The Stem Cell Niche in Leaf Axils Is Established by Auxin and Cytokinin in Arabidopsis

Plants differ from most animals in their ability to initiate new cycles of growth and development, which relies on the establishment and activity of branch meristems harboring new stem cell niches. In seed plants, this is achieved by axillary meristems, which are established in the axil of each leaf base and develop into lateral branches. Here, we describe the initial processes of Arabidopsis thaliana axillary meristem initiation. Using reporter gene expression analysis, we find that axillary meristems initiate from leaf axil cells with low auxin through stereotypical stages. Consistent with this, ectopic overproduction of auxin in the leaf axil efficiently inhibits axillary meristem initiation. Furthermore, our results demonstrate that auxin efflux is required for the leaf axil auxin minimum and axillary meristem initiation. After lowering of auxin levels, a subsequent cytokinin signaling pulse is observed prior to axillary meristem initiation. Genetic analysis suggests that cytokinin perception and signaling are both required for axillary meristem initiation. Finally, we show that cytokinin overproduction in the leaf axil partially rescue axillary meristem initiation-deficient mutants. These results define a mechanistic framework for understanding axillary meristem initiation

Hormonal control of cell identity and growth in the shoot apical meristem

International audienceHow cells acquire their identities and grow coordinately within a tissue is a fundamental question to understand plant development. In angiosperms, the shoot apical meristem (SAM) is a multicellular tissue containing a stem cell niche, which activity allows for a dynamic equilibrium between maintenance of stem cells and production of differentiated cells that are incorporated in new aerial tissues and lateral organs produced in the SAM. Plant hormones are small-molecule signals controlling many aspects of plant development and physiology. Several hormones are essential regulators of SAM activities. This review highlights current advances that are starting to decipher the complex mechanisms underlying the hormonal control of cell identity and growth in the SAM

Feedback from lateral organs controls shoot apical meristem growth by modulating auxin transport

International audienceStem cells must balance self-renewal and differentiation; thus, their activities are precisely controlled. In plants, the control circuits that underlie division and differentiation within meristems have been well studied, but those that underlie feedback on meristems from lateral organs remain largely unknown. Here we show that long-distance auxin transport mediates this feedback in a non-cell-autonomous manner. A low-auxin zone is associated with the shoot apical meristem (SAM) organization center, and auxin levels negatively affect SAM size. Using computational model simulations, we show that auxin transport from lateral organs can inhibit auxin transport from the SAM through an auxin transport switch and thus maintain SAM auxin homeostasis and SAM size. Genetic and microsurgical analyses confirmed the model's predictions. In addition, the model explains temporary change in SAM size of yabby mutants. Our study suggests that the canalization-based auxin flux can be widely adapted as a feedback control mechanism in plants

A gene expression map of shoot domains reveals regulatory mechanisms

The shoot apical meristem (SAM) maintains stem cells and generates new leaves and flowers from its periphery. Here via spatially resolved translatome profiling, Tian et al. define distinct molecular signatures of different SAM and leaf domains and identify regulators of axillary meristem initiation

AUXIN RESPONSE FACTOR 3 integrates the functions of AGAMOUS and APETALA2 in floral meristem determinacy

In Arabidopsis, AUXIN RESPONSE FACTOR 3 (ARF3) belongs to the auxin response factor (ARF) family that regulates the expression of auxin-responsive genes. ARF3 is known to function in leaf polarity specification and gynoecium patterning. In this study, we discovered a previously unknown role for ARF3 in floral meristem (FM) determinacy through the isolation and characterization of a mutant of ARF3 that enhanced the FM determinacy defects of agamous (ag)-10, a weak ag allele. Central players in FM determinacy include WUSCHEL (WUS), a gene critical for FM maintenance, and AG and APETALA2 (AP2), which regulate FM determinacy by repression and promotion of WUS expression, respectively. We showed that ARF3 confers FM determinacy through repression of WUS expression, and associates with the WUS locus in part in an AG-dependent manner. We demonstrated that ARF3 is a direct target of AP2 and partially mediates AP2's function in FM determinacy. ARF3 exhibits dynamic and complex expression patterns in floral organ primordia; altering the patterns spatially compromised FM determinacy. This study uncovered a role for ARF3 in FM determinacy and revealed relationships among genes in the genetic network governing FM determinacy

Direct up-regulation of <i>STM</i> expression by REV.

<p>(A) RT-qPCR analysis of <i>STM</i> expression in <i>pREV</i>::<i>REV-GR-HA rev-6</i> vegetative shoot apex tissues (with leaves removed) before and after simultaneous Dex and CHX treatment. The vertical axis indicates relative mRNA amount compared with the amount before treatment. Error bars indicate SD. (B) Schematic diagram of the <i>STM</i> genomic region. Vertical red lines indicate the sites containing the consensus REV binding sequence (ATGAT box). ATG denotes the translation start site. The underlying lines represent the DNA fragments amplified in ChIP assays, or used for plant protoplast assays. (C and D) ChIP enrichment test by PCR shows binding of REV-GR-HA to the ATGAT box-containing regions, especially the ones near the start site, in vegetative shoot apex tissues enriched with leaf axil (C) but not mature leaves (>P₁₀) without the leaf axil region from 30-d old plants (D) of <i>pREV</i>::<i>REV-GR-HA rev-6</i> plants. A paired design was used, in which each measurement was paired with a corresponding control without antibody. Error bars indicate SD. More controls are shown in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1006168#pgen.1006168.s004" target="_blank">S4I Fig</a>. (E) Transcriptional activity assays in <i>Arabidopsis</i> protoplasts. A <i>p35S</i>::<i>GFP</i> empty vector was the negative control, and a <i>p35S</i>::<i>GUS</i> line was the internal control. Relative <i>LUC</i> reporter gene expression is shown in the lower panel. The p1-p5 regions (indicated as in B) were assayed. Data are mean ± SD. Error bars are derived from three independent biological experiments, each run in triplicate.</p

Two-Step Regulation of a Meristematic Cell Population Acting in Shoot Branching in <i>Arabidopsis</i>

<div><p>Shoot branching requires the establishment of new meristems harboring stem cells; this phenomenon raises questions about the precise regulation of meristematic fate. In seed plants, these new meristems initiate in leaf axils to enable lateral shoot branching. Using live-cell imaging of leaf axil cells, we show that the initiation of axillary meristems requires a meristematic cell population continuously expressing the meristem marker <i>SHOOT MERISTEMLESS</i> (<i>STM</i>). The maintenance of <i>STM</i> expression depends on the leaf axil auxin minimum. Ectopic expression of <i>STM</i> is insufficient to activate axillary buds formation from plants that have lost leaf axil <i>STM</i> expressing cells. This suggests that some cells undergo irreversible commitment to a developmental fate. In more mature leaves, <i>REVOLUTA</i> (<i>REV</i>) directly up-regulates <i>STM</i> expression in leaf axil meristematic cells, but not in differentiated cells, to establish axillary meristems. Cell type-specific binding of REV to the <i>STM</i> region correlates with epigenetic modifications. Our data favor a threshold model for axillary meristem initiation, in which low levels of <i>STM</i> maintain meristematic competence and high levels of <i>STM</i> lead to meristem initiation.</p></div

A quantitative gibberellin signalling biosensor reveals a role for gibberellins in internode specification at the shoot apical meristem

Growth at the shoot apical meristem (SAM) is essential for shoot architecture construction. The phytohormones gibberellins (GA) play a pivotal role in coordinating plant growth, but their role in the SAM remains mostly unknown. Here, we developed a ratiometric GA signalling biosensor by engineering one of the DELLA repressors, to suppress its master regulatory function in GA transcriptional responses while preserving its degradation upon GA sensing. We demonstrate that this novel degradation-based biosensor accurately reports on cellular changes in GA levels and perception during development. We used this biosensor to map GA signalling activity in the SAM. We show that high GA signalling is found primarily in cells located between organ primordia that are the precursors of internodes. By gain- and loss-of-function approaches, we further demonstrate that GAs regulate cell division plane orientation to establish the typical cellular organisation of internodes, thus contributing to internode specification in the SAM