Diversification of fruit shape in the Brassicaceae family

Unraveling the Hormonal and Molecular Mechanisms Shaping Fruit Morphology in Plants

The importance of fruit shape studies extends beyond fundamental plant biology, as it holds significant implications for breeding. Understanding the genetic and hormonal regulation of fruit morphology can facilitate targeted breeding strategies to enhance yield, quality, and stress resistance, ultimately contributing to sustainable farming and nutrition security. The diversity in fruit shapes is the result of complex hormone regulation and molecular pathways that affect key traits, including carpel number, fruit length, and weight. Fruit shape is a quality attribute that directly influences consumer preference, marketability and the ease of post-harvest processing. This article focuses on investigations carried out on molecular, genetic and hormonal regulation mechanisms of fruit shape, color, maturation in fruit plants and key genetic pathways such as CLV-WUS and OVATE, as well as their roles in shaping non-climacteric fruits such as strawberries, grapes and raspberries. Plant hormones, especially abscisic acid (ABA) and indole-3-acetic acid (IAA), play a crucial role in enhancing desirable traits such as color and taste, while regulating anthocyanin synthesis and growth time. In addition, the dynamic interactions between auxin, gibberellin, and ethylene are crucial for the ripening process. Jasmonate enhances stress response, brassinosteroids promote ripening and cytokinins promote early fruit development. In addition, this review also studied the fruit morphology of species such as tomatoes and cucumbers, emphasizing the importance of the CLV-WUS pathway, which regulates the number of carpels through genes such as WUSCHEL (WUS), FRUITFULL1 (FUL1), and auxin response factor 14 (ARF14). The weight of fresh fruit is affected by microRNAs such as miRNA156, which emphasizes the importance of post transcriptional regulation. The involvement of transcription factors such as SISHN1, CaOvate, and CISUN25-26-27a further emphasizes the complexity of hormone regulation. Understanding these regulatory mechanisms can enhance our understanding of fruit development and have a profound impact on agricultural practices and crop improvement strategies aimed at meeting the growing global demand for high-quality agricultural products.

Conserved roles of ETT and ARF4 in gynoecium development in Brassicaceae with distinct fruit shapes

Gynoecium patterning is dependent on the dynamic distribution of auxin, the signalling of which is transduced through several distinct pathways. ETTIN (ETT)-mediated signalling occurs independently of the canonical auxin pathway, and ETT shares partial redundancy with Auxin Response Factor 4 (ARF4) in the gynoecium. ETT and ARF4 were previously hypothesized to translate auxin gradients into patterns of tissue polarity alongside other ARFs. As ARF repressors, ETT/ARF were assumed to antagonistically regulate targets shared with ARF activators of the canonical pathway. Here, comparative transcriptomics identified the distinct and overlapping targets of ETT/ARF4 in the Arabidopsis gynoecium. However, ETT/ARF4 targets with known roles in gynoecium development did not conform to models of A-B ARF antagonism, leaving the relationship with the canonical pathway unclear. Mutants in tir1 afb2 ett were therefore generated in Arabidopsis and Capsella to assess the relationship between the two pathways, and their conservation in species with distinct fruit shapes. The data presented indicate conserved synergism between the two pathways in gynoecium development and suggest a role for ARF4 in the integration of these pathways in Brassicaceae with distinct fruit shapes.

Cellular mechanism of polarized auxin transport on fruit shape determination revealed by time-lapse live imaging

KEY MESSAGE

Polarized auxin transport regulates fruit shape determination by promoting anisotropic cell growth. Angiosperms produce organs with distinct shape resultant from adaptive evolution. Understanding the cellular basis underlying the development of plant organ has been a central topic in plant biology as it is key to unlock the mechanisms leading to the diversification of plants. Variations in the location of synthesis, polarized auxin transport (PAT) have been proposed to account for the development of diverse organ shapes, but the exact cellular mechanism has yet to be elucidated. The Capsella rubella develops a perfect heart-shaped fruit from an ovate shape gynoecium that is tightly linked to the localized auxin synthesis in the valve tips and provides a unique opportunity to address this question. In this study, we studied auxin movement in the fruits and the cellular effect of N-1-Naphthylphthalamic Acid (NPA) on the fruit shape determination by constructing the pCrPIN3:PIN3:GFP reporter and live-imaging. We found PAT in the valve epidermis is in congruent with fruit shape development and NPA treatment disrupts the heat-shaped fruit development mainly by repressing cell anisotropic growth with minor effect on division. As the Capsella fruit is unusually big in size, we also included a detailed step-by-step protocol on how to conduct live-imaging experiment. We further test the utility of this protocol by conducting a live-imaging analysis of the gynophore in Arachis hypogaea. Collectively, the results of this study elucidated the mechanism on how auxin signal was translated into instructions guiding cell growth during organ shape determination. In addition, the description of the detailed live-imaging protocol will encourage further studies of the cellular mechanisms underlying shape diversification in angiosperms.

Heat shock protein 70 is associated with duration of cell proliferation in early pod development of soybean

Pod is an important organ for seed production in soybean. Pod size varies among soybean cultivars, but the mechanism is largely unknown. Here we reveal one of the factors for pod size regulation. We investigate pod size differences between two cultivars. The longer pod of 'Tachinagaha' is due to more cell number than in the short pod of 'Iyodaizu'. POD SIZE OF SOYBEAN 8 (GmPSS8), a member of the heat shock protein 70 (HSP70) family, is identified as a candidate gene for determining pod length in a major QTL for pod length. Expression of GmPSS8 in pods is higher in 'Tachinagaha' than 'Iyodaizu' and is highest in early pod development. The difference in expression is the result of an in/del polymorphism　which includes an enhancer motif. Treatment with an HSP70 inhibitor reduces pod length and cell number in the pod. Additionally, shorter pods in Arabidopsis hsp70-1/-4 double mutant are rescued by overexpression of GmPSS8. Our results identify GmPSS8 as a target gene for pod length, which regulates cell number during early pod development through regulation of transcription in soybean. Our findings provide the mechanisms of pod development and suggest possible strategies enhancing yield potential in soybean.

Identification of CaPCR1, an OFP gene likely involved in pointed versus concave fruit tip regulation in pepper (Capsicum annuum L.) using recombinant inbred lines

KEY MESSAGE

CaPCR1 (Capana12g002165) was a candidate gene regulating fruit concave/pointed tip shape in pepper. The concave shape of the fruit tip in pepper plants is highly susceptible to drought and low temperature stresses, resulting in the appearance of a pointed tip fruit, which affects its commercial value. However, few studies on the process of fruit tip development and regulatory genes in pepper have been reported. Herein, the developmental process of the ovary before anthesis, especially changes in the shape of the ovary tip, was studied in detail. The results showed that the final fruit tip shape was consistent with the ovary tip shape before anthesis, and a concave tip shape gradually developed. F4 recombinant inbred lines (RILs) were constructed to map the genes regulating fruit tip shape through hybridization of the LRS and SBS pepper inbred lines. CaPCR1 (Capana12g002165), an OFP (OVATE Family Protein) family gene, was located in the candidate region on chr12. Three SNPs were found in the protein coding sequence of CaPCR1 between SBS and LRS, but only one SNP led to amino acid variation. Sequence variations, including base replacements, deletions and insertions, were also detected in the gene promoter region. The relative expression level of the CaPCR1 gene was significantly greater in the concave tip ovary than in the pointed tip ovary. qRT‒PCR analysis revealed that the CaPCR1 gene was expressed mainly in the gynoecium, placenta and green fruit pericarp, which was consistent with its function in ovary and fruit development. Taken together, these results suggested that CaPCR1 is a candidate gene involved in fruit tip shape determination in pepper.

Gynoecium and fruit development in Arabidopsis

Flowering plants produce flowers and one of the most complex floral structures is the pistil or the gynoecium. All the floral organs differentiate from the floral meristem. Various reviews exist on molecular mechanisms controlling reproductive development, but most focus on a short time window and there has been no recent review on the complete developmental time frame of gynoecium and fruit formation. Here, we highlight recent discoveries, including the players, interactions and mechanisms that govern gynoecium and fruit development in Arabidopsis. We also present the currently known gene regulatory networks from gynoecium initiation until fruit maturation.

Genetic and Molecular Regulation Mechanisms in the Formation and Development of Vegetable Fruit Shape

Vegetable crops have a long history of cultivation worldwide and rich germplasm resources. With its continuous development and progress, molecular biology technology has been applied to various fields of vegetable crop research. Fruit is an important organ in vegetable crops, and fruit shape can affect the yield and commercialization of vegetables. In nature, fruits show differences in size and shape. Based on fruit shape diversity, the growth direction and coordination mechanism of fruits remain unclear. In this review, we discuss the latest research on fruit shape. In addition, we compare the current theories on the molecular mechanisms that regulate fruit growth, size, and shape in different vegetable families.

Ascophyllum nodosum Extract (SealicitTM) Boosts Soybean Yield Through Reduction of Pod Shattering-Related Seed Loss and Enhanced Seed Production

Soybean is one of the most valuable commercial crops because of its high protein, carbohydrate, and oil content. The land area cultivated with soybean in subtropical regions, such as Brazil, is continuously expanding, in some instances at the expense of carbon storing natural habitats. Strategies to decrease yield/seed losses and increase production efficiency are urgently required to meet global demand for soybean in a sustainable manner. Here, we evaluated the effectiveness of an Ascophyllum nodosum extract (ANE), Sealicit^TM, in increasing yields of different soybean varieties, in two geographical regions (Canada and Brazil). In addition, we investigated the potential of Sealicit^TM to reduce pod shattering at the trials in Brazil. Three different concentrations of Sealicit^TM were applied to pod shatter-susceptible (SS) UFUS 6901 and shatter-resistant (SR) UFUS 7415 varieties to assess their impact on pod firmness. SS variety demonstrated a significant decrease in pod shattering, which coincided with deregulation of GmPDH1.1 and GmSHAT1-5 expression, genes that determine pod dehiscence, and higher seed weight per pod. Sealicit^TM application to the SR variety did not significantly alter its inherent pod shatter resistance, but provided higher increases in seed yield at harvest. This yield increase maybe associated with to other yield components stimulated by the biostimulant. This work demonstrates that Sealicit^TM, which has previously been shown to improve pod firmness in Arabidopsis and selected commercial oilseed rape varieties through IND gene down-regulation, also has the potential to improve pod resistance and seed productivity in soybean, a member of the legume family sharing a similar strategy for seed dispersal.

HEARTBREAK Controls Post-translational Modification of INDEHISCENT to Regulate Fruit Morphology in Capsella

Morphological variation is the basis of natural diversity and adaptation. For example, angiosperms (flowering plants) evolved during the Cretaceous period more than 100 mya and quickly colonized terrestrial habitats [1]. A major reason for their astonishing success was the formation of fruits, which exist in a myriad of different shapes and sizes [2]. Evolution of organ shape is fueled by variation in expression patterns of regulatory genes causing changes in anisotropic cell expansion and division patterns [3, 4, 5]. However, the molecular mechanisms that alter the polarity of growth to generate novel shapes are largely unknown. The heart-shaped fruits produced by members of the Capsella genus comprise an anatomical novelty, making it particularly well suited for studies on morphological diversification [6, 7, 8]. Here, we show that post-translational modification of regulatory proteins provides a critical step in organ-shape formation. Our data reveal that the SUMO protease, HEARTBREAK (HTB), from Capsella rubella controls the activity of the key regulator of fruit development, INDEHISCENT (CrIND in C. rubella), via de-SUMOylation. This post-translational modification initiates a transduction pathway required to ensure precisely localized auxin biosynthesis, thereby facilitating anisotropic cell expansion to ultimately form the heart-shaped Capsella fruit. Therefore, although variation in the expression of key regulatory genes is known to be a primary driver in morphological evolution, our work demonstrates how other processes—such as post-translational modification of one such regulator—affects organ morphology.

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HTB encodes a SUMO protease required for fruit shape in Capsella

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Anisotropic cell growth is suppressed in the fruit valves of the htb mutant

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HTB stabilizes CrIND through de-SUMOylation to facilitate local auxin biosynthesis

Photonic paper: Multiscale assembly of reflective cellulose sheets in Lunaria annua

Bright, iridescent colors observed in nature are often caused by light interference within nanoscale periodic lattices, inspiring numerous strategies for coloration devoid of inorganic pigments. Here, we describe and characterize the septum of the Lunaria annua plant that generates large (multicentimeter), freestanding iridescent sheets, with distinctive silvery-white reflective appearance. This originates from the thin-film assembly of cellulose fibers in the cells of the septum that induce thin-film interference-like colors at the microscale, thus accounting for the structure's overall silvery-white reflectance at the macroscale. These cells further assemble into two thin layers, resulting in a mechanically robust, iridescent septum, which is also significantly light due to its high air porosity (>70%) arising from the cells' hollow-core structure. This combination of hierarchical structure comprising mechanical and optical function can inspire technological classes of devices and interfaces based on robust, light, and spectrally responsive natural substrates.

Growth dynamics of the Arabidopsis fruit is mediated by cell expansion

Fruit have evolved a sophisticated tissue and cellular architecture to secure plant reproductive success. Postfertilization growth is perhaps the most dramatic event during fruit morphogenesis. Several studies have proposed that fertilized ovules and developing seeds initiate signaling cascades to coordinate and promote the growth of the accompanying fruit tissues. This dynamic process allows the fruit to conspicuously increase its size and acquire its final shape and means for seed dispersal. All these features are key for plant survival and crop yield. Despite its importance, we lack a high-resolution spatiotemporal map of how postfertilization fruit growth proceeds at the cellular level. In this study, we have combined live imaging, mutant backgrounds in which fertilization can be controlled, and computational modeling to monitor and predict postfertilization fruit growth in Arabidopsis We have uncovered that, unlike leaves, sepals, or roots, fruit do not exhibit a spatial separation of cell division and expansion domains; instead, there is a separation into temporal stages with fertilization as the trigger for transitioning to cell expansion, which drives postfertilization fruit growth. We quantified the coordination between fertilization and fruit growth by imaging no transmitting tract (ntt) mutants, in which fertilization fails in the bottom half of the fruit. By combining our experimental data with computational modeling, we delineated the mobility properties of the seed-derived signaling cascades promoting growth in the fruit. Our study provides the basis for generating a comprehensive understanding of the molecular and cellular mechanisms governing fruit growth and shape.

A plant biostimulant from the seaweed Ascophyllum nodosum (Sealicit) reduces podshatter and yield loss in oilseed rape through modulation of IND expression

The yield of podded crops such as oilseed rape (OSR) is limited by evolutionary adaptations of the plants for more efficient and successful seed dispersal for survival. These plants have evolved dehiscent dry fruits that shatter along a specifically developed junction at carpel margins. A number of strategies such as pod sealants, GMOs and hybrids have been developed to mitigate the impact of pod shatter on crop yield with limited success. Plant biostimulants have been shown to influence plant development. A challenge in plant biostimulant research is elucidating the mechanisms of action. Here we have focused on understanding the effect of an Ascophyllum nodosum based biostimulant (Sealicit) on fruit development and seed dispersal trait in Arabidopsis and OSR at genetic and physiological level. The results indicate that Sealicit is affecting the expression of the major regulator of pod shattering, INDEHISCENT, as well as disrupting the auxin minimum. Both factors influence the formation of the dehiscence zone and consequently reduce pod shattering. Unravelling the mode of action of this unique biostimulant provides data to support its effectiveness in reducing pod shatter and highlights its potential for growers to increase seed yield in a number of OSR varieties.

How to build a fruit: Transcriptomics of a novel fruit type in the Brassiceae

Comparative gene expression studies are invaluable for predicting how existing genetic pathways may be modified or redeployed to produce novel and variable phenotypes. Fruits are ecologically important organs because of their impact on plant fitness and seed dispersal, modifications in which results in morphological variation across species. A novel fruit type in the Brassicaceae known as heteroarthrocarpy enables distinct dispersal methods in a single fruit through segmentation via a lateral joint and variable dehiscence at maturity. Given the close relationship to Arabidopsis, species that exhibit heteroarthrocarpy are powerful models to elucidate how differences in gene expression of a fruit patterning pathway may result in novel fruit types. Transcriptomes of distal, joint, and proximal regions from Erucaria erucarioides and Cakile lanceolata were analyzed to elucidate within fruit and between species differences in whole transcriptome, gene ontology, and fruit patterning expression profiles. Whole transcriptome expression profiles vary between fruit regions in patterns that are consistent with fruit anatomy. These transcriptomic variances do not correlate with changes in gene ontology, as they remain generally stable within and between both species. Upstream regulators in the fruit patterning pathway, FILAMENTOUS FLOWER and YABBY3, are expressed in the distal and proximal regions of E. erucarioides, but not in the joint, implicating alterations in the pathway in heteroarthrocarpic fruits. Downstream gene, INDEHISCENT, is significantly upregulated in the abscissing joint region of C. lanceolata, which suggests repurposing of valve margin genes for novel joint disarticulation in an otherwise indehiscent fruit. In summary, these data are consistent with modifications in fruit patterning genes producing heteroarthrocarpic fruits through different components of the pathway relative to other indehiscent, non-heteroarthrocarpic, species within the family. Our understanding of fruit development in Arabidopsis is now extended to atypical siliques within the Brassicaceae, facilitating future studies on seed shattering in important Brassicaceous crops and pernicious weeds.

Fruit ripening: the role of hormones, cell wall modifications, and their relationship with pathogens

Fruits result from complex biological processes that begin soon after fertilization. Among these processes are cell division and expansion, accumulation of secondary metabolites, and an increase in carbohydrate biosynthesis. Later fruit ripening is accomplished by chlorophyll degradation and cell wall lysis. Fruit maturation is an essential step to optimize seed dispersal, and is controlled by a complex network of transcription factors and genetic regulators that are strongly influenced by phytohormones. Abscisic acid (ABA) and ethylene are the major regulators of ripening and senescence in both dry and fleshy fruits, as demonstrated by numerous ripening-defective mutants, effects of exogenous hormone application, and transcriptome analyses. While ethylene is the best characterized player in the final step of a fruit's life, ABA also has a key regulatory role, promoting ethylene production and acting as a stress-related hormone in response to drought and pathogen attack. In this review, we focus on the role of ABA and ethylene in relation to the interconnected biotic and abiotic phenomena that affect ripening and senescence. We integrate and discuss the most recent data available regarding these biological processes, which are crucial for post-harvest fruit conservation and for food safety.

Brassicaceae flowers: diversity amid uniformity

The mustard family Brassicaceae, which includes the model plant Arabidopsis thaliana, exhibits morphological stasis and significant uniformity of floral plan. Nonetheless, there is untapped diversity in almost every aspect of floral morphology in the family that lends itself to comparative study, including organ number, shape, form, and color. Studies on the genetic basis of morphological diversity, enabled by extensive genetic tools and genomic resources and the close phylogenetic distance among mustards, have revealed a mosaic of conservation and divergence in numerous floral traits. Here I review the morphological diversity of the flowers of Brassicaceae and discuss studies addressing the underlying genetic and developmental mechanisms shaping floral diversity. To put flowers in the context of the floral display, I describe diversity in inflorescence morphology and the variation that exists in the structures preceding the floral organs. Reconstructing the floral morphospace in Brassicaceae coupled with next-generation sequencing data and unbiased approaches to interrogate gene function in species throughout the mustard phylogeny offers promising ways to understand how developmental mechanisms originate and diversify.

Fruit Development: Turning Sticks into Hearts

Fruit morphological diversity reflects the versatility of these angiosperm-specific structures, which facilitate plant progeny dispersal from their sessile parents. A recent study links regulatory changes in a key genetic network for fruit patterning with the origin of heart-shaped pods in Brassicaceae.

Regulatory Diversification of INDEHISCENT in the Capsella Genus Directs Variation in Fruit Morphology

Evolution of gene-regulatory sequences is considered the primary driver of morphological variation [1-3]. In animals, the diversity of body plans between distantly related phyla is due to the differential expression patterns of conserved "toolkit" genes [4]. In plants, variation in expression domains similarly underlie most of the reported diversity of organ shape both in natural evolution and in the domestication of crops [5-9]. The heart-shaped fruit from members of the Capsella genus is a morphological novelty that has evolved after Capsella diverged from Arabidopsis ∼8 mya [10]. Comparative studies of fruit growth in Capsella and Arabidopsis revealed that the difference in shape is caused by local control of anisotropic growth [11]. Here, we show that sequence variation in regulatory domains of the fruit-tissue identity gene, INDEHISCENT (IND), is responsible for expansion of its expression domain in the heart-shaped fruits from Capsella rubella. We demonstrate that expression of this CrIND gene in the apical part of the valves in Capsella contributes to the heart-shaped appearance. While studies on morphological diversity have revealed the importance of cis-regulatory sequence evolution, few examples exist where the downstream effects of such variation have been characterized in detail. We describe here how CrIND exerts its function on Capsella fruit shape by binding sequence elements of auxin biosynthesis genes to activate their expression and ensure auxin accumulation into highly localized maxima in the fruit valves. Thus, our data provide a direct link between changes in expression pattern and altered hormone homeostasis in the evolution of morphological novelty.

Image-based phenotyping for identification of QTL determining fruit shape and size in American cranberry (Vaccinium macrocarpon L.)

Image-based phenotyping methodologies are powerful tools to determine quality parameters for fruit breeders and processors. The fruit size and shape of American cranberry (Vaccinium macrocarpon L.) are particularly important characteristics that determine the harvests’ processing value and potential end-use products (e.g., juice vs. sweetened dried cranberries). However, cranberry fruit size and shape attributes can be difficult and time consuming for breeders and processors to measure, especially when relying on manual measurements and visual ratings. Therefore, in this study, we implemented image-based phenotyping techniques for gathering data regarding basic cranberry fruit parameters such as length, width, length-to-width ratio, and eccentricity. Additionally, we applied a persistent homology algorithm to better characterize complex shape parameters. Using this high-throughput artificial vision approach, we characterized fruit from 351 progeny from a full-sib cranberry population over three field seasons. Using a covariate analysis to maximize the identification of well-supported quantitative trait loci (QTL), we found 252 single QTL in a 3-year period for cranberry fruit size and shape descriptors from which 20% were consistently found in all years. The present study highlights the potential for the identified QTL and the image-based methods to serve as a basis for future explorations of the genetic architecture of fruit size and shape in cranberry and other fruit crops.

When the BRANCHED network bears fruit: how carpic dominance causes fruit dimorphism in Aethionema

Life in unpredictably changing habitats is a great challenge, especially for sessile organisms like plants. Fruit and seed heteromorphism is one way to cope with such variable environmental conditions. It denotes the production of distinct types of fruits and seeds that often mediate distinct life-history strategies in terms of dispersal, germination and seedling establishment. But although the phenomenon can be found in numerous species and apparently evolved several times independently, its developmental time course or molecular regulation remains largely unknown. Here, we studied fruit development in Aethionema arabicum, a dimorphic member of the Brassicaceae family. We characterized fruit morph differentiation by comparatively analyzing discriminating characters like fruit growth, seed abortion and dehiscence zone development. Our data demonstrate that fruit morph determination is a 'last-minute' decision happening in flowers after anthesis directly before the first morphotypical differences start to occur. Several growth experiments in combination with hormone and gene expression analyses further indicate that an accumulation balance of the plant hormones auxin and cytokinin in open flowers together with the transcript abundance of the Ae. arabicum ortholog of BRANCHED1, encoding a transcription factor known for its conserved function as a branching repressor, may guide fruit morph determination. Thus, we hypothesize that the plasticity of the fruit morph ratio in Ae. arabicum may have evolved through the modification of a preexisting network known to govern correlative dominance between shoot organs.

A molecular framework controlling style morphology in Brassicaceae

Organ formation in multicellular organisms depends on the coordinated activities of regulatory components that integrate developmental and hormonal cues to control gene expression and mediate cell-type specification. For example, development of the Arabidopsis gynoecium is tightly controlled by distribution and synthesis of the plant hormone auxin. The functions of several transcription factors (TFs) have been linked with auxin dynamics during gynoecium development; yet how their activities are coordinated is not known. Here, we show that five such TFs function together to ensure polarity establishment at the gynoecium apex. The auxin response factor ETTIN (ARF3; herein, ETT) is a central component of this framework. Interaction of ETT with TF partners is sensitive to the presence of auxin and our results suggest that ETT forms part of a repressive gene-regulatory complex. We show that this function is conserved between members of the Brassicaceae family and that variation in an ETT subdomain affects interaction strengths and gynoecium morphology. These results suggest that variation in affinities between conserved TFs can lead to morphological differences and thus contribute to the evolution of diverse organ shapes.

Summary: Variation in interaction affinity between transcription factors of an ETTIN-containing complex underlies diversity of gynoecium style structure among members of the Brassicacea family.

Auxin signaling: a big question to be addressed by small molecules

Providing a mechanistic understanding of the crucial roles of the phytohormone auxin has been an important and coherent aspect of plant biology research. Since its discovery more than a century ago, prominent advances have been made in the understanding of auxin action, ranging from metabolism and transport to cellular and transcriptional responses. However, there is a long road ahead before a thorough understanding of its complex effects is achieved, because a lot of key information is still missing. The availability of an increasing number of technically advanced scientific tools has boosted the basic discoveries in auxin biology. A plethora of bioactive small molecules, consisting of the synthetic auxin-like herbicides and the more specific auxin-related compounds, developed as a result of the exploration of chemical space by chemical biology, have made the tool box for auxin research more comprehensive. This review mainly focuses on the compounds targeting the auxin co-receptor complex, demonstrates the various ways to use them, and shows clear examples of important basic knowledge obtained by their usage. Application of these precise chemical tools, together with an increasing amount of structural information for the major components in auxin action, will certainly aid in strengthening our insights into the complexity and diversity of auxin response.

Shaping a fruit: Developmental pathways that impact growth patterns

Angiosperms produce seeds as their progeny enclosed in maternally-derived structures called fruits. Evolutionarily, fruits have contributed enormously to the success of the Angiosperms phylum by providing protection and nutrition to the developing seeds, while ensuring the efficient dispersal upon maturity. Fruits vary massively in both size and shape and certain species have been targeted for domestication due to their nutritional value and delicious taste. Among the vast array of 3D fruit shapes that exist in nature, the mechanism by which growth is oriented and coordinated to generate this diversity of forms is unclear. In this review, we discuss the latest results in identifying components that control fruit morphology and their effect on isotropic and anisotropic growth. Moreover, we will compare the current knowledge on the mechanisms that control fruit growth, size and shape between the domesticated Solanaceae species, tomato and members of the large family of Brassicaceae.

Shattering fruits: variations on a dehiscent theme

Fruits are seed dispersal units, and for that they have evolved different strategies to facilitate separation and dispersal of the progeny from the mother plant. A great proportion of fruits from different clades are dry and dehiscent, opening upon maturity to disperse the seeds. In the last two decades, intense research mainly in Arabidopsis has uncovered the basic network that controls the differentiation of the Arabidopsis fruit dehiscence zone. This review focuses on recent discoveries that have helped to complete the picture, as well as the insights from evo-devo and crop domestication studies that show how the conservation/variation of the elements of this network across species accounts for its evolutionary plasticity and the origin of evolutionary innovations.

Fruit shape diversity in the Brassicaceae is generated by varying patterns of anisotropy

Fruits exhibit a vast array of different 3D shapes, from simple spheres and cylinders to more complex curved forms; however, the mechanism by which growth is oriented and coordinated to generate this diversity of forms is unclear. Here, we compare the growth patterns and orientations for two very different fruit shapes in the Brassicaceae: the heart-shaped Capsella rubella silicle and the near-cylindrical Arabidopsis thaliana silique. We show, through a combination of clonal and morphological analyses, that the different shapes involve different patterns of anisotropic growth during three phases. These experimental data can be accounted for by a tissue-level model in which specified growth rates vary in space and time and are oriented by a proximodistal polarity field. The resulting tissue conflicts lead to deformation of the tissue as it grows. The model allows us to identify tissue-specific and temporally specific activities required to obtain the individual shapes. One such activity may be provided by the valve-identity gene FRUITFULL, which we show through comparative mutant analysis to modulate fruit shape during post-fertilisation growth of both species. Simple modulations of the model presented here can also broadly account for the variety of shapes in other Brassicaceae species, thus providing a simplified framework for fruit development and shape diversity.