Leaf shape diversity with an emphasis on leaf contour variation, developmental background, and adaptation

Formation mechanism and regulation analysis of trumpet leaf in Ginkgo biloba L

Introduction

The research on plant leaf morphology is of great significance for understanding the development and evolution of plant organ morphology. As a relict plant, the G. biloba leaf morphology typically exhibits bifoliate and peltate forms. However, throughout its long evolutionary history, Ginkgo leaves have undergone diverse changes.

Methods

This study focuses on the distinct "trumpet" leaves and normal fan-shaped leaves of G. biloba for analysis of their phenotypes, photosynthetic activity, anatomical observations, as well as transcriptomic and metabolomic analyses.

Results

The results showed that trumpet-shaped G. biloba leaves have fewer cells, significant morphological differences between dorsal and abaxial epidermal cells, leading to a significantly lower net photosynthetic rate. Additionally, this study found that endogenous plant hormones such as GA, auxin, and JA as well as metabolites such as flavonoids and phenolic acids play roles in the formation of trumpet-shaped G. biloba leaves. Moreover, the experiments revealed the regulatory mechanisms of various key biological processes and gene expressions in the trumpet-shaped leaves of G. biloba.

Discussion

Differences in the dorsal and abdominal cells of G. biloba leaves can cause the leaf to curl, thus reducing the overall photosynthetic efficiency of the leaves. However, the morphology of plant leaves is determined during the primordia leaf stage. In the early stages of leaf development, the shoot apical meristem (SAM) determines the developmental morphology of dicotyledonous plant leaves. This process involves the activity of multiple gene families and small RNAs. The establishment of leaf morphology is complexly regulated by various endogenous hormones, including the effect of auxin on cell walls. Additionally, changes in intracellular ion concentrations, such as fluctuations in Ca2+ concentration, also affect cell wall rigidity, thereby influencing leaf growth morphology.

Diving into the Water: Amphibious Plants as a Model for Investigating Plant Adaptations to Aquatic Environments

Amphibious plants can grow and survive in both aquatic and terrestrial environments. This review explores the diverse adaptations that enable them to thrive in such contrasting habitats. Plants with amphibious lifestyles possess fascinating traits, and their phenotypic plasticity plays an important role in adaptations. Heterophylly, the ability to produce different leaf forms, is one such trait, with submerged leaves generally being longer, narrower, and thinner than aerial leaves. In addition to drastic changes in leaf contours, amphibious plants display significant anatomical and physiological changes, including a reduction in stomatal number and cuticle thickness and changes in photosynthesis mode. This review summarizes and compares the regulatory mechanisms and evolutionary origins of amphibious plants based on molecular biology studies actively conducted in recent years using novel model amphibious plant species. Studying amphibious plants will enhance our understanding of plant adaptations to aquatic environments.

Leaf form diversity and evolution: a never-ending story in plant biology

Leaf form can vary at different levels, such as inter/intraspecies, and diverse leaf shapes reflect their remarkable ability to adapt to various environmental conditions. Over the past two decades, considerable progress has been made in unraveling the molecular mechanisms underlying leaf form diversity, particularly the regulatory mechanisms of leaf complexity. However, the mechanisms identified thus far are only part of the entire process, and numerous questions remain unanswered. This review aims to provide an overview of the current understanding of the molecular mechanisms driving leaf form diversity while highlighting the existing gaps in our knowledge. By focusing on the unanswered questions, this review aims to shed light on areas that require further research, ultimately fostering a more comprehensive understanding of leaf form diversity.

Cooperation of an external carbonic anhydrase and HCO3- transporter supports underwater photosynthesis in submerged leaves of the amphibious plant Hygrophila difformis

BACKGROUND AND AIMS

HCO3- can be a major carbon resource for photosynthesis in underwater environments. Here we investigate the underlying mechanism of uptake and membrane transport of HCO3- in submerged leaves of Hygrophila difformis, a heterophyllous amphibious plant. To characterize these mechanisms, we evaluated the sensitivity of underwater photosynthesis to an external carbonic anhydrase (CA) inhibitor and an anion exchanger protein inhibitor, and we attempted to identify components of the mechanism of HCO3- utilization.

METHODS

We evaluated the effects of the external CA inhibitor and anion exchanger protein inhibitor on the NaHCO3 response of photosynthetic O2 evolution in submerged leaves of H. difformis. Furthermore, we performed a comparative transcriptomic analysis between terrestrial and submerged leaves.

KEY RESULTS

Photosynthesis in the submerged leaves was decreased by both the external CA inhibitor and anion exchanger protein inhibitor, but no additive effect was observed. Among upregulated genes in submerged leaves, two α-CAs, Hdα-CA1 and Hdα-CA2, and one β-carbonic anhydrase, Hdβ-CA1, were detected. Based on their putative amino acid sequences, the α-CAs are predicted to be localized in the apoplastic region. Recombinant Hdα-CA1 and Hdβ-CA1 showed dominant CO2 hydration activity over HCO3- dehydration activity.

CONCLUSIONS

We propose that the use of HCO3- for photosynthesis in submerged leaves of H. difformis is driven by the cooperation between an external CA, Hdα-CA1, and an unidentified HCO3- transporter.

Study on the Interactions of Cyclins with CDKs Involved in Auxin Signal during Leaf Development by WGCNA in Populus alba

Cell division plays an indispensable role in leaf morphogenesis, which is regulated via the complexes formed by cyclin and cyclin-dependent kinase (CDK). In this study, gene family analysis, exogenous auxin stimulation, RNA-seq and WGCNA analysis were all used to investigate the molecular mechanisms by which cell-cycle-related factors participated in the auxin signaling pathway on leaf morphogenesis. Sixty-three cyclin members and seventeen CDK members in Populus alba were identified and systematically analyzed. During the evolution, WGD was the main reason that resulted in the expansion of cyclin and CDK genes. Firstly, after a short time treating with auxin to matured leaves of seedlings, genes related to cell division including GRF and ARGOS were both upregulated to restart the transition of cells from G1-to-S phase. Secondly, with three days of continuous auxin stimulation to leaves at different developmental stages, leaves area variation, transcriptomes and hormones were analyzed. By PCA, PCoA and WGCNA analyses, the turquoise module was both positively related to leaf development and auxin. Based on the co-expression analysis and Y2H experiment, PoalbCYCD1;4, PoalbCYCD3;3 and PoalbCYCD3;5 were supposed to interact with PoalbCDKA;1, which could be the trigger to promote the G1-to-S phase transition. The ARF transcription factor might play the key role of connecting the auxin signaling pathway and cell division in leaf morphogenesis by affecting CYC-CDK complexes.

Geographical variation in functional traits of leaves of Caryopteris mongholica and the role of climate

BACKGROUND

Quantifying intra-specific variation in leaf functional traits along environmental gradients is important for understanding species' responses to climate change. In this study, we assessed the degree of among and within populations variation in leaf functional traits and explored leaf response to geographic and climate change using Caryopteris mongholica as material, which has a wide range of distribution environments.

RESULTS

We selected 40 natural populations of C. mongholica, measured 8 leaf functional traits, analyzed the extent of trait variation among and within populations, and developed geographic and climatic models to explain trait variation between populations. Our results showed that the variation in leaf functional traits of C. mongholica was primarily lower within populations compared to among populations. Specifically, the leaf area (LA) exhibited higher variability both among and within populations, whereas leaf carbon content (LC) exhibited lower variation within populations but greater variation among populations. We observed a specific covariation pattern among traits and a strong linkage between morphological, economic, and mechanical traits. Increasing minimum temperature, precipitation of month, and seasonal precipitation differences all limited the growth and development of C. mongholica. However, it was observed that an increase in mean annual precipitation positively influenced the morphological development of its leaf.

CONCLUSIONS

These results demonstrate the response of intra-specific trait variation to the environment and provide valuable insights into the adaptation of intra-specific leaf functional traits under changing climatic conditions.

Diversity of tomato leaf form provides novel insights into breeding

Tomato (Solanum lycopersicum L.) is cultivated widely globally. The crop exhibits tremendous morphological variations because of its long breeding history. Apart from the commercial tomato varieties, wild species and heirlooms are grown in certain regions of the world. Since the fruit constitutes the edible part, much of the agronomical research is focused on it. However, recent studies have indicated that leaf morphology influences fruit quality. As leaves are specialized photosynthetic organs and the vascular systems transport the photosynthetic products to sink organs, the architectural characteristics of the leaves have a strong influence on the final fruit quality. Therefore, comprehensive research focusing on both the fruit and leaf morphology is required for further tomato breeding. This review summarizes an overview of knowledge of the basic tomato leaf development, morphological diversification, and molecular mechanisms behind them and emphasizes its importance in breeding. Finally, we discuss how these findings and knowledge can be applied to future tomato breeding.

Leaf-size control beyond transcription factors: Compensatory mechanisms

Plant leaves display abundant morphological richness yet grow to characteristic sizes and shapes. Beginning with a small number of undifferentiated founder cells, leaves evolve via a complex interplay of regulatory factors that ultimately influence cell proliferation and subsequent post-mitotic cell enlargement. During their development, a sequence of key events that shape leaves is both robustly executed spatiotemporally following a genomic molecular network and flexibly tuned by a variety of environmental stimuli. Decades of work on Arabidopsis thaliana have revisited the compensatory phenomena that might reflect a general and primary size-regulatory mechanism in leaves. This review focuses on key molecular and cellular events behind the organ-wide scale regulation of compensatory mechanisms. Lastly, emerging novel mechanisms of metabolic and hormonal regulation are discussed, based on recent advances in the field that have provided insights into, among other phenomena, leaf-size regulation.

Position of meristems and the angles of the cell division plane regulate the uniqueness of lateral organ shape

Leaf meristem is a cell proliferative zone present in the lateral organ primordia. In this study, we examined how cell proliferative zones in primordia of planar floral organs and polar auxin transport inhibitor (PATI)-treated leaf organs differ from those of non-treated foliage leaves of Arabidopsis thaliana, with a focus on the accumulation pattern of ANGUSTIFOLIA3 (AN3) protein, a key element for leaf meristem positioning. We found that PATI-induced leaf shape changes were correlated with cell division angle but not with meristem positioning/size or AN3 localisation. In contrast, different shapes between sepals and petals compared with foliage leaves were associated with both altered meristem position, due to altered AN3 expression patterns, and different distributions of cell division angles. A numerical simulation showed that meristem position majorly affected the final shape but biased cell division angles had a minor effect. Taken together, these results suggest that the unique shapes of different lateral organs depend on the position of the meristem in the case of floral organs and cell division angles in the case of leaf organs with different auxin flow.

The upregulated LsKN1 gene transforms pinnately to palmately lobed leaves through auxin, gibberellin, and leaf dorsiventrality pathways in lettuce

Leaf shape represents a vital agronomic trait for leafy vegetables such as lettuce. Some lettuce cultivars produce lobed leaves, varying from pinnately to palmately lobed, but the genetic mechanisms remain unclear. In this study, we cloned one major quantitative trait locus (QTL) controlling palmately lobed leaves. The candidate gene, LsKN1, encodes a homeobox transcription factor, and has been shown previously to be critical for the development of leafy heads in lettuce. The LsKN1 allele that is upregulated by the insertion of a transposon promotes the development of palmately lobed leaves. We demonstrated that LsKN1 upregulated LsCUC2 and LsCUC3 through different mechanisms, and their upregulation was critical for the development of palmately lobed leaves. LsKN1 binds the promoter of LsPID to promote auxin biosynthesis, which positively contributes to the development of palmately lobed leaves. In contrast, LsKN1 suppresses GA biosynthesis to promote palmately lobed leaves. LsKN1 also binds to the promoter of LsAS1, a dorsiventrality gene, to downregulate its expression. Overexpression of the LsAS1 gene compromised the effects of the LsKN1 gene changing palmately to pinnately lobed leaves. Our study illustrated that the upregulated LsKN1 gene led to palmately lobed leaves in lettuce by integrating several downstream pathways, including auxin, gibberellin, and leaf dorsiventrality pathways.

Tomato SlBES1.8 Influences Leaf Morphogenesis by Mediating Gibberellin Metabolism and Signaling

Leaf morphogenetic activity determines its shape diversity. However, our knowledge of the regulatory mechanism in maintaining leaf morphogenetic capacity is still limited. In tomato, gibberellin (GA) negatively regulates leaf complexity by shortening the morphogenetic window. We here report a tomato BRI1-EMS-suppressor 1 transcription factor, SlBES1.8, that promoted the simplification of leaf pattern in a similar manner as GA functions. OE-SlBES1.8 plants exhibited reduced sensibility to exogenous GA3 treatment whereas showed increased sensibility to the application of GA biosynthesis inhibitor, paclobutrazol. In line with the phenotypic observation, the endogenous bioactive GA contents were increased in OE-SlBES1.8 lines, which certainly promoted the degradation of the GA signaling negative regulator, SlDELLA. Moreover, transcriptomic analysis uncovered a set of overlapping genomic targets of SlBES1.8 and GA, and most of them were regulated in the same way. Expression studies showed the repression of SlBES1.8 to the transcriptions of two GA-deactivated genes, SlGA2ox2 and SlGA2ox6, and one GA receptor, SlGID1b-1. Further experiments confirmed the direct regulation of SlBES1.8 to their promoters. On the other hand, SlDELLA physically interacted with SlBES1.8 and further inhibited its transcriptional regulation activity by abolishing SlBES1.8-DNA binding. Conclusively, by mediating GA deactivation and signaling, SlBES1.8 greatly influenced tomato leaf morphogenesis.

Looking beyond the gene network - metabolic and mechanical cell drivers of leaf morphogenesis

The above-ground organs in plants display a rich diversity, yet they grow to characteristic sizes and shapes. Organ morphogenesis progresses through a sequence of key events, which are robustly executed spatiotemporally as an emerging property of intrinsic molecular networks while adapting to various environmental cues. This Review focuses on the multiscale control of leaf morphogenesis. Beyond the list of known genetic determinants underlying leaf growth and shape, we focus instead on the emerging novel mechanisms of metabolic and biomechanical regulations that coordinate plant cell growth non-cell-autonomously. This reveals how metabolism and mechanics are not solely passive outcomes of genetic regulation but play instructive roles in leaf morphogenesis. Such an integrative view also extends to fluctuating environmental cues and evolutionary adaptation. This synthesis calls for a more balanced view on morphogenesis, where shapes are considered from the standpoints of geometry, genetics, energy and mechanics, and as emerging properties of the cellular expression of these different properties.

PdeHCA2 affects biomass in Populus by regulating plant architecture, the transition from primary to secondary growth, and photosynthesis

PdeHCA2 regulates the transition from primary to secondary growth, plant architecture, and affects photosynthesis by targeting PdeBRC1 and controlling the anatomy of the mesophyll, and intercellular space, respectively. Branching, secondary growth, and photosynthesis are vital developmental processes of woody plants that determine plant architecture and timber yield. However, the mechanisms underlying these processes are unknown. Here, we report that the Populus transcription factor High Cambium Activity 2 (PdeHCA2) plays a role in the transition from primary to secondary growth, vascular development, and branching. In Populus, PdeHCA2 is expressed in undifferentiated provascular cells during primary growth, in phloem cells during secondary growth, and in leaf veins, which is different from the expression pattern of its homolog in Arabidopsis. Overexpression of PdeHCA2 has pleiotropic effects on shoot and leaf development; overexpression lines showed delayed growth of shoots and leaves, reduced photosynthesis, and abnormal shoot branching. In addition, auxin-, cytokinin-, and photosynthesis-related genes were differentially regulated in these lines. Electrophoretic mobility shift assays and transcriptome analysis indicated that PdeHCA2 directly up-regulates the expression of BRANCHED1 and the MADS-box gene PdeAGL9, which regulate plant architecture, by binding to cis-elements in the promoters of these genes. Taken together, our findings suggest that HCA2 regulates several processes in woody plants including vascular development, photosynthesis, and branching by affecting the proliferation and differentiation of parenchyma cells.

Population Variability of Almond-Leaved Willow (Salix triandra L.) Based on the Leaf Morphometry: Isolation by Distance and Environment Explain Phenotypic Diversity

Almond-leaved willow (Salix triandra L., Salicaceae) is a dioecious shrub, rarely a small tree that grows under various environmental conditions. We examined the population structure of 12 populations of almond-leaved willow using nine leaf morphological traits and specific leaf area. Populations were selected from a range of habitats, from continental to the sub-Mediterranean zone, to examine the influence of environmental conditions (climate and altitude) and geographic distance on leaf variability. Significant differences were confirmed among all populations for all traits, with significant correlations between geographic location of populations and morphological traits, and between environmental conditions and morphological traits. Large-leaved populations were found in continental and sub-Mediterranean climates, while small-leaved populations were found in higher elevations and smaller karstic rivers. In addition, populations from floodplains showed greater variability than populations from the karstic habitats, indicating a positive influence of lowland habitats and possible underlying differences in gene pool size. In conclusion, we found that environmental conditions and geographical distances in addition to genetic drift, are the main influences on the variability in almond-leaved willow, with the species showing a high level of plasticity and adaptation to local environmental conditions.

The Potential Roles of Unique Leaf Structure for the Adaptation of Rheum tanguticum Maxim. ex Balf. in Qinghai-Tibetan Plateau

Leaves are essential plant organs with numerous variations in shape and size. The leaf size is generally smaller in plants that thrive in areas of higher elevation and lower annual mean temperature. The Qinghai-Tibetan Plateau is situated at an altitude of >4000 m with relatively low annual average temperatures. Most plant species found on the Qinghai-Tibetan Plateau have small leaves, with Rheum tanguticum Maxim. ex Balf. being an exception. Here, we show that the large leaves of R. tanguticum with a unique three-dimensional (3D) shape are potentially an ideal solution for thermoregulation with little energy consumption. With the increase in age, the shape of R. tanguticum leaves changed from a small oval plane to a large palmatipartite 3D shape. Therefore, R. tanguticum is a highly heteroblastic species. The leaf shape change during the transition from the juvenile to the adult phase of the development in R. tanguticum is a striking example of the manifestation of plant phenotypic plasticity. The temperature variation in different parts of the leaf was a distinct character of leaves of over-5-year-old plants. The temperature of single-plane leaves under strong solar radiation could accumulate heat rapidly and resulted in temperatures much higher than the ambient temperature. However, leaves of over-5-year-old plants could lower leaf temperature by avoiding direct exposure to solar radiation and promoting local airflow to prevent serious tissue damage by sunburn. Furthermore, the net photosynthesis rate was correlated with the heterogeneity of the leaf surface temperature. Our results demonstrate that the robust 3D shape of the leaf is a strategy that R. tanguticum has developed evolutionarily to adapt to the strong solar radiation and low temperature on the Qinghai-Tibetan Plateau.

Gigantic floating leaves occupy a large surface area at an economical material cost

The giant Amazonian waterlily (genus Victoria) produces the largest floating leaves in the plant kingdom. The leaves' notable vasculature has inspired artists, engineers, and architects for centuries. Despite the aesthetic appeal and scale of this botanical enigma, little is known about the mechanics of these extraordinary leaves. For example, how do these leaves achieve gigantic proportions? We show that the geometric form of the leaf is structurally more efficient than those of other smaller species of waterlily. In particular, the spatially varying thickness and regular branching of the primary veins ensures the structural integrity necessary for extensive coverage of the water surface, enabling optimal light capture despite a relatively low leaf biomass. Leaf gigantism in waterlilies may have been driven by selection pressures favoring a large surface area at an economical material cost, for outcompeting other plants in fast-drying ephemeral pools.

Phenotypic Variation in European Wild Pear (Pyrus pyraster (L.) Burgsd.) Populations in the North-Western Part of the Balkan Peninsula

Leaves play a central role in plant fitness, allowing efficient light capture, gas exchange and thermoregulation, ensuring optimal growing conditions for the plant. Phenotypic variability in leaf shape and size has been linked to environmental heterogeneity and habitat characteristics. Therefore, the study of foliar morphology in plant populations can help us to identify the environmental factors that may have influenced the process of species diversification. In this study, we used European wild pear (Pyrus pyraster (L.) Burgsd., Rosaceae) as a model species to investigate the phenotypic variability of leaves under different environmental conditions. Using leaf morphometric data from 19 natural populations from the north-western part of the Balkan Peninsula, a high level of variability among and within populations were found. Leaf traits related to leaf size were more variable compared to leaf shape traits, with both influenced by geographic and environmental factors. Consequently, patterns of isolation by environment (IBE) and distance (IBD) were identified, with IBE showing a stronger influence on leaf variability. Multivariate statistical analysis revealed that European wild pear populations from the north-western part of the Balkan Peninsula can be divided into two morphological clusters, consistent with their geographical distance and environmental conditions. Our results confirm a high level of phenotypic variability in European wild pear populations, providing additional data on this poorly studied species, emphasizing phenotypic plasticity as a major driver in the adaptation of this noble hardwood species to rapid climate change.

Identification of the unique molecular framework of heterophylly in the amphibious plant Callitriche palustris L

Heterophylly is the development of different leaf forms in a single plant depending on the environmental conditions. It is often observed in amphibious aquatic plants that can grow under both aerial and submerged conditions. Although heterophylly is well recognized in aquatic plants, the associated developmental mechanisms and the molecular basis remain unclear. To clarify these underlying developmental and molecular mechanisms, we analyzed heterophyllous leaf formation in an aquatic plant, Callitriche palustris. Morphological analyses revealed extensive cell elongation and the rearrangement of cortical microtubules in the elongated submerged leaves of C. palustris. Our observations also suggested that gibberellin, ethylene, and abscisic acid all regulate the formation of submerged leaves. However, the perturbation of one or more of the hormones was insufficient to induce the formation of submerged leaves under aerial conditions. Finally, we analyzed gene expression changes during aerial and submerged leaf development and narrowed down the candidate genes controlling heterophylly via transcriptomic comparisons, including a comparison with a closely related terrestrial species. We discovered that the molecular mechanism regulating heterophylly in C. palustris is associated with hormonal changes and diverse transcription factor gene expression profiles, suggesting differences from the corresponding mechanisms in previously investigated amphibious plants.

The leaf meristem enigma: The relationship between the plate meristem and the marginal meristem

Leaf organogenesis is governed by the spatiotemporal activity of the leaf meristem, which has far greater mitotic activity than the shoot apical meristem. The two types of leaf meristems, the plate meristem and the marginal meristem, are distinguished by the location and longevity of their cell proliferative activity. Most leaf lamina outgrowth depends on the plate meristem. The presence of the marginal meristem was a matter of debate in classic anatomy, but recent genetic analyses of leaf growth in Arabidopsis thaliana confirmed its short-lived activity. Several genes key for the regulation of the two meristem types have been identified, and at least superficially, the systems appear to function independently, as they are regulated by different transcription factors and microRNAs. However, many of the details of these regulatory systems, including how the expression of these key factors is spatially regulated, remain unclear. One major unsolved question is the relationship between the plate meristem and the marginal meristem. Here, I present an overview of our current understanding of this topic and discuss questions that remain to be answered.

Age-Dependent Abiotic Stress Resilience in Plants

Developmental age is a strong determinant of stress responses in plants. Differential susceptibility to various environmental stresses is widely observed at both the organ and whole-plant level. While it is clear that age determines stress susceptibility, the causes, regulatory mechanisms, and functions are only now beginning to emerge. Compared with concepts on age-related biotic stress resilience, advancements in the abiotic stress field are relatively limited. In this review, we focus on current knowledge of ontogenic resistance to abiotic stresses, highlighting examples at the organ (leaf) and plant level, preceded by an overview of the relevant concepts in plant aging. We also discuss age-related abiotic stress resilience mechanisms, speculate on their functional relevance, and outline outstanding questions.

The diverse roles of cytokinins in regulating leaf development

Leaves provide energy for plants, and consequently for animals, through photosynthesis. Despite their important functions, plant leaf developmental processes and their underlying mechanisms have not been well characterized. Here, we provide a holistic description of leaf developmental processes that is centered on cytokinins and their signaling functions. Cytokinins maintain the growth potential (pluripotency) of shoot apical meristems, which provide stem cells for the generation of leaf primordia during the initial stage of leaf formation; cytokinins and auxins, as well as their interaction, determine the phyllotaxis pattern. The activities of cytokinins in various regions of the leaf, especially at the margins, collectively determine the final leaf morphology (e.g., simple or compound). The area of a leaf is generally determined by the number and size of the cells in the leaf. Cytokinins promote cell division and increase cell expansion during the proliferation and expansion stages of leaf cell development, respectively. During leaf senescence, cytokinins reduce sugar accumulation, increase chlorophyll synthesis, and prolong the leaf photosynthetic period. We also briefly describe the roles of other hormones, including auxin and ethylene, during the whole leaf developmental process. In this study, we review the regulatory roles of cytokinins in various leaf developmental stages, with a focus on cytokinin metabolism and signal transduction processes, in order to shed light on the molecular mechanisms underlying leaf development.

GhARF16-1 modulates leaf development by transcriptionally regulating the GhKNOX2-1 gene in cotton

The leaf is a crucial organ evolved with remarkable morphological diversity to maximize plant photosynthesis. The leaf shape is a key trait that affects photosynthesis, flowering rates, disease resistance and yield. Although many genes regulating leaf development have been identified in the past years, the precise regulatory architecture underlying the generation of diverse leaf shapes remains to be elucidated. We used cotton as a reference model to probe the genetic framework underlying divergent leaf forms. Comparative transcriptome analysis revealed that the GhARF16-1 and GhKNOX2-1 genes might be potential regulators of leaf shape. We functionally characterized the auxin-responsive factor ARF16-1 acting upstream of GhKNOX2-1 to determine leaf morphology in cotton. The transcription of GhARF16-1 was significantly higher in lobed-leaved cotton than in smooth-leaved cotton. Furthermore, the overexpression of GhARF16-1 led to the up-regulation of GhKNOX2-1 and resulted in more and deeper serrations in cotton leaves, similar to the leaf shape of cotton plants overexpressing GhKNOX2-1. We found that GhARF16-1 specifically bound to the promoter of GhKNOX2-1 to induce its expression. The heterologous expression of GhARF16-1 and GhKNOX2-1 in Arabidopsis led to lobed and curly leaves, and a genetic analysis revealed that GhKNOX2-1 is epistatic to GhARF16-1 in Arabidopsis, suggesting that the GhARF16-1 and GhKNOX2-1 interaction paradigm also functions to regulate leaf shape in Arabidopsis. To our knowledge, our results uncover a novel mechanism by which auxin, through the key component ARF16-1 and its downstream-activated gene KNOX2-1, determines leaf morphology in eudicots.

Tissue growth constrains root organ outlines into an isometrically scalable shape

Organ morphologies are diverse but also conserved under shared developmental constraints among species. Any geometrical similarities in the shape behind diversity and the underlying developmental constraints remain unclear. Plant root tip outlines commonly exhibit a dome shape, which likely performs physiological functions, despite the diversity in size and cellular organization among distinct root classes and/or species. We carried out morphometric analysis of the primary roots of ten angiosperm species and of the lateral roots (LRs) of Arabidopsis, and found that each root outline was isometrically scaled onto a parameter-free catenary curve, a stable structure adopted for arch bridges. Using the physical model for bridges, we analogized that localized and spatially uniform occurrence of oriented cell division and expansion force the LR primordia (LRP) tip to form a catenary curve. These growth rules for the catenary curve were verified by tissue growth simulation of developing LRP development based on time-lapse imaging. Consistently, LRP outlines of mutants compromised in these rules were found to deviate from catenary curves. Our analyses demonstrate that physics-inspired growth rules constrain plant root tips to form isometrically scalable catenary curves.

Highlighted Article: The dome-shaped outlines of plant root tips converge to a parameter-free catenary curve seen in arch bridges, owing to a constraint from anisotropic and localized tissue growth.

Mechanisms of the Morphological Plasticity Induced by Phytohormones and the Environment in Plants

Plants adapt to environmental changes by regulating their development and growth. As an important interface between plants and their environment, leaf morphogenesis varies between species, populations, or even shows plasticity within individuals. Leaf growth is dependent on many environmental factors, such as light, temperature, and submergence. Phytohormones play key functions in leaf development and can act as molecular regulatory elements in response to environmental signals. In this review, we discuss the current knowledge on the effects of different environmental factors and phytohormone pathways on morphological plasticity and intend to summarize the advances in leaf development. In addition, we detail the molecular mechanisms of heterophylly, the representative of leaf plasticity, providing novel insights into phytohormones and the environmental adaptation in plants.

Leaflet initiation and blade expansion are separable in compound leaf development

Compound leaves are composed of multiple separate blade units termed leaflets. In tomato (Solanum lycopersicum) compound leaves, auxin promotes both leaflet initiation and blade expansion. However, it is unclear how these two developmental processes interact. With highly variable complexity, tomato compound leaves provide an ideal system to address this question. In this study, we obtained and analyzed mutants of the WUSCHEL-RELATED HOMEOBOX (WOX) family gene SlLAM1 from tomato, whose orthologs in tobacco (Nicotiana sylvestris) and other species are indispensable for blade expansion. We show that SlLAM1 is expressed in the middle and marginal domains of leaves, and is required for blade expansion in leaflets. We demonstrate that sllam1 mutants cause a delay of leaflet initiation and slightly alter the arrangement of first-order leaflets, whereas the overall leaflet number is comparable to that of wild-type leaves. Analysis of the genetic interactions between SlLAM1 and key auxin signaling components revealed an epistatic effect of SlLAM1 in determining the final leaf form. Finally, we show that SlLAM1 is also required for floral organ growth and affects the fertility of gametophytes. Our data suggest that SlLAM1 promotes blade expansion in multiple leaf types, and leaflet initiation can be largely uncoupled from blade expansion during compound leaf morphogenesis.

Dimorphic Leaf Development of the Aquatic Plant Callitriche palustris L. Through Differential Cell Division and Expansion

Heterophylly, or phenotypic plasticity in leaf form, is a remarkable feature of amphibious plants. When the shoots of these plants grow underwater, they often develop surprisingly different leaves from those that emerge in air. Among aquatic plants, it is typical for two or more distinct leaf development processes to be observed in the same individual exposed to different environments. Here, we analyze the developmental processes of heterophylly in the amphibious plant Callitriche palustris L. (Plantaginaceae). First, we reliably cultured this species under laboratory conditions and established a laboratory strain. We also established a framework for molecular-based developmental analyses, such as whole-mount in situ hybridization. We observed several developmental features of aerial and submerged leaves, including changes in form, stomata and vein formation, and transition of the meristematic zone. Then we defined developmental stages for C. palustris leaves. We found that in early stages, aerial and submerged leaf primordia had similar forms, but became discriminable through cell divisions with differential direction, and later became highly distinct via extensive cell elongation in submerged leaf primordia.

Morphological characterization of domatium development in Callicarpa saccata

BACKGROUND AND AIMS

Domatia are plant structures within which organisms reside. Callicarpa saccata (Lamiaceae) is the sole myrmecophyte, or 'ant plant', that develops foliar (leaf-borne) myrmeco-domatia in this genus. In this work we examined domatium development in C. saccata to understand the developmental processes behind pouch-like domatia.

METHODS

Scanning electron microscopy, sectioning and microcomputed tomography were carried out to compare the leaves of C. saccata with those of the closely related but domatia-less myrmecophyte Callicarpa subaequalis, both under cultivation without ants.

KEY RESULTS

Callicarpa saccata domatia are formed as a result of excess cell proliferation at the blade/petiole junctions of leaf primordia. Blade/petiole junctions are important meristematic sites in simple leaf organogenesis. We also found that the mesophyll tissue of domatia does not clearly differentiate into palisade and spongy layers.

CONCLUSIONS

Rather than curling of the leaf margins, a perturbation of the normal functioning of the blade/petiole junction results in the formation of domatium tissue. Excess cell proliferation warps the shape of the blade and disturbs the development of the proximal-distal axis. This process leads to the generation of distinct structures that facilitate interaction between C. saccata and ants.

Ectopic expression of the transcription factor CUC2 restricts growth by cell cycle inhibition in Arabidopsis leaves

Plant leaf margins produce small outgrowths or teeth causing serration in a regular arrangement, which is specified by auxin maxima. In Arabidopsis, the spatiotemporal pattern of auxin dependents on both, the transcription factor CUC2 and the signal peptide EPFL2, a ligand of the growth-promoting receptor kinase ERECTA (ER). Ectopic expression of CUC2 can have contrary effects on leaf growth. Ubiquitous expressed CUC2 suppresses growth in the whole leaf, whereas cuc2-1D mutants have enlarged leaves, through ER-dependent cell proliferation in the teeth. Here we investigated the growth dynamics of cuc2-1D leaves and the growth restricting the function of CUC2 using the ubiquitous inducible CUC2-GR transgene. In time courses, we dissected the serration promoting the function of CUC2 in the leaf margin and ectopic growth inhibition by CUC2 in the leaf plate. We found that CUC2 limits growth rather by cell cycle inhibition than by cell size control. Furthermore, endogenous CUC2 was rapidly induced by CUC2-GR indicating a possible auto-inducible feedback. In contrast, EPFL2 was quickly decreased by transient CUC2 induction but increased in cuc2-3 mutant leaves suggesting that CUC2 can also counteract the EPFL2-ER pathway. Therefore, tooth growth promotion and growth inhibition by CUC2 involve partially the same mechanism but in contrary ways.

Morphogenesis of flattened unifacial leaves in Juncus prismatocarpus (Juncaceae)

To reveal the mode of morphogenesis of flattened unifacial leaves, we analysed the cell division direction and distribution on the leaf blade of Juncus prismatocarpus. Using the pulse-chase 5-ethynyl-2'-deoxyuridine method, we quantified and mapped the cell division direction on the leaf blade of J. prismatocarpus and compared the distribution of thickening cell divisions with the expression pattern of DROOPING LEAF (DL), a key gene involved in leaf blade thickening. Thickening cell divisions were the most abundant (> 45%) among all cell division directions on the leaf blade of J. prismatocarpus from the early plastochron 2 stage through the plastochron 3 stage. Mapping of cell divisions indicated that cell divisions in a particular direction were not restricted to a particular domain but were distributed diffusely throughout the entire cross-sectional area of the leaf blade. Gradient analysis indicated that the distribution of thickening cell divisions of the adaxial domain was denser than that of the abaxial domain. Contrary to the prolonged and diffuse distribution of thickening cell divisions, DL expression was transient and restricted in a narrow band. Our results suggest that a diffuse 'thickening meristem' plays the key role in the development of flattened unifacial leaves.

One-leaf plants in the Gesneriaceae: Natural mutants of the typical shoot system

The aerial part of seed plants is called the shoot, which is composed of stems, leaves, and axial buds. These are produced by indeterminate activity in the shoot apical meristem (SAM), whereas the morphogenesis of leaves depends on determinate activity of leaf meristems. However, one-leaf plants in the Gesneriaceae family (eudicots) do not have a typical SAM and do not produce new organs when in the vegetative phase. Instead, they have one cotyledon whose growth is indeterminate. This peculiar development is supported by the groove meristem, which corresponds to the canonical SAM, and the basal meristem, which corresponds to the typical leaf meristem. However, the former does not produce any organ and the latter is active indeterminately. Gene expression and physiological analyses have been conducted in an effort to determine the molecular nature of this peculiar organogenesis. This review summarizes the current understanding of the development of one-leaf plants to provide future perspectives in this field of research.

Mechanisms Underlying the Environmentally Induced Plasticity of Leaf Morphology

The primary function of leaves is to provide an interface between plants and their environment for gas exchange, light exposure and thermoregulation. Leaves have, therefore a central contribution to plant fitness by allowing an efficient absorption of sunlight energy through photosynthesis to ensure an optimal growth. Their final geometry will result from a balance between the need to maximize energy uptake while minimizing the damage caused by environmental stresses. This intimate relationship between leaf and its surroundings has led to an enormous diversification in leaf forms. Leaf shape varies between species, populations, individuals or even within identical genotypes when those are subjected to different environmental conditions. For instance, the extent of leaf margin dissection has, for long, been found to inversely correlate with the mean annual temperature, such that Paleobotanists have used models based on leaf shape to predict the paleoclimate from fossil flora. Leaf growth is not only dependent on temperature but is also regulated by many other environmental factors such as light quality and intensity or ambient humidity. This raises the question of how the different signals can be integrated at the molecular level and converted into clear developmental decisions. Several recent studies have started to shed the light on the molecular mechanisms that connect the environmental sensing with organ-growth and patterning. In this review, we discuss the current knowledge on the influence of different environmental signals on leaf size and shape, their integration as well as their importance for plant adaptation.

Molecular Mechanisms of Leaf Morphogenesis

Plants maintain the ability to form lateral appendages throughout their life cycle and form leaves as the principal lateral appendages of the stem. Leaves initiate at the peripheral zone of the shoot apical meristem and then develop into flattened structures. In most plants, the leaf functions as a solar panel, where photosynthesis converts carbon dioxide and water into carbohydrates and oxygen. To produce structures that can optimally fulfill this function, plants precisely control the initiation, shape, and polarity of leaves. Moreover, leaf development is highly flexible but follows common themes with conserved regulatory mechanisms. Leaves may have evolved from lateral branches that are converted into determinate, flattened structures. Many other plant parts, such as floral organs, are considered specialized leaves, and thus leaf development underlies their morphogenesis. Here, we review recent advances in the understanding of how three-dimensional leaf forms are established. We focus on how genes, phytohormones, and mechanical properties modulate leaf development, and discuss these factors in the context of leaf initiation, polarity establishment and maintenance, leaf flattening, and intercalary growth.

Conserved functional control, but distinct regulation, of cell proliferation in rice and Arabidopsis leaves revealed by comparative analysis of GRF-INTERACTING FACTOR 1 orthologs

Regulation of cell proliferation is crucial for establishing the shape of plant leaves. We have identified MAKIBA3 (MKB3), a loss-of-function mutant of which exhibits a narrowed- and rolled-leaf phenotype in rice. MKB3 was found to be an ortholog of Arabidopsis ANGUSTIFOLIA3 (AN3), which positively regulates cell proliferation. The reduced leaf size of mkb3 plants with enlarged cells and the increased size of MKB3-overexpressing leaves with normal-sized cells indicate that MKB3 is a positive regulator of leaf proliferation and that mkb3 mutation triggers a compensation syndrome, as does Arabidopsis an3 Expression analysis revealed that MKB3 is predominantly expressed on the epidermis of leaf primordia, which is different from the location of AN3 A protein movement assay demonstrated that MKB3 moves from an MKB3-expressing domain to a non-expressing domain, which is required for normal leaf development. Our results suggest that rice MKB3 and Arabidopsis AN3 have conserved functions and effects on leaf development. However, the expression pattern of MKB3 and direction of protein movement are different between rice and Arabidopsis, which might reflect differences in leaf primordia development in these two species.

OLIGOCELLULA1/HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 Promotes Cell Proliferation With HISTONE DEACETYLASE9 and POWERDRESS During Leaf Development in Arabidopsis thaliana

Organ size regulation is dependent on the precise spatial and temporal regulation of cell proliferation and cell expansion. A number of transcription factors have been identified that play a key role in the determination of aerial lateral organ size, but their functional relationship to various chromatin modifiers has not been well understood. To understand how leaf size is regulated, we previously isolated the oligocellula1 (oli1) mutant of Arabidopsis thaliana that develops smaller first leaves than the wild type (WT) mainly due to a reduction in the cell number. In this study, we further characterized oli1 leaf phenotypes and identified the OLI1 gene as well as interaction partners of OLI1. Detailed characterizations of leaf development suggested that the cell proliferation rate in oli1 leaf primordia is lower than that in the WT. In addition, oli1 was associated with a slight delay of the progression from the juvenile to adult phases of leaf traits. A classical map-based approach demonstrated that OLI1 is identical to HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES15 (HOS15). HOS15/OLI1 encodes a homolog of human transducin β-like protein1 (TBL1). TBL1 forms a transcriptional repression complex with the histone deacetylase (HDAC) HDAC3 and either nuclear receptor co-repressor (N-CoR) or silencing mediator for retinoic acid and thyroid receptor (SMRT). We found that mutations in HISTONE DEACETYLASE9 (HDA9) and a switching-defective protein 3, adaptor 2, N-CoR, and transcription factor IIIB-domain protein gene, POWERDRESS (PWR), showed a small-leaf phenotype similar to oli1. In addition, hda9 and pwr did not further enhance the oli1 small-leaf phenotype, suggesting that these three genes act in the same pathway. Yeast two-hybrid assays suggested physical interactions, wherein PWR probably bridges HOS15/OLI1 and HDA9. Earlier studies suggested the roles of HOS15, HDA9, and PWR in transcriptional repression. Consistently, transcriptome analyses showed several genes commonly upregulated in the three mutants. From these findings, we propose a possibility that HOS15/OLI1, PWR, and HDA9 form an evolutionary conserved transcription repression complex that plays a positive role in the regulation of final leaf size.