Nectar

Nectar is the sweet fluid that many dicotyledenous plants use to attract insect pollinators. The composition of nectar varies from plant species to plant species, but a general composition is presented below.

The secretion of nectar is usually under developmental control beginning when the flowers open. After pollination, the nectar is frequently resorbed (1). In addition, nectar secretion increases as the flower is visited by pollinators (2).

Hummingbird moth

 

Carbohydrates
Carbohydrates make up the largest fraction in nectar by weight. Other carbohydrates including arabinose, galactose, mannose, gentiobiose, lactose, maltose, melibiose, trehelose, melezitose, raffinose, and stachyose have also been identified in nectars of some flowers (3). The various types of nectars can be ordered into three groups according to sugar content: sucrose prevalent, glucose and fructose prevalent, and equal amounts of sucrose, glucose, and fructose (4). Interestingly, sugar concentrations vary greatly depending on the type and location of the nectary (4). A relationship exists between the sugar composition of the nectar and the amount of vascular tissue underlying the nectary (5). If phloem makes up most of the vascular tissue, the nectar may contain up to 50 percent sugar. On the other hand, if xylem predominates, the sugar content may fall to as little as 8 percent (6).

Amino Acids
Some nectars also contain amino acids (7). All twenty of the normal amino acids found in protein have been identified in various nectars, with alanine, arginine, serine, proline, glycine, isoleucine, threonine, and valine being the most prevalent.

Other Substances
Other substances reported in nectar include organic acids (8), terpenes (9), alkaloids (10), flavonoids (11), glycosides (12), vitamins (4), phenolics (13), and oils (14).

Metal Ions
Using laser mass-spectroscopic microanalysis, Heinrich (15) found that the major cation of most nectars was K+, making up 35 to 74 percent of the total cation content. Averages of other notable cations were Na+ (17.9%), Ca2+ (12.8%), Mg2+ (5.9%), Al3+ (4.6%), Fe3+ (1.2%), and Mn2+ (0.8%).

sundew nectar
Connecticut Botanical Society

Proteins in Nectar
Notable among these analyses of the composition of nectar is the lack of studies characterizing the presence of proteins in nectar. Although the presence of proteins in nectar has been long reported (16), there has not been to date a focused examination of the presence of proteins in nectar. Indeed other than secreted digestive enzymes in carnivorous plants (17, 18), only a single report (19) has characterized any nectar proteins.

Recently we have reported (20) that tobacco plants secrete a limited array of proteins into their nectar. Subsequently, we identified the major nectar protein, Nectarin I, as a novel germin-like protein (GLP). We have characterized its expression and have isolated the gene encoding Nectarin I.

 


In 1994, we discovered that the nectar of tobacco plants also contains a limited array of nectar proteins. These proteins are called Nectarins to indicate their source. We have isolated and characterized several of these proteins. The figure at right shows an SDS PAGE analysis of raw nectar proteins. Careful analysis reveals that there are 10 to 12 proteins in these nectars.

Nectarin I is the most highly expressed nectar protein and has a monomer molecular mass of 29 kDa. The other major nectar proteins are expressed at lower levels and have monomer molecular masses of 41, 54, and 65 kDa respectively. Nectarin I was purified and antiserum was raised against the protein. Under nondenaturing conditions, Nectarin I has an apparent molecular mass of >120 kDa. The expression of Nectarin I was restricted to nectary tissues and to a much lower level in the ovary. No Nectarin I was found in petals, stems, leaves, or roots. The expression of Nectarin I was also developmentally regulated. It is expressed in nectary tissues only while nectar is being actively secreted. Subsequently, the N-terminus of purified Nectarin I was sequenced. Sequence identity showed Nectarin I is related to wheat germin.

A brief description of these proteins is presented below. Further information about these nectar proteins can be found in NecGEx.


Nectar Proteins
Nectarin I is the most abundant of the nectar proteins.
 

 Nectarin I Findings

  1. Nectarin I is not immunologically similar to any of the other Nectar proteins.
  2. Nectarin I belongs to a family of proteins termed Germin-like Proteins (GLPs)
  3. The enzymatic activity associated with Nectarin I is Superoxide Dismutase. This activity generates hydrogen peroxide in nectar for anti-microbial purposes.
  4. Nectarin I is widely conserved among dicotyledenous plants
 

 References relating to Nectarin I

  1. Carter & Thornburg
  2. Carter & Thornburg
 

The Nectarin I gene was recently isolated and characterized. It can be accessed from the GenBank

  1. GenBank Accession

Nectarin II
  Nectarin II is a 41 kDa protein. Its function and identity remain unknown.

Nectarin III
  Nectarin III is a 54 kDa protein. Its function and identity remain unknown.

Nectarin IV
  Nectarin IV is a 64 kDa protein. Although Nectarin IV has been characterized, we have not yet published that result. Once the manuscript is accepted, a link to it will be available here.
  A partial cDNA encoding Nectarin IV gene has been isolated. Once we obtain the full length sequence and characterize it, a link to it will be available here.

References:

1. Burquez, A. and Corbet, S.A. (1991) Do flowers reabsorb nectar? Funct. Ecol. 5:369-379.

2. Smith, L.L., Lanza, J. and Smith, G.C. (1990) Amino acid concentrations in extrafloral nectar of Impatiens sultani increase after simulated herbivory. Ecol. Publ. Ecol. Soc. Am. 71:107-115.

3. Baker, H.G. and Baker, I. (1981) Chemical constituents of nectar in relation to pollination mechanisms and phylogeny. In Biochemical aspects of evolutionary biology. 131-171.

4. Roshchina, V.V. and Roshchina, V.D. (1993) The excretory function of higher plants. Springer-Verlag, Berlin.

5. Esau, K. (1977) Anatomy of seed plants. John Wiley & Sons, New York.

6. Frey-Wyssling, A. (1955) The phloem supply to the nectaries. Acta Bot. Neerl. 4:358-369.

7. Baker, H.G. and Baker, I. (1973) Amino acids in nectar and their evolutionary significance. Nature 241:543-545.

8. Baker, H.G. and Baker, I. (1975) Studies of nectar-constitution and pollinator-plant coevolution. In Coevolution of animals and plants. Gilbert, L.E. and Raven, P.H. ed. Univ. of Texas Press, Austin, 100-140.

9. Ecroyd, C.E., Franich, R.A., Kroese, H.W. and Steward, D. (1995) Volatile constituents of Dactylanthus taylorii flower nectar in relation to flower pollination and browsing by animals. Phytochemistry 40:1387-1389.

10. Deinzer, M.L., Thompson, P.A., Burgett, D.M. and Isaacson, D.L. (1977) Pyrrolizidine alkaloids: Their occurance in honey from tansy ragwort (Senecio jacobaea L.). Science 195:497-499.

11. Rodriguez-Arce, A.L. and Diaz, N. (1992) The stability of beta-carotene in mango nectar. J. Agric. Univ. P.R. Rio Piedras, P.R. 76:101-102.

12. Griebel, C. and Hess, G. (1940) The vitamin C content of flower nectar of certain Labiatae. Zeit. Untersuch. Lebensmitt. 79:168-171.

13. Ferreres, F., Andrade, P., Gil, M.I. and Tomas Barberan, F.A. (1996) Floral nectar phenolics as biochemical markers for the botanical origin of heather honey. Zeitschrift fur Lebensmittel Untersuchung und Forschung. 202:40-44.

14. Vogel, S. (1969) Flowers offering fatty oil instead of nectar. Abstracts XIth Internatl. Bot. Congr. Seattle.

15. Heinrich, G. (1989) Analysis of cations in nectars by means of a laser microprobe mass analyser (LAMMA). Beitr. Biol. Pflanz 64:293-308.

16. Beutler, R. (1935) Nectar. Bee World 24:106-116, 128-136, 156-162.

17. Scala, J., Iott, K., Schwab, W. and Semersky, F.E. (1969) Digestive secretion of Dionaea muscipula (Venus's-Flytrap). Plant Physiol. 44:367-371.

18. Heslop-Harrison, Y. and Knox, R.B. (1971) A cytochemical study of the leaf-gland enzymes of insectivorus plants of the genus Pinguicula. Planta 96:183-211.

19. Peumans, W.J., Smeets, K., Van Nerum, K., Van Leuven, F. and Van Damme, E.J.M. (1997) Lectin and alliinase are the predominant proteins in nectar from leek (Allium porrum L.) flowers. Planta 201:298-302.

20. Carter, C., Graham, R. and Thornburg, R.W. (1999) Nectarin I is a novel, soluble germin-like protein expressed in the nectar of Nicotiana sp. Plant Mol. Biol. 41:207-216.

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Last updated 4/6/01 -- Robert Thornburg -- thorn@iastate.edu