Minisymposium 22: Development

Abs # 36001: The nectar redox cycle: A novel floral defense mechanism

Presenter:	Thornburg, Robert W., thorn@iastate.edu
Authors	Thornburg, Robert W. (A)   Carter, Clay J (A) (B)
Affiliations:	(A): Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011 (B): 2130 Batchelor Hall, University of California, Riverside, CA 92521
Web Site:	http://www.bb.iastate.edu/necgex/

The development of floral nectaries was one of the defining processes in the evolution of angiosperms. Although highly variable in structure, these remarkable floral organs are often associated with the gynoecium at the base of the flower. At floral maturity, they secrete a metabolically-rich, complex, biological fluid termed nectar that entices insect, avian, and mammalian pollinators to visit the flower. These visitors inadvertently transfer pollen between flowers thereby improving plant fecundity by increasing both out-crossing and seed set. The nectar of ornamental tobacco is composed of sugars, amino acids, vitamins, ions, and five proteins that we have termed nectarins. Although this rich milieu is promiscuously visited by non-sterile insects, nectar infections appear to be rare. To understand this disparity, we have cloned cDNAs encoding four of the five tobacco nectarins. Characterization of these cDNAs and isolation of the nectarin proteins has enabled us to define their functions. The major nectar protein, Nectarin 1, is a germin-like protein (GLP) that has superoxide dismutase activity. It functions to generate high levels of hydrogen peroxide. The hydrogen peroxide readily reaches levels (up to 4 mM) that are antimicrobial. These levels of hydrogen peroxide also create problems by generating significant levels of hydroxyl free radicals. Ascorbate, present in nectar at 0.9 mM, can reduce the free radicals generating monodehydroascorbate as a result. Nectar contains two enzyme systems that function to regenerate ascorbate. Nectarin 3 is a dioscorin-like polyfunctional protein that has carbonic anhydrase activity as well as monodehydroascorbate reductase activity. The carbonic anhydrase activity functions to maintain the pH of nectar at physiological levels. Nectarin 2 is a proteolytic product of Nectarin 3. Nectarin 5 is a flavin-containing berberine bridge-like enzyme with both dehydroascorbate reductase and glucose oxidase activities. Together, these enzymes function to maintain a redox cycle in nectar that protects the gynoecium by maintaining nectar in an axenic state. Expression of Nectarins 1 and 5 is limited exclusively to the nectary gland and is intimately linked to floral maturity. Expression begins just prior to anthesis and continues while the flower is receptive to pollination. Nectarin 3 is expressed throughout nectary development.