Initiation Of Lateral Organs Research Articles

Evolutionary origins of Fibonacci phyllotaxis in land plants

Fibonacci phyllotaxis is commonly seen in all major groups of land plants. While a precise correlation is found between the internal pattern of the primary vascular system and the external pattern of appendages on the stem surface, it remains a big question how this regularity of Fibonacci phyllotaxis came into being in the course of evolution. Here I address this problem with a model describing phylogenetic and ontogenetic changes in vascular phyllotaxis based on two hypotheses. The first is that the Fibonacci pattern of vascular connection is uniquely determined by the primary arrangement of incipient primordia, the sources of the primary signal system in vascular tissue differentiation. The second is that the surface-area-to-volume ratio of primary vascular tissues serves as a measure of fitness in evolution. The model explains the empirical rule on the manner in which vascular connection is reconfigured during ontogeny, especially during juvenile development. Fossil and phylogenetic evidence suggests that Fibonacci phyllotaxis appeared shortly after the innovation of indefinite lateral organ initiation in a regular sequence.

The Arabidopsis SHORTROOT network coordinates shoot apical meristem development with auxin-dependent lateral organ initiation.

Plants produce new organs post-embryonically throughout their entire life cycle. This is due to stem cells present in the shoot and root apical meristems, the SAM and RAM, respectively. In the SAM, stem cells are located in the central zone where they divide slowly. Stem cell daughters are displaced laterally and enter the peripheral zone, where their mitotic activity increases and lateral organ primordia are formed. How the spatial arrangement of these different domains is initiated and controlled during SAM growth and development, and how sites of lateral organ primordia are determined in the peripheral zone is not yet completely understood. We found that the SHORTROOT (SHR) transcription factor together with its target transcription factors SCARECROW (SCR), SCARECROW-LIKE23 (SCL23) and JACKDAW (JKD), promotes formation of lateral organs and controls shoot meristem size. SHR, SCR, SCL23, and JKD are expressed in distinct, but partially overlapping patterns in the SAM. They can physically interact and activate expression of key cell cycle regulators such as CYCLIND6;1 (CYCD6;1) to promote the formation of new cell layers. In the peripheral zone, auxin accumulates at sites of lateral organ primordia initiation and activates SHR expression via the auxin response factor MONOPTEROS (MP) and auxin response elements in the SHR promoter. In the central zone, the SHR-target SCL23 physically interacts with the key stem cell regulator WUSCHEL (WUS) to promote stem cell fate. Both SCL23 and WUS expression are subject to negative feedback regulation from stem cells through the CLAVATA signaling pathway. Together, our findings illustrate how SHR-dependent transcription factor complexes act in different domains of the shoot meristem to mediate cell division and auxin dependent organ initiation in the peripheral zone, and coordinate this activity with stem cell maintenance in the central zone of the SAM.

GRAS family member LATERAL SUPPRESSOR regulates the initiation and morphogenesis of watermelon lateral organs.

The lateral organs of watermelon (Citrullus lanatus), including lobed leaves, branches, flowers, and tendrils, together determine plant architecture and yield. However, the genetic controls underlying lateral organ initiation and morphogenesis remain unclear. Here, we found that knocking out the homologous gene of shoot branching regulator LATERAL SUPPRESSOR in watermelon (ClLs) repressed the initiation of branches, flowers, and tendrils and led to developing round leaves, indicating that ClLs undergoes functional expansion compared with its homologs in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and tomato (Solanum lycopersicum). Using ClLs as the bait to screen against the cDNA library of watermelon, we identified several ClLs-interacting candidate proteins, including TENDRIL (ClTEN), PINOID (ClPID), and APETALA1 (ClAP1). Protein-protein interaction assays further demonstrated that ClLs could directly interact with ClTEN, ClPID, and ClAP1. The mRNA in situ hybridization assay revealed that the transcriptional patterns of ClLs overlapped with those of ClTEN, ClPID, and ClAP1 in the axillary meristems and leaf primordia. Mutants of ClTEN, ClPID, and ClAP1 generated by the CRISPR/Cas9 gene editing system lacked tendrils, developed round leaves, and displayed floral diapause, respectively, and all these phenotypes could be observed in ClLs knockout lines. Our findings indicate that ClLs acts as lateral organ identity protein by forming complexes with ClTEN, ClPID, and ClAP1, providing several gene targets for transforming the architecture of watermelon.

Size regulation of the lateral organ initiation zone and its role in determining cotyledon number in conifers.

Unlike monocots and dicots, many conifers, particularly Pinaceae, form three or more cotyledons. These are arranged in a whorl, or ring, at a particular distance from the embryo tip, with cotyledons evenly spaced within the ring. The number of cotyledons, nc, varies substantially within species, both in clonal cultures and in seed embryos. nc variability reflects embryo size variability, with larger diameter embryos having higher nc. Correcting for growth during embryo development, we extract values for the whorl radius at each nc. This radius, corresponding to the spatial pattern of cotyledon differentiation factors, varies over three-fold for the naturally observed range of nc. The current work focuses on factors in the patterning mechanism that could produce such a broad variability in whorl radius. Molecularly, work in Arabidopsis has shown that the initiation zone for leaf primordia occurs at a minimum between inhibitor zones of HD-ZIP III at the shoot apical meristem (SAM) tip and KANADI (KAN) encircling this farther from the tip. PIN1-auxin dynamics within this uninhibited ring form auxin maxima, specifying primordia initiation sites. A similar mechanism is indicated in conifer embryos by effects on cotyledon formation with overexpression of HD-ZIP III inhibitors and by interference with PIN1-auxin patterning. We develop a mathematical model for HD-ZIP III/KAN spatial localization and use this to characterize the molecular regulation that could generate (a) the three-fold whorl radius variation (and associated nc variability) observed in conifer cotyledon development, and (b) the HD-ZIP III and KAN shifts induced experimentally in conifer embryos and in Arabidopsis. This quantitative framework indicates the sensitivity of mechanism components for positioning lateral organs closer to or farther from the tip. Positional shifting is most readily driven by changes to the extent of upstream (meristematic) patterning and changes in HD-ZIP III/KAN mutual inhibition, and less efficiently driven by changes in upstream dosage or the activation of HD-ZIP III. Sharper expression boundaries can also be more resistant to shifting than shallower expression boundaries. The strong variability seen in conifer nc (commonly from 2 to 10) may reflect a freer variation in regulatory interactions, whereas monocot (nc = 1) and dicot (nc = 2) development may require tighter control of such variation. These results provide direction for future quantitative experiments on the positional control of lateral organ initiation, and consequently on plant phyllotaxy and architecture.

Expression characterization and cross-species complementation uncover the functional conservation of YABBY genes for leaf abaxial polarity and carpel polarity establishment in Saccharum spontaneum

BackgroundCell polarity establishment and maintenance is indispensable for plant growth and development. In plants, the YABBY transcription factor family has a distinct role in leaf asymmetric polarity establishment and lateral organ initiation. However, for the important sugar crop Saccharum, little information on YABBY genes is available.ResultsIn this study, a total of 20 sequences for 7 SsYABBY genes were identified in the sugarcane genome, designated as SsYABBY1-7 based on their chromosome locations, and characterized by phylogenetic analysis. We provided a high-resolution map of SsYABBYs’ global expression dynamics during vegetative and reproductive organ morphogenesis and revealed that SsYABBY3/4/5 are predominately expressed at the seedling stage of stem and leaf basal zone; SsYABBY2/5/7 are highly expressed in ovules. Besides, cross-species overexpression and/or complementation verified the conserved function of SsYABBY2 in establishing leaf adaxial-abaxial polarity and ovules development. We found that the SsYABBY2 could successfully rescue the leaves curling, carpel dehiscence, and ovule abortion defects in Arabidopsis crc mutant.ConclusionsCollectively, our study demonstrates that SsYABBY genes retained a conserved function in establishing and preserving leaf adaxial-abaxial polarity and lateral organ development during evolution.

The Arabidopsis HDZIP class II transcription factor ABA INSENSITIVE TO GROWTH 1 functions in leaf development.

Leaf laminar growth and adaxial-abaxial boundary formation are fundamental outcomes of plant development. Boundary and laminar growth coordinate the further patterning and growth of the leaf, directing the differentiation of cell types within the top and bottom domains and promoting initiation of lateral organs along their adaxial or abaxial axis. Leaf adaxial-abaxial polarity specification and laminar outgrowth are regulated by two transcription factors, REVOLUTA (REV) and KANADI (KAN). ABA INSENSITIVE TO GROWTH 1 (ABIG1) encodes a HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP) class II transcription factor and is a direct target of the adaxial-abaxial regulators REV and KAN. To investigate the role of ABIG1 in leaf development and in the establishment of polarity, we examined the phenotypes of both gain-of-function and loss-of-function mutants. Through genetic interaction analysis with REV and KAN mutants, we determined that ABIG1 plays a role in leaf laminar growth as well as in adaxial-abaxial polarity establishment. Genetic and physical interaction assays showed that ABIG1 interacts with the transcriptional TOPLESS corepressor. This study provides new evidence that ABIG1, another HD-ZIP II, facilitates growth through the corepressor TOPLESS.

A phylogenetically conserved APETALA2/ETHYLENE RESPONSE FACTOR, ERF12, regulates Arabidopsis floral development

Key messageArabidopsis ETHYLENE RESPONSE FACTOR12 (ERF12), the rice MULTIFLORET SPIKELET1 orthologue pleiotropically affects meristem identity, floral phyllotaxy and organ initiation and is conserved among angiosperms.Reproductive development necessitates the coordinated regulation of meristem identity and maturation and lateral organ initiation via positive and negative regulators and network integrators. We have identified ETHYLENE RESPONSE FACTOR12 (ERF12) as the Arabidopsis orthologue of MULTIFLORET SPIKELET1 (MFS1) in rice. Loss of ERF12 function pleiotropically affects reproductive development, including defective floral phyllotaxy and increased floral organ merosity, especially supernumerary sepals, at incomplete penetrance in the first-formed flowers. Wildtype floral organ number in early formed flowers is labile, demonstrating that floral meristem maturation involves the stabilisation of positional information for organogenesis, as well as appropriate identity. A subset of erf12 phenotypes partly defines a narrow developmental time window, suggesting that ERF12 functions heterochronically to fine-tune stochastic variation in wild type floral number and similar to MFS1, promotes meristem identity. ERF12 expression encircles incipient floral primordia in the inflorescence meristem periphery and is strong throughout the floral meristem and intersepal regions. ERF12 is a putative transcriptional repressor and genetically opposes the function of its relatives DORNRÖSCHEN, DORNRÖSCHEN-LIKE and PUCHI and converges with the APETALA2 pathway. Phylogenetic analysis suggests that ERF12 is conserved among all eudicots and appeared in angiosperm evolution concomitant with the generation of floral diversity.

Organogenesis at the Shoot Apical Meristem.

Lateral organ initiation at the shoot apical meristem involves complex changes in growth rates and directions, ultimately leading to the formation of leaves, stems and flowers. Extensive molecular analysis identifies auxin and downstream transcriptional regulation as major elements in this process. This molecular regulatory network must somehow interfere with the structural elements of the cell, in particular the cell wall, to induce specific morphogenetic events. The cell wall is composed of a network of rigid cellulose microfibrils embedded in a matrix composed of water, polysaccharides such as pectins and hemicelluloses, proteins, and ions. I will discuss here current views on how auxin dependent pathways modulate wall structure to set particular growth rates and growth directions. This involves complex feedbacks with both the cytoskeleton and the cell wall.

A Molecular Framework for Auxin-Controlled Homeostasis of Shoot Stem Cells in Arabidopsis

The classic phytohormone auxin plays an essential role in priming meristematic cell differentiation in the shoot apical meristem to promote lateral organ initiation. Recently, several lines of evidence have suggested that auxin is not only transported to new primordia but also descends to the stem cells in the central zone. However, the function of auxin in stem cell regulation has remained elusive. Here, we show that auxin signaling in stem cells is mediated, at least in part, by AUXIN RESPONSE FACTOR 5/MONOPTEROS (ARF5/MP), which directly represses the transcription of DORNROSCHEN/ENHANCER OF SHOOT REGENERATION 1 (DRN/ESR1). DRN expressed in stem cells positively regulates CLAVATA3 (CLV3) expression and has important meristematic functions. Our results provide a mechanistic framework for auxin control of shoot stem cell homeostasis and demonstrate how auxin differentially controls plant stem cell maintenance and differentiation.

The founder-cell transcriptome in the Arabidopsis apetala1 cauliflower inflorescence meristem.

BackgroundAlthough the pattern of lateral organ formation from apical meristems establishes species-specific plant architecture, the positional information that confers cell fate to cells as they transit to the meristem flanks where they differentiate, remains largely unknown. We have combined fluorescence-activated cell sorting and RNA-seq to characterise the cell-type-specific transcriptome at the earliest developmental time-point of lateral organ formation using DORNRÖSCHEN-LIKE::GFP to mark founder-cell populations at the periphery of the inflorescence meristem (IM) in apetala1 cauliflower double mutants, which overproliferate IMs.ResultsWithin the lateral organ founder-cell population at the inflorescence meristem, floral primordium identity genes are upregulated and stem-cell identity markers are downregulated. Additional differentially expressed transcripts are involved in polarity generation and boundary formation, and in epigenetic and post-translational changes. However, only subtle transcriptional reprogramming within the global auxin network was observed.ConclusionsThe transcriptional network of differentially expressed genes supports the hypothesis that lateral organ founder-cell specification involves the creation of polarity from the centre to the periphery of the IM and the establishment of a boundary from surrounding cells, consistent with bract initiation. However, contrary to the established paradigm that sites of auxin response maxima pre-pattern lateral organ initiation in the IM, auxin response might play a minor role in the earliest stages of lateral floral initiation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3189-x) contains supplementary material, which is available to authorized users.

Targeted cell elimination reveals an auxin-guided biphasic mode of lateral root initiation.

To sustain a lifelong ability to initiate organs, plants retain pools of undifferentiated cells with a preserved proliferation capacity. The root pericycle represents a unique tissue with conditional meristematic activity, and its tight control determines initiation of lateral organs. Here we show that the meristematic activity of the pericycle is constrained by the interaction with the adjacent endodermis. Release of these restraints by elimination of endodermal cells by single-cell ablation triggers the pericycle to re-enter the cell cycle. We found that endodermis removal substitutes for the phytohormone auxin-dependent initiation of the pericycle meristematic activity. However, auxin is indispensable to steer the cell division plane orientation of new organ-defining divisions. We propose a dual, spatiotemporally distinct role for auxin during lateral root initiation. In the endodermis, auxin releases constraints arising from cell-to-cell interactions that compromise the pericycle meristematic activity, whereas, in the pericycle, auxin defines the orientation of the cell division plane to initiate lateral roots.

Conserved Functions of the MATE Transporter BIG EMBRYO1 in Regulation of Lateral Organ Size and Initiation Rate.

Genetic networks that determine rates of organ initiation and organ size are key regulators of plant architecture. Whereas several genes that influence the timing of lateral organ initiation have been identified, the regulatory pathways in which these genes operate are poorly understood. Here, we identify a class of genes implicated in regulation of the lateral organ initiation rate. Loss-of-function mutations in the MATE transporter encoded by maize (Zea mays) Big embryo 1 (Bige1) cause accelerated leaf and root initiation as well as enlargement of the embryo scutellum. BIGE1 is localized to trans-Golgi, indicating a possible role in secretion of a signaling molecule. Interestingly, phenotypes of bige1 bear striking similarity to cyp78a mutants identified in diverse plant species. We show that a CYP78A gene is upregulated in bige1 mutant embryos, suggesting a role for BIGE1 in feedback regulation of a CYP78A pathway. We demonstrate that accelerated leaf formation and early flowering phenotypes conditioned by mutants of Arabidopsis thaliana BIGE1 orthologs are complemented by maize Bige1, showing that the BIGE1 transporter has a conserved function in regulation of lateral organ initiation in plants. We propose that BIGE1 is required for transport of an intermediate or product associated with the CYP78A pathway.

Transcriptomic analyses indicate that maize ligule development recapitulates gene expression patterns that occur during lateral organ initiation.

Development of multicellular organisms proceeds via the correct interpretation of positional information to establish boundaries that separate developmental fields with distinct identities. The maize (Zea mays) leaf is an ideal system to study plant morphogenesis as it is subdivided into a proximal sheath and a distal blade, each with distinct developmental patterning. Specialized ligule and auricle structures form at the blade-sheath boundary. The auricles act as a hinge, allowing the leaf blade to project at an angle from the stem, while the ligule comprises an epidermally derived fringe. Recessive liguleless1 mutants lack ligules and auricles and have upright leaves. We used laser microdissection and RNA sequencing to identify genes that are differentially expressed in discrete cell/tissue-specific domains along the proximal-distal axis of wild-type leaf primordia undergoing ligule initiation and compared transcript accumulation in wild-type and liguleless1-R mutant leaf primordia. We identified transcripts that are specifically upregulated at the blade-sheath boundary. A surprising number of these "ligule genes" have also been shown to function during leaf initiation or lateral branching and intersect multiple hormonal signaling pathways. We propose that genetic modules utilized in leaf and/or branch initiation are redeployed to regulate ligule outgrowth from leaf primordia.

Simulation of Organ Patterning on the Floral Meristem Using a Polar Auxin Transport Model

An intriguing phenomenon in plant development is the timing and positioning of lateral organ initiation, which is a fundamental aspect of plant architecture. Although important progress has been made in elucidating the role of auxin transport in the vegetative shoot to explain the phyllotaxis of leaf formation in a spiral fashion, a model study of the role of auxin transport in whorled organ patterning in the expanding floral meristem is not available yet. We present an initial simulation approach to study the mechanisms that are expected to play an important role. Starting point is a confocal imaging study of Arabidopsis floral meristems at consecutive time points during flower development. These images reveal auxin accumulation patterns at the positions of the organs, which strongly suggests that the role of auxin in the floral meristem is similar to the role it plays in the shoot apical meristem. This is the basis for a simulation study of auxin transport through a growing floral meristem, which may answer the question whether auxin transport can in itself be responsible for the typical whorled floral pattern. We combined a cellular growth model for the meristem with a polar auxin transport model. The model predicts that sepals are initiated by auxin maxima arising early during meristem outgrowth. These form a pre-pattern relative to which a series of smaller auxin maxima are positioned, which partially overlap with the anlagen of petals, stamens, and carpels. We adjusted the model parameters corresponding to properties of floral mutants and found that the model predictions agree with the observed mutant patterns. The predicted timing of the primordia outgrowth and the timing and positioning of the sepal primordia show remarkable similarities with a developing flower in nature.

Plant Development: Light Exposure Directs Meristem Fate

Leaf initiation was previously thought to be self-regulated and not reliant on environmental cues. However, a recent study has revealed that light redirects meristem fate from maintenance to lateral organ initiation, through the regulation of the plant hormones auxin and cytokinin.

Stem cell activation by light guides plant organogenesis

Leaves originate from stem cells located at the shoot apical meristem. The meristem is shielded from the environment by older leaves, and leaf initiation is considered to be an autonomous process that does not depend on environmental cues. Here we show that light acts as a morphogenic signal that controls leaf initiation and stabilizes leaf positioning. Leaf initiation in tomato shoot apices ceases in the dark but resumes in the light, an effect that is mediated through the plant hormone cytokinin. Dark treatment also affects the subcellular localization of the auxin transporter PIN1 and the concomitant formation of auxin maxima. We propose that cytokinin is required for meristem propagation, and that auxin redirects cytokinin-inducible meristem growth toward organ formation. In contrast to common wisdom over the last 150 years, the light environment controls the initiation of lateral organs by regulating two key hormones: auxin and cytokinin.

Molecular dissection of the pea shoot apical meristem.

The shoot apical meristem (SAM) is responsible for the development of all the above-ground parts of a plant. Our understanding of the SAM at the molecular level is incomplete. This study investigates the gene expression repertoire of SAMs in the garden pea (Pisum sativum). To this end, 10 346 EST sequences representing 7610 unique genes were generated from SAM cDNA libraries. These sequences, together with previously reported pea ESTs, were used to construct a 12K oligonucleotide array to identify genes with differential SAM expression, as compared to axillary meristems, root apical meristems, or non-meristematic tissues. A number of genes were identified, predominantly expressed in specific cell layers or domains of the SAM and thus are likely components of the gene networks involved in stem cell maintenance or the initiation of lateral organs. Further in situ hybridization analysis confirmed the spatial localization of some of these genes within the SAM. Our data also indicate the diversification of some gene expression patterns and hence functions in legume crop plants. A number of transcripts highly expressed in all three meristems have also been uncovered and these candidates may provide valuable insight into molecular networks that underpin the maintenance of meristematic functionality.

Translational Biology: From Arabidopsis Flowers to Grass Inflorescence Architecture

One of the key events in plant development is the initiation of lateral organs from the flanks of the meristem. In grasses, the inflorescence meristem (IM) reiteratively initiates a series of lateral meristems with slightly different fates. Our understanding of the genes and networks that regulate

Functional conservation of wheat orthologs of maize rough sheath1 and rough sheath2 genes

Maize rough sheath2 (RS2) and Arabidopsis ASYMMETRIC LEAVES1 (AS1) both encode a Myb transcription factor and repress Knotted1-type homeobox (KNOX) genes. The RS2/AS1-KNOX relationship is functionally conserved between maize and Arabidopsis. Here, we cloned wheat orthologs of RS2/AS1 and of a maize rough sheath1 (rs1) KNOX gene and named them WRS2 and WRS1, respectively. WRS1 mRNA was detected at leaf insertion points of the vegetative shoot meristem but was missing in differentiating floral organs. Conversely, WRS2 transcripts accumulated in initiating and developing floral organs. Transgenic tobacco plants expressing WRS1 showed morphological alterations typically observed due to expression of other KNOX genes. WRS2 with a deletion of the Myb domain could interact with NtPHAN to form a heterodimer, and expression of the truncated WRS2 gene conferred a dominant-negative phenotype similar to that expected and induced ectopic expression of an endogenous KNOX gene. Moreover, WRS2 expression alleviated morphological alterations in tobacco plants expressing the wheat KNOX gene. Therefore, the WRS2 gene product represses KNOX expression. These results indicate that the WRS2-KNOX relationship plays a fundamentally important role in lateral organ initiation and differentiation of meristems in wheat development. The antagonistic relationship between WRS2 and KNOX around meristematic tissues has been functionally conserved during wheat evolution.

Sparse inflorescence1 encodes a monocot-specific YUCCA-like gene required for vegetative and reproductive development in maize.

The plant growth hormone auxin plays a critical role in the initiation of lateral organs and meristems. Here, we identify and characterize a mutant, sparse inflorescence1 (spi1), which has defects in the initiation of axillary meristems and lateral organs during vegetative and inflorescence development in maize. Positional cloning shows that spi1 encodes a flavin monooxygenase similar to the YUCCA (YUC) genes of Arabidopsis, which are involved in local auxin biosynthesis in various plant tissues. In Arabidopsis, loss of function of single members of the YUC family has no obvious effect, but in maize the mutation of a single yuc locus causes severe developmental defects. Phylogenetic analysis of the different members of the YUC family in moss, monocot, and eudicot species shows that there have been independent expansions of the family in monocots and eudicots. spi1 belongs to a monocot-specific clade, within which the role of individual YUC genes has diversified. These observations, together with expression and functional data, suggest that spi1 has evolved a dominant role in auxin biosynthesis that is essential for normal maize inflorescence development. Analysis of the interaction between spi1 and genes regulating auxin transport indicate that auxin transport and biosynthesis function synergistically to regulate the formation of axillary meristems and lateral organs in maize.