Submitted to: Naturwissenschaften
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2005
Publication Date: 2/8/2006
Citation: Carter, C., Shafir, S., Yehonatan, L., Palmer, R.G., Thornburg, R. 2006. A novel role for proline in plant floral nectars. Naturwissenschaften. 93:72-79. Interpretive Summary: Insect pollinators are attracted to plants (flowers) by a number of morphological and chemical traits. Pollinators return to flowers because they obtain a reward (food), e.g. pollen or nectar. Pollen is usually carried back to their "home" where it is further processed for use. Nectar, however, is very rich in sugars and free amino acids. In studies to determine the free amino acid profile in Nicotiana and soybean, it was observed that proline was the most abundant free amino acid. A literature review, indicated that this was a common phenomenon among insect pollinated plants. To determine whether the insect pollinator, honeybees, showed any preference for nectars rich in proline, an insect preference test was conducted. Honeybees did prefer synthetic nectars rich in the amino acid proline. Because proline has favorable energetics of metabolism, particularly during the initial phases of insect flight, we hypothesize that some plants have evolved to express high levels of proline in their nectar. This observation of pollinator preference for proline in nectar could be used to select plants (geneotypes), that are more attractive (offer better rewards), for insects to produce hybrid seed more efficiently. Pollination ecologists, evolutionists, and plant breeders will benefit from these studies. Scientists doing basic insect-plant research to breeders who want to increase seed-set will find these results useful.
Technical Abstract: We have examined the amino acid content of the nectar of ornamental tobacco and found that it is extremely rich (2 mM) in the free amino acid proline. Insect pollinators are reported to be able to taste proline. To determine whether honeybees showed any preference for nectars rich in proline, we established an insect preference test and found that honeybees did prefer synthetic 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. Because proline has favorable energetics of metabolism, particularly during the initial phases of insect flight, we hypothesize that some plants have evolved to express high levels of proline in their nectar thereby facilitating insect visitation by providing an energetically favorable nutrient to the insect visitors.