Carbohydrate Metabolism and Signaling in Squash Nectaries and Nectar Throughout Floral Maturation

Histochemical, metabolic and ultrastructural changes in leaf patelliform nectaries explain extrafloral nectar synthesis and secretion in Clerodendrum chinense

BACKGROUND AND AIMS

Extrafloral nectaries are nectar-secreting structures present on vegetative parts of plants which provide indirect defences against herbivore attack. Extrafloral nectaries in Clerodendrum chinense are patelliform-shaped specialized trichomatous structures. However, a complete understanding of patelliform extrafloral nectaries in general, and of C. chinense in particular, has not yet been established to provide fundamental insight into the cellular physiological machinery involved in nectar biosynthesis and secretory processes.

METHODS

We studied temporal changes in the morphological, anatomical and ultrastructural features in the architectures of extrafloral nectaries. We also compared metabolite profiles of extrafloral nectar, nectary tissue, non-nectary tissue and phloem sap. Further, both in situ histolocalization and normal in vitro activities of enzymes related to sugar metabolism were examined.

KEY RESULTS

Four distinct tissue regions in the nectar gland were revealed from histochemical characterization, among which the middle nectariferous tissue was found to be the metabolically active region, while the intermediate layer was found to be lipid-rich. Ultrastructural study showed the presence of a large number of mitochondria along with starch-bearing chloroplasts in the nectariferous region. However, starch depletion was noted with progressive maturation of nectaries. Metabolite analysis revealed compositional differences among nectar, phloem sap, nectary and non-nectary tissue. Invertase activity was higher in secretory stages and localized in nectariferous tissue and adjacent region.

CONCLUSIONS

Our study suggests extrafloral nectar secretion in C. chinense to be both eccrine and merocrine in nature. A distinct intermediate lipid-rich layer that separates the epidermis from nectary parenchyma was revealed, which possibly acts as a barrier to water flow in nectar. This study also revealed a distinction between nectar and phloem sap, and starch could act as a nectar precursor, as evidenced from enzymatic and ultrastructural studies. Thus, our findings on changing architecture of extrafloral nectaries with temporal secretion revealed a cell physiological process involved in nectar biosynthesis and secretion.

Nectar guides and floral nectary in Lamium album L. subsp. album: structure and histochemistry in light, fluorescence, and electron microscopy

Lamium album is a native common plant growing in Eurasia. It is used in medicine and cosmetics and is highly valued in apiculture. The aim of the study was to investigate the structure of the floral nectary in three stages of flower development. Additionally, histochemical studies of the nectary and nectar guides present on the lower corolla lobe were carried out. No detailed analyses of nectary tissues in this species have been conducted to date. The present analyses were performed with the use of light, fluorescence, and scanning electron microscopy. The nectary gland in the flowers of Lamium album subsp. album formed an incomplete ring at the ovary base. The nectarostomata were arranged in clusters only in the adaxial epidermis of the anterior part of the nectary. During the secretory activity of the nectary (1st day of flowering), numerous small vacuoles and cells with large lobulate nuclei with surrounding plastid clusters were observed in the epidermis and glandular parenchyma cells. The vascular bundles contained xylem and phloem elements. Corolla wilting (3rd day of flowering) was accompanied by destructive changes in the nectary parenchyma, leading to the formation of empty spaces and appearance of cell remnants on the nectary surface. The histochemical analyses revealed the presence of starch and phenolic compounds as well as acidic and neutral lipids, which are characteristic of essential oils, in the nectary tissues. The nectar guides were composed of large yellow papillae containing phenolic compounds and acidic and neutral lipids, which were also present in glandular trichomes and abaxial parenchyma cells. The present study has demonstrated that the scent of Lamium album subsp. album flowers is produced with the involvement of essential oils contained in adaxial and abaxial epidermis cells, glandular trichomes, and nectary tissues.

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.

Comparative analyses of the metabolite and ion concentrations in nectar, nectaries, and leaves of 36 bromeliads with different photosynthesis and pollinator types

Floral nectar contains mainly sugars as well as smaller amounts of amino acids and further compounds. The nectar composition varies between different plant species and it is related to the pollination type of the plant. In addition to this, other factors can influence the composition. Nectar is produced in and secreted from nectaries. A few models exist to explain the origin of nectar for dicotyl plant species, a complete elucidation of the processes, however, has not yet been achieved. This is particularly true for monocots or plant species with CAM photosynthesis. To get closer to such an elucidation, nectar, nectaries, and leaves of 36 bromeliad species were analyzed for sugars, starch, amino acids, and inorganic ions. The species studied include different photosynthesis types (CAM/C3), different pollination types (trochilophilous/chiropterophilous), or different live forms. The main sugars in nectar and nectaries were glucose, fructose, and sucrose, the total sugar concentration was about twofold higher in nectar than in nectaries, which suggests that sugars are actively transported from the nectaries into the nectar. The composition of amino acids in nectar is already determined in the nectaries, but the concentration is much lower in nectar than in nectaries, which suggests selective retention of amino acids during nectar secretion. The same applies to inorganic ions. Statistical analyses showed that the photosynthesis type and the pollination type can explain more data variation in nectar than in nectaries and leaves. Furthermore, the pollinator type has a stronger influence on the nectar or nectary composition than the photosynthesis type. Trochilophilous C3 plants showed significant correlations between the nitrate concentration in leaves and the amino acid concentration in nectaries and nectar. It can be assumed that the more nitrate is taken up, the more amino acids are synthesized in leaves and transported to the nectaries and nectar. However, chiropterophilous C3 plants show no such correlation, which means that the secretion of amino acids into the nectar is regulated by further factors. The results help understand the physiological properties that influence nectaries and nectar as well as the manner of metabolite and ion secretion from nectaries to nectar.

Plant growth: An active or passive role for starch reserves?

Starch metabolism is linked to plant growth, yet blocking its biosynthesis has species-specific consequences. In a new study, plastidial phosphoglucomutase is knocked out in aspen trees using CRISPR-Cas9, limiting starch production and altering photosynthesis, but growth, bud break and wood production proceed unaffected.

The Role of Sugar Transporter CsSWEET7a in Apoplasmic Phloem Unloading in Receptacle and Nectary During Cucumber Anthesis

During anthesis, there is an increased demand for carbohydrates due to pollen maturation and nectary secretion that warrants a systematic phloem unloading strategy for sugar partitioning. Sugar transporters are key components of the apoplasmic phloem unloading strategy and control the sugar flux needed for plant development. Currently, the phloem unloading strategy during anthesis has not been explored in cucumber, and the question of which sugar transporters are active during flower anthesis is poorly understood. In this study, a study utilizing the phloem-mobile symplasmic tracer carboxyfluorescein (CF) suggested that the phloem unloading was symplasmically isolated in the receptacle and nectary of cucumber flowers at anthesis. We also identified a hexose transporter that is highly expressed in cucumber flower, Sugar Will Eventually be Exported Transporter 7a (SWEET7a). CsSWEET7a was mainly expressed in receptacle and nectary tissues in both male and female flowers, where its expression level increased rapidly right before anthesis. At anthesis, the CsSWEET7a protein was specifically localized to the phloem region of the receptacle and nectary, indicating that CsSWEET7a may function in the apoplasmic phloem unloading during flower anthesis. Although cucumber mainly transports raffinose family oligosaccharides (RFOs) in the phloem, sucrose, glucose, and fructose are the major sugars in the flower receptacle and the nectary as well as in nectar at anthesis. In addition, the transcript levels of genes encoding soluble sugar hydrolases (α-galactosidase, sucrose synthase, cytoplasmic invertase, and cell wall invertase) were correlated with that of CsSWEET7a. These results indicated that CsSWEET7a may be involved in sugar partitioning as an exporter in the phloem of the receptacle and nectary to supply carbohydrates for flower anthesis and nectar secretion in cucumber.

Nectary photosynthesis contributes to the production of mānuka (Leptospermum scoparium) floral nectar

Current models of floral nectar production do not include a contribution from photosynthesis by green nectary tissue, even though many species have green nectaries. Mānuka (Leptospermum scoparium) floral nectaries are green, and in addition to sugars, their nectar contains dihydroxyacetone (DHA), the precursor of the antimicrobial agent in the honey. We investigated causes of variation in mānuka floral nectar production, particularly the effect of light incident on the nectary. Flower gas exchange, chlorophyll fluorescence, and the effects on nectar of age, temperature, light, sucrose, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), pyridoxal phosphate, and ¹³ CO₂ , were measured for attached and excised flowers. Flower age affected all nectar traits, whilst temperature affected total nectar sugar only. Increased light reduced floral CO₂ efflux, increased nectar sugar production, and affected the ratio of DHA to other nectar sugars. DCMU, an inhibitor of photosystem II, reduced nectar sugar production. Pyridoxal phosphate, an inhibitor of the chloroplast envelope triose phosphate transporter, reduced nectar DHA content. Incubation of excised flowers with ¹³ CO₂ in the light resulted in enrichment of nectar sugars, including DHA. Photosynthesis within green nectaries contributes to nectar sugars and influences nectar composition. Mānuka nectar DHA arises from pools of triose phosphate that are modulated by nectary photosynthesis.

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.

Phosphorus fertilization induces nectar secretion for honeybee visitation and cross-pollination of almond trees

Precise phosphorus (P) application requires a mechanistic understanding of mineral effects on crop biology and physiology. Photosynthate assimilation, metabolism, and transport require phosphorylation, and we postulated that P is critical for the bloom and fruit-set of almond trees that rely on stored carbohydrate reserves. Hence, we studied the growth, physiology and carbohydrate dynamics in 2-year-old almond trees irrigated with P concentrations between 1 mg l-1 and 20 mg l-1. Almond trees attained maximal photosynthesis, transpiration, and growth by 6 mg P l-1 irrigation. Nevertheless, almond trees continued to extract P in 10 mg P l-1 and 15 mg P l-1 irrigations, which corresponded to larger yields. We attributed the augmented productivity to increased fruit-set (59% between 6 mg P l-1 and 15 mg P l-1), caused by more frequent (29%) honeybee visits. High P improved pollinator visitation by enabling almond trees to utilize more of their starch reserves for nectar secretion (which increased by ~140% between 6 mg P l-1 and 15 mg P l-1). This work elucidates the benefits of P fertilization to plant-pollinator mutualism, critical to almond productivity, and reveals novel indices for optimal P application in almond orchards.

The role of alanine synthesis and nitrate-induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre-stored and direct phloem-derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N-containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia-related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.

Influence of light, dark, temperature and drought on metabolite and ion composition in nectar and nectaries of an epiphytic bromeliad species (Aechmea fasciata)

Research into the influence of stress factors, such as drought, different temperatures and/or varied light conditions, on plants due to climate changes is becoming increasingly important. Epiphytes, like many species of the Bromeliaceae, are particularly affected by this, but little is known about impacts on nectar composition and nectary metabolism. We investigated the influence of drought, different temperatures and light-dark regimes on nectar and nectaries of the epiphytic bromeliad species, Aechmea fasciata, and also the influence of drought with the terrestrial bromeliad, Billbergia nutans. The content of sugars, amino acids and ions in nectar and nectaries was analysed using HPLC. In addition, the starch content and the activities of different invertases in nectaries were determined. Compositions of nectar and nectaries were hardly influenced, neither by light nor dark, nor by different temperatures. In contrast, drought revealed changes in nectar volumes and nectar sugar compositions in the epiphytic bromeliad as well as in the terrestrial bromeliad. In both species, the sucrose-to-hexose ratio in nectar decreased considerably during the drought period. These changes in nectar sugar composition do not correlate with changes in the nectaries. The total sugar, amino acid and ion concentrations remained constant in nectar as well as in nectaries during the drought period. Changes in nectar composition or in the production of floral pollinator rewards are likely to affect plant-pollinator interactions. It remains questionable how far the adaptations of the bromeliads to drought and diverse light or temperature conditions are still sufficient.

Outstanding questions in flower metabolism

The great diversity of flowers, their color, odor, taste, and shape, is mostly a result of the metabolic processes that occur in this reproductive organ when the flower and its tissues develop, grow, and finally die. Some of these metabolites serve to advertise flowers to animal pollinators, other confer protection towards abiotic stresses, and a large proportion of the molecules of the central metabolic pathways have bioenergetic and signaling functions that support growth and the transition to fruits and seeds. Although recent studies have advanced our general understanding of flower metabolism, several questions still await an answer. Here, we have compiled a list of open questions on flower metabolism encompassing molecular aspects, as well as topics of relevance for agriculture and the ecosystem. These questions include the study of flower metabolism through development, the biochemistry of nectar and its relevance to promoting plant-pollinator interaction, recycling of metabolic resources after flowers whiter and die, as well as the manipulation of flower metabolism by pathogens. We hope with this review to stimulate discussion on the topic of flower metabolism and set a reference point to return to in the future when assessing progress in the field.