, Volume 93, Issue 2, pp 72–79 | Cite as

A novel role for proline in plant floral nectars

  • Clay Carter
  • Sharoni Shafir
  • Lia Yehonatan
  • Reid G. Palmer
  • Robert ThornburgEmail author
Original Article


Plants offer metabolically rich floral nectar to attract visiting pollinators. The composition of nectar includes not only sugars, but also amino acids. We have examined the amino acid content of the nectar of ornamental tobacco and found that it is extremely rich (2 mM) in proline. Because insect pollinators preferentially utilize proline during the initial phases of insect flight and can reportedly taste proline, we determined whether honeybees showed a preference for synthetic nectars rich in proline. We therefore established an insect preference test and found that honeybees indeed prefer nectars rich in the amino acid proline. To determine whether this was a general phenomenon, we also examined the nectars of two insect-pollinated wild perennial species of soybean. These species also showed high levels of proline in their nectars demonstrating that plants often produce proline-rich floral nectar. Because insects such as honeybees prefer proline-rich nectars, we hypothesize that some plants offer proline-rich nectars as a mechanism to attract visiting pollinators.


Proline Proline Concentration Floral Nectar PITC Plant Nectar 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



The authors would like to acknowledge the National Science Foundation for funding to Robert Thornburg (NSF#IBN-0235645) and the Israel Science Foundation for funding to Sharoni Shafir (ISF#513/01) for support of this work. This is a joint contribution of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Project No. 3769 and the USDA-ARS, Corn Insects and Crop Genetics Research Unit, and supported by Hatch Act and State of Iowa. RGP is most grateful to Dr. A.H.D. Brown and to CSIRO Plant Industry, Canberra, Australia for maintenance of the Glycine species and for their hospitality during his sabbatical visit. The mention of a trademark or proprietary product does not constitute a guarantee or warranty of the project by Iowa State University or the USDA, and the use of the name by Iowa State University or the USDA implies no approval of the product to the exclusion of others that may also be suitable. All experiments conducted in this manuscript comply with the current laws of the country in which they were performed.


  1. Alm J, Ohnmeiss TE, Lanza J, Vriesenga L (1990) Preference of cabbage white butterflies and honey bees for nectar that contains amino acids. Oecologia 84:53–57CrossRefGoogle Scholar
  2. Auerswald L, Schneider P, Gade G (1998) Utilisation of substrates during tethered flight with and without lift generation in the African fruit beetle Pachnoda sinuata (Cetoniinae). J Exp Biol 201:2333–2342PubMedGoogle Scholar
  3. Baker HG (1978) Chemical aspects of the pollination biology of woody plants in the tropics. In: Tomlinson P, Zimmerman MH (eds) Tropical trees as living systems: the proceedings of the fourth Cabot Symposium, Harvard Forest, Petersham, Massachusetts, 26–30 April, 1976. Cambridge University Press, New York, pp 57–82Google Scholar
  4. Baker HG, Baker I (1971) Amino acids in nectar and their evolutionary significance. Nature 241:543–545CrossRefGoogle Scholar
  5. Baker HG, Baker I (1973) Some anthecological aspects of the evolution of nectar-producing flowers, particularly amino acid production in the nectar. In: Heywood VH (ed) Taxonomy and ecology: proceedings of an international symposium, Department of Botany, University of Reading, vol 5. Academic, London, pp 243–264Google Scholar
  6. Baker HG, Baker I (1975) Studies of nectar-constitution and pollinator-plant coevolution. In: Gilbert LE, Raven PH (eds) Coevolution of animals and plants: symposium V, first international congress of systematic and evolutionary biology, Boulder, Colorado, August 1973. University of Texas Press, Austin, pp 100–140Google Scholar
  7. Baker HG, Baker I (1981) Chemical constituents of nectar in relation to pollination mechanisms and phylogeny. In: Nitecki MH (ed) Biochemical aspects of evolutionary biology. University of Chicago Press, Chicago, pp 131–171Google Scholar
  8. Balboni E (1978) A proline shuttle in insect flight muscle. Biochem Biophys Res Commun 85:1090–1096PubMedCrossRefGoogle Scholar
  9. Brosemer RW, Veerabhadrappa PS (1965) Pathway of proline oxidation in insect flight muscle. Biochem Biophys Acta 110:102–112PubMedGoogle Scholar
  10. Burquez A, Corbet SA (1991) Do flowers reabsorb nectar? Funct Ecol 5:369–379CrossRefGoogle Scholar
  11. Carter C, Thornburg RW (2000) Tobacco nectarin I: purification and characterization as a germin-like, manganese superoxide dismutase implicated in the defense of floral reproductive tissues. J Biol Chem 275:36726–36733CrossRefPubMedGoogle Scholar
  12. Carter C, Thornburg R (2004a) Tobacco nectarin V is a flavin-containing berberine bridge enzyme-like protein with glucose oxidase activity. Plant Physiol 134:460–469CrossRefPubMedGoogle Scholar
  13. Carter C, Thornburg RW (2004b) Is the nectar redox cycle a floral defense against microbial attack? Trends Plant Sci 9:320–324CrossRefPubMedGoogle Scholar
  14. Carter C, Thornburg RW (2004c) Tobacco nectarin III is a bifunctional enzyme with monodehydroascorbate reductase and carbonic anhydrase activities. Plant Mol Biol 54:415–425CrossRefPubMedGoogle Scholar
  15. Carter C, Graham R, Thornburg RW (1999) Nectarin I is a novel, soluble germin-like protein expressed in the nectar of Nicotiana sp. Plant Mol Biol 41:207–216CrossRefPubMedGoogle Scholar
  16. Crabtree B, Newsholme EA (1970) The activities of proline dehydrogenase, glutamate dehydrogenase, aspartate-oxoglutarate aminotransferase and alanine-oxoglutarate aminotransferase in some insect flight muscles. Biochem J 117:1019–1021PubMedGoogle Scholar
  17. Deinzer ML, Thompson PA, Burgett DM, Isaacson DL (1977) Pyrrolizidine alkaloids: their occurrence in honey from tansy ragwort (Senecio jacobaea L.). Science 195:497–499PubMedCrossRefGoogle Scholar
  18. Ecroyd CE, Franich RA, Kroese HW, Steward D (1995) Volatile constituents of Dactylanthus taylorii flower nectar in relation to flower pollination and browsing by animals. Phytochemistry 40:1387–1389CrossRefGoogle Scholar
  19. Ferreres F, Andrade P, Gil MI, Tomas Barberan FA (1996) Floral nectar phenolics as biochemical markers for the botanical origin of heather honey. Zeit Lebensmitt Untersuch Forsch 202:40–44CrossRefGoogle Scholar
  20. Gardener MC, Gillman MP (2001a) Analyzing variability in nectar amino acids: composition is less variable than concentration. J Chem Ecol 27:2545–2558CrossRefPubMedGoogle Scholar
  21. Gardener MC, Gillman MP (2001b) The effects of soil fertilizer on amino acids in the floral nectar of corncockle, Agrostemma githago L. (Caryolhyllaceae). Oikos 92:101–106CrossRefGoogle Scholar
  22. Gardener MC, Gillman MP (2002) The taste of nectar—a neglected area of pollination. Oikos 98:552–557CrossRefGoogle Scholar
  23. Gottsberger G, Schrauwen J, Linskens HF (1984) Amino acids and sugars in nectar and their putative evolutionary significance. Plant Syst Evol 145:55–77CrossRefGoogle Scholar
  24. Griebel C, Hess G (1940) The vitamin C content of flower nectar of certain Labiatae. Zeit Untersuch Lebensmitt 79:168–171CrossRefGoogle Scholar
  25. Hansen K, Wacht S, Seebauer H, Schnuch M (1998) New aspects of chemoreception in flies. Ann NY Acad Sci 855:143–147PubMedCrossRefGoogle Scholar
  26. Hrassnigg N, Leonhard B, Crailsheim K (2003) Free amino acids in the haemolymph of honey bee queens (Apis mellifera L.). Amino Acids 24:205–212PubMedGoogle Scholar
  27. Inouye D, Waller G (1984) Responses of honey bees (Apis mellifera) to amino acid solutions mimicking nectars. Ecology 65:618–625CrossRefGoogle Scholar
  28. Jackson S, Nicolson SW (2002) Xylose as a nectar sugar: from biochemistry to ecology. Comp Biochem Physiol B 131B:613–620CrossRefGoogle Scholar
  29. Jouve L, Hoffmann L, Hausman JF (2004) Polyamine, carbohydrate, and proline content changes during salt stress exposure of aspen (Populus tremula L.): involvement of oxidation and osmoregulation metabolism. Plant Biol (Stuttg) 6:74–80CrossRefGoogle Scholar
  30. Kaczorowski RL, Gardener MC, Holtsford TP (2005) Nectar traits in Nicotiana section Alatae (Solanaceae) in relation to floral traits, pollinators and mating system. Am J Bot 92:1270–1283Google Scholar
  31. Kim Y, Smith B (2000) Effect of an amino acid on feeding preferences and learning behavior in the honey bee, Apis mellifera. J Insect Physiol 46:793–801CrossRefPubMedGoogle Scholar
  32. Kishor P, Hong Z, Miao GH, Hu C, Verma D (1995) Overexpression of [delta]-pyrroline-5-carboxylate synthetase increases proline production and confers osmotolerance in transgenic plants. Plant Physiol 108:1387–1394PubMedGoogle Scholar
  33. Kornaga T (1993) Genetic and biochemical characterization of a “lost” unstable flower color phenotype in interspecific crosses of Nicotiana sp. MS. Iowa State University, AmesGoogle Scholar
  34. Kornaga T, Zyzak DV, Kintinar A, Baynes J, Thornburg R (1997) Genetic and biochemical characterization of a “lost” unstable flower color phenotype in interspecific crosses of Nicotiana sp. WWW J Biol 2:8Google Scholar
  35. Lüttge U (1961) Über die Zusammensetzung des Nektars und den Mechanismus seiner Sekretion. I. Planta 56:189–212CrossRefGoogle Scholar
  36. Lüttge U (1962) Über die Zusammensetzung des Nektars und den Mechanismus seiner Sekretion. II. Planta 59:108–114CrossRefGoogle Scholar
  37. Micheu S, Crailsheim K, Leonhard B (2000) Importance of proline and other amino acids during honeybee flight (Apis mellifera carnica Pollmann). Amino Acids 18:157–175CrossRefPubMedGoogle Scholar
  38. Mostowska I (1964) Amino acids of nectars and honeys. Zeszyty Kauk Wyzszej Szkoly Rolniczej Olsztynie 20:417–432 (Chem. Abstr. 464: No. 20529, 21966)Google Scholar
  39. Motulsky H, Christopoulos A (2003) Fitting models to biological data using linear and nonlinear regression: a practical guide to curve fitting. GraphPad Software Inc., San DiegoGoogle Scholar
  40. Nair A, Nagarajan S, Subramanian S (1964) Chemical compositions of nectar in Thunbergia grandiflora. Current Sci (Bangalore) 33:401Google Scholar
  41. Naqvi SMS, Harper A, Carter CJ, Ren G, Guirgis A, York WS, Thornburg RW (2005) Nectarin IV, a potent endoglucanase inhibitor secreted into the nectar of ornamental tobacco plants. Isolation, cloning and characterization. Plant Physiol 139:1389–1400CrossRefPubMedGoogle Scholar
  42. Njagi EN, Olembo NK, Pearson DJ (1992) Proline transport by tsetse fly Glossina morsitans flight muscle mitochondria. Comp Biochem Physiol B 102:579–584CrossRefPubMedGoogle Scholar
  43. O’Brien D, Boggs CL, Fogel Ml (2003) Pollen feeding in the butterfly Heliconius charitonia: isotopic evidence for essential amino acid transfer from pollen to eggs. Proc R Soc Lond B 270:2631–2636CrossRefGoogle Scholar
  44. Parvanova D, Ivanov S, Konstantinova T, Karanov E, Atanassov A, Tsvetkov T, Alexieva V, Djilianov D (2004) Transgenic tobacco plants accumulating osmolytes show reduced oxidative damage under freezing stress. Plant Physiol Biochem 42:57–63CrossRefPubMedGoogle Scholar
  45. Peumans WJ, Smeets K, Van Nerum K, Van Leuven F, Van Damme EJM (1997) Lectin and alliinase are the predominant proteins in nectar from leek (Allium porrum L.) flowers. Planta 201:298–302CrossRefPubMedGoogle Scholar
  46. Raubenheimer D, Simpson S (1999) Integrating nutrition: a geometrical approach. Entomol Exp Appl 91:67–82CrossRefGoogle Scholar
  47. Rodriguez-Arce AL, Diaz N (1992) The stability of beta-carotene in mango nectar. J Agric Univ PR 76:101–102Google Scholar
  48. Roshchina VV, Roshchina VD (1993) The excretory function of higher plants. Springer, Berlin Heidelberg New YorkGoogle Scholar
  49. Rusterholz HP, Erhardt A (1998) Effects of elevated CO2 on flowering phenology and nectar production of nectar plants important for butterflies of calcareous grasslands. Oecologia 113:341–349CrossRefGoogle Scholar
  50. Schwacke R, Grallath S, Breitkreuz KE, Stransky E, Stransky H, Frommer WB, Rentsch D (1999) LeProT1, a transporter for proline, glycine betaine, and gamma-amino butyric acid in tomato pollen. Plant Cell 11:377–392CrossRefPubMedGoogle Scholar
  51. Shiraishi A, Kuwabara M (1970) The effects of amino acids on the labellar hair chemosensory cells of the fly. J Gen Physiol 56:768–782CrossRefPubMedGoogle Scholar
  52. Simpson S, Raubenheimer D (1993) A multi-level analysis of feeding behaviour: the geometry of nutritional decision. Philos Trans Roy Soc B 342:381–402CrossRefGoogle Scholar
  53. Thornburg RW, Carter C, Powell A, Rizhsky L, Mittler R, Horner HT (2003) A major function of the tobacco floral nectary is defense against microbial attack. Plant Syst Evol 238:211–218Google Scholar
  54. Verbruggen N, Villarroel R, Van Montagu M (1993) Osmoregulation of a pyrroline-5-carboxylate reductase gene in Arabidopsis thaliana. Plant Physiol 103:771–781CrossRefPubMedGoogle Scholar
  55. Verbruggen N, Hua XJ, May M, Van Montagu M (1996) Environmental and developmental signals modulate proline homeostasis: evidence for a negative transcriptional regulator. Proc Natl Acad Sci USA 93:8787–8791CrossRefPubMedGoogle Scholar
  56. Vogel S (1969) Flowers offering fatty oil instead of nectar (Abstract No. 229). In: Abstracts of the papers presented at the XI International Botanical Congress, August 24–September 2, 1969 and the International Wood Chemistry Symposium, September 2–4, 1969 Seattle, WA (USA), pp 260Google Scholar
  57. Wacht S, Lunau K, Hansen K (2000) Chemosensory control of pollen ingestion in the hoverfly Eristalis tenax by labellar taste hairs. J Comp Physiol A 186:193–203CrossRefPubMedGoogle Scholar
  58. Yamada M, Morishita H, Urano K, Shiozaki N, Yamaguchi-Shinozaki K, Shinozaki K, Yoshiba Y (2005) Effects of free proline accumulation in petunias under drought stress. J Exp Bot 56:1975–1981CrossRefPubMedGoogle Scholar
  59. Yoshiba Y, Kiyosue T, Katagiri T, Ueda H, Mizoguchi T, Yamaguchi-Shinozaki K, Wada K, Harada Y, Shinozaki K (1995) Correlation between the induction of a gene for delta 1-pyrroline-5-carboxylate synthetase and the accumulation of proline in Arabidopsis thaliana under osmotic stress. Plant J 7:751–760CrossRefPubMedGoogle Scholar
  60. Zhang H, Croes A, Linskens H (1982) Protein synthesis in germinating pollen of petunia: Role of proline. Planta 154:199–203CrossRefGoogle Scholar
  61. Ziegler H (1956) Untersuchungen über die Leitung und Sekretion der Assimilate. Planta 47:447–500CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Clay Carter
    • 1
    • 2
  • Sharoni Shafir
    • 3
  • Lia Yehonatan
    • 3
  • Reid G. Palmer
    • 4
  • Robert Thornburg
    • 1
    Email author
  1. 1.Department of Biochemistry, Biophysics and Molecular Biology 2212 Molecular Biology BuildingIowa State UniversityAmesUSA
  2. 2.Department of BiologyUniversity of Minnesota DuluthDuluthUSA
  3. 3.B. Triwaks Bee Research Center, Department of Entomology, Faculty of Agricultural, Food and Environmental Quality SciencesThe Hebrew University of JerusalemRehovotIsrael
  4. 4.United States Department of Agriculture-Agricultural Research Service G301 Agronomy HallIowa State UniversityAmesUSA

Personalised recommendations