An epigenetic timer regulates the transition from cell division to cell expansion during Arabidopsis petal organogenesis.

A number of studies have demonstrated that epigenetic factors regulate plant developmental timing in response to environmental changes. However, we still have an incomplete view of how epigenetic factors can regulate developmental events such as organogenesis, and the transition from cell division to cell expansion, in plants. The small number of cell types and the relatively simple developmental progression required to form the Arabidopsis petal makes it a good model to investigate the molecular mechanisms driving plant organogenesis. In this study, we investigated how the RABBIT EARS (RBE) transcriptional repressor maintains the downregulation of its downstream direct target, TCP5, long after RBE expression dissipates. We showed that RBE recruits the Groucho/Tup1-like corepressor TOPLESS (TPL) to repress TCP5 transcription in petal primordia. This process involves multiple layers of changes such as remodeling of chromatin accessibility, alteration of RNA polymerase activity, and histone modifications, resulting in an epigenetic memory that is maintained through multiple cell divisions. This memory functions to maintain cell divisions during the early phase of petal development, and its attenuation in a cell division-dependent fashion later in development enables the transition from cell division to cell expansion. Overall, this study unveils a novel mechanism by which the memory of an epigenetic state, and its cell-cycle regulated decay, acts as a timer to precisely control organogenesis.

Cellulose assembles into helical bundles of uniform handedness in cell walls with abnormal pectin composition.

Plant cells and organs grow into a remarkable diversity of shapes, as directed by cell walls composed primarily of polysaccharides such as cellulose and multiple structurally distinct pectins. The properties of the cell wall that allow for precise control of morphogenesis are distinct from those of the individual polysaccharide components. For example, cellulose, the primary determinant of cell morphology, is a chiral macromolecule that can self-assemble in vitro into larger-scale structures of consistent chirality, and yet most plant cells do not display consistent chirality in their growth. One interesting exception is the Arabidopsis thaliana rhm1 mutant, which has decreased levels of the pectin rhamnogalacturonan-I and causes conical petal epidermal cells to grow with a left-handed helical twist. Here, we show that in rhm1 the cellulose is bundled into large macrofibrils, unlike the evenly distributed microfibrils of the wild type. This cellulose bundling becomes increasingly severe over time, consistent with cellulose being synthesized normally and then self-associating into macrofibrils. We also show that in the wild type, cellulose is oriented transversely, whereas in rhm1 mutants, the cellulose forms right-handed helices that can account for the helical morphology of the petal cells. Our results indicate that when the composition of pectin is altered, cellulose can form cellular-scale chiral structures in vivo, analogous to the helicoids formed in vitro by cellulose nano-crystals. We propose that an important emergent property of the interplay between rhamnogalacturonan-I and cellulose is to permit the assembly of nonbundled cellulose structures, providing plants flexibility to orient cellulose and direct morphogenesis.

Do Epigenetic Timers Control Petal Development?

Epigenetic modifications include histone modifications and DNA methylation; such modifications can induce heritable changes in gene expression by altering DNA accessibility and chromatin structure. A number of studies have demonstrated that epigenetic factors regulate plant developmental timing in response to environmental changes. However, we still have an incomplete picture of how epigenetic factors can regulate developmental events such as organogenesis. The small number of cell types and the relatively simple developmental progression required to form the Arabidopsis petal makes it a good model to investigate the molecular mechanisms driving plant organogenesis. In this minireview, we summarize recent studies demonstrating the epigenetic control of gene expression during various developmental transitions, and how such regulatory mechanisms can potentially act in petal growth and differentiation.

CENTRORADIALIS maintains shoot meristem indeterminacy by antagonizing THORN IDENTITY1 in Citrus.

Differential regulation of stem cell activity in shoot meristems contributes to the wide variation in shoot architecture.1-3 In most Citrus species, a thorn meristem and a dormant axillary meristem co-localize at each leaf base, offset from each other in a spiral phyllotactic pattern. We recently identified THORN IDENTITY1 (TI1) and THORN IDENTITY2 (TI2), encoding TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors, as necessary for the termination of meristem proliferation and concomitant thorn production in Citrus.4 However, how the dormant axillary meristem at the same leaf axil maintains stem cell activity is still unknown. The phosphatidylethanolamine-binding protein (PEBP)-type transcription factors CENTRORADIALIS (CEN) and TERMINAL FLOWER1 (TFL1) maintain inflorescence meristem indeterminacy in many plant species by antagonizing floral meristem identity regulators.5-9 Here, we show that, in Citrus, Citrus CEN (CsCEN) maintains vegetative axillary meristem indeterminacy by antagonizing TI1. CsCEN is expressed in the axillary meristem, but not in the thorn meristem. Disruption of CsCEN function results in termination of the stem cell activity and conversion of dormant axillary meristems into thorns, although ectopic overexpression of CsCEN represses TI1 expression and converts thorns into dormant buds, a phenotype similar to the ti1 mutant. We further show that CsCEN interacts with Citrus FD (CsFD) to repress TI1 expression. CsCEN activity depends on the function of TI1 and TI2, as mutations in TI1 and TI2 rescue the cscen mutant phenotype. We suggest that the antagonistic roles of CsCEN and TI1 define the pattern of axillary meristem determinacy, which shapes vegetative Citrus tree shoot architecture.

TCP5 controls leaf margin development by regulating KNOX and BEL-like transcription factors in Arabidopsis.

Development of leaf margins is an important process in leaf morphogenesis. CIN-clade TCP (TEOSINTE BRANCHED1/CYCLOIDEA/PCF) transcription factors are known to have redundant roles in specifying leaf margins, but the specific mechanisms through which individual TCP genes function remain elusive. In this study, we report that the CIN-TCP gene TCP5 is involved in repressing the initiation and outgrowth of leaf serrations by activating two key regulators of margin development, the Class II KNOX factor KNAT3 and BEL-like SAW1. Specifically, TCP5 directly promotes the transcription of KNAT3 and indirectly activates the expression of SAW1. We also show that TCP5 regulates KNAT3 and SAW1 in a temporal- and spatial- specific manner that is largely in accordance with the progress of formation of serrations. This regulation might serve as a key mechanism in patterning margin morphogenesis and in sculpting the final form of the leaf.

Reprogramming of Stem Cell Activity to Convert Thorns into Branches.

Thorns arise from axillary shoot apical meristems that proliferate for a time and then terminally differentiate into a sharp tip. Like other meristems, thorn meristems contain stem cells but, in the case of thorns, these stem cells undergo a programmed cessation of proliferative activity. Using Citrus, we characterize a gene network necessary for thorn development. We identify two Citrus genes, THORN IDENTITY1 (TI1) and THORN IDENTITY2 (TI2), encoding TCP transcription factors, as necessary for stem cell quiescence and thorn identity. Disruption of TI1 and TI2 function results in reactivation of stem cells and concomitant conversion of thorns to branches. Expression of WUSCHEL (WUS) defines the shoot stem cell niche in the apical meristems of many angiosperm species; we show that TI1 binds to the Citrus WUS promoter and negatively regulates its expression to terminate stem cell proliferation. We propose that shifts in the timing and function of components of this gene network can account for the evolution of Citrus thorn identity. Modulating this pathway can significantly alter plant architecture and could be leveraged to improve crop yields.

Phytochrome B Induces Intron Retention and Translational Inhibition of PHYTOCHROME-INTERACTING FACTOR3.

The phytochrome B (phyB) photoreceptor stimulates light responses in plants in part by inactivating repressors of light responses, such as PHYTOCHROME-INTERACTING FACTOR3 (PIF3). Activated phyB inhibits PIF3 by rapid protein degradation and decreased transcription. PIF3 protein degradation is mediated by EIN3-BINDING F-BOX PROTEIN (EBF) and LIGHT-RESPONSE BTB (LRB) E3 ligases, the latter of which simultaneously targets phyB for degradation. In this study, we show that PIF3 levels are additionally regulated by alternative splicing and protein translation in Arabidopsis (Arabidopsis thaliana). Overaccumulation of photo-activated phyB, which occurs in the mutant defective for LRB genes under continuous red light, induces a specific alternative splicing of PIF3 that results in retention of an intron in the 5' untranslated region of PIF3 mRNA. In turn, the upstream open reading frames contained within this intron inhibit PIF3 protein synthesis. The phyB-dependent alternative splicing of PIF3 is diurnally regulated under the short-day light cycle. We hypothesize that this reversible regulatory mechanism may be utilized to fine tune the level of PIF3 protein in light-grown plants and may contribute to the oscillation of PIF3 protein abundance under the short-day environment.

The Transcription Factors TCP4 and PIF3 Antagonistically Regulate Organ-Specific Light Induction of SAUR Genes to Modulate Cotyledon Opening during De-Etiolation in Arabidopsis.

Light elicits different growth responses in different organs of plants. These organ-specific responses are prominently displayed during de-etiolation. While major light-responsive components and early signaling pathways in this process have been identified, this information has yet to explain how organ-specific light responses are achieved. Here, we report that members of the TEOSINTE BRANCHED1, CYCLOIDEA, and PCF (TCP) transcription factor family participate in photomorphogenesis and facilitate light-induced cotyledon opening in Arabidopsis (Arabidopsis thaliana). Chromatin immunoprecipitation sequencing and RNA sequencing analyses indicated that TCP4 targets a number of SMALL AUXIN UPREGULATED RNA (SAUR) genes that have previously been shown to exhibit organ-specific, light-responsive expression. We demonstrate that TCP4-like transcription factors, which are predominantly expressed in the cotyledons of both light- and dark-grown seedlings, activate SAUR16 and SAUR50 expression in response to light. Light regulates the binding of TCP4 to the promoters of SAUR14, SAUR16, and SAUR50 through PHYTOCHROME-INTERACTING FACTORs (PIFs). PIF3, which accumulates in etiolated seedlings and its levels rapidly decline upon light exposure, also binds to the SAUR16 and SAUR50 promoters, while suppressing the binding of TCP4 to these promoters in the dark. Our study reveals that the interplay between light-responsive factors PIFs and the developmental regulator TCP4 determines the cotyledon-specific light regulation of SAUR16 and SAUR50, which contributes to cotyledon closure and opening before and after de-etiolation.

Flavonol rhamnosylation indirectly modifies the cell wall defects of RHAMNOSE BIOSYNTHESIS1 mutants by altering rhamnose flux.

Rhamnose is required in Arabidopsis thaliana for synthesizing pectic polysaccharides and glycosylating flavonols. RHAMNOSE BIOSYNTHESIS1 (RHM1) encodes a UDP-l-rhamnose synthase, and rhm1 mutants exhibit many developmental defects, including short root hairs, hyponastic cotyledons, and left-handed helically twisted petals and roots. It has been proposed that the hyponastic cotyledons observed in rhm1 mutants are a consequence of abnormal flavonol glycosylation, while the root hair defect is flavonol-independent. We have recently shown that the helical twisting of rhm1 petals results from decreased levels of rhamnose-containing cell wall polymers. In this study, we found that flavonols indirectly modify the rhm1 helical petal phenotype by altering rhamnose flux to the cell wall. Given this finding, we further investigated the relationship between flavonols and the cell wall in rhm1 cotyledons. We show that decreased flavonol rhamnosylation is not responsible for the cotyledon phenotype of rhm1 mutants. Instead, blocking flavonol synthesis or rhamnosylation can suppress rhm1 defects by diverting UDP-l-rhamnose to the synthesis of cell wall polysaccharides. Therefore, rhamnose is required in the cell wall for normal expansion of cotyledon epidermal cells. Our findings suggest a broad role for rhamnose-containing cell wall polysaccharides in the morphogenesis of epidermal cells.

The COP9 Signalosome regulates seed germination by facilitating protein degradation of RGL2 and ABI5.

The control of seed germination and seed dormancy are critical for the successful propagation of plant species, and are important agricultural traits. Seed germination is tightly controlled by the balance of gibberellin (GA) and abscisic acid (ABA), and is influenced by environmental factors. The COP9 Signalosome (CSN) is a conserved multi-subunit protein complex that is best known as a regulator of the Cullin-RING family of ubiquitin E3 ligases (CRLs). Multiple viable mutants of the CSN showed poor germination, except for csn5b-1. Detailed analyses showed that csn1-10 has a stronger seed dormancy, while csn5a-1 mutants exhibit retarded seed germination in addition to hyperdormancy. Both csn5a-1 and csn1-10 plants show defects in the timely removal of the germination inhibitors: RGL2, a repressor of GA signaling, and ABI5, an effector of ABA responses. We provide genetic evidence to demonstrate that the germination phenotype of csn1-10 is caused by over-accumulation of RGL2, a substrate of the SCF (CRL1) ubiquitin E3 ligase, while the csn5a-1 phenotype is caused by over-accumulation of RGL2 as well as ABI5. The genetic data are consistent with the hypothesis that CSN5A regulates ABI5 by a mechanism that may not involve CSN1. Transcriptome analyses suggest that CSN1 has a more prominent role than CSN5A during seed maturation, but CSN5A plays a more important role than CSN1 during seed germination, further supporting the functional distinction of these two CSN genes. Our study delineates the molecular targets of the CSN complex in seed germination, and reveals that CSN5 has additional functions in regulating ABI5, thus the ABA signaling pathway.

Increased efficiency of targeted mutagenesis by CRISPR/Cas9 in plants using heat stress.

The CRISPR/Cas9 system has greatly improved our ability to engineer targeted mutations in eukaryotic genomes. While CRISPR/Cas9 appears to work universally, the efficiency of targeted mutagenesis and the adverse generation of off-target mutations vary greatly between different organisms. In this study, we report that Arabidopsis plants subjected to heat stress at 37°C show much higher frequencies of CRISPR-induced mutations compared to plants grown continuously at the standard temperature (22°C). Using quantitative assays relying on green fluorescent protein (GFP) reporter genes, we found that targeted mutagenesis by CRISPR/Cas9 in Arabidopsis is increased by approximately 5-fold in somatic tissues and up to 100-fold in the germline upon heat treatment. This effect of temperature on the mutation rate is not limited to Arabidopsis, as we observed a similar increase in targeted mutations by CRISPR/Cas9 in Citrus plants exposed to heat stress at 37°C. In vitro assays demonstrate that Cas9 from Streptococcus pyogenes (SpCas9) is more active in creating double-stranded DNA breaks at 37°C than at 22°C, thus indicating a potential contributing mechanism for the in vivo effect of temperature on CRISPR/Cas9. This study reveals the importance of temperature in modulating SpCas9 activity in eukaryotes, and provides a simple method to increase on-target mutagenesis in plants using CRISPR/Cas9.

Isolation of mutants with abnormal petal epidermal cell morphology.

Plants consist of many different cell types with specific shapes optimized for their particular functions. For example, most flowering plants have conically shaped epidermal cells on the upper surface of their petals that are important for pollinator attraction. The control of cell morphology in organs such as roots and leaves has been extensively studied, but much less is known about the genes that promote conical expansion of petal epidermal cells. We have developed a technique to rapidly assay the morphology of conical petal epidermal cells, and we employed this method in an unbiased genetic screen to identify mutants that alter the development of these cells. Mutants isolated in this screen affected cell shape, cell size, cuticle synthesis, and cellular chirality. This approach allowed for the identification of novel cellular components that are critical for the morphology of conical petal cells, and demonstrates the usefulness of petal epidermal cells as a model system for studying cellular morphogenesis.

The ABC model of floral development.

Flowers are organized into concentric whorls of sepals, petals, stamens and carpels, with each of these floral organ types having a unique role in reproduction (Figure 1). Sepals enclose and protect the flower bud, while petals can be large and showy so as to attract pollinators (or people!). Stamens produce pollen grains that contain male gametes, while the carpels contain the ovules that when fertilized will produce the seeds. While the size, shape, number and elaboration of each of these organ types can be quite different, the same general organization of four floral organ types arranged in concentric whorls exists across all flowering plant (angiosperm) species. As I shall explain in this Primer, the 'ABC model' is a simple and satisfying explanation for how this conserved floral architecture is genetically specified.

Type-B ARABIDOPSIS RESPONSE REGULATORs Directly Activate WUSCHEL.

The WUSCHEL (WUS) gene is necessary for the maintenance of stem cells in the shoot apical meristem. Four recent reports show that cytokinin responsive type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) directly activate WUS expression, providing a long-awaited explanation for how cytokinin influences the maintenance of the stem cell niche.

Rhamnose-Containing Cell Wall Polymers Suppress Helical Plant Growth Independently of Microtubule Orientation.

Although specific organs in some plant species exhibit helical growth patterns of fixed or variable handedness, most plant organs are not helical. Here we report that mutations in Arabidopsis RHAMNOSE BIOSYNTHESIS 1 (RHM1) cause dramatic left-handed helical growth of petal epidermal cells, leading to left-handed twisted petals. rhm1 mutant roots also display left-handed growth. Furthermore, we find that RHM1 is required to promote epidermal cell expansion. RHM1 encodes a UDP-L-rhamnose synthase, and rhm1 mutations affect synthesis of the pectic polysaccharide rhamnogalacturonan-I. Unlike other mutants that exhibit helical growth of fixed handedness, the orientation of cortical microtubule arrays is unaltered in rhm1 mutants. Our findings reveal a novel source of left-handed plant growth caused by changes in cell wall composition that is independent of microtubule orientation. We propose that an important function of rhamnose-containing cell wall polymers is to suppress helical twisting of expanding plant cells.