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).
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).
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 reported in nectar include
organic acids (8), terpenes (9),
alkaloids (10), flavonoids (11),
glycosides (12), vitamins (4),
phenolics (13), and oils (14).
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%).
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.
|Nectarin I is the most abundant
of the nectar proteins.
Nectarin I Findings
- Nectarin I is not immunologically similar
to any of the other Nectar proteins.
- Nectarin I belongs to a family of proteins
termed Germin-like Proteins (GLPs)
- The enzymatic activity associated with Nectarin
I is Superoxide Dismutase. This activity generates hydrogen peroxide
in nectar for anti-microbial purposes.
- Nectarin I is widely conserved among dicotyledenous
References relating to Nectarin I
- Carter & Thornburg
- Carter & Thornburg
The Nectarin I gene was recently isolated
and characterized. It can be accessed from the GenBank
- GenBank Accession
||Nectarin II is a 41 kDa protein. Its function
and identity remain unknown.
||Nectarin III is a 54 kDa protein. Its function
and identity remain unknown.
||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.
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.
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.
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,
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.