Poster: Reproductive Development
Abs #
488: The nectar redox cycle: A novel floral defense mechanism
The nectar of ornamental tobacco is a complex biological fluid that is composed of sugars, amino acids, vitamins, ions, and proteins (nectarins). We have cloned cDNAs encoding four of the five nectarins present in tobacco nectar. Characterization of these cDNAs and isolation of the nectarin proteins has enabled us to define the functions of these proteins. The major nectar protein (Nectarin 1) is a novel manganese-containing superoxide dismutase that generates high levels of hydrogen peroxide. The hydrogen peroxide readily reaches levels (up to 4 mM) that are antimicrobial. These levels of hydrogen peroxide also generate significant levels of hydroxyl free radicles. Ascorbate, present in nectar at 0.9 mM, can reduce the free radicals generating monodehydroascorbate as a result. Two enzyme systems function to regenerate ascorbate. Nectarin 3 is a dioscroin-like polyfunctional protein that has carbonic anhydrase activity as well as monodehydroascorbate reductase activity. Nectarin 2 is a proteolytic product of Nectarin 3. The carbonic anhydrase activity functions to maintain a physiological pH of nectar and the monodehydroascorbate reductase activity functions in the regneration of ascorbate. Nectarin 5 is a flavin-containing berberine bridge-like enzyme that has both dehydroascorbate reductase and glucose oxidase activities. Together, these enzymes function to maintain a redox cycle in nectar that tolerates high levels of hydrogen peroxide. Expression of Nectarins 1 and 5 is limited exclusively to the nectary gland and is intimately linked to floral maturity with expression betinning just prior to anthesis. Nectarin 3 is expressed during nectary development as well as in mature nectaries.
