The Role of Starch in Nectar Production

Funded by the National Science Foundation

Many plants produce a rich floral nectar whose function is to entice animals to the flower. This attraction of visitors (bats, birds, insects) benefits not only the animals that drink the nectar, but it also benefits plants by increasing cross-pollination between plants and also increasing seed production. The nectar in flowers is produced by a specialized floral organ termed the nectary gland. The nectaries of ornamental tobacco are unique, not only in their structural simplicity, but also in their extremely large size. Further, ornamental tobacco produces hundreds of flowers per plant, so isolation of nectary tissues for cloning and biochemical analyses is greatly facilitated. Thus, ornamental tobacco is the ideal system to study nectary function and development. The understanding the mechanisms of nectary function in this simple system will facilitate interpretation of nectary function in more complex and less tractable species that are more difficult to study because of the small size and multiplicity of nectary types. Therefore, these studies will facilitate a global understanding of angiosperm genomics more readily than a strict reliance on the Arabidopsis genome.

Because nectar sugars are one of the most important factors that attract insect pollinators to flowers and because increased pollinator visitation results in greater success of pollination and higher yields, understanding the metabolic source of sugars in nectar will provide new targets for genetic manipulation that will significantly improve plant yield for many species of fruits, nuts, berries, vegetables, forages, fiber and oilseed crops as well as flowers. These products contribute directly to the quality of both human and animal diets as well as to human psychological well-being. Such increases in food, fiber, and floral production will be required in the coming decades as the world's population approaches 10 billion.

While it is clear that nectar is particularly rich in sugar, the metabolic source of the sugars in nectar remains unknown. This proposal hypothesizes that starch is deposited in the nectary gland during development and is degraded at floral maturity to produce sugars that flow into nectar. This proposal seeks first to identify specific structural changes in nectary deposited starch that occur during the transition from developing to secreting nectaries. This will provide insight into the biochemical processes required for nectary starch formation and usage. Next, this proposal seeks to quantify the flow of transported photosynthetic sugars both into starch and directly into nectar. This will determine whether starch is the sole source of nectar sugars. Third, this proposal will identify starch metabolic genes that are expressed at various stages in the nectary gland and will finally identify proteins that are actively involved in starch biosynthesis and catabolism in nectaries. Identification of these proteins and the genes that encode them will provide potential targets for manipulation to improve pollinator attraction. Together, the results of the proposed research will permit us to fully evaluate the role that starch plays in nectar secretion, and provide insight into the mechanisms that underlie starch deposition and catabolism during nectary development.