The Arabidopsis nectary is an ABC-independent floral structure

The developmental basis of floral nectary diversity and evolution

Nectar is a central bridge between angiosperms and animal mutualists. It is produced by specialized structures termed nectaries, which can be found on different plant organs. Consumption of floral nectar by pollinators and the subsequent transfer of pollen contribute to the reproductive success of both angiosperms and their pollinators. Floral nectaries have evolved many times independently, feature diverse structural organizations, and produce nectars with various compositions, which cater to a wide range of pollinators. While the nectary and its nectar have been documented for two millennia, many aspects of nectary biology are still unknown. Recent advances in genetics, genomics, and comparative analyses across diverse species have accelerated our understanding of floral nectary structures and the genetic circuits behind their formation and evolution. In this review, we summarize the recent breakthroughs in nectary research and provide a macroevolutionary framework of floral nectary evolution, focusing on the genetic mechanisms that drive nectary development and shape nectary diversity.

Morphological and molecular mechanisms of floral nectary development in Chinese Jujube

BACKGROUND

Chinese jujube (Ziziphus jujuba Mill.), also called Chinese date, is one of the oldest and widely cultivated fruit trees with great economic values, which, at least, can be attributed to the melliferous flower with highly developed nectary that can secret huge amount of nectar in a rather tiny floral size. However, the morphological nature, metabolic products, developmental process, as well as molecular and regulatory mechanisms of jujube nectary remain largely unknown.

RESULTS

Here, we selected Z. jujuba 'Dongzao' as a system to address these questions. We uncovered that the jujube nectary is an annular or donut-shaped secretory protrusion that surrounds the base of the carpels, along with emerald and glistening hues, which can produce a bulk honey with many metabolic compounds (e.g. saccharides and flavonoids) that has a high nutritional value and benefit for human health. The development of jujube nectary is a dynamic process of earlier cell division followed by later cell expansion. We also identified putative genes associated with the nectary development and found that the CRABS CLAW (CRC) ortholog (ZjCRC) is the key to nectary development: the gene is highly expressed in nectary; ectopic expression of it in the Arabidopsis crc-1 mutant rescued the lost nectary (also the carpel and silique defects). We also demonstrated that a MADS-box transcription factor ZjAGAMOUS1 (ZjAG1) is required for the direct activation of ZjCRC expression.

CONCLUSIONS

Taken together, our results not only provide a comprehensive portrait of the jujube nectary, but also pave the way to effective utilization of jujube and other woody crops.

Reflections on the ABC model of flower development

The formulation of the ABC model by a handful of pioneer plant developmental geneticists was a seminal event in the quest to answer a seemingly simple question: how are flowers formed? Fast forward 30 years and this elegant model has generated a vibrant and diverse community, capturing the imagination of developmental and evolutionary biologists, structuralists, biochemists and molecular biologists alike. Together they have managed to solve many floral mysteries, uncovering the regulatory processes that generate the characteristic spatio-temporal expression patterns of floral homeotic genes, elucidating some of the mechanisms allowing ABC genes to specify distinct organ identities, revealing how evolution tinkers with the ABC to generate morphological diversity, and even shining a light on the origins of the floral gene regulatory network itself. Here we retrace the history of the ABC model, from its genesis to its current form, highlighting specific milestones along the way before drawing attention to some of the unsolved riddles still hidden in the floral alphabet.

Thinking outside the F-box: how UFO controls angiosperm development

The formation of inflorescences and flowers is essential for the successful reproduction of angiosperms. In the past few decades, genetic studies have identified the LEAFY transcription factor and the UNUSUAL FLORAL ORGANS (UFO) F-box protein as two major regulators of flower development in a broad range of angiosperm species. Recent research has revealed that UFO acts as a transcriptional cofactor, redirecting the LEAFY floral regulator to novel cis-elements. In this review, we summarize the various roles of UFO across species, analyze past results in light of new discoveries and highlight the key questions that remain to be solved.

Post-secretory synthesis of a natural analog of iron-gall ink in the black nectar of Melianthus spp

The black nectar produced by Melianthus flowers is thought to serve as a visual attractant to bird pollinators, but the chemical identity and synthesis of the black pigment are unknown. A combination of analytical biochemistry, transcriptomics, proteomics, and enzyme assays was used to identify the pigment that gives Melianthus nectar its black color and how it is synthesized. Visual modeling of pollinators was also used to infer a potential function of the black coloration. High concentrations of ellagic acid and iron give the nectar its dark black color, which can be recapitulated through synthetic solutions containing only ellagic acid and iron(iii). The nectar also contains a peroxidase that oxidizes gallic acid to form ellagic acid. In vitro reactions containing the nectar peroxidase, gallic acid, hydrogen peroxide, and iron(iii) fully recreate the black color of the nectar. Visual modeling indicates that the black color is highly conspicuous to avian pollinators within the context of the flower. Melianthus nectar contains a natural analog of iron-gall ink, which humans have used since at least medieval times. This pigment is derived from an ellagic acid-Fe complex synthesized in the nectar and is likely involved in the attraction of passerine pollinators endemic to southern Africa.

BELL1 interacts with CRABS CLAW and INNER NO OUTER to regulate ovule and seed development in pomegranate

Pomegranate (Punica granatum) flowers are classified as bisexual flowers and functional male flowers. Functional male flowers have sterile pistils that show abnormal ovule development. In previous studies, we identified INNER NO OUTER (INO), CRABS CLAW (CRC), and BELL1 (BEL1), which were specifically expressed in bisexual and functional male flowers. However, the functions of ovule identity genes and the mechanism underlying ovule sterility in pomegranate remain unknown. Here, we found that the integument primordia formed and then ceased developing in the ovules of functional male flowers with a vertical diameter of 8.1-13.0 mm. Megaspore mother cells were observed in bisexual flowers when the vertical diameters of flowers were 10.1-13.0 mm, but not in functional male flowers. We analyzed the expression patterns of ovule-related genes in pomegranate ovule sterility and found that PgCRC mRNA was highly expressed at a critical stage of ovule development in bisexual flowers. Ectopic expression of PgCRC and PgINO was sufficient to increase seed number in transgenic lines. PgCRC partially complemented the Arabidopsis (Arabidopsis thaliana) crc mutant, and PgINO successfully rescued the seeds set in the Arabidopsis ino mutant. The results of yeast two-hybrid assays, bimolecular fluorescence complementation assays, and genetic data analyses showed that PgCRC and PgINO directly interact with PgBEL1. Our results also showed that PgCRC and PgINO could not interact directly with MADS-box proteins and that PgBEL1 interacted with SEPALLATA proteins. We report the function of PgCRC and PgINO in ovule and seed development and show that PgCRC and PgINO interact with PgBEL1. Thus, our results provide understanding of the genetic regulatory networks underlying ovule development in pomegranate.

Nectary development in Cleome violacea

Nectaries are a promising frontier for plant evo-devo research, and are particularly fascinating given their diversity in form, position, and secretion methods across angiosperms. Emerging model systems permit investigations of the molecular basis for nectary development and nectar secretion across a range of taxa, which addresses fundamental questions about underlying parallelisms and convergence. Herein, we explore nectary development and nectar secretion in the emerging model taxa, Cleome violacea (Cleomaceae), which exhibits a prominent adaxial nectary. First, we characterized nectary anatomy and quantified nectar secretion to establish a foundation for quantitative and functional gene experiments. Next, we leveraged RNA-seq to establish gene expression profiles of nectaries across three key stages of development: pre-anthesis, anthesis, and post-fertilization. We then performed functional studies on five genes that were putatively involved in nectary and nectar formation: CvCRABSCLAW (CvCRC), CvAGAMOUS (CvAG), CvSHATTERPROOF (CvSHP), CvSWEET9, and a highly expressed but uncharacterized transcript. These experiments revealed a high degree of functional convergence to homologues from other core Eudicots, especially Arabidopsis. CvCRC, redundantly with CvAG and CvSHP, are required for nectary initiation. Concordantly, CvSWEET9 is essential for nectar formation and secretion, which indicates that the process is eccrine based in C. violacea. While demonstration of conservation is informative to our understanding of nectary evolution, questions remain. For example, it is unknown which genes are downstream of the developmental initiators CvCRC, CvAG, and CvSHP, or what role the TCP gene family plays in nectary initiation in this family. Further to this, we have initiated a characterization of associations between nectaries, yeast, and bacteria, but more research is required beyond establishing their presence. Cleome violacea is an excellent model for continued research into nectary development because of its conspicuous nectaries, short generation time, and close taxonomic distance to Arabidopsis.

Flower Development in Arabidopsis

Like in other angiosperms, the development of flowers in Arabidopsis starts right after the floral transition, when the shoot apical meristem (SAM) stops producing leaves and makes flowers instead. On the flanks of the SAM emerge the flower meristems (FM) that will soon differentiate into the four main floral organs, sepals, petals, stamens, and pistil, stereotypically arranged in concentric whorls. Each phase of flower development-floral transition, floral bud initiation, and floral organ development-is under the control of specific gene networks. In this chapter, we describe these different phases and the gene regulatory networks involved, from the floral transition to the floral termination.

Genetic and Phenotypic Analyses of Carpel Development in Arabidopsis

Carpels are the female reproductive organs of the flower, organized in a gynoecium, which is likely the most complex organ of the plant. The gynoecium provides protection for the ovules, helps to discriminate between male gametophytes, and facilitates successful pollination. After fertilization, it develops into a fruit, a specialized organ for seed protection and dispersal. To carry out all these functions, coordinated patterning and tissue specification within the developing gynoecium has to be achieved. In this chapter, we provide different methods to characterize defects in carpel morphogenesis and patterning associated with developmental mutations, as well as a list of reporter lines that can be used to facilitate genetic analyses.

Phenotypes of Floral Nectaries in Developmental Mutants of Legumes and What They May Tell about Genetic Control of Nectary Formation

Simple Summary

The third largest angiosperm family, Leguminosae, is remarkable with the outstanding diversity of its flowers, usually monosymmetric and adapted to different pollination strategies. A key attractant of leguminous flowers is nectar. Compared with Arabidopsis (Brassicaceae), very little is known about regulation of floral nectaries development in legumes. This work aimed to investigate details of these nectaries’ morphology in flowers of mutants of different legume species. It was found that the changes in identity of petals and stamens usually do not affect a proper structure and position of nectaries in leguminous flowers, thus suggesting a high stability of attracting structures versus the pronounced plasticity of perianth and stamens. Some of genes involved in regulation of nectary development in Arabidopsis seem to have the same functions in legumes. The principal difference between Arabidopsis and legumes is connected with a flower monosymmetry in most representatives of the latter taxon, which is also reflected in structure of their floral nectaries.

Abstract

The vast majority of angiosperms attracts animal pollinators with the nectar secreted through specialized floral nectaries (FNs). Although there is evidence that principal patterns of regulation of FN development are conserved in large angiosperm clades, these structures are very diverse considering their morphology and position within a flower. Most data on genetic control of FN formation were obtained in surveys of a model plant species, Arabidopsis thaliana (Brassicaceae). There are almost no data on genetic factors affecting FN development in Leguminosae, the plant family of a high agricultural value and possessing outstandingly diverse flowers. In this work, the morphology of FNs was examined in a set of leguminous species, both wild-type and developmental mutants, by the means of a scanning electron microscopy. Unlike Brassicaceae, FNs in legumes are localized between stamens and a carpel instead of being associated with a certain floral organ. FNs were found stable in most cases of mutants when perianth and/or androecium morphology was affected. However, regulation of FN development by BLADE-ON-PETIOLE-like genes seems to be a shared feature between legumes (at least Pisum) and Arabidopsis. In some legumes, the adaxial developmental program (most probably CYCLOIDEA-mediated) suppresses the FN development. The obtained results neither confirm the role of orthologues of UNUSUAL FLORAL ORGANS and LEAFY in FN development in legumes nor reject it, as two studied pea mutants were homozygous at the weakest alleles of the corresponding loci and possessed FNs similar to those of wild-type.

Modularity and selection of nectar traits in the evolution of the selfing syndrome in Ipomoea lacunosa (Convolvulaceae)

Although the evolution of the selfing syndrome often involves reductions in floral size, pollen and nectar, few studies of selfing syndrome divergence have examined nectar. We investigate whether nectar traits have evolved independently of other floral size traits in the selfing syndrome, whether nectar traits diverged due to drift or selection, and the extent to which quantitative trait locus (QTL) analyses predict genetic correlations. We use F5 recombinant inbred lines (RILs) generated from a cross between Ipomoea cordatotriloba and Ipomoea lacunosa. We calculate genetic correlations to identify evolutionary modules, test whether trait divergence was due to selection, identify QTLs and perform correlation analyses to evaluate how well QTL properties reflect genetic correlations. Nectar and floral size traits form separate evolutionary modules. Selection has acted to reduce nectar traits in the selfing I. lacunosa. Genetic correlations predicted from QTL properties are consistent with observed genetic correlations. Changes in floral traits associated with the selfing syndrome reflect independent evolution of at least two evolutionary modules: nectar and floral size traits. We also demonstrate directional selection on nectar traits, which is likely to be independent of selection on floral size traits. Our study also supports the expected mechanistic link between QTL properties and genetic correlations.

Integrative analysis of transcriptome and proteome revealed nectary and nectar traits in the plant-pollinator interaction of Nitraria tangutorum Bobrov

BACKGROUND

Nitraria tangutorum is an important desert shrub that shows resistance to drought, salt and wind erosion stresses. It is a central ecological species in its area. Here, we have studied how N. tangutorum has adapted to achieve a successful reproduction strategy.

RESULTS

We found that N. tangutorum is mainly pollinated by insects of the Hymenoptera, Diptera and Coleoptera orders. Nitraria tangutorum has very small flowers, with the nectary composed of secretive epidermal cells from which nectar is secreted, located within the inner petals. In addition, analyzing the transcriptome of four successive flower developmental stages revealed that mainly differentially expressed genes associated with flower and nectary development, nectar biosynthesis and secretion, flavonoid biosynthesis, plant hormone signal transduction and plant-pathogen interaction show dynamic expression. From the nectar, we could identify seven important proteins, of which the L-ascorbate oxidase protein was first found in plant nectar. Based on the physiological functions of these proteins, we predict that floral nectar proteins of N. tangutorum play an important role in defending against microbial infestation and scavenging active oxygen.

CONCLUSIONS

This study revealed that N. tangutorum is an insect-pollinated plant and its nectary is composed of secretive epidermal cells that specialized into secretive trichomes. We identified a large number of differentially expressed genes controlling flower and nectary development, nectar biosynthesis and secretion, flavonoid biosynthesis, plant hormone signal transduction and plant-pathogen interaction. We suggest that proteins present in N. tangutorum nectar may have both an antibacterial and oxygen scavenging effect. These results provide a scientific basis for exploring how the reproductive system of N. tangutorum and other arid-desert plants functions.

GoNe encoding a class VIIIb AP2/ERF is required for both extrafloral and floral nectary development in Gossypium

The floral nectary, first recognized and described by Carl Linnaeus, is a remarkable organ that serves to provide carbohydrate-rich nectar to visiting pollinators in return for gamete transfer between flowers. Therefore, the nectary has indispensable biological significance in plant reproduction and even in evolution. Only two genes, CRC and STY, have been reported to regulate floral nectary development. However, it is still unknown what genes contribute to extrafloral nectary development. Here, we report that a nectary development gene in Gossypium (GoNe), annotated as an APETALA 2/ethylene-responsive factor (AP2/ERF), is responsible for the formation of both floral and extrafloral nectaries. GoNe plants that are silenced via virus-induced gene silencing technology and/or knocked out by Cas9 produce a nectariless phenotype. Point mutation and gene truncation simultaneously in duplicated genes Ne₁ Ne₂ lead to impaired nectary development in tetraploid cotton. There is no difference in the expression of the CRC and STY genes between the nectary TM-1 and the nectariless MD90ne in cotton. Therefore, the GoNe gene responsible for the formation of floral and extrafloral nectaries may be independent of CRC and STY. A complex mechanism might exist that restricts the nectary to a specific position with different genetic factors. Characterization of these target genes regulating nectary production has provided insights into the development, evolution, and function of nectaries and insect-resistant breeding.

Evolutionary and Ecological Considerations on Nectar-Mediated Tripartite Interactions in Angiosperms and Their Relevance in the Mediterranean Basin

The Mediterranean basin hosts a high diversity of plants and bees, and it is considered one of the world's biodiversity hotspots. Insect pollination, i.e., pollen transfer from male reproductive structures to conspecific female ones, was classically thought to be a mutualistic relationship that links these two groups of organisms, giving rise to an admirable and complex network of interactions. Although nectar is often involved in mediating these interactions, relatively little is known about modifications in its chemical traits during the evolution of plants. Here, we examine how the current sucrose-dominated floral nectar of most Mediterranean plants could have arisen in the course of evolution of angiosperms. The transition from hexose-rich to sucrose-rich nectar secretion was probably triggered by increasing temperature and aridity during the Cretaceous period, when most angiosperms were radiating. This transition may have opened new ecological niches for new groups of insects that were co-diversifying with angiosperms and for specific nectar-dwelling yeasts that originated later (i.e., Metschnikowiaceae). Our hypothesis embeds recent discoveries in nectar biology, such as the involvement of nectar microbiota and nectar secondary metabolites in shaping interactions with pollinators, and it suggests a complex, multifaceted ecological and evolutionary scenario that we are just beginning to discover.

The Genetic Control of Nectary Development

Nectar is the most important reward offered by flowering plants to pollinators for pollination services. Since pollinator decline has emerged as a major threat for agriculture, and the food demand is growing globally, studying the nectar gland is of utmost importance. Although the genetic mechanisms that control the development of angiosperm flowers have been quite well understood for many years, the development and maturation of the nectar gland and the secretion of nectar in synchrony with the maturation of the sexual organs appears to be one of the flower's best-kept secrets. Here we review key findings controlling these processes. We also raise key questions that need to be addressed to develop crop ecological functions that take into consideration pollinators' needs.

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.

Can the anatomy of abnormal flowers elucidate relationships of the androecial members in the ginger (Zingiberaceae)?

BACKGROUND

Interpretation of the floral structure of Zingiberaceae has long concentrated on the relationships of the androecial members. It suggested that labellum is composed of two structures rather than three or five, and glands are interpreted either as gynoecial part or as androecial members.

METHODS

Serial sections were used to observe the vasculature of normal and two-staminate flowers in Alpinia intermedia 'shengzhen'. Floral diagrams were drawn to interpret the morphological category of the floral organs and the relationships of the androecial members. Androecial vascular bundles were associated with carpellary dorsal bundles (CDBs) and parietal bundles (PBs) in a Zingiberales phylogeny setting using ancestral state reconstruction.

RESULTS

Anatomical observations demonstrate that the fertile stamen(s) incorporate parietal bundles both in normal and two-staminate flowers. The three appendages represent the three members of the outer whorl of the androecium, while the labellum represents the inner whorl of the androecium in the two-staminate flower. Reconstruction of the origin of the vascular system in the androecium suggests that the outer whorl of androecium receives its vascular supply from the CDBs, and the inner whorl of androecium receives from the PBs in both the basal banana group and the more derived ginger clade.

CONCLUSIONS

The present study adds to a growing body of literature suggesting that the anatomy of abnormal flowers may not provide enough evidence for elucidating the relationships of the androecial members, and help us to better understand how the vascular system is constructed during the androecial petaloidy evolution.

Floral Scent Emission from Nectaries in the Adaxial Side of the Innermost and Middle Petals in Chimonanthus praecox

Wintersweet (Chimonanthus praecox) is a well-known traditional fragrant plant and a winter-flowering deciduous shrub that originated in China. The five different developmental stages of wintersweet, namely, flower-bud period (FB), displayed petal stage (DP), open flower stage (OF), later blooming period (LB), and wilting period (WP) were studied using a scanning electron microscope (SEM) to determine the distribution characteristics of aroma-emitting nectaries. Results showed that the floral scent was probably emitted from nectaries distributed on the adaxial side of the innermost and middle petals, but almost none on the abaxial side. The nectaries in different developmental periods on the petals differ in numbers, sizes, and characteristics. Although the distribution of nectaries on different rounds of petals showed a diverse pattern at the same developmental periods, that of the nectaries on the same round of petals showed some of regularity. The nectary is concentrated on the adaxial side of the petals, especially in the region near the axis of the lower part of the petals. Based on transcriptional sequence and phylogenetic analysis, we report one nectary development related gene CpCRC (CRABS CLAW), and the other four YABBY family genes, CpFIL (FILAMENTOUS FLOWER), CpYABBY2, CpYABBY5-1, and CpYABBY5-2 in C. praecox (accession no. MH718960-MH718964). Quantitative RT-PCR (qRT-PCR) results showed that the expression characteristics of these YABBY family genes were similar to those of 11 floral scent genes, namely, CpSAMT, CpDMAPP, CpIPP, CpGPPS1, CpGPPS2, CpGPP, CpLIS, CpMYR1, CpFPPS, CpTER3, and CpTER5. The expression levels of these genes were generally higher in the lower part of the petals than in the upper halves in different rounds of petals, the highest being in the innermost petals, but the lowest in the outer petals. Relative expression level of CpFIL, CpCRC, CpYABBY5-1, and CpLIS in the innermost and middle petals in OF stages is significant higher than that of in outer petals, respectively. SEM and qRT-PCR results in C. praecox showed that floral scent emission is related to the distribution of nectaries.

The Octadecanoid Pathway, but Not COI1, Is Required for Nectar Secretion in Arabidopsis thaliana

Over 75% of crop species produce nectar and are dependent on pollinators to achieve maximum seed set, yet little is known about the mechanisms regulating nectar secretion. The phytohormone jasmonic acid (JA) is recognized to be involved in several plant processes including development and defense. JA was also recently shown to positively influence nectar secretion in both floral and extrafloral nectaries. For example, endogenous JA levels peak in flowers just prior to nectar secretion, but the details of how JA regulates nectar secretion have yet to be elucidated. We have found that the octadecanoid pathway does indeed play a role in the production and regulation of floral nectar in Arabidopsis. Null alleles for several JA biosynthesis and response genes had significantly reduced amounts of nectar, as well as altered expression of genes known to be involved in nectar production. We additionally identified crosstalk between the JA and auxin response pathways in nectaries. For example, the nectar-less JA synthesis mutant aos-2 showed no auxin response in nectaries, but both nectar production and the auxin response were restored upon exogenous JA and auxin treatment. Conversely, coi1-1, a JA-Ile-insensitive receptor mutant, displayed no auxin response in nectaries under any circumstance, even in older flowers that produced nectar. Surprisingly, opr3-1, a mutant for 12-oxophytodienoate reductase 3 [an enzyme further down the JA biosynthetic pathway that reduces 12-oxo phytodienoic acid (OPDA)], produced no nectar in newly opened flowers, but did secrete nectar in older flowers. Furthermore, a similar phenotype was observed in coi1-1. Cumulatively, these observations strongly suggest an indispensable role for an octadecanoic acid- and auxin-dependent, but JA- and COI1-dispensible, pathway in regulating nectar production in Arabidopsis.

Genome-wide identification, phylogeny, evolution, and expression patterns of MtN3/saliva/SWEET genes and functional analysis of BcNS in Brassica rapa

Background

Members of the MtN3/saliva/SWEET gene family are present in various organisms and are highly conserved. Their precise biochemical functions remain unclear, especially in Chinese cabbage. Based on the whole genome sequence, this study aims to identify the MtN3/saliva/SWEETs family members in Chinese cabbage and to analyze their classification, gene structure, chromosome distribution, phylogenetic relationship, expression pattern, and biological functions.

Results

We identified 34 SWEET genes in Chinese cabbage and analyzed their localization on chromosomes and transmembrane regions of their corresponding proteins. Analysis of a phylogenetic tree indicated that there were at least 17 supposed ancestor genes before the separation in Brassica rapa and Arabidopsis. The expression patterns of these genes in different tissues and flower developmental stages of Chinese cabbage showed that they are mainly involved in reproductive development. The Ka/Ks ratio between paralogous SWEET gene pairs of B. rapa were far less than 1. In our previous study, At2g39060 homologous gene Bra000116 (BraSWEET9, also named BcNS, Brassica Nectary and Stamen) played an important role during flower development in Chinese cabbage. Instantaneous expression experiments in onion epidermal cells showed that the gene encoding this protein is localized to the plasma membrane. A basal nectary split is the phenotype of transgenic plants transformed with the antisense expression vector.

Conclusion

This study is the first to perform a sequence analysis, structures analysis, physiological and biochemical characteristics analysis of the MtN3/saliva/SWEETs gene in Chinese cabbage and to verify the function of BcNS. A total of 34 SWEET genes were identified and they are distributed among ten chromosomes and one scaffold. The Ka/Ks ratio implies that the duplication genes suffered strong purifying selection for retention. These genes were differentially expressed in different floral organs. The phenotypes of the transgenic plants indicated that BcNs participates in the development of the floral nectary. This study provides a basis for further functional analysis of the MtN3/saliva/SWEETs gene family.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4554-8) contains supplementary material, which is available to authorized users.

Review: Nectar biology: From molecules to ecosystems

Plants attract mutualistic animals by offering a reward of nectar. Specifically, floral nectar (FN) is produced to attract pollinators, whereas extrafloral nectar (EFN) mediates indirect defenses through the attraction of mutualist predatory insects to limit herbivory. Nearly 90% of all plant species, including 75% of domesticated crops, benefit from animal-mediated pollination, which is largely facilitated by FN. Moreover, EFN represents one of the few defense mechanisms for which stable effects on plant health and fitness have been demonstrated in multiple systems, and thus plays a crucial role in the resistance phenotype of plants producing it. In spite of its central role in plant-animal interactions, the molecular events involved in the development of both floral and extrafloral nectaries (the glands that produce nectar), as well as the synthesis and secretion of the nectar itself, have been poorly understood until recently. This review will cover major recent developments in the understanding of (1) nectar chemistry and its role in plant-mutualist interactions, (2) the structure and development of nectaries, (3) nectar production, and (4) its regulation by phytohormones.

Transcriptional changes during ovule development in two genotypes of litchi (Litchi chinensis Sonn.) with contrast in seed size

Litchi chinensis is a subtropical fruit crop, popular for its nutritional value and taste. Fruits with small seed size and thick aril are desirable in litchi. To gain molecular insight into gene expression that leads to the reduction in the size of seed in Litchi chinensis, transcriptomes of two genetically closely related genotypes, with contrasting seed size were compared in developing ovules. The cDNA library constructed from early developmental stages of ovules (0, 6, and 14 days after anthesis) of bold- and small-seeded litchi genotypes yielded 303,778,968 high quality paired-end reads. These were de-novo assembled into 1,19,939 transcripts with an average length of 865 bp. A total of 10,186 transcripts with contrast in expression were identified in developing ovules between the small- and large- seeded genotypes. A majority of these differences were present in ovules before anthesis, thus suggesting the role of maternal factors in seed development. A number of transcripts indicative of metabolic stress, expressed at higher level in the small seeded genotype. Several differentially expressed transcripts identified in such ovules showed homology with Arabidopsis genes associated with different stages of ovule development and embryogenesis.

Floral nectar production and carbohydrate composition and the structure of receptacular nectaries in the invasive plant Bunias orientalis L. (Brassicaceae)

The data relating to the nectaries and nectar secretion in invasive Brassicacean taxa are scarce. In the present paper, the nectar production and nectar carbohydrate composition as well as the morphology, anatomy and ultrastructure of the floral nectaries in Bunias orientalis were investigated. Nectary glands were examined using light, fluorescence, scanning electron and transmission electron microscopy. The quantities of nectar produced by flowers and total sugar mass in nectar were relatively low. Total nectar carbohydrate production per 10 flowers averaged 0.3 mg. Nectar contained exclusively glucose (G) and fructose (F) with overall G/F ratio greater than 1. The flowers of B. orientalis have four nectaries placed at the base of the ovary. The nectarium is intermediate between two nectary types: the lateral and median nectary type (lateral and median glands stay separated) and the annular nectary type (both nectaries are united into one). Both pairs of glands represent photosynthetic type and consist of epidermis and glandular tissue. However, they differ in their shape, size, secretory activity, dimensions of epidermal and parenchyma cells, thickness of secretory parenchyma, phloem supply, presence of modified stomata and cuticle ornamentation. The cells of nectaries contain dense cytoplasm, plastids with starch grains and numerous mitochondria. Companion cells of phloem lack cell wall ingrowths. The ultrastructure of secretory cells indicates an eccrine mechanism of secretion. Nectar is exuded throughout modified stomata.

Evolution of the YABBY gene family in seed plants

Members of the YABBY gene family of transcription factors in angiosperms have been shown to be involved in the initiation of outgrowth of the lamina, the maintenance of polarity, and establishment of the leaf margin. Although most of the dorsal-ventral polarity genes in seed plants have homologs in non-spermatophyte lineages, the presence of YABBY genes is restricted to seed plants. To gain insight into the origin and diversification of this gene family, we reconstructed the evolutionary history of YABBY gene lineages in seed plants. Our findings suggest that either one or two YABBY genes were present in the last common ancestor of extant seed plants. We also examined the expression of YABBY genes in the gymnosperms Ephedra distachya (Gnetales), Ginkgo biloba (Ginkgoales), and Pseudotsuga menziesii (Coniferales). Our data indicate that some YABBY genes are expressed in a polar (abaxial) manner in leaves and female cones in gymnosperms. We propose that YABBY genes already acted as polarity genes in the last common ancestor of extant seed plants.

Annular floral nectary with oil-producing trichomes in Salvia farinacea (Lamiaceae): Anatomy, histochemistry, ultrastructure, and significance

PREMISE OF THE STUDY

Many angiosperms produce nectar that entices pollinator visits. Each floral nectary tends to embody a singular form, such as the receptacular ring arising beneath the ovary in mint flowers (Lamiaceae). Exceptionally, the annular floral nectary in Salvia farinacea possesses modified stomata plus secretory trichomes. This first study of nectary ultrastructure within the largest genus of Lamiaceae examined this unusual condition.

METHODS

Nectary anatomy, histochemistry, and ultrastructure were investigated from fresh and fixed material using light microscopy and scanning electron and transmission electron microscopy.

KEY RESULTS

The annular nectary encircled the ovary plus extended ventrally as a projection. Modified stomata occurred only in the projection's abaxial epidermis. Conversely, peltate trichomes with a basal cell, a stalk cell, and 4-7 head cells were interspersed among the ovary lobes and covered the projection's adaxial surface. Phloem and xylem supplied the nectary interior, where parenchyma cells had numerous mitochondria and plastids with little starch, but few dictyosomes and little endoplasmic reticulum. Nectar accumulated as a drop opposite the projection's abaxial surface, escaping through stomatal pores and probably the cuticle. However, the annular nectary's glistening trichomes secreted a Sudan-positive product largely retained below the distended cuticle, but not nectar.

CONCLUSIONS

This first ultrastructural study of co-occurring secretory trichomes and modified stomata on a mint nectary suggests multiple interactive functions for this atypical structure. These trichomes-possibly generating a substance informative to pollinators or as an ovarian defense against phytophagy-produced oil in an aqueous milieu, rather than contributing fluid to nectar.

BLADE-ON-PETIOLE genes: setting boundaries in development and defense

BLADE-ON-PETIOLE (BOP) genes encode an ancient and conserved subclade of BTB-ankryin transcriptional co-activators, divergent in the NPR1 family of plant defense regulators. Arabidopsis BOP1/2 were originally characterized as regulators of leaf and floral patterning. Recent investigation of BOP activity in a variety of land plants provides a more complete picture of their conserved functions at lateral organ boundaries in the determination of leaf, flower, inflorescence, and root nodule architecture. BOPs exert their function in part through promotion of lateral organ boundary genes including ASYMMETRIC LEAVES2, KNOTTED1-LIKE FROM ARABIDOPSIS6, and ARABIDOPSIS THALIANA HOMEOBOX GENE1 whose products restrict growth, promote differentiation, and antagonize meristem activity in various developmental contexts. Mutually antagonistic interactions between BOP and meristem factors are important in maintaining a border between meristem-organ compartments and in controlling irreversible transitions in cell fate associated with differentiation. We also examine intriguing new evidence for BOP function in plant defense. Comparisons to NPR1 highlight previously unexplored mechanisms for co-ordination of development and defense in land plants.

MADS reloaded: evolution of the AGAMOUS subfamily genes

AGAMOUS subfamily proteins are encoded by MADS-box family genes. They have been shown to play key roles in the determination of reproductive floral organs such as stamens, carpels and ovules. However, they also play key roles in ensuring a fixed number of floral organs by controlling floral meristem determinacy. Recently, an enormous amount of sequence data for nonmodel species have become available together with functional data on AGAMOUS subfamily members in many species. Here, we give a detailed overview of the most important information about this interesting gene subfamily and provide new insights into its evolution.

Meristem maturation and inflorescence architecture--lessons from the Solanaceae

Plant apical meristems (AMs) grow continuously by delicately balancing cells leaving at the periphery to form lateral organs with slowly dividing central domain cells that replenish reservoirs of pluripotent cells. This balance can be modified by signals originating from within and outside the meristem, and their integration results in a gradual maturation process that often culminates with the meristem differentiating into a flower. Accompanying this 'meristem maturation' are changes in spacing and size of lateral organs and in rates at which lateral meristems are released from apical dominance. Modulation of distinct meristem maturation parameters through environmental and genetic changes underlies the remarkable diversity of shoot architectures. Here, we discuss recent studies relating the dynamics of meristem maturation with organization of floral branching systems--inflorescences--in the nightshades. From this context, we suggest general principles on how factors coordinating meristem maturation impact shoot organization more broadly.

Genetic and phenotypic analyses of carpel development in Arabidopsis

Carpels are the female reproductive organs of the flower, organized in a gynoecium, which is arguably the most complex organ of a plant. The gynoecium provides protection for the ovules, helps to discriminate between male gametophytes, and facilitates successful pollination. After fertilization, it develops into a fruit, a specialized organ for seed protection and dispersal. To carry out all these functions, coordinated patterning and tissue specification within the developing gynoecium have to be achieved. In this chapter, we describe different methods to characterize defects in carpel patterning and morphogenesis associated with developmental mutations as well as a list of reporter lines that can be used to facilitate genetic analyses.

The evolution of floral nectaries in Disa (Orchidaceae: Disinae): recapitulation or diversifying innovation?

BACKGROUND AND AIMS

The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. However, orchids exhibit considerable diversity in nectary type, position and morphology, indicating that this convergence arose from alternative adaptive solutions. Using the genus Disa, this study asks whether repeated evolution of floral nectaries involved recapitulation of the same nectary type or diversifying innovation. Epidermis morphology of closely related nectar-producing and nectarless species is also compared in order to identify histological changes that accompanied the gain or loss of nectar production.

METHODS

The micromorphology of nectaries and positionally equivalent tissues in nectarless species was examined with light and scanning electron microscopy. This information was subjected to phylogenetic analyses to reconstruct nectary evolution and compare characteristics of nectar-producing and nectarless species.

KEY RESULTS

Two nectary types evolved in Disa. Nectar exudation by modified stomata in floral spurs evolved twice, whereas exudation by a secretory epidermis evolved six times in different perianth segments. The spur epidermis of nectarless species exhibited considerable micromorphological variation, including strongly textured surfaces and non-secreting stomata in some species. Epidermis morphology of nectar-producing species did not differ consistently from that of rewardless species at the magnifications used in this study, suggesting that transitions from rewardlessness to nectar production are not necessarily accompanied by visible morphological changes but only require sub-cellular modification.

CONCLUSIONS

Independent nectary evolution in Disa involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. Such functional convergence generated by morphologically diverse solutions probably also underlies the extensive diversity of nectary types and positions in the Orchidaceae.

Diversity and evolution of a trait mediating ant-plant interactions: insights from extrafloral nectaries in Senna (Leguminosae)

BACKGROUND AND AIMS

Plants display a wide range of traits that allow them to use animals for vital tasks. To attract and reward aggressive ants that protect developing leaves and flowers from consumers, many plants bear extrafloral nectaries (EFNs). EFNs are exceptionally diverse in morphology and locations on a plant. In this study the evolution of EFN diversity is explored by focusing on the legume genus Senna, in which EFNs underwent remarkable morphological diversification and occur in over 80 % of the approx. 350 species.

METHODS

EFN diversity in location, morphology and plant ontogeny was characterized in wild and cultivated plants, using scanning electron microscopy and microtome sectioning. From these data EFN evolution was reconstructed in a phylogenetic framework comprising 83 Senna species.

KEY RESULTS

Two distinct kinds of EFNs exist in two unrelated clades within Senna. 'Individualized' EFNs (iEFNs), located on the compound leaves and sometimes at the base of pedicels, display a conspicuous, gland-like nectary structure, are highly diverse in shape and characterize the species-rich EFN clade. Previously overlooked 'non-individualized' EFNs (non-iEFNs) embedded within stipules, bracts, and sepals are cryptic and may represent a new synapomorphy for clade II. Leaves bear EFNs consistently throughout plant ontogeny. In one species, however, early seedlings develop iEFNs between the first pair of leaflets, but later leaves produce them at the leaf base. This ontogenetic shift reflects our inferred diversification history of iEFN location: ancestral leaves bore EFNs between the first pair of leaflets, while leaves derived from them bore EFNs either between multiple pairs of leaflets or at the leaf base.

CONCLUSIONS

EFNs are more diverse than previously thought. EFN-bearing plant parts provide different opportunities for EFN presentation (i.e. location) and individualization (i.e. morphology), with implications for EFN morphological evolution, EFN-ant protective mutualisms and the evolutionary role of EFNs in plant diversification.

The phylogenetic distribution of extrafloral nectaries in plants

BACKGROUND AND AIMS

Understanding the evolutionary patterns of ecologically relevant traits is a central goal in plant biology. However, for most important traits, we lack the comprehensive understanding of their taxonomic distribution needed to evaluate their evolutionary mode and tempo across the tree of life. Here we evaluate the broad phylogenetic patterns of a common plant-defence trait found across vascular plants: extrafloral nectaries (EFNs), plant glands that secrete nectar and are located outside the flower. EFNs typically defend plants indirectly by attracting invertebrate predators who reduce herbivory.

METHODS

Records of EFNs published over the last 135 years were compiled. After accounting for changes in taxonomy, phylogenetic comparative methods were used to evaluate patterns of EFN evolution, using a phylogeny of over 55 000 species of vascular plants. Using comparisons of parametric and non-parametric models, the true number of species with EFNs likely to exist beyond the current list was estimated.

KEY RESULTS

To date, EFNs have been reported in 3941 species representing 745 genera in 108 families, about 1-2 % of vascular plant species and approx. 21 % of families. They are found in 33 of 65 angiosperm orders. Foliar nectaries are known in four of 36 fern families. Extrafloral nectaries are unknown in early angiosperms, magnoliids and gymnosperms. They occur throughout monocotyledons, yet most EFNs are found within eudicots, with the bulk of species with EFNs being rosids. Phylogenetic analyses strongly support the repeated gain and loss of EFNs across plant clades, especially in more derived dicot families, and suggest that EFNs are found in a minimum of 457 independent lineages. However, model selection methods estimate that the number of unreported cases of EFNs may be as high as the number of species already reported.

CONCLUSIONS

EFNs are widespread and evolutionarily labile traits that have repeatedly evolved a remarkable number of times in vascular plants. Our current understanding of the phylogenetic patterns of EFNs makes them powerful candidates for future work exploring the drivers of their evolutionary origins, shifts, and losses.

A novel indirect defence in Brassicaceae: structure and function of extrafloral nectaries in Brassica juncea

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket-like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above- and belowground herbivores and sap-sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.

Asymmetric morphogenetic cues along the transverse plane: shift from disymmetry to zygomorphy in the flower of Fumarioideae

PREMISE OF THE STUDY

Zygomorphy has evolved multiple times in angiosperms. Near-actinomorphy is the ancestral state in the early diverging eudicot family Papaveraceae. Zygomorphy evolved once in the subfamily Fumarioideae from a disymmetric state. Unusual within angiosperms, zygomorphy takes place along the transverse plane of the flower.

METHODS

We investigated floral development to understand the developmental bases of the evolution of floral symmetry in Papaveraceae. We then assessed the expression of candidate genes for the key developmental events responsible for the shift from disymmetry to transverse zygomorphy, namely CrabsClaw for nectary formation (PapCRC), ShootMeristemless (PapSTL) for spur formation, and Cycloidea (PapCYL) for growth control.

KEY RESULTS

We found that an early disymmetric groundplan is common to all species studied, and that actinomorphy was acquired after sepal initiation in Papaveroideae. The shift from disymmetry to zygomorphy in Fumarioideae was associated with early asymmetric growth of stamen filaments, followed by asymmetric development of nectary outgrowth and spur along the transverse plane. Patterns of PapSTL expression could not be clearly related to spur formation. PapCRC and PapCYL genes were expressed in the nectary outgrowths, with a pattern of expression correlated with asymmetric nectary development in the zygomorphic species. Additionally, PapCYL genes were found asymmetrically expressed along the transverse plane in the basal region of outer petals in the zygomorphic species.

CONCLUSION

Genes of PapCRC and PapCYL families could be direct or indirect targets of the initial transversally asymmetric cue responsible for the shift from disymmetry to zygomorphy in Fumarioideae.

Stomatal development in Arabidopsis

Stomata consist of two guard cells that function as turgor-operated valves that regulate gas exchange in plants. In Arabidopsis, a dedicated cell lineage is initiated and undergoes a series of cell divisions and cell-state transitions to produce a stoma. A set of basic helix-loop-helix (bHLH) transcription factors regulates the transition and differentiation events through the lineage, while the placement of stomata relative to each other is controlled by intercellular signaling via peptide ligands, transmembrane receptors, and mitogen-activated protein kinase (MAPK) modules. Some genes involved in regulating stomatal differentiation or density are also involved in hormonal and environmental stress responses, which may provide a link between modulation of stomatal development or function in response to changes in the environment. Premitotic polarlylocalized proteins provide an added layer of regulation, which can be addressed more thoroughly with the identification of additional proteins in this pathway. Linking the networks that control stomatal development promises to bring advances to our understanding of signal transduction, cell polarity, and cell-fate specification in plants.

Knockdown of MYB305 disrupts nectary starch metabolism and floral nectar production

MYB transcription factors have important roles during floral organ development. In this study, we generated myb305 RNAi knockdown tobacco plants and studied the role of MYB305 in the growth of the floral nectary. We have previously shown the MYB305 regulates the expression of flavonoid metabolic genes as well as of nectar proteins (nectarins); however, the myb305 plants showed other floral phenotypes that we investigate in these studies. The nectaries of myb305 plants show juvenile character at late stages of development and secrete reduced levels of nectar. Because starch metabolism is intimately involved in nectar secretion and is strongly regulated during normal nectary development, we examined the accumulation of starch in the nectaries of the myb305 plants. The myb305 plants accumulated lower levels of starch in their nectaries than did wild-type plants. The reduced starch correlated with the reduced expression of the ATP-glucose pyrophosphorylase (small subunit) gene in nectaries of the myb305 plants during the starch biosynthetic phase. Expression of genes encoding several starch-degrading enzymes including β-amylase, isoamylase 3, and α-amylase was also reduced in the myb305 plants. In addition to regulating nectarin and flavonoid metabolic gene expression, these results suggest that MYB305 may also function in the tobacco nectary maturation program by controlling the expression of starch metabolic genes.

Temporal and spatial regulation of DROOPING LEAF gene expression that promotes midrib formation in rice

Genes involved in the differentiation and development of tissues and organs are temporally and spatially regulated in plant development. The DROOPING LEAF (DL) gene, a member of the YABBY gene family, promotes midrib formation in the leaf and carpel specification in the flower. Consistent with these functions, DL is initially expressed in the central region of the leaf primordia (presumptive midrib) and in the presumptive carpel primordia in the meristem. To understand the regulatory mechanism underlying DL expression, we tried to identify cis-regulatory regions required for temporal and spatial expression of this gene. We found that the cis region responsible for the presumptive midrib-specific expression in the leaf primordia is located in intron 2. Next, we confined the region to a sequence of about 200bp, which corresponds to a conserved non-coding sequence (CNS) identified by phylogenetic footprinting. In addition, a sequence termed DG1, incorporating a 5' upstream region of about 7.4kb, and introns 1 and 2, was shown to be sufficient to induce DL in the presumptive midrib, and to suppress it in other regions in the leaf primordia. By contrast, the regulatory region required for carpel-specific expression was not included in the DG1 sequence. We modified Oryza sativa (rice) plant architecture by expressing an activated version of DL (DL-VP16) in a precise manner using the DG1 sequence: the resulting transgenic plant produced a midrib in the distal region of the leaf blade, where there is no midrib in wild type, and formed more upright leaves compared with the wild type.

Expression patterns of AaDL, a CRABS CLAW ortholog in Asparagus asparagoides (Asparagaceae), demonstrate a stepwise evolution of CRC/DL subfamily of YABBY genes

Understanding the evolutionary path of transcription factors is essential for the elucidation of plant evolution. The CRC/DL subfamily of the YABBY gene family are functionally diverse, plant-specific, putative transcription factors. In Arabidopsis thaliana, CRABS CLAW (CRC) is expressed in the abaxial region of carpel primordia and in floral nectaries, where it regulates carpel morphology and nectary development. By contrast, in Oryza sativa, DROOPING LEAF (DL) is expressed in the entire carpel primordium and in the central undifferentiated cells of leaves, where it regulates carpel identity and midrib development. Recent studies suggest that abaxial expression and functional roles in the carpel are ancestral characters, although when and how neo-functionalizations occurred remains unclear. To elucidate the evolutionary processes of the CRC/DL subfamily, we examined in situ expression patterns of a CRC ortholog (AaDL) in Asparagus asparagoides (Asparagales). Like CRC in Arabidopsis thaliana, AaDL was clearly expressed in the abaxial region of the ovary wall. Expression was also detected in the phloem of leaves, but not in the septal nectary. Thus, expression in the entire carpel primordium might have been acquired after the divergence of Asparagus, with expression competence in the leaves acquired before the divergence of Asparagus in monocots. Our data indicate that the evolution of CRC/DL subfamily genes occurred in a stepwise manner.

Flower development

Flowers are the most complex structures of plants. Studies of Arabidopsis thaliana, which has typical eudicot flowers, have been fundamental in advancing the structural and molecular understanding of flower development. The main processes and stages of Arabidopsis flower development are summarized to provide a framework in which to interpret the detailed molecular genetic studies of genes assigned functions during flower development and is extended to recent genomics studies uncovering the key regulatory modules involved. Computational models have been used to study the concerted action and dynamics of the gene regulatory module that underlies patterning of the Arabidopsis inflorescence meristem and specification of the primordial cell types during early stages of flower development. This includes the gene combinations that specify sepal, petal, stamen and carpel identity, and genes that interact with them. As a dynamic gene regulatory network this module has been shown to converge to stable multigenic profiles that depend upon the overall network topology and are thus robust, which can explain the canalization of flower organ determination and the overall conservation of the basic flower plan among eudicots. Comparative and evolutionary approaches derived from Arabidopsis studies pave the way to studying the molecular basis of diverse floral morphologies.

The spatial expression patterns of DROOPING LEAF orthologs suggest a conserved function in grasses

The DROOPING LEAF (DL) gene regulates carpel specification in the flower and midrib formation in the leaf in Oryza sativa (rice). Loss-of-function mutations in the dl locus cause homeotic transformation of carpels into stamens and lack of midrib, resulting in the drooping leaf phenotype. DL is a member of the YABBY gene family and is closely related to the CRABS CLAW (CRC) gene in Arabidopsis thaliana. The function of Arabidopsis CRC, however, differs from that of rice DL: it is responsible for nectary development and is partially involved in carpel identity. Thus, genes related to DL/CRC seem to have functionally diversified during angiosperm evolution. To assess the conservation of DL function in related species, here we examined the in situ expression patterns of DL orthologs in three grass species, i.e., maize, wheat and sorghum, which is assigned to subfamilies different from Ehrhartoideae including O. sativa. The results clearly show that the temporal and spatial expression patterns of DL orthologs in the three species are identical to those of rice DL in both flower and leaf development, suggesting that DL-related genes are functionally conserved within the grass family. It is likely that DL may have been recruited to carpel specification and midrib formation within the lineage of the grass family after divergence of their ancestor from that of eudicots.

Nectar and pollination drops: how different are they?

BACKGROUND

Pollination drops and nectars (floral nectars) are secretions related to plant reproduction. The pollination drop is the landing site for the majority of gymnosperm pollen, whereas nectar of angiosperm flowers represents a common nutritional resource for a large variety of pollinators. Extrafloral nectars also are known from all vascular plants, although among the gymnosperms they are restricted to the Gnetales. Extrafloral nectars are not generally involved in reproduction but serve as 'reward' for ants defending plants against herbivores (indirect defence).

SCOPE

Although very different in their task, nectars and pollination drops share some features, e.g. basic chemical composition and eventual consumption by animals. This has led some authors to call these secretions collectively nectar. Modern techniques that permit chemical analysis and protein characterization have very recently added important information about these sugary secretions that appear to be much more than a 'reward' for pollinating (floral nectar) and defending animals (extrafloral nectar) or a landing site for pollen (pollination drop).

CONCLUSIONS

Nectar and pollination drops contain sugars as the main components, but the total concentration and the relative proportions are different. They also contain amino acids, of which proline is frequently the most abundant. Proteomic studies have revealed the presence of common functional classes of proteins such as invertases and defence-related proteins in nectar (floral and extrafloral) and pollination drops. Invertases allow for dynamic rearrangement of sugar composition following secretion. Defence-related proteins provide protection from invasion by fungi and bacteria. Currently, only few species have been studied in any depth. The chemical composition of the pollination drop must be investigated in a larger number of species if eventual phylogenetic relationships are to be revealed. Much more information can be provided from further proteomic studies of both nectar and pollination drop that will contribute to the study of plant reproduction and evolution.

Uncovering the Arabidopsis thaliana nectary transcriptome: investigation of differential gene expression in floral nectariferous tissues

BACKGROUND

Many flowering plants attract pollinators by offering a reward of floral nectar. Remarkably, the molecular events involved in the development of nectaries, the organs that produce nectar, as well as the synthesis and secretion of nectar itself, are poorly understood. Indeed, to date, no genes have been shown to directly affect the de novo production or quality of floral nectar. To address this gap in knowledge, the ATH1 Affymetrix GeneChip array was used to systematically investigate the Arabidopsis nectary transcriptome to identify genes and pathways potentially involved in nectar production.

RESULTS

In this study, we identified a large number of genes differentially expressed between secretory lateral nectaries and non-secretory median nectary tissues, as well as between mature lateral nectaries (post-anthesis) and immature lateral nectaries (pre-anthesis). Expression within nectaries was also compared to thirteen non-nectary reference tissues, from which 270 genes were identified as being significantly upregulated in nectaries. The expression patterns of 14 nectary-enriched genes were also confirmed via RT PCR. Upon looking into functional groups of upregulated genes, pathways involved in gene regulation, carbohydrate metabolism, and lipid metabolism were particularly enriched in nectaries versus reference tissues.

CONCLUSION

A large number of genes preferentially expressed in nectaries, as well as between nectary types and developmental stages, were identified. Several hypotheses relating to mechanisms of nectar production and regulation thereof are proposed, and provide a starting point for reverse genetics approaches to determine molecular mechanisms underlying nectar synthesis and secretion.

Independence and interaction of regions of the INNER NO OUTER protein in growth control during ovule development

The outer integument of the Arabidopsis (Arabidopsis thaliana) ovule develops asymmetrically, with growth and cell division occurring primarily along the region of the ovule facing the base of the gynoecium (gynobasal). This process is altered in the mutants inner no outer (ino) and superman (sup), which lead to absent or symmetrical growth of the outer integument, respectively. INO encodes a member of the YABBY family of putative transcription factors, and its expression is restricted to the gynobasal side of developing ovules via negative regulation by the transcription factor SUP. Other YABBY proteins (e.g. CRABS CLAW [CRC] and YABBY3 [YAB3]) can substitute for INO in promotion of integument growth, but do not respond to SUP regulation. In contrast, YAB5 fails to promote integument growth. To separately investigate the growth-promotive effects of INO and its inhibition by SUP, domain swaps between INO and YAB3, YAB5, or CRC were assembled. The ability of chimeric YABBY proteins to respond to SUP restriction showed a quantitative response proportional to the amount of INO protein and was more dependent on C-terminal regions of INO. A different response was seen when examining growth promotion where the number and identity of regions of INO in chimeric YABBY proteins were not the primary influence on promotion of outer integument growth. Instead, promotion of growth required a coordination of features along the entire length of the INO protein, suggesting that intramolecular interactions between regions of INO may coordinately facilitate the intermolecular interactions necessary to promote formation of the outer integument.

The Arabidopsis zinc finger-homeodomain genes encode proteins with unique biochemical properties that are coordinately expressed during floral development

Arabidopsis (Arabidopsis thaliana) contains approximately 100 homeobox genes, many of which have been shown to play critical roles in various developmental processes. Here we characterize the zinc finger-homeodomain (ZF-HD) subfamily of homeobox genes, consisting of 14 members in Arabidopsis. We demonstrate that the HDs of the ZF-HD proteins share some similarities with other known HDs in Arabidopsis, but they contain distinct features that cluster them as a unique class of plant HD-containing proteins. We have carried out mutational analyses to show that the noncanonical residues present in the HDs of this family of proteins are important for function. Yeast (Saccharomyces cerevisiae) two-hybrid matrix analyses of the ZF-HD proteins reveal that these proteins both homo- and heterodimerize, which may contribute to greater selectivity in DNA binding. These assays also show that most of these proteins do not contain an intrinsic activation domain, suggesting that interactions with other factors are required for transcriptional activation. We also show that the family members are all expressed predominantly or exclusively in floral tissue, indicating a likely regulatory role during floral development. Furthermore, we have identified loss-of-function mutations for six of these genes that individually show no obvious phenotype, supporting the idea that the encoded proteins have common roles in floral development. Based on these results, we propose the ZF-HD gene family encodes a group of transcriptional regulators with unique biochemical activities that play overlapping regulatory roles in Arabidopsis floral development.

Role of auxin in regulating Arabidopsis flower development

To elucidate the role of auxin in flower morphogenesis, its distribution patterns were studied during flower development in Arabidopsis thaliana (L.) Heynh. Expression of DR5::GUS was regarded to reflect sites of free auxin, while immunolocalization with auxin polyclonal antibodies visualized conjugated auxin distribution. The youngest flower bud was loaded with conjugated auxin. During development, the apparent concentration of free auxin increased in gradual patterns starting at the floral-organ tip. Anthers are major sites of high concentrations of free auxin that retard the development of neighboring floral organs in both the acropetal and basipetal directions. The IAA-producing anthers synchronize flower development by retarding petal development and nectary gland activity almost up to anthesis. Tapetum cells of young anthers contain free IAA which accumulates in pollen grains, suggesting that auxin promotes pollen-tube growth towards the ovules. High amounts of free auxin in the stigma induce a wide xylem fan immediately beneath it. After fertilization, the developing embryos and seeds show elevated concentrations of auxin, which establish their axial polarity. This developmental pattern of auxin production during floral-bud development suggests that young organs which produce high concentrations of free IAA inhibit or retard organ-primordium initiation and development at the shoot tip.

Multiple protein regions contribute to differential activities of YABBY proteins in reproductive development

Members of the YABBY family of putative transcription factors participate in abaxial-adaxial identity determination in lateral organs in Arabidopsis (Arabidopsis thaliana). Two YABBY genes specifically expressed in reproductive structures, CRABS CLAW (CRC) and INNER NO OUTER (INO), have additional activities, with CRC promoting nectary development and carpel fusion, and INO responding to spatial regulation by SUPERMAN during ovule development. All YABBY coding regions, except YABBY5, were able to restore outer integument growth in ino-1 mutants when expressed from the INO promoter (PRO(INO)). However, INO was the only YABBY family member that responded correctly to SUPERMAN to maintain the wild-type gynoapical-gynobasal asymmetry of the outer integument. By contrast, INO, FILAMENTOUS FLOWER, and YABBY3 failed to complement crc-1 when expressed from PRO(CRC). Roles of individual regions of CRC and INO in these effects were assessed using chimeric proteins with PRO(INO) and PRO(CRC) and the relatively constitutive cauliflower mosaic virus PRO(35S). Regions of CRC were found to contribute additively to CRC-specific functions in nectary and carpel formation, with a nearly direct relationship between the amount of CRC included and the degree of complementation of crc-1. When combined with INO sequences, the central and carboxyl-terminal regions of CRC were individually sufficient to overcome inhibitory effects of SUPERMAN within the outer integument. Reproductive phenotypes resulting from constitutive expression were dependent on the nature of the central region with some contributions from the amino terminus. Thus, the YABBY family members have both unique and common functional capacities, and residues involved in differential activities are distributed throughout the protein sequences.