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DEFECTIVE EMBRYO AND MERISTEMS1 (DEM1) Is Essential for Cell Proliferation and Cell Differentiation in Tomato

Most flowering plant species contain at least two copies of the DEFECTIVE EMBRYO AND MERISTEMS (DEM) gene with the encoded DEM proteins lacking homology to proteins of known biochemical function. In tomato (Sl; Solanum lycopersicum), stable mutations in the SlDEM1 locus result in shoot and root meristem defects with the dem1 mutant failing to progress past the cotyledon stage of seedling development. Generation of a Somatic Mutagenesis of DEM1 (SMD) transformant line in tomato allowed for the characterization of SlDEM1 gene function past the seedling stage of vegetative development with SMD plants displaying a range of leaf development abnormalities. Further, the sectored or stable in planta expression of specific regions of the SlDEM1 coding sequence also resulted in the generation of tomato transformants that displayed a range of vegetative development defects, which when considered together with the dem1 mutant seedling and SMD transformant line phenotypic data, allowed for the assignment of SlDEM1 gene function to early embryo development, adaxial epidermis cell development, lateral leaf blade expansion, and mesophyll cell proliferation and differentiation.

Association between Petal Form Variation and CYC2-like Genotype in a Hybrid Line of Sinningia speciosa

This study used three-dimensional (3D) micro-computed tomography (μCT) imaging to examine petal form variation in a hybrid cross of Sinningia speciosa between a cultivar with actinomorphic flowers and a variety with zygomorphic flowers. The major objectives were to determine the genotype-phenotype associations between the petal form variation and CYCLOIDEA2-like alleles in S. speciosa (SsCYC) and to morphologically investigate the differences in petal types between actinomorphic and zygomorphic flowers. In this study, μCT was used to accurately acquire 3D floral images. Landmark-based geometric morphometrics (GM) was applied to evaluate the major form variations of the petals. Nine morphological traits of the petals were defined according to the form variations quantified through the GM analysis. The results indicated that the outward curvature of dorsal petals, the midrib asymmetry of lateral petals, and the dilation of ventral region of the tube were closely associated with the SsCYC genotype. Multiple analyses of form similarity between the petals suggested that the dorsal and ventral petals of actinomorphic plants resembled the ventral petals of zygomorphic plants. This observation indicated that the transition from zygomorphic to actinomorphic flowers in S. speciosa might be caused by the ventralization of the dorsal petals. We demonstrated that the 3D-GM approach can be used to determine genotype-phenotype associations and to provide morphological evidence for the transition of petal types between actinomorphic and zygomorphic flowers in S. speciosa.

The complex of ASYMMETRIC LEAVES (AS) proteins plays a central role in antagonistic interactions of genes for leaf polarity specification in Arabidopsis

Leaf primordia are born around meristem‐containing stem cells at shoot apices, grow along three axes (proximal–distal, adaxial–abaxial, medial–lateral), and develop into flat symmetric leaves with adaxial–abaxial polarity. Axis development and polarity specification of Arabidopsis leaves require a network of genes for transcription factor‐like proteins and small RNAs. Here, we summarize present understandings of adaxial‐specific genes, ASYMMETRIC LEAVES1 (AS1) and AS2. Their complex (AS1–AS2) functions in the regulation of the proximal–distal leaf length by directly repressing class 1 KNOX homeobox genes (BP, KNAT2) that are expressed in the meristem periphery below leaf primordia. Adaxial–abaxial polarity specification involves antagonistic interaction of adaxial and abaxial genes including AS1 and AS2 for the development of two respective domains. AS1–AS2 directly represses the abaxial gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3) and indirectly represses ETT/ARF3 and ARF4 through tasiR‐ARF. Modifier mutations have been identified that abolish adaxialization and enhance the defect in the proximal–distal patterning in as1 and as2. AS1–AS2 and its modifiers synergistically repress both ARFs and class 1 KNOXs. Repression of ARFs is critical for establishing adaxial–abaxial polarity. On the other hand, abaxial factors KANADI1 (KAN1) and KAN2 directly repress AS2 expression. These data delineate a molecular framework for antagonistic gene interactions among adaxial factors, AS1, AS2, and their modifiers, and the abaxial factors ARFs as key regulators in the establishment of adaxial–abaxial polarity. Possible AS1–AS2 epigenetic repression and activities downstream of ARFs are discussed. WIREs Dev Biol 2015, 4:655–671. doi: 10.1002/wdev.196

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ASYMMETRIC LEAVES2 gene, a member of LOB/AS2 family of Arabidopsis thaliana, causes an abaxializing leaves in transgenic cockscomb

The leaf primordium derives from the peripheral zone of shoot apical meristem. During the formation of leaf primordia, they need to establish the proximodistal, mediolateral, and ab/adaxial axes. Among these axes, the ab/adaxial axis might be the most important. ASYMMETRIC LEAVES2 (AS2) gene is a member of AS2/LATERAL ORGAN BOUNDARY (LOB) family of Arabidopsis thaliana. In this work, we transformed 35S:AS2 transgene constructs to cockscomb (Celosia cristata) via Agrobacterium tumefaciens. All primary transformants subsequently obtained were placed into phenotypic categories and self-pollinated. As a whole, a total of 44 T1 35S:AS2 cockscomb plants obtained were grouped into two major categories: (I) slightly wrinkled leaves (28/44), (II) extremely curved leaves (16/44), on the basis of their leaf phenotypes. Furthermore, we characterized the anatomical features of these malformed leaves; and found the transformation of adaxial cell types into abaxial cell ones. A series of data suggest that AS2 might be involved in the determination of abaxial polarity in cockscomb plants. However, a few research teams have reported that AS2 might be involved in the determination of adaxial polarity in leaf primodia of Arabidopsis thaliana. These data above indicate that the roles of the same ab/adaxial determinant might differ between distinct species. At last, the different function of AS2 in distinct species was discussed.

HYL1 regulates the balance between adaxial and abaxial identity for leaf flattening via miRNA-mediated pathways

HYPONASTIC LEAVES1 (HYL1) is an important regulator of microRNA (miRNA) biogenesis. Incurvature of rosette leaves in loss-of-function mutants of HYL1 implicates the regulation of leaf flatness by HYL1 via miRNA pathways. Recent studies have identified jba-1D, jaw-1D, and oe-160c, the dominant mutants of MIR166g, MIR319a, and MIR160c genes, respectively, which display three types of leaf curvature. However, it remains unclear whether or how HYL1 controls leaf flatness through the pathways mediated by these miRNAs. To define which miRNAs and target genes are relevant to the hyl1 phenotype in terms of leaf incurvature, the effects of three mutated MIRNA genes and their targets on the direction and extent of leaf curvature in hyl1 mutants were examined. The genetic analysis shows that the hyl1 phenotype is strongly rescued by jba-1D, but not by jaw-1D or oe-160c, whereas the mutant phenotypes of jba-1D, jaw-1D, or oe-160c leaves are compromised by the hyl1 allele. Expression analysis indicates that reduced accumulation of miR166, rather than of miR319a or miR160, causes incurvature of hyl1 leaves, and that miR319a-targeted TCP3 positively regulates the adaxial identity gene PHABULOSA while miR160-targeted ARF16 negatively regulates the abaxial identity gene FILAMENTOUS FLOWER. In these cases, the direction and extent of leaf incurvature are associated with the expression ratio of adaxial to abaxial genes (adaxial to abaxial ratio). HYL1 regulates the balance between adaxial and abaxial identity and modulates leaf flatness by preventing leaf incurvature, wavy margins, and downward curvature. It is concluded that HYL1 monitors the roles of miR165/166, miR319a, and miR160 in leaf flattening through the relative activities of adaxial and abaxial identity genes, thus playing an essential role in leaf development.

A novel allele of FILAMENTOUS FLOWER reveals new insights on the link between inflorescence and floral meristem organization and flower morphogenesis

BACKGROUND

The Arabidopsis FILAMENTOUS FLOWER (FIL) gene encodes a YABBY (YAB) family putative transcription factor that has been implicated in specifying abaxial cell identities and thus regulating organ polarity of lateral organs. In contrast to double mutants of fil and other YAB genes, fil single mutants display mainly floral and inflorescence morphological defects that do not reflect merely a loss of abaxial identity. Recently, FIL and other YABs have been shown to regulate meristem organization in a non-cell-autonomous manner. In a screen for new mutations affecting floral organ morphology and development, we have identified a novel allele of FIL, fil-9 and characterized its floral and meristem phenotypes.

RESULTS

The fil-9 mutation results in highly variable disruptions in floral organ numbers and size, partial homeotic transformations, and in defective inflorescence organization. Examination of meristems indicates that both fil-9 inflorescence and floral meristems are enlarged as a result of an increase in cell number, and deformed. Furthermore, primordia emergence from these meristems is disrupted such that several primordia arise simultaneously instead of sequentially. Many of the organs produced by the inflorescence meristems are filamentous, yet they are not considered by the plant as flowers. The severity of both floral organs and meristem phenotypes is increased acropetally and in higher growth temperature.

CONCLUSIONS

Detailed analysis following the development of fil-9 inflorescence and flowers throughout flower development enabled the drawing of a causal link between multiple traits of fil-9 phenotypes. The study reinforces the suggested role of FIL in meristem organization. The loss of spatial and temporal organization of fil-9 inflorescence and floral meristems presumably leads to disrupted cell allocation to developing floral organs and to a blurring of organ whorl boundaries. This disruption is reflected in morphological and organ identity aberrations of fil-9 floral organs and in the production of filamentous organs that are not perceived as flowers. Here, we show the role of FIL in reproductive meristem development and emphasize the potential of using fil mutants to study mersitem organization and the related effects on flower morphogenesis.

Classification and quantification of leaf curvature

Various mutants of Arabidopsis thaliana deficient in polarity, cell division, and auxin response are characterized by certain types of leaf curvature. However, comparison of curvature for clarification of gene function can be difficult without a quantitative measurement of curvature. Here, a novel method for classification and quantification of leaf curvature is reported. Twenty-two mutant alleles from Arabidopsis mutants and transgenic lines deficient in leaf flatness were selected. The mutants were classified according to the direction, axis, position, and extent of leaf curvature. Based on a global measure of whole leaves and a local measure of four regions in the leaves, the curvature index (CI) was proposed to quantify the leaf curvature. The CI values accounted for the direction, axis, position, and extent of leaf curvature in all of the Arabidopsis mutants grown in growth chambers. Comparison of CI values between mutants reveals the spatial and temporal variations of leaf curvature, indicating the strength of the mutant alleles and the activities of the corresponding genes. Using the curvature indices, the extent of curvature in a complicated genetic background becomes quantitative and comparable, thus providing a useful tool for defining the genetic components of leaf development and to breed new varieties with leaf curvature desirable for the efficient capture of sunlight for photosynthesis and high yields.

LeafAnalyser: a computational method for rapid and large-scale analyses of leaf shape variation

A comprehensive understanding of leaf shape is important in many investigations in plant biology. Techniques to assess variation in leaf shape are often time-consuming, labour-intensive and prohibited by complex calculation of large data sets. We have developed LeafAnalyser, software that uses image-processing techniques to greatly simplify the measurement of leaf shape variation. LeafAnalyser places a large number of evenly distributed landmarks along leaf margins and records the position of each automatically. We used LeafAnalyser to analyse the variation in 3000 leaves from 400 plants of Antirrhinum majus. We were able to summarise the major trends in leaf shape variation using a principal components (PC) analysis and assess the changes in size, width and tip-to-base asymmetry within our leaf library. We demonstrate how this information can be used to develop a model that describes the range and variation of leaf shape within standard wild-type lines, and illustrate the shape transformations that occur between leaf nodes. We also show that information from LeafAnalyser can be used to identify novel trends in shape variation, as low-variance PCs that only affect a subset of position landmarks. These results provide a high-throughput method to calculate leaf shape variation that allows a large number of leaves to be visualised in higher-dimensional phenotypic space. To illustrate the applicability of LeafAnalyser we also calculated the leaf shape variation in 300 leaves from Arabidopsis thaliana.

Histone deacetylases and ASYMMETRIC LEAVES2 are involved in the establishment of polarity in leaves of Arabidopsis

We show that two Arabidopsis thaliana genes for histone deacetylases (HDACs), HDT1/HD2A and HDT2/HD2B, are required to establish leaf polarity in the presence of mutant ASYMMETRIC LEAVES2 (AS2) or AS1. Treatment of as1 or as2 plants with inhibitors of HDACs resulted in abaxialized filamentous leaves and aberrant distribution of microRNA165 and/or microRNA166 (miR165/166) in leaves. Knockdown mutations of these two HDACs by RNA interference resulted in phenotypes like those observed in the as2 background. Nuclear localization of overproduced AS2 resulted in decreased levels of mature miR165/166 in leaves. This abnormality was abolished by HDAC inhibitors, suggesting that HDACs are required for AS2 action. A loss-of-function mutation in HASTY, encoding a positive regulator of miRNA levels, and a gain-of-function mutation in PHABULOSA, encoding a determinant of adaxialization, suppressed the generation of abaxialized filamentous leaves by inhibition of HDACs in the as1 or as2 background. AS2 and AS1 were colocalized in subnuclear bodies adjacent to the nucleolus where HDT1/HD2A and HDT2/HD2B were also found. Our results suggest that these HDACs and both AS2 and AS1 act independently to control levels and/or patterns of miR165/166 distribution and the development of adaxial-abaxial leaf polarity and that there may be interactions between HDACs and AS2 (AS1) in the generation of those miRNAs.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

Signalling between the shoot apical meristem and developing lateral organs

A characteristic feature of plant development is the extensive role played by cell-cell signalling in regulating patterns of growth and cell fate. This is particularly apparent in the shoot apical meristem (SAM) where signalling is involved in the maintenance of a central undifferentiated stem cell population and the formation of a regular and predictable pattern of leaves, from the meristem periphery. Although these two functions occur in different regions of the meristem, their activity must be coordinated to maintain meristem integrity. The role of signalling in the SAM was first characterised over 60 years ago by elegant surgical experiments. These studies showed that existing leaf primordia determine future sites of organ formation in adjacent regions of the SAM, a finding that laid the foundation for subsequent studies into the mechanisms controlling phyllotaxy. Recent studies have identified auxin as a likely signal promoting organ formation and shown that young primordia play an important role in determining its distribution in the SAM. These pioneering surgical experiments also revealed that signals from the meristem regulate the development of organ primordia. In this case a meristem signal promotes the formation of cell types found in the top/adaxial half of the emerging leaf. While the identity of this signal remains elusive, the recent characterisation of a small family of PHABULOSA-like (PHB-like) transcription factor genes has provided important clues to its nature. These genes, which promote adaxial cell identity, are regulated by microRNAs (miRNAs) raising the exciting possibility that the meristem signal is either a miRNA or part of a pathway regulating the distribution of miRNAs.

Keeping it together: co-ordinating plant growth

One of the most important demands of multicellularity is the co-ordination of cell proliferation and cell growth to allow the ultimate differentiation of functional organs and tissues. In plants, endogenously and exogenously generated developmental signals hone a basic patterning plan to the demands of a changing environment throughout the lifecycle. Recent advances have started to identify many signalling pathways and intermediates that are potentially implicated in controlling plant growth in response to developmentally important signals. These include pathways that are conserved in other eukaryotes, such as the Target Of Rapamycin (TOR) pathway and lipid signalling via S6-Kinases, as well as pathways that contain plant-specific elements, such as ERECTA-class receptor kinases and TCP-class transcription factors. Understanding how these elements are integrated to give co-ordinated growth remains one of the major challenges in plant biology.

CORKSCREW1 defines a novel mechanism of domain specification in the maize shoot

In higher plants, determinate leaf primordia arise in regular patterns on the flanks of the indeterminate shoot apical meristem (SAM). The acquisition of leaf form is then a gradual process, involving the specification and growth of distinct domains within the three leaf axes. The recessive corkscrew1 (cks1) mutation of maize (Zea mays) disrupts both leaf initiation patterns in the SAM and domain specification within the mediolateral and proximodistal leaf axes. Specifically, cks1 mutant leaves exhibit multiple midribs and leaf sheath tissue differentiates in the blade domain. Such perturbations are a common feature of maize mutants that ectopically accumulate KNOTTED1-like homeobox (KNOX) proteins in leaf tissue. Consistent with this observation, at least two knox genes are ectopically expressed in cks1 mutant leaves. However, ectopic KNOX proteins cannot be detected. We therefore propose that CKS1 primarily functions within the SAM to establish boundaries between meristematic and leaf zones. Loss of gene function disrupts boundary formation, impacts phyllotactic patterns, and leads to aspects of indeterminate growth within leaf primordia. Because these perturbations arise independently of ectopic KNOX activity, the cks1 mutation defines a novel component of the developmental machinery that facilitates leaf-versus-shoot development in maize.

Pattern formation in the vascular system of monocot and dicot plant species

Plant vascular tissues are organised in continuous strands, the longitudinal and radial patterns of which are intimately linked to the signals that direct plant architecture as a whole. Therefore, understanding the mechanisms underlying vascular tissue patterning is expected to shed light on patterning events beyond those that organise the vascular system, and thus represents a central issue in plant developmental biology. A number of recent advances, reviewed here, are leading to a more precise definition of the signals that control the formation of vascular tissues and their integration into a larger organismal context. Contents Summary 209 I. Introduction 209 II. The plant vascular system 210 III. Ontogeny of the vascular tissues 210 IV. Procambium development 210 V. The organisation of the vascular tissues 212 VI. The regulation of longitudinal vascular pattern formation 214 VII. The regulation of radial vascular pattern formation 220 VIII. Genetic screens for vascular development mutants 231 IX. Genes involved in vascular development identified through reverse genetics approaches 235 X. Conclusions and perspectives 235 Note added at the revision stage 236 Acknowledgements 236 References 236.

The wandering carpel mutation of Zea mays (Gramineae) causes misorientation and loss of zygomorphy in flowers and two-seeded kernels

We have isolated a new mutation, wandering carpel (wcr), which affects polarity of the maize flower, altering its orientation or converting it from zygomorphy to radial symmetry. These changes result in the development of embryos on locations other than the normal, acropetal side of the kernel. More than two carpels can develop into silks. More rarely, two ovules develop in a single ovary, giving rise to kernels with two seeds. The wcr mutation is a maternal-sporophyte-effect, semidominant mutation whose expression is background dependent. As spikelets with abnormal flowers are almost always paired with a normal spikelet, we hypothesize that WCR+ is required for establishing polarity in spikelet meristems during inflorescence development.

Establishment of polarity in angiosperm lateral organs

In seed plants, lateral organs such as leaves and floral organs are formed from the flanks of apical meristems. Therefore, they have an inherent positional relationship: organ primordia have an adaxial side next to the meristem, and an abaxial one away from the meristem. Surgical and genetic studies suggest that a morphogenetic gradient, which originates in the meristem, converts the inherent polarity into a functional one. Once an adaxial-abaxial axis of polarity is established within organ primordia, it provides cues for proper lamina growth and asymmetrical development. Several key participants in this process have been identified, and analyses of these genes support and refine our views of axis formation in plants. The complex relationships between and within various members of these plant-specific gene families (class III HD-ZIPs, YABBYs and KANADIs) might account for a substantial part of the morphological variation in lateral organs of seed plants.