Adaxial-abaxial polarity: the developmental basis of leaf shape diversity

Modulation of cell differentiation and growth underlies the shift from bud protection to light capture in cauline leaves

Plant organs have evolved into diverse shapes for specialized functions despite emerging as simple protrusions at the shoot apex. Cauline leaves serve as photosynthetic organs and protective structures for emerging floral buds. However, the growth patterns underlying this dual function remain unknown. Here, we investigate the developmental dynamics shaping Arabidopsis (Arabidopsis thaliana) cauline leaves underlying their functional diversification from other laminar organs. We show that cauline leaves display a significant delay in overall elongation compared with rosette leaves. Using live imaging, we reveal that their functional divergence hinges on early modulation of the timing of cell differentiation and cellular growth rates. In contrast to rosette leaves and sepals, cell differentiation is delayed in cauline leaves, fostering extended proliferation, prolonged morphogenetic activity, and growth redistribution within the organ. Notably, cauline leaf growth is transiently suppressed during the early stages, keeping the leaf small and unfolded during the initiation of the first flowers. Our findings highlight the unique developmental timing of cauline leaves, underlying their shift from an early protective role to a later photosynthetic function.

Single-cell RNA-sequencing provides new insights into the cell-specific expression patterns and transcriptional regulation of photosynthetic genes in bermudagrass leaf blades

As an important warm-season turfgrass species, bermudagrass (Cynodon dactylon L.) flourishes in warm areas around the world due to the existence of the C4 photosynthetic pathway. However, how C4 photosynthesis operates in bermudagrass leaves is still poorly understood. In this study, we performed single-cell RNA-sequencing on 5296 cells from bermudagrass leaf blades. Eight cell clusters corresponding to mesophyll, bundle sheath, epidermis and vascular bundle cells were successfully identified using known cell marker genes. Expression profiling indicated that genes encoding NADP-dependent malic enzymes (NADP-MEs) were highly expressed in bundle sheath cells, whereas NAD-ME genes were weakly expressed in all cell types, suggesting C4 photosynthesis of bermudagrass leaf blades might be NADP-ME type rather than NAD-ME type. The results also indicated that starch synthesis-related genes showed preferential expression in bundle sheath cells, whereas starch degradation-related genes were highly expressed in mesophyll cells, which agrees with the observed accumulation of starch-filled chloroplasts in bundle sheath cells. Gene co-expression analysis further revealed that different families of transcription factors were co-expressed with multiple C4 photosynthesis-related genes, suggesting a complex transcription regulatory network of C4 photosynthesis might exist in bermudagrass leaf blades. These findings collectively provided new insights into the cell-specific expression patterns and transcriptional regulation of photosynthetic genes in bermudagrass.

PeNAC67-PeKAN2-PeSCL23 and B-class MADS-box transcription factors synergistically regulate the specialization process from petal to lip in Phalaenopsis equestris

The molecular basis of orchid flower development involves a specific regulatory program in which MADS-box transcription factors play a central role. The recent 'perianth code' model hypothesizes that two types of higher-order heterotetrameric complexes, namely SP complex and L complex, play pivotal roles in the orchid perianth organ formation. Therefore, we explored their roles and searched for other components of the regulatory network.Through the combined analysis for transposase-accessible chromatin with high-throughput sequencing and RNA sequencing of the lip-like petal and lip from Phalaenopsis equestris var.trilip, transcription factor-(TF) genes involved in lip development were revealed. PeNAC67 encoding a NAC-type TF and PeSCL23 encoding a GRAS-type TF were differentially expressed between the lip-like petal and the lip. PeNAC67 interacted with and stabilized PeMADS3, which positively regulated the development of lip-like petal to lip. PeSCL23 and PeNAC67 competitively bound with PeKAN2 and positively regulated the development of lip-like petal to petal by affecting the level of PeMADS3. PeKAN2 as an important TF that interacts with PeMADS3 and PeMADS9 can promote lip development. These results extend the 'perianth code' model and shed light on the complex regulation of orchid flower development.

Growth directions and stiffness across cell layers determine whether tissues stay smooth or buckle

From smooth to buckled, nature exhibits organs of various shapes and forms. How cellular growth patterns produce smooth organ shapes such as leaves and sepals remains unclear. Here we show that unidirectional growth and comparable stiffness across both epidermal layers of Arabidopsis sepals are essential for smoothness. We identified a mutant with ectopic ASYMMETRIC LEAVES 2 (AS2) expression on the outer epidermis. Our analysis reveals that ectopic AS2 expression causes outer epidermal buckling at early stages of sepal development, due to conflicting growth directions and unequal epidermal stiffnesses. Aligning growth direction and increasing stiffness of the outer epidermis restores smoothness. Furthermore, buckling influences auxin efflux transporter protein PIN-FORMED 1 polarity to generate outgrowth in the later stages, suggesting that buckling is sufficient to initiate outgrowths. Our findings suggest that in addition to molecular cues influencing tissue mechanics, tissue mechanics can also modulate molecular signals, giving rise to well-defined shapes.

Rhubarb: A novel model plant to study the conundrum of calcium oxalate synthesis

The study investigated calcium oxalate (CaOx) crystal composition, accumulation, synthesis, and degradation in five rhubarb species from the North-Western Indian Himalayas. Techniques like optical and scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy (SEM-EDS), inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray diffraction spectroscopy (XRD), and real-time (qRT-PCR) expression analysis of strategic genes were used to understand the processes of oxalate synthesis and precipitation. Results showed crystals tend to accumulate around vascular bundles in all species, irrespective of size, indicating a consistent pattern. Crystal synthesis and accumulation were stress-driven, linked to substrate composition, and in planta soluble oxalate and calcium levels, paralleling oxalate precursors. Based on their availability, CaOx crystals precipitated heavy metals mostly associated with its weddellite form. Crystal content correlated positively with mRNA levels of calcium/oxalate/ascorbate-related and stress-responsive genes, and negatively with oxalate oxidation/decarboxylation genes. CaOx crystals were suggested as potential biominerals for addressing heavy metal stress in agriculture.

Carnivorous Plant Biology: From Gene to Traps

Carnivorous plants (approximately 850 species) are specific mixotrophic plants which all perform photosynthesis but need mainly nitrogen and phosphorous from animal or protist bodies [...].

Differences in the Occurrence of Cell Wall Components between Distinct Cell Types in Glands of Drosophyllum lusitanicum

Carnivorous plants are mixotrophs that have developed the ability to lure, trap, and digest small organisms and utilize components of the digested bodies. Leaves of Drosophyllum lusitanicum have two kinds of glands (emergences): stalked mucilage glands and sessile digestive glands. The stalked mucilage glands perform the primary role in prey lure and trapping. Apart from their role in carnivory, they absorb water condensed from oceanic fog; thus, plants can survive in arid conditions. To better understand the function of carnivorous plant emergences, the molecular composition of their cell walls was investigated using immunocytochemical methods. In this research, Drosophyllum lusitanicum was used as a study system to determine whether cell wall immunocytochemistry differs between the mucilage and digestive glands of other carnivorous plant species. Light and electron microscopy were used to observe gland structure. Fluorescence microscopy revealed the localization of carbohydrate epitopes associated with the major cell wall polysaccharides and glycoproteins. The mucilage gland (emergence) consists of a glandular head, a connecting neck zone, and stalk. The gland head is formed by an outer and inner layer of glandular (secretory) cells and supported by a layer of endodermoid (barrier) cells. The endodermoid cells have contact with a core of spongy tracheids with spiral-shaped thickenings. Lateral tracheids are surrounded by epidermal and parenchymal neck cells. Different patterns of cell wall components were found in the various cell types of the glands. Cell walls of glandular cells generally are poor in both low and highly esterified homogalacturonans (HGs) but enriched with hemicelluloses. Cell walls of inner glandular cells are especially rich in arabinogalactan proteins (AGPs). The cell wall ingrowths in glandular cells are significantly enriched with hemicelluloses and AGPs. In the case of cell wall components, the glandular cells of Drosophyllum lusitanicum mucilage glands are similar to the glandular cells of the digestive glands of Aldrovanda vesiculosa and Dionaea muscipula.

How to Grow a Flat Leaf

Growing a flat lamina such as a leaf is almost impossible without some feedback to stabilize long wavelength modes that are easy to trigger since they are energetically cheap. Here we combine the physics of thin elastic plates with feedback control theory to explore how a leaf can remain flat while growing. We investigate both in-plane (metric) and out-of-plane (curvature) growth variation and account for both local and nonlocal feedback laws. We show that a linearized feedback theory that accounts for both spatially nonlocal and temporally delayed effects suffices to suppress long wavelength fluctuations effectively and explains recently observed statistical features of growth in tobacco leaves. Our work provides a framework for understanding the regulation of the shape of leaves and other leaflike laminar objects.

Adaptive evolution of the enigmatic Takakia now facing climate change in Tibet

The most extreme environments are the most vulnerable to transformation under a rapidly changing climate. These ecosystems harbor some of the most specialized species, which will likely suffer the highest extinction rates. We document the steepest temperature increase (2010-2021) on record at altitudes of above 4,000 m, triggering a decline of the relictual and highly adapted moss Takakia lepidozioides. Its de-novo-sequenced genome with 27,467 protein-coding genes includes distinct adaptations to abiotic stresses and comprises the largest number of fast-evolving genes under positive selection. The uplift of the study site in the last 65 million years has resulted in life-threatening UV-B radiation and drastically reduced temperatures, and we detected several of the molecular adaptations of Takakia to these environmental changes. Surprisingly, specific morphological features likely occurred earlier than 165 mya in much warmer environments. Following nearly 400 million years of evolution and resilience, this species is now facing extinction.

DWARF AND ROBUST PLANT regulates plant height via modulating gibberellin biosynthesis in chrysanthemum

YABBY (YAB) genes are specifically expressed in abaxial cells of lateral organs and determine abaxial cell fate. However, most studies have focused on few model plants, and the molecular mechanisms of YAB genes are not well understood. Here, we identified a YAB transcription factor in chrysanthemum (Chrysanthemum morifolium), Dwarf and Robust Plant (CmDRP), that belongs to a distinct FILAMENTOUS FLOWER (FlL)/YAB3 sub-clade lost in Brassicaceae. CmDRP was expressed in various tissues but did not show any polar distribution in chrysanthemum. Overexpression of CmDRP resulted in a semi-dwarf phenotype with a significantly decreased active GA3 content, while reduced expression generated the opposite phenotype. Furthermore, plant height of transgenic plants was partially rescued through the exogenous application of GA3 and Paclobutrazol, and expression of the GA biosynthesis gene CmGA3ox1 was significantly altered in transgenic plants. Yeast one-hybrid, luciferase, and chromatin immunoprecipitation-qPCR analyses showed that CmDRP could directly bind to the CmGA3ox1 promoter and suppress its expression. Our research reveals a nonpolar expression pattern of a YAB family gene in dicots and demonstrates it regulates plant height through the GA pathway, which will deepen the understanding of the genetic and molecular mechanisms of YAB genes.

The Genetic Control of the Compound Leaf Patterning in Medicago truncatula

Simple and compound which are the two basic types of leaves are distinguished by the pattern of the distribution of blades on the petiole. Compared to simple leaves comprising a single blade, compound leaves have multiple blade units and exhibit more complex and diverse patterns of organ organization, and the molecular mechanisms underlying their pattern formation are receiving more and more attention in recent years. Studies in model legume Medicago truncatula have led to an improved understanding of the genetic control of the compound leaf patterning. This review is an attempt to summarize the current knowledge about the compound leaf morphogenesis of M. truncatula, with a focus on the molecular mechanisms involved in pattern formation. It also includes some comparisons of the molecular mechanisms between leaf morphogenesis of different model species and offers useful information for the molecular design of legume crops.

Case not closed: the mystery of the origin of the carpel

The carpel is a fascinating structure that plays a critical role in flowering plant reproduction and contributed greatly to the evolutionary success and diversification of flowering plants. The remarkable feature of the carpel is that it is a closed structure that envelopes the ovules and after fertilization develops into the fruit which protects, helps disperse, and supports seed development into a new plant. Nearly all plant-based foods are either derived from a flowering plant or are a direct product of the carpel. Given its importance it's no surprise that plant and evolutionary biologists have been trying to explain the origin of the carpel for a long time. Before carpel evolution seeds were produced on open leaf-like structures that are exposed to the environment. When the carpel evolved in the stem lineage of flowering plants, seeds became protected within its closed structure. The evolutionary transition from that open precursor to the closed carpel remains one of the greatest mysteries of plant evolution. In recent years, we have begun to complete a picture of what the first carpels might have looked like. On the other hand, there are still many gaps in our understanding of what the precursor of the carpel looked like and what changes to its developmental mechanisms allowed for this evolutionary transition. This review aims to present an overview of existing theories of carpel evolution with a particular emphasis on those that account for the structures that preceded the carpel and/or present testable developmental hypotheses. In the second part insights from the development and evolution of diverse plant organs are gathered to build a developmental hypothesis for the evolutionary transition from a hypothesized laminar open structure to the closed structure of the carpel.

Coordination of biradial-to-radial symmetry and tissue polarity by HD-ZIP II proteins

Symmetry establishment is a critical process in the development of multicellular organs and requires careful coordination of polarity axes while cells actively divide within tissues. Formation of the apical style in the Arabidopsis gynoecium involves a bilateral-to-radial symmetry transition, a stepwise process underpinned by the dynamic distribution of the plant morphogen auxin. Here we show that SPATULA (SPT) and the HECATE (HEC) bHLH proteins mediate the final step in the style radialisation process and synergistically control the expression of adaxial-identity genes, HOMEOBOX ARABIDOPSIS THALIANA 3 (HAT3) and ARABIDOPSIS THALIANA HOMEOBOX 4 (ATHB4). HAT3/ATHB4 module drives radialisation of the apical style by promoting basal-to-apical auxin flow and via a negative feedback mechanism that finetune auxin distribution through repression of SPT expression and cytokinin sensitivity. Thus, this work reveals the molecular basis of axes-coordination and hormonal cross-talk during the sequential steps of symmetry transition in the Arabidopsis style.

The apical style in Arabidopsis is formed following a bilateral-to-radial symmetry transition in the gynoecium. Here the authors show that the final step in style radialization is coordinated by the adaxial regulators HAT3 and ATHB4, which are induced by the SPT and HEC transcription factors.

On the Origin of Carnivory: Molecular Physiology and Evolution of Plants on an Animal Diet

Charles Darwin recognized that carnivorous plants thrive in nutrient-poor soil by capturing animals. Although the concept of botanical carnivory has been known for nearly 150 years, its molecular mechanisms and evolutionary origins have not been well understood until recently. In the last decade, technical advances have fueled the genome and transcriptome sequencings of active and passive hunters, leading to a better understanding of the traits associated with the carnivorous syndrome, from trap leaf development and prey digestion to nutrient absorption, exemplified, for example, by the Venus flytrap (Dionaea muscipula), pitcher plant (Cephalotus follicularis), and bladderwort (Utricularia gibba). The repurposing of defense-related genes is an important trend in the evolution of plant carnivory. In this review, using the Venus flytrap as a representative of the carnivorous plants, we summarize the molecular mechanisms underlying their ability to attract, trap, and digest prey and discuss the origins of plant carnivory in relation to their genomic evolution.

Leaf Shape Diversity: From Genetic Modules to Computational Models

Plant leaves display considerable variation in shape. Here, we introduce key aspects of leaf development, focusing on the morphogenetic basis of leaf shape diversity. We discuss the importance of the genetic control of the amount, duration, and direction of cellular growth for the emergence of leaf form. We highlight how the combined use of live imaging and computational frameworks can help conceptualize how regulated cellular growth is translated into different leaf shapes. In particular, we focus on the morphogenetic differences between simple and complex leaves and how carnivorous plants form three-dimensional insect traps. We discuss how evolution has shaped leaf diversity in the case of complex leaves, by tinkering with organ-wide growth and local growth repression, and in carnivorous plants, by modifying the relative growth of the lower and upper sides of the leaf primordium to create insect-digesting traps.

A discordance of seasonally covarying cues uncovers misregulated phenotypes in the heterophyllous pitcher plant Cephalotus follicularis

Organisms withstand normal ranges of environmental fluctuations by producing a set of phenotypes genetically programmed as a reaction norm; however, extreme conditions can expose a misregulation of phenotypes called a hidden reaction norm. Although an environment consists of multiple factors, how combinations of these factors influence a reaction norm is not well understood. To elucidate the combinatorial effects of environmental factors, we studied the leaf shape plasticity of the carnivorous pitcher plant Cephalotus follicularis. Clonally propagated plants were subjected to 12-week-long growth experiments in different conditions controlled by growth chambers. Here, we show that the dimorphic response of forming a photosynthetic flat leaf or an insect-trapping pitcher leaf is regulated by two covarying environmental cues: temperature and photoperiod. Even within the normal ranges of temperature and photoperiod, unusual combinations of the two induced the production of malformed leaves that were rarely observed under the environmentally typical combinations. We identified such cases in combinations of a summer temperature with a short-to-neutral day length, whose average frequency in the natural Cephalotus habitats corresponded to a once-in-a-lifetime event for this perennial species. Our results suggest that even if individual cues are within the range of natural fluctuations, a hidden reaction norm can be exposed under their discordant combinations. We anticipate that climate change may challenge organismal responses through not only extreme cues but also through uncommon combinations of benign cues.

Fine Mapping of a Locus Underlying the Ectopic Blade-Like Outgrowths on Leaf and Screening Its Candidate Genes in Rapeseed (Brassica napus L.)

Leaf is an important organ for higher plants, and the shape of it is one of the crucial traits of crops. In this study, we investigated a unique aberrant leaf morphology trait in a mutational rapeseed material, which displayed ectopic blade-like outgrowths on the adaxial side of leaf. The abnormal line 132000B-3 was crossed with the normal line 827-3. Based on the F_{2 : 3} family, we constructed two DNA pools (normal pool and abnormal pool) by the bulked segregant analysis (BSA) method and performed whole genome re-sequencing (WGR), obtaining the single-nucleotide polymorphism (SNP) and insertion/deletion (InDel) data. The SNP-index method was used to calculate the Δ(SNP/InDel-index), and then an association region was identified on chromosome A10 with a length of 5.5 Mbp, harboring 1048 genes totally. Subsequently, the fine mapping was conducted by using the penta-primer amplification refractory mutation system (PARMS), and the associated region was narrowed down to a 35.1-kbp segment, containing only seven genes. These seven genes were then analyzed according to their annotations and finally, BnA10g0422620 and BnA10g0422610, orthologs of LATE MERISTEM IDENTITY1 (LMI1) gene from Arabidopsis and REDUCED COMPLEXITY (RCO) gene from its relative Cardamine hirsuta, respectively, were identified as the candidate genes responding to this blade-like outgrowth trait in rapeseed. This study provides a novel perspective into the leaf formation in Brassica plants.

Roles of YABBY transcription factors in the modulation of morphogenesis, development, and phytohormone and stress responses in plants

The YABBY family is a class of plant-specific transcription factors comprising a typical N-terminal C2C2-type zinc finger domain and a C-terminal helix-loop-helix YABBY domain. YABBY transcription factors play important roles in multiple biological processes, including polarity establishment in plant leaves, the formation and development of reproductive organs, the response to plant hormone signals, resistance to stress, crop breeding and agricultural production. The aim of this review is to summarize our current understanding of the roles, functions and value of the YABBY family in plants, with particular emphasis on new insights into the molecular and physiological mechanisms involved in the YABBY-mediated modulation of polarity establishment, morphogenesis and development, and phytohormone and stress responses in plants. In addition, we propose that this transcription factor family presents great value and potential for research, application and development in crop breeding and agricultural production in the future.

The flattened and needlelike leaves of the pine family (Pinaceae) share a conserved genetic network for adaxial-abaxial polarity but have diverged for photosynthetic adaptation

Background

Leaves have highly diverse morphologies. However, with an evolutionary history of approximately 200 million years, leaves of the pine family are relatively monotonous and often collectively called “needles”, although they vary in length, width and cross-section shapes. It would be of great interest to determine whether Pinaceae leaves share similar morpho-physiological features and even consistent developmental and adaptive mechanisms.

Results

Based on a detailed morpho-anatomical study of leaves from all 11 Pinaceae genera, we particularly investigated the expression patterns of adaxial-abaxial polarity genes in two types of leaves (needlelike and flattened) and compared their photosynthetic capacities. We found that the two types of leaves share conserved spatial patterning of vasculatures and genetic networks for adaxial-abaxial polarity, although they display different anatomical structures in the mesophyll tissue differentiation and distribution direction. In addition, the species with needlelike leaves exhibited better photosynthetic capacity than the species with flattened leaves.

Conclusions

Our study provides the first evidence for the existence of a conserved genetic module controlling adaxial-abaxial polarity in the development of different Pinaceae leaves.

Chemical profiles of cuticular waxes on various organs of Sorghum bicolor and their antifungal activities

Sorghum bicolor is widely cultivated in arid and semi-arid areas. This paper reports the chemical profiles of cuticular waxes on adaxial and abaxial sides of common leaf, flag leaf, sheath and stem from six sorghum cultivars and the variations of leaf cuticular waxes at seedling, jointing and filling stages. Then, the bioassay of leaf and sheath wax were evaluated against Penicillium sp and Alternaria alternata. The six sorghum cultivars had similar wax profiles. In total, eight wax compounds were identified, including fatty acids, aldehydes, primary alcohols, alkanes, secondary alcohols, ketones, sterols and minor triterpenoids. Leaf wax coverage increased from 2.2 to 3.1 μg/cm² at seedling stages to 6.5-14.0 μg/cm² at jointing and filling stages, respectively. The relative abundance of primary alcohols decreased from 51 to 62% at seedling stage to 17-33% at jointing stage whereas alkanes increased from 5-9% to 19-33%. Leaf was dominated with alkanes (28.4%) and aldehydes (28.4%), sheath with acids (42.8%), and stem with aldehydes (80.8%). Epicuticular wax of leaf and sheath contained higher proportions of alkanes whereas the intracuticular waxes contained higher proportions of sterols. The leaf wax improved the growth of Penicillium but reduced that of A. alternaria, whereas sheath wax reduced the growth of Penicillium but unchanged A. alternaria. The detailed sorghum wax profiles improve our understanding of the physiological roles of these waxes and their diversified potential usages in industries.

Irregular adaxial-abaxial polarity rearrangement contributes to the monosymmetric-to-asymmetric transformation of Canna indica stamen

In flowering plants, lateral organs including stamens develop according to the precise regulation of adaxial-abaxial polarity. However, the polarity establishment process is poorly understood in asymmetric stamens. Canna indica (Zingiberales: Cannaceae) is a common ornamental plant with an asymmetric stamen comprising a one-theca anther and a petaloid appendage. In this study, we depicted the monosymmetric-to-asymmetric morphogenesis of C. indica stamen, and the morphogenesis of the monosymmetric stamen of a sister species was used as a contrast. We chose a HD-ZIP III gene family member and a YABBY family member as the adaxial and abaxial polarity marker genes, respectively, and tested their expression using mRNA in situ hybridization. The expression patterns of the two genes changed dynamically and asymmetrically during the stamen development process. Compared with their homologues in Arabidopsis thaliana, these two genes exhibited some specific expression patterns. We hypothesize that the distinctive adaxial-abaxial polarity participates in the irregular morphogenesis of C. indica stamen, which mediates the putative stamen-to-petaloid staminode conversion in this species.

Identification of the target genes of AqAPETALA3-3 (AqAP3-3) in Aquilegia coerulea (Ranunculaceae) helps understand the molecular bases of the conserved and nonconserved features of petals

Identification and comparison of the conserved and variable downstream genes of floral organ identity regulators are critical to understanding the mechanisms underlying the commonalities and peculiarities of floral organs. Yet, because of the lack of studies in nonmodel species, a general picture of the regulatory evolution between floral organ identity genes and their targets is still lacking. Here, by conducting extensive chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-seq), electrophoretic mobility shift assay and bioinformatic analyses, we identify and predict the target genes of a petal identity gene, AqAPETALA3-3 (AqAP3-3), in Aquilegia coerulea (Ranunculaceae) and compare them with those of its counterpart in Arabidopsis thaliana, AP3. In total, 7049 direct target genes are identified for AqAP3-3, of which 2394 are highly confident and 1085 are shared with AP3. Gene Ontology enrichment analyses further indicate that conserved targets are largely involved in the formation of identity-related features, whereas nonconserved targets are mostly required for the formation of species-specific features. These results not only help understand the molecular bases of the conserved and nonconserved features of petals, but also pave the way to studying the regulatory evolution between floral organ identity genes and their targets.

Developmental and molecular characterization of novel staminodes in Aquilegia

BACKGROUND AND AIMS

The ranunculid model system Aquilegia is notable for the presence of a fifth type of floral organ, the staminode, which appears to be the result of sterilization and modification of the two innermost whorls of stamens. Previous studies have found that the genetic basis for the identity of this new organ is the result of sub- and neofunctionalization of floral organ identity gene paralogues; however, we do not know the extent of developmental and molecular divergence between stamens and staminodes.

METHODS

We used histological techniques to describe the development of the Aquilegia coerulea 'Origami' staminode relative to the stamen filament. These results have been compared with four other Aquilegia species and the closely related genera Urophysa and Semiaquilegia. As a complement, RNA sequencing has been conducted at two developmental stages to investigate the molecular divergence of the stamen filaments and staminodes in A. coerulea 'Origami'.

KEY RESULTS

Our developmental study has revealed novel features of staminode development, most notably a physical interaction along the lateral margin of adjacent organs that appears to mediate their adhesion. In addition, patterns of abaxial/adaxial differentiation are observed in staminodes but not stamen filaments, including asymmetric lignification of the adaxial epidermis in the staminodes. The comparative transcriptomics are consistent with the observed lignification of staminodes and indicate that stamen filaments are radialized due to overexpression of adaxial identity, while the staminodes are expanded due to the balanced presence of abaxial identity.

CONCLUSIONS

These findings suggest a model in which the novel staminode identity programme interacts with the abaxial/adaxial identity pathways to produce two whorls of laterally expanded organs that are highly differentiated along their abaxial/adaxial axis. While the ecological function of Aquilegia staminodes remains to be determined, these data are consistent with a role in protecting the early carpels from herbivory and/or pathogens.

The morphology, molecular development and ecological function of pseudonectaries on Nigella damascena (Ranunculaceae) petals

Pseudonectaries, or false nectaries, the glistening structures that resemble nectaries or nectar droplets but do not secrete nectar, show considerable diversity and play important roles in plant-animal interactions. The morphological nature, optical features, molecular underpinnings and ecological functions of pseudonectaries, however, remain largely unclear. Here, we show that pseudonectaries of Nigella damascena (Ranunculaceae) are tiny, regional protrusions covered by tightly arranged, non-secretory polygonal epidermal cells with flat, smooth and reflective surface, and are clearly visible even under ultraviolet light and bee vision. We also show that genes associated with cell division, chloroplast development and wax formation are preferably expressed in pseudonectaries. Specifically, NidaYABBY5, an abaxial gene with ectopic expression in pseudonectaries, is indispensable for pseudonectary development: knockdown of it led to complete losses of pseudonectaries. Notably, when flowers without pseudonectaries were arrayed beside those with pseudonectaries, clear differences were observed in the visiting frequency, probing time and visiting behavior of pollinators (i.e., honey bees), suggesting that pseudonectaries serve as both visual attractants and nectar guides.

Interspecies interactions, including those between plants and pollinators, can involve deception. The authors characterize the molecular development of Nigella damascena pseudonectaries, and their adaptive function in attracting specific pollinators to concealed nectaries with visual cues.

Form Follows Function: How to Build a Deadly Trap

The carnivorous plant Utricularia gibba forms cup-shaped leaflets to capture prey. Whitewoods et al. (2020) use computational modeling to simulate the formation of the trap's 3D geometry. Directional expansion of the young leaflet is proposed to be a crucial morphogenetic driver, pointing at a fundamental principle of plant development.

Shaping of a three-dimensional carnivorous trap through modulation of a planar growth mechanism

Leaves display a remarkable range of forms, from flat sheets with simple outlines to cup-shaped traps. Although much progress has been made in understanding the mechanisms of planar leaf development, it is unclear whether similar or distinctive mechanisms underlie shape transformations during development of more complex curved forms. Here, we use 3D imaging and cellular and clonal analysis, combined with computational modelling, to analyse the development of cup-shaped traps of the carnivorous plant Utricularia gibba. We show that the transformation from a near-spherical form at early developmental stages to an oblate spheroid with a straightened ventral midline in the mature form can be accounted for by spatial variations in rates and orientations of growth. Different hypotheses regarding spatiotemporal control predict distinct patterns of cell shape and size, which were tested experimentally by quantifying cellular and clonal anisotropy. We propose that orientations of growth are specified by a proximodistal polarity field, similar to that hypothesised to account for Arabidopsis leaf development, except that in Utricularia, the field propagates through a highly curved tissue sheet. Independent evidence for the polarity field is provided by the orientation of glandular hairs on the inner surface of the trap. Taken together, our results show that morphogenesis of complex 3D leaf shapes can be accounted for by similar mechanisms to those for planar leaves, suggesting that simple modulations of a common growth framework underlie the shaping of a diverse range of morphologies.

Many plant and animal organs derive from tissue sheets, but how are they shaped to create the diversity of forms observed in nature? This study uses a combination of imaging and mathematical modelling to show how carnivorous plant traps shape themselves in 3D by a growth framework oriented by tissue polarity, similar to that found in planar leaves.

Developmental mechanisms involved in the diversification of flowers

We all appreciate the fantastic diversity of flowers. How flowers diversified, however, remains largely enigmatic. The mechanisms underlying the diversification of flowers are complex because the overall appearance of a flower is determined by many factors, such as the shape and size of its receptacle, and the arrangement, number, type, shape and colour of floral organs. Modifications of the developmental trajectories of a flower and its components, therefore, can lead to the generation of new floral types. In this Review, by summarizing the recent progress in studying the initiation, identity determination, morphogenesis and maturation of floral organs, we present our current understanding of the mechanisms underlying the diversification of flowers.

The Evolution of the KANADI Gene Family and Leaf Development in Lycophytes and Ferns

Leaves constitute the main photosynthetic plant organ and even though their importance is not debated, the origin and development of leaves still is. The leaf developmental network has been elucidated for angiosperms, from genes controlling leaf initiation, to leaf polarity and shape. There are four KANADI (KAN) paralogs in Arabidopsisthaliana needed for organ polarity with KAN1 and KAN2 specifying abaxial leaf identity. Yet, studies of this gene lineage outside angiosperms are required to better understand the evolutionary patterns of leaf development and the role of KAN homologs. We studied the evolution of KAN genes across vascular plants and their expression by in situ hybridization in the fern, Equisetum hyemale and the lycophyte Selaginella moellendorffii. Our results show that the expression of KAN genes in leaves is similar between ferns and angiosperms. However, the expression patterns observed in the lycophyte S. moellendorffii are significantly different compared to all other vascular plants, suggesting that the KAN function in leaf polarity is likely only conserved across ferns, gymnosperms, and angiosperms. This study indicates that mechanisms for leaf development are different in lycophytes compared to other vascular plants.

Mangroves in the Leaves: Anatomy, Physiology, and Immunity of Epithemal Hydathodes

Hydathodes are organs found on aerial parts of a wide range of plant species that provide almost direct access for several pathogenic microbes to the plant vascular system. Hydathodes are better known as the site of guttation, which is the release of droplets of plant apoplastic fluid to the outer leaf surface. Because these organs are only described through sporadic allusions in the literature, this review aims to provide a comprehensive view of hydathode development, physiology, and immunity by compiling a historic and contemporary bibliography. In particular, we refine the definition of hydathodes.We illustrate their important roles in the maintenance of plant osmotic balance, nutrient retrieval, and exclusion of deleterious chemicals from the xylem sap. Finally, we present our current understanding of the infection of hydathodes by adapted vascular pathogens and the associated plant immune responses.

A mutation in class III homeodomain-leucine zipper (HD-ZIP III) transcription factor results in curly leaf (cul) in cucumber (Cucumis sativus L.)

We identified two curly-leaf (cul) mutants in cucumber. Map-based cloning revealed that both mutants are due to allelic mutations in the CsPHB gene, a homolog of the Arabidopsis PHABULOSA which encodes a class III homeodomain-leucine zipper (HD-ZIP III) transcription factor. Leaf rolling is an important agronomic trait in crop breeding. Moderate leaf rolling minimizes shadowing between leaves, leading to improved photosynthetic efficiency. Although a number of genes controlling rolled leaf have been identified from rice and other plant species, none have been mapped or cloned in cucurbit crops. In this study, we identified and characterized two curly leaf (cul) mutants, cul-1 and cul-2 in cucumber. With map-based cloning, we show that cul-1 and cul-2 are allelic mutations and CsPHB (Csa6G525430) was the candidate gene for both mutants. The CsPHB gene encoded a class III homeodomain-leucine zipper (HD-ZIP III) transcription factor. A single non-synonymous mutation in the fourth and fifth exons of the CsPHB was responsible for the cul-1 and cul-2 mutant phenotypes, respectively. The single-nucleotide substitutions in cul-1 and cul-2 were both located in cs-miRNA165/166 complementary sites of CsPHB. The expression level of CsPHB gene in multiple organs of cul-1 and cul-2 mutants was higher than that in the wild type, while the expression of cs-miRNA165/166 in the two genotypes showed the opposite trend. We speculate that disruption of the binding between the mutant allele of CsPHB and cs-miRNA165/166 leads to the curly-leaf phenotype. This is the first report to clone and characterize the CsPHB gene in the family Cucurbitaceae. Taken together, these results support CsPHB as an important player in the modulation of leaf shape development in cucumber.

Catalytic Scavenging of Plant Reactive Oxygen Species In Vivo by Anionic Cerium Oxide Nanoparticles

Reactive oxygen species (ROS) accumulation is a hallmark of plant abiotic stress response. ROS play a dual role in plants by acting as signaling molecules at low levels and damaging molecules at high levels. Accumulation of ROS in stressed plants can damage metabolites, enzymes, lipids, and DNA, causing a reduction of plant growth and yield. The ability of cerium oxide nanoparticles (nanoceria) to catalytically scavenge ROS in vivo provides a unique tool to understand and bioengineer plant abiotic stress tolerance. Here, we present a protocol to synthesize and characterize poly (acrylic) acid coated nanoceria (PNC), interface the nanoparticles with plants via leaf lamina infiltration, and monitor their distribution and ROS scavenging in vivo using confocal microscopy. Current molecular tools for manipulating ROS accumulation in plants are limited to model species and require laborious transformation methods. This protocol for in vivo ROS scavenging has the potential to be applied to wild type plants with broad leaves and leaf structure like Arabidopsis thaliana.

Getting leaves into shape: a molecular, cellular, environmental and evolutionary view

Leaves arise from groups of undifferentiated cells as small primordia that go through overlapping phases of morphogenesis, growth and differentiation. These phases are genetically controlled and modulated by environmental cues to generate a stereotyped, yet plastic, mature organ. Over the past couple of decades, studies have revealed that hormonal signals, transcription factors and miRNAs play major roles during leaf development, and more recent findings have highlighted the contribution of mechanical signals to leaf growth. In this Review, we discuss how modulating the activity of some of these regulators can generate diverse leaf shapes during development, in response to a varying environment, or between species during evolution.

Genomic methylation and transcriptomic profiling provides insights into heading depression in inbred Brassica rapa L. ssp. pekinensis

Inbreeding depression is the reduction in fitness observed in inbred populations. In plants, it leads to disease, weaker resistance to adverse environmental conditions, inhibition of growth, and decrease of yield. To elucidate molecular mechanisms behind inbreeding depression, we compared global DNA methylation and transcriptome profiles of a normal and a highly inbred heading degenerated variety of the Chinese cabbage (Brassica rapa L. ssp. pekinensis). DNA methylation was reduced in inbred plants, suggesting a change in the epigenetic landscape. Transcriptome analysis by RNA-Seq revealed that genes in auxin-response and synthesis pathways were differentially expressed in the inbreeding depression lines. Interestingly, methylation levels of some of those genes were also changed. Furthermore, endogenous IAA content was decreased in inbred plants, in agreement with expression and methylation data. Chemical inhibition of auxin also replicated the degenerated phenotype in normal plants, while exogenous IAA application had no effect in inbred depression plants, suggesting a more complex mechanism. These data indicate DNA methylation-regulated auxin pathways play a role in establishing inbred depression phenotypes in plants. Our findings reveal new insights into inbreeding depression and leafy head development in Chinese cabbage.

Genome-Wide Identification and Analysis of TCP Transcription Factors Involved in the Formation of Leafy Head in Chinese Cabbage

Chinese cabbage (Brassica rapa L. ssp. pekinensis) is a widely cultivated and economically important vegetable crop with typical leaf curvature. The TCP (Teosinte branched1, Cycloidea, Proliferating cell factor) family proteins are plant-specific transcription factors (TFs) and play important roles in many plant biological processes, especially in the regulation of leaf curvature. In this study, 39 genes encoding TCP TFs are detected on the whole genome of B. rapa. Based on the phylogenetic analysis of TCPs between Arabidopsis thaliana and Brassica rapa, TCP genes of Chinese cabbage are named from BrTCP1a to BrTCP24b. Moreover, the chromosomal location; phylogenetic relationships among B. rapa, A. thaliana, and rice; gene structures and protein conserved sequence alignment; and conserved domains are analyzed. The expression profiles of BrTCPs are analyzed in different tissues. To understand the role of Chinese cabbage TCP members in regulating the curvature of leaves, the expression patterns of all BrTCP genes are detected at three development stages essential for leafy head formation. Our results provide information on the classification and details of BrTCPs and allow us to better understand the function of TCPs involved in leaf curvature of Chinese cabbage.

The STENOFOLIA gene from Medicago alters leaf width, flowering time and chlorophyll content in transgenic wheat

Molecular genetic analyses revealed that the WUSCHEL-related homeobox (WOX) gene superfamily regulates several programs in plant development. Many different mechanisms are reported to underlie these alterations. The WOX family member STENOFOLIA (STF) is involved in leaf expansion in the eudicot Medicago truncutula. Here, we report that when this gene was ectopically expressed in a locally adapted hard red winter wheat cultivar (Triticum aestivum), the transgenic plants showed not only widened leaves but also accelerated flowering and increased chlorophyll content. These desirable traits were stably inherited in the progeny plants. STF binds to wheat genes that have the (GA)_n /(CT)_n DNA cis element, regardless of sequences flanking the DNA repeats, suggesting a mechanism for its pleiotropic effects. However, the amino acids between position 91 and 262 in the STF protein that were found to bind with the (GA)_n motif have no conserved domain with any other GAGA-binding proteins in animals or plants. We also found that STF interacted with a variety of proteins in wheat in yeast 2 hybrid assays. We conclude that the eudicot STF gene binds to (GA)_n /(CT)_n DNA elements and can be used to regulate leaf width, flowering time and chlorophyll content in monocot wheat.

Coupling Seq-BSA and RNA-Seq Analyses Reveal the Molecular Pathway and Genes Associated with Heading Type in Chinese Cabbage

In Chinese cabbage, heading type is a key agricultural trait of significant economic importance. Using a natural microspore-derived doubled haploid plant, we generated self-crossed progeny with overlapping or outward curling head morphotypes. Sequencing-based bulked segregant analysis (Seq-BSA) revealed a candidate region of 0.52 Mb (A06: 1,824,886~2,347,097 bp) containing genes enriched for plant hormone signal transduction. RNA Sequencing (RNA-Seq) analysis supported the hormone pathway enrichment leading to the identification of two key candidate genes, BrGH3.12 and BrABF1. The regulated homologous genes and the relationship between genes in this pathway were also revealed. Expression of BrGH3.12 varied significantly in the apical portion of the leaf, consistent with the morphological differences between overlapping and outward curling leaves. Transcript levels of BrABF1 in the top, middle and basal segments of the leaf were significantly different between the two types. The two morphotypes contained different concentrations of IAA in the apical portion of their leaves while levels of ABA differed significantly between plant types in the top, middle, and basal leaf segments. Results from Seq-BSA, RNA-Seq and metabolite analyses all support a role for IAA and ABA in heading type formation. These findings increase our understanding of the molecular basis for pattern formation of the leafy head in Chinese cabbage and will contribute to future work developing more desirable leafy head patterns.

The Aquilegia JAGGED homolog promotes proliferation of adaxial cell types in both leaves and stems

In order to explore the functional conservation of JAGGED, a key gene involved in the sculpting of lateral organs in several model species, we identified its ortholog AqJAG in the lower eudicot species Aquilegia coerulea. We analyzed the expression patterns of AqJAG in various tissues and developmental stages, and used RNAi-based methods to generate knockdown phenotypes of AqJAG. AqJAG was strongly expressed in shoot apices, floral meristems, lateral root primordia and all lateral organ primordia. Silencing of AqJAG revealed a wide range of defects in the developing stems, leaves and flowers; strongest phenotypes include severe reduction of leaflet laminae due to a decrease in cell size and number, change of adaxial cell identity, outgrowth of laminar-like tissue on the inflorescence stem, and early arrest of floral meristems and floral organ primordia. Our results indicate that AqJAG plays a critical role in controlling primordia initiation and distal growth of floral organs, and laminar development of leaflets. Most strikingly, we demonstrated that AqJAG disproportionally controls the behavior of cells with adaxial identity in vegetative tissues, providing evidence of how cell proliferation is controlled in an identity-specific manner.

AUXIN RESPONSE FACTOR3 Regulates Compound Leaf Patterning by Directly Repressing PALMATE-LIKE PENTAFOLIATA1 Expression in Medicago truncatula

Diverse leaf forms can be seen in nature. In Medicago truncatula, PALM1 encoding a Cys(2)His(2) transcription factor is a key regulator of compound leaf patterning. PALM1 negatively regulates expression of SGL1, a key regulator of lateral leaflet initiation. However, how PALM1 itself is regulated is not yet known. To answer this question, we used promoter sequence analysis, yeast one-hybrid tests, quantitative transcription activity assays, ChIP-PCR analysis, and phenotypic analyses of overexpression lines and mutant plants. The results show that M. truncatula AUXIN RESPONSE FACTOR3 (MtARF3) functions as a direct transcriptional repressor of PALM1. MtARF3 physically binds to the PALM1 promoter sequence in yeast cells. MtARF3 selectively interacts with specific auxin response elements (AuxREs) in the PALM1 promoter to repress reporter gene expression in tobacco leaves and binds to specific sequences in the PALM1 promoter in vivo. Upregulation of MtARF3 or removal of both PHANTASTICA (PHAN) and ARGONAUTE7 (AGO7) pathways resulted in compound leaves with five narrow leaflets arranged in a palmate-like configuration. These results support that MtARF3, in addition as an adaxial-abaxial polarity regulator, functions to restrict spatiotemporal expression of PALM1, linking auxin signaling to compound leaf patterning in the legume plant M. truncatula.

Breaking the mold: understanding the evolution and development of lateral organs in diverse plant models

The formation of complex three-dimensional shape differs significantly between plants and animals due to the presence of the cell wall in the former, which prevents all cell migration. Instead, in lateral plant organs such as leaves or petals, shape is controlled by a series of developmental phases in which the organ acquires polarity, cells undergo proliferation, and, lastly, cells expand to their final shape and size. Although these processes were first described based on mutagenesis approaches in major model systems like Arabidopsis thaliana, further insight into their complexity is best provided by studies of natural variation in organ shape in alternative model systems that sample a broader range of plant form. Weaving together work from both forward and evolutionary genetics, this review focuses on how modification in polarity establishment, cell proliferation and cell expansion drives modifications in the fundamental lateral organ developmental program to create diversity in shape.

Morphological analysis of the 6b oncogene-induced enation syndrome

MAIN CONCLUSION

The T-DNA 6b oncogene induces complex and partly unprecedented phenotypic changes in tobacco stems and leaves, which result from hypertrophy and hyperplasia with ectopic spot-like, ridge-like and sheet-like meristems. The Agrobacterium T-DNA oncogene 6b causes complex growth changes in tobacco including enations; this unusual phenotype has been called "6b enation syndrome". A detailed morphological and anatomical analysis of the aerial part of Nicotiana tabacum plants transformed with a dexamethasone-inducible dex-T-6b gene revealed several striking growth phenomena. Among these were: uniform growth of ectopic photosynthetic cells on the abaxial leaf side, gutter-like petioles with multiple parallel secondary veins, ectopic leaf primordia emerging behind large glandular trichomes, corniculate structures emerging from distal ends of secondary veins, pin-like structures with remarkable branching patterns, ectopic vascular strands in midveins and petioles extending down along the stem, epiascidia and hypoascidia, double enations and complete inhibition of leaf outgrowth. Ectopic stipule-like leaves and inverted leaves were found at the base of the petioles. Epinastic and hyponastic growth of petioles and midveins yielded complex but predictable leaf folding patterns. Detailed anatomical analysis of over sixty different 6b-induced morphological changes showed that the different modifications are derived from hypertrophy and abaxial hyperplasia, with ectopic photosynthetic cells forming spot-like, ridge-like and sheet-like meristems and ectopic vascular strands forming regular patterns in midveins, petioles and stems. Part of the enation syndrome is due to an unknown phloem-mobile enation factor. Graft experiments showed that the 6b mRNA is mobile and could be the enation factor. Our work provides a better insight in the basic effects of the 6b oncogene.

Role of transcriptional regulation in the evolution of plant phenotype: A dynamic systems approach

A growing body of evidence suggests that alterations in transcriptional regulation of genes involved in modulating development are an important part of phenotypic evolution, and this can be documented among species and within populations. While the effects of differential transcriptional regulation in organismal development have been preferentially studied in animal systems, this phenomenon has also been addressed in plants. In this review, we summarize evidence for cis-regulatory mutations, trans-regulatory changes and epigenetic modifications as molecular events underlying important phenotypic alterations, and thus shaping the evolution of plant development. We postulate that a mechanistic understanding of why such molecular alterations have a key role in development, morphology and evolution will have to rely on dynamic models of complex regulatory networks that consider the concerted action of genetic and nongenetic components, and that also incorporate the restrictions underlying the genotype to phenotype mapping process. Developmental Dynamics 244:1074-1095, 2015. © 2015 Wiley Periodicals, Inc.

Oriented cell division shapes carnivorous pitcher leaves of Sarracenia purpurea

Complex morphology is an evolutionary outcome of phenotypic diversification. In some carnivorous plants, the ancestral planar leaf has been modified to form a pitcher shape. However, how leaf development was altered during evolution remains unknown. Here we show that the pitcher leaves of Sarracenia purpurea develop through cell division patterns of adaxial tissues that are distinct from those in bifacial and peltate leaves, subsequent to standard expression of adaxial and abaxial marker genes. Differences in the orientation of cell divisions in the adaxial domain cause bifacial growth in the distal region and adaxial ridge protrusion in the middle region. These different growth patterns establish pitcher morphology. A computer simulation suggests that the cell division plane is critical for the pitcher morphogenesis. Our results imply that tissue-specific changes in the orientation of cell division underlie the development of a morphologically complex leaf.

The pitcher-shaped leaf of the carnivorous plant Sarracenia purpurea acts as a pitfall trap to capture small animals. Here, Fukushima et al. analyse pitcher leaf development and propose that this unusual shape evolved from ancestral planar leaves through changes in the orientation of cell division.

Analysis of global gene expression profiles to identify differentially expressed genes critical for embryo development in Brassica rapa

Embryo development represents a crucial developmental period in the life cycle of flowering plants. To gain insights into the genetic programs that control embryo development in Brassica rapa L., RNA sequencing technology was used to perform transcriptome profiling analysis of B. rapa developing embryos. The results generated 42,906,229 sequence reads aligned with 32,941 genes. In total, 27,760, 28,871, 28,384, and 25,653 genes were identified from embryos at globular, heart, early cotyledon, and mature developmental stages, respectively, and analysis between stages revealed a subset of stage-specific genes. We next investigated 9,884 differentially expressed genes with more than fivefold changes in expression and false discovery rate ≤ 0.001 from three adjacent-stage comparisons; 1,514, 3,831, and 6,633 genes were detected between globular and heart stage embryo libraries, heart stage and early cotyledon stage, and early cotyledon and mature stage, respectively. Large numbers of genes related to cellular process, metabolism process, response to stimulus, and biological process were expressed during the early and middle stages of embryo development. Fatty acid biosynthesis, biosynthesis of secondary metabolites, and photosynthesis-related genes were expressed predominantly in embryos at the middle stage. Genes for lipid metabolism and storage proteins were highly expressed in the middle and late stages of embryo development. We also identified 911 transcription factor genes that show differential expression across embryo developmental stages. These results increase our understanding of the complex molecular and cellular events during embryo development in B. rapa and provide a foundation for future studies on other oilseed crops.