reallocate resources and meet the energy demands required for survival. Here, we have challenged this view by mutating the cyclin-dependent kinase inhibitor SMR1 to reverse the growth arrest imposed by high DELLA levels. These plants continue growing under limited water availability but maintain the same oxidative stress tolerance and survival rates as the parental line that halted growth. However, shoot and root meristematic cells that keep dividing under drought or genotoxic stress accumulate DNA damage, frequently leading to cell death. Since the DNA lesions are observed in the apical stem cells that give rise to all plant organs, including flowers, we propose that systemic growth arrest acts as a defense strategy that plants employ not only to maximize individual fitness, but also to ensure the accurate transmission of genetic information to their progeny.

Starting from pools of undifferentiated cells, plants generate new organs postembryonically in response to external and endogenous signals. This requires a dynamic coordination of cell division with cellular growth and differentiation regulatory programs. However, little is known about how this coordination is achieved at the molecular level during flower development. We used time-series single-nucleus RNA sequencing (snRNA-seq) experiments of synchronized Arabidopsis thaliana flower developmental stages to characterize the transcriptome dynamics and the connections between cell cycle and developmental regulatory programs during early flower development. The results show a bifurcation between transcriptional trajectories corresponding to cell cycle progression and floral development. We identify the regulation of the cell cycle inhibitor KIP-RELATED PROTEIN 2 (KRP2) by FRUITFULL (FUL) as a key regulatory point on this bifurcation point and validate the importance of this regulation in vivo. Our work illustrates how time-series snRNA-seq experiments can be used to identify bifurcation points between regulatory programs and to identify candidate regulators on these bifurcations. In particular, we identify the regulation of KRP2 by FUL as an important regulatory point to balance cell division and developmental differentiation in plants.

Fruit morphogenesis is determined by the coordination of cell division and expansion, which are fundamental processes required for the development of all plant organs. Here, we show that the regulation of TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) LANCEOLATE (TCP2/LA) by miR319 is crucial for tomato fruit morphology. The loss of miR319 regulation in the semi-dominant La mutant led to a premature SlTCP2/LA expression during gynoecium patterning, which results in modified cell division during carpel development. As a consequence, La mutants exhibited elongated ovary and fruit shape, and a reduced number of ovules and seeds. Elongated fruit shape in La may be partially due to the SlTCP2/LA-mediated repression of OVATE activity in young floral buds. Further analysis showed that the de-repression of SlTCP2/LA decreases auxin responses in young floral buds by directly repressing SlYUCCA4 expression, but SlTCP2/LA also acts in parallel with ENTIRE (E) to orchestrate fruit morphology and seed production. Our study defines a novel miRNA-based molecular link between the domestication-associated OVATE gene and auxin responses. Given the striking variation in fruit morphology among members of the Solanaceae family, fine-tuning regulation of gene expression by miRNA coupled with modulation of auxin dynamics may be a common driver in the evolution of fruit shape diversity.

Cell size affects many processes, including exchange of nutrients and external signals, cell division and tissue mechanics. Across eukaryotes, cells have evolved mechanisms that assess their own size to inform processes such as cell cycle progression or gene expression. Here, we review recent progress in understanding plant cell size regulation and its implications, relating these findings to work in other eukaryotes. Highlights include use of DNA contents as reference point to control the cell cycle in shoot meristems, a size-dependent cell fate decision during stomatal development and insights into the interconnection between ploidy, cell size and cell wall mechanics.

Interview with Elliot Meyerowitz, who studies plant growth and development at Caltech.

ABSTRACT Many developmental processes associated with fruit development take place at the floral meristem (FM). Age-regulated microRNA156 (miR156) and gibberellins (GA) interact to control flowering time, but their interplay in subsequent stages of reproductive development is poorly understood. Here, we show that GA and miR156 function in tomato FM and fruit patterning. High GA responses or overexpression of miR156 (156OE), which leads to low levels of miR156-targeted SQUAMOSA PROMOTER BINDING PROTEIN– LIKE ( SPL/SBP ), resulted in enlarged FMs, defects in FM determinacy and fruits with increased locule number. Conversely, low GA responses reduced fruit indeterminacy and locule number, and overexpression of a miR156-resistant SlSBP15 allele ( rSBP15 ) reduced cell number and size in the FM, as well as locule number. GA responses were partially required for the fruit defects observed in 156OE and rSBP15 plants. Transcriptome analysis and genetic interactions revealed shared and divergent functions of miR156-targeted SlSBPs, PROCERA/DELLA and the classical WUSCHEL/CLAVATA pathway, which has been previously associated with meristem size and determinacy. Our findings reveal that the miR156/ SlSBP /GA regulatory module is deployed differently depending on developmental stage and create novel opportunities to genetically fine-tune aspects of fruit development that have been important for tomato domestication.

Many developmental processes associated with fruit development occur at the floral meristem (FM). Age-regulated microRNA156 (miR156) and gibberellins (GAs) interact to control flowering time, but their interplay in subsequent stages of reproductive development is poorly understood. Here, in tomato (Solanum lycopersicum), we show that GA and miR156-targeted SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL or SBP) genes interact in the tomato FM and ovary patterning. High GA responses or overexpression of miR156 (156OE), which leads to low expression levels of miR156-silenced SBP genes, resulted in enlarged FMs, ovary indeterminacy and fruits with increased locule number. Conversely, low GA responses reduced indeterminacy and locule number, and overexpression of a S. lycopersicum (Sl)SBP15 allele that is miR156 resistant (rSBP15) reduced FM size and locule number. GA responses were partially required for the defects observed in 156OE and rSBP15 fruits. Transcriptome analysis and genetic interactions revealed shared and divergent functions of miR156-targeted SlSBP genes, PROCERA/DELLA and the classical WUSCHEL/CLAVATA pathway, which has been previously associated with meristem size and determinacy. Our findings reveal that the miR156/SlSBP/GA regulatory module is deployed differently depending on developmental stage and create novel opportunities to fine-tune aspects of fruit development that have been important for tomato domestication.

Diversity in fruit morphology is one of the hallmarks of varietal differences among modern cultivars of fruit-bearing crops. As evolutionarily related organs, fruits and leaves share developmental processes, but there are surprisingly few connections between regulatory pathways for fruit and leaf development. Here, we show the regulation of the leaf development-associated TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) TCP4/LANCEOLATE (TCP4/LA) transcription factor by the microRNA319 (miR319) is crucial for gynoecium patterning and establishment of fruit morphology. Loss of miR319 regulation leads to a premature, ectopic TCP4/LA expression during gynoecium patterning, which results in elongated fruits, resembling ovate mutants. TCP4/LA modulates tomato fruit development and morphology partially by directly repressing OVATE expression as early as 5-8 days post-inflorescence (dpi) flower buds. Furthermore, miR319-targeted CINCINNATA-like TCP4/LANCEOLATE controls auxin responses in developing flower buds by directly binding to the SlYUCCA4 promoter. Modulation of auxin biosynthesis by TCP4/LA is shared with other CINCINNATA-like TCPs during Arabidopsis gynoecium patterning. Our study defines a novel miRNA-based molecular link between OVATE , a fundamental gene associated with tomato domestication, and auxin responses in the control of fruit development and morphology. Given the striking variation in fruit shape among members of the Solanaceae family, fine-tuning regulation of gene expression by miRNA coupled with modulation of hormone dynamics may be a common driver in the evolution of fruit-shape diversity. ### Competing Interest Statement The authors have declared no competing interest.