Identification of differential gene expression in Brassica rapa nectaries through expressed sequence tag analysis

A cell wall invertase controls nectar volume and sugar composition

Nectar volume and sugar composition are key determinants of the strength of plant-pollinator mutualisms. The main nectar sugars are sucrose, glucose and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose-dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa. Moreover, a brcwinv4a null mutant: (i) has greatly reduced CWINV activity in the nectaries; (ii) produces a sucrose-rich nectar; but (iii) with significantly less volume. These results definitively demonstrate that CWINV activity is not only essential for the production of a hexose-rich nectar, but also support a hypothetical model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular-to-extracellular sucrose ratio that facilitates the continuous export of sucrose into the nectary apoplast. The extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. These results cumulatively indicate that modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition. Finally, honeybees prefer nectars with some sucrose, but wild-type B. rapa flowers were much more heavily visited than flowers of brcwinv4a, suggesting that the potentially attractive sucrose-rich nectar of brcwinv4a could not compensate for its low volume.

Nectar biosynthesis is conserved among floral and extrafloral nectaries

Nectar is a primary reward mediating plant-animal mutualisms to improve plant fitness and reproductive success. Four distinct trichomatic nectaries develop in cotton (Gossypium hirsutum), one floral and three extrafloral, and the nectars they secrete serve different purposes. Floral nectar attracts bees for promoting pollination, while extrafloral nectar attracts predatory insects as a means of indirect protection from herbivores. Cotton therefore provides an ideal system for contrasting mechanisms of nectar production and nectar composition between different nectary types. Here, we report the transcriptome and ultrastructure of the four cotton nectary types throughout development and compare these with the metabolomes of secreted nectars. Integration of these datasets supports specialization among nectary types to fulfill their ecological niche, while conserving parallel coordination of the merocrine-based and eccrine-based models of nectar biosynthesis. Nectary ultrastructures indicate an abundance of rough endoplasmic reticulum positioned parallel to the cell walls and a profusion of vesicles fusing to the plasma membranes, supporting the merocrine model of nectar biosynthesis. The eccrine-based model of nectar biosynthesis is supported by global transcriptomics data, which indicate a progression from starch biosynthesis to starch degradation and sucrose biosynthesis and secretion. Moreover, our nectary global transcriptomics data provide evidence for novel metabolic processes supporting de novo biosynthesis of amino acids secreted in trace quantities in nectars. Collectively, these data demonstrate the conservation of nectar-producing models among trichomatic and extrafloral nectaries.

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.

Liquid profiling in plants: identification and analysis of extracellular metabolites and miRNAs in pollination drops of Ginkgo biloba

The pollination drop (PD), also known as an ovular secretion, is a critical feature of most wind-pollinated gymnosperms and function as an essential component of pollination systems. However, the metabolome and small RNAs of gymnosperm PDs are largely unknown. We employed gas chromatography-mass spectrometry to identify a total of 101 metabolites in Ginkgo biloba L. PDs. The most abundant metabolites were sugars (45.70%), followed by organic acids (15.94%) and alcohols (15.39%) involved in carbohydrate metabolism, glycine, serine and threonine metabolism. Through pollen culture of the PDs, we further demonstrated that the metabolic components of PDs are indispensable for pollen germination and growth; in particular, organic acids and fatty acids play defensive roles against microbial activity. In addition, we successfully constructed a small RNA library and detected 45 known and 550 novel miRNAs in G. biloba PDs. Interestingly, in a comparative analysis of miRNA expression between PDs and ovules, we found that most of the known miRNAs identified in PDs were also expressed in the ovules, implying that miRNAs in PDs may originate from ovules. Further, combining with potential target prediction, degradome validation and transcriptome sequencing, we identified that the interactions of several known miRNAs and their targets in PDs are involved in carbohydrate metabolism, hormone signaling and defense response pathways, consistent with the metabolomics results. Our results broaden the knowledge of metabolite profiling and potential functional roles in gymnosperm PDs and provide the first evidence of extracellular miRNA functions in ovular secretions from gymnosperms.

QTLian breeding for climate resilience in cereals: progress and prospects

The ever-rising population of the twenty-first century together with the prevailing challenges, such as deteriorating quality of arable land and water, has placed a big challenge for plant breeders to satisfy human needs for food under erratic weather patterns. Rice, wheat, and maize are the major staple crops consumed globally. Drought, waterlogging, heat, salinity, and mineral toxicity are the key abiotic stresses drastically affecting crop yield. Conventional plant breeding approaches towards abiotic stress tolerance have gained success to limited extent, due to the complex (multigenic) nature of these stresses. Progress in breeding climate-resilient crop plants has gained momentum in the last decade, due to improved understanding of the physiochemical and molecular basis of various stresses. A good number of genes have been characterized for adaptation to various stresses. In the era of novel molecular markers, mapping of QTLs has emerged as viable solution for breeding crops tolerant to abiotic stresses. Therefore, molecular breeding-based development and deployment of high-yielding climate-resilient crop cultivars together with climate-smart agricultural practices can pave the path to enhanced crop yields for smallholder farmers in areas vulnerable to the climate change. Advances in fine mapping and expression studies integrated with cheaper prices offer new avenues for the plant breeders engaged in climate-resilient plant breeding, and thereby, hope persists to ensure food security in the era of climate change.

An integrated transcriptomics and metabolomics analysis of the Cucurbita pepo nectary implicates key modules of primary metabolism involved in nectar synthesis and secretion

Nectar is the main reward that flowers offer to pollinators to entice repeated visitation. Cucurbita pepo (squash) is an excellent model for studying nectar biology, as it has large nectaries that produce large volumes of nectar relative to most other species. Squash is also monoecious, having both female and male flowers on the same plant, which allows comparative analyses of nectary function in one individual. Here, we report the nectary transcriptomes from both female and male nectaries at four stages of floral maturation. Analysis of these transcriptomes and subsequent confirmatory experiments revealed a metabolic progression in nectaries leading from starch synthesis to starch degradation and to sucrose biosynthesis. These results are consistent with previously published models of nectar secretion and also suggest how a sucrose-rich nectar can be synthesized and secreted in the absence of active transport across the plasma membrane. Nontargeted metabolomic analyses of nectars also confidently identified 40 metabolites in both female and male nectars, with some displaying preferential accumulation in nectar of either male or female flowers. Cumulatively, this study identified gene targets for reverse genetics approaches to study nectary function, as well as previously unreported nectar metabolites that may function in plant-biotic interactions.

The major nectar protein of Brassica rapa is a non-specific lipid transfer protein, BrLTP2.1, with strong antifungal activity

Nectar is one of the key rewards mediating plant-mutualist interactions. In addition to sugars, nectars often contain many other compounds with important biological functions, including proteins. This study was undertaken to assess the proteinaceous content of Brassica rapa nectar. SDS-PAGE analysis of raw B. rapa nectar revealed the presence of ~10 proteins, with a major band at ~10 kDa. This major band was found to contain a non-specific lipid transfer protein encoded by B. rapa locus Bra028980 and subsequently termed BrLTP2.1. Sequence analysis of BrLTP2.1 predicted the presence of a signal peptide required for secretion from the cell, eight cysteines, and a mature molecular mass of 7.3 kDa. Constitutively expressed BrLTP2.1-GFP in Arabidopsis displayed accumulation patterns consistent with secretion from nectary cells. BrLTP2.1 was also found to have relatively high sequence similarity to non-specific lipid-transfer proteins with known functions in plant defense, including Arabidopsis DIR1. Heterologously expressed and purified BrLTP2.1 was extremely heat stable and bound strongly to saturated free fatty acids, but not methyl jasmonate. Recombinant BrLTP2.1 also had direct antimicrobial activity against an extensive range of plant pathogens, being particularly effective against necrotrophic fungi. Taken together, these results suggest that BrLTP2.1 may function to prevent microbial growth in nectars.

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.

Improvement of Drought Tolerance in Rice (Oryza sativa L.): Genetics, Genomic Tools, and the WRKY Gene Family

Drought tolerance is an important quantitative trait with multipart phenotypes that are often further complicated by plant phenology. Different types of environmental stresses, such as high irradiance, high temperatures, nutrient deficiencies, and toxicities, may challenge crops simultaneously; therefore, breeding for drought tolerance is very complicated. Interdisciplinary researchers have been attempting to dissect and comprehend the mechanisms of plant tolerance to drought stress using various methods; however, the limited success of molecular breeding and physiological approaches suggests that we rethink our strategies. Recent genetic techniques and genomics tools coupled with advances in breeding methodologies and precise phenotyping will likely reveal candidate genes and metabolic pathways underlying drought tolerance in crops. The WRKY transcription factors are involved in different biological processes in plant development. This zinc (Zn) finger protein family, particularly members that respond to and mediate stress responses, is exclusively found in plants. A total of 89 WRKY genes in japonica and 97 WRKY genes in O. nivara (OnWRKY) have been identified and mapped onto individual chromosomes. To increase the drought tolerance of rice (Oryza sativa L.), research programs should address the problem using a multidisciplinary strategy, including the interaction of plant phenology and multiple stresses, and the combination of drought tolerance traits with different genetic and genomics approaches, such as microarrays, quantitative trait loci (QTLs), WRKY gene family members with roles in drought tolerance, and transgenic crops. This review discusses the newest advances in plant physiology for the exact phenotyping of plant responses to drought to update methods of analysing drought tolerance in rice. Finally, based on the physiological/morphological and molecular mechanisms found in resistant parent lines, a strategy is suggested to select a particular environment and adapt suitable germplasm to that environment.

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.

The pennycress (Thlaspi arvense L.) nectary: structural and transcriptomic characterization

BACKGROUND

Pennycress [Thlaspi arvense L (Brassicaceae)] is being domesticated as a renewable biodiesel feedstock that also provides crucial ecosystems services, including as a nutritional resource for pollinators. However, its flowers produce significantly less nectar than other crop relatives in the Brassicaceae. This study was undertaken to understand the basic biology of the pennycress nectary as an initial step toward the possibility of enhancing nectar output from its flowers.

RESULTS

Pennycress flowers contain four equivalent nectaries located extrastaminally at the base of the insertion sites of short and long stamens. Like other Brassicaceae, the nectaries have open stomates on their surface, which likely serve as the sites of nectar secretion. The nectaries produce four distinct nectar droplets that accumulate in concave structures at the base of each of the four petals. To understand the molecular biology of the pennycress nectary, RNA was isolated from 'immature' (pre-secretory) and 'mature' (secretory) nectaries and subjected to RNA-seq. Approximately 184 M paired-end reads (368 M total reads) were de novo assembled into a total of 16,074 independent contigs, which mapped to 12,335 unique genes in the pennycress genome. Nearly 3700 genes were found to be differentially expressed between immature and mature nectaries and subjected to gene ontology and metabolic pathway analyses. Lastly, in silico analyses identified 158 pennycress orthologs to Arabidopsis genes with known enriched expression in nectaries. These nectary-enriched expression patterns were verified for select pennycress loci by semi-quantitative RT-PCR.

CONCLUSIONS

Pennycress nectaries are unique relative to those of other agriculturally important Brassicaceae, as they contain four equivalent nectaries that present their nectar in specialized cup-shaped structures at the base of the petals. In spite of these morphological differences, the genes underlying the regulation and production of nectar appear to be largely conserved between pennycress and Arabidopsis thaliana. These results provide a starting point for using forward and reverse genetics approaches to enhance nectar synthesis and secretion in pennycress.

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.

Comparison of the structure of floral nectaries in two Euonymus L. species (Celastraceae)

The inconspicuous Euonymus L. flowers are equipped with open receptacular floral nectaries forming a quadrilateral green disc around the base of the superior ovary. The morphology and anatomy of the nectaries in Euonymus fortunei (Turcz.) Hand.-Mazz. and Euonymus europaeus L. flowers were analysed under a bright-field light microscope as well as stereoscopic and scanning electron microscopes. Photosynthetic nectaries devoid of the vascular tissue were found in both species. Nectar was exuded through typical nectarostomata (E. fortunei) or nectarostomata and secretory cell cuticle (E. europaeus). The nectaries of the examined species differed in their width and height, number of layers and thickness of secretory parenchyma, and the height of epidermal cells. Moreover, there were differences in the location and abundance of nectarostomata and the content of starch and phenolic compounds.

The F-box protein COI1 functions upstream of MYB305 to regulate primary carbohydrate metabolism in tobacco (Nicotiana tabacum L. cv. TN90)

Jasmonate (JA) plays an important role in regulating plant male fertility and secondary metabolism, but its role in regulating primary metabolism remains unclear. The F-box protein CORONATINE INSENSITIVE 1 (COI1) is a critical component of the JA receptor, and mediates JA-signalling by targeting JASMONATE ZIM-domain (JAZ) proteins for proteasomal degradation in response to JA perception. Here, we found that RNA interference-mediated knockdown of NtCOI1 in tobacco (Nicotiana tabacum L. cv. TN90) recapitulated many previously observed phenotypes in coi1 mutants, including male sterility, JA insensitivity, and loss of floral anthocyanin production. It also affected starch metabolism in the pollen, anther wall, and floral nectary, leading to pollen abortion and loss of floral nectar. Transcript levels of genes encoding starch metabolism enzymes were significantly altered in the pollen, anther wall, and floral nectary of NtCOI1-silenced tobacco. Changes in leaf primary metabolism were also observed in the NtCOI1-silenced tobacco. The expression of NtMYB305, an orthologue of MYB305 previously identified as a flavonoid metabolic regulator in Antirrhinum majus flowers and as a floral-nectar regulator mediating starch synthesis in ornamental tobacco, was extremely downregulated in NtCOI1-silenced tobacco. These findings suggest that NtCOI1 functions upstream of NtMYB305 and plays a fundamental role in coordinating plant primary carbohydrate metabolism and correlative physiological processes.

Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9

Angiosperms developed floral nectaries that reward pollinating insects. Although nectar function and composition have been characterized, the mechanism of nectar secretion has remained unclear. Here we identify SWEET9 as a nectary-specific sugar transporter in three eudicot species: Arabidopsis thaliana, Brassica rapa (extrastaminal nectaries) and Nicotiana attenuata (gynoecial nectaries). We show that SWEET9 is essential for nectar production and can function as an efflux transporter. We also show that sucrose phosphate synthase genes, encoding key enzymes for sucrose biosynthesis, are highly expressed in nectaries and that their expression is also essential for nectar secretion. Together these data are consistent with a model in which sucrose is synthesized in the nectary parenchyma and subsequently secreted into the extracellular space via SWEET9, where sucrose is hydrolysed by an apoplasmic invertase to produce a mixture of sucrose, glucose and fructose. The recruitment of SWEET9 for sucrose export may have been a key innovation, and could have coincided with the evolution of core eudicots and contributed to the evolution of nectar secretion to reward pollinators.

Natural variation in floral nectar proteins of two Nicotiana attenuata accessions

BACKGROUND

Floral nectar (FN) contains not only energy-rich compounds to attract pollinators, but also defense chemicals and several proteins. However, proteomic analysis of FN has been hampered by the lack of publically available sequence information from nectar-producing plants. Here we used next-generation sequencing and advanced proteomics to profile FN proteins in the opportunistic outcrossing wild tobacco, Nicotiana attenuata.

RESULTS

We constructed a transcriptome database of N. attenuata and characterized its nectar proteome using LC-MS/MS. The FN proteins of N. attenuata included nectarins, sugar-cleaving enzymes (glucosidase, galactosidase, and xylosidase), RNases, pathogen-related proteins, and lipid transfer proteins. Natural variation in FN proteins of eleven N. attenuata accessions revealed a negative relationship between the accumulation of two abundant proteins, nectarin1b and nectarin5. In addition, microarray analysis of nectary tissues revealed that protein accumulation in FN is not simply correlated with the accumulation of transcripts encoding FN proteins and identified a group of genes that were specifically expressed in the nectary.

CONCLUSIONS

Natural variation of identified FN proteins in the ecological model plant N. attenuata suggests that nectar chemistry may have a complex function in plant-pollinator-microbe interactions.

PIN6 is required for nectary auxin response and short stamen development

The PIN family of proteins is best known for its involvement in polar auxin transport and tropic responses. PIN6 (At1g77110) is one of the remaining PIN family members in Arabidopsis thaliana to which a biological function has not yet been ascribed. Here we report that PIN6 is a nectary-enriched gene whose expression level is positively correlated with total nectar production in Arabidopsis, and whose function is required for the proper development of short stamens. PIN6 accumulates in internal membranes consistent with the ER, and multiple lines of evidence demonstrate that PIN6 is required for auxin-dependent responses in nectaries. Wild-type plants expressing auxin-responsive DR5:GFP or DR5:GUS reporters displayed intense signal in lateral nectaries, but pin6 lateral nectaries showed little or no signal for these reporters. Further, exogenous auxin treatment increased nectar production more than tenfold in wild-type plants, but nectar production was not increased in pin6 mutants when treated with auxin. Conversely, the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) reduced nectar production in wild-type plants by more than twofold, but had no significant effect on pin6 lines. Interestingly, a MYB57 transcription factor mutant, myb57-2, closely phenocopied the loss-of-function mutant pin6-2. However, PIN6 expression was not dependent on MYB57, and RNA-seq analyses of pin6-2 and myb57-2 mutant nectaries showed little overlap in terms of differentially expressed genes. Cumulatively, these results demonstrate that PIN6 is required for proper auxin response and nectary function in Arabidopsis. These results also identify auxin as an important factor in the regulation of nectar production, and implicate short stamens in the maturation of lateral nectaries.

Integrated genomics, physiology and breeding approaches for improving drought tolerance in crops

Drought is one of the most serious production constraint for world agriculture and is projected to worsen with anticipated climate change. Inter-disciplinary scientists have been trying to understand and dissect the mechanisms of plant tolerance to drought stress using a variety of approaches; however, success has been limited. Modern genomics and genetic approaches coupled with advances in precise phenotyping and breeding methodologies are expected to more effectively unravel the genes and metabolic pathways that confer drought tolerance in crops. This article discusses the most recent advances in plant physiology for precision phenotyping of drought response, a vital step before implementing the genetic and molecular-physiological strategies to unravel the complex multilayered drought tolerance mechanism and further exploration using molecular breeding approaches for crop improvement. Emphasis has been given to molecular dissection of drought tolerance by QTL or gene discovery through linkage and association mapping, QTL cloning, candidate gene identification, transcriptomics and functional genomics. Molecular breeding approaches such as marker-assisted backcrossing, marker-assisted recurrent selection and genome-wide selection have been suggested to be integrated in crop improvement strategies to develop drought-tolerant cultivars that will enhance food security in the context of a changing and more variable climate.

Nectar: generation, regulation and ecological functions

Nectar contains water, sugars and amino acids to attract pollinators and defenders and is protected from nectar robbers and microorganisms by secondary compounds and antimicrobial proteins. Floral and extrafloral nectar secretion can be induced by jasmonic acid, it is often adjusted to consumer identity and consumption rate and depends on invertase activity. Invertases are likely to play at least three roles: the uploading of sucrose from the phloem, carbohydrate mobilization during active secretion and the postsecretory adjustment of the sucrose:hexose ratio of nectar. However, it remains to be studied how plants produce and secrete non-carbohydrate components. More research is needed to understand how plants produce nectar, the most important mediator of their interactions with mutualistic animals.