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.
Bibliographical noteFunding Information:
Acknowledgements 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.