| A small set of proteins, termed Nectarins, are secreted into the nectar of ornamental tobacco (Nicotiana langsdorffii x Nicotiana sanderae). The most highly expressed protein, Nectarin I (29 kD monomer), was originally identified as a germin-like protein (GLP) of unknown function. This remarkably stable protein was later shown to possess manganese dependent superoxide dismutase activity. Hydrogen peroxide, the product of SOD activity, was found to accumulate in nectar at levels up to 4 mM. The promoter (1.2 kb) of the Nectarin I gene was fused to the reporter gene chloramphenicol acetyl transferase (CAT) and shown to be expressed in mature nectary tissues only at times when nectar is actively being secreted.;Nectarin II (35 kD) was identified as a dioscorin-like protein and found to be a degradation product of Nectarin III (40 kD). These proteins were shown to posses monodehydroascorbate reductase and carbonic anhydrase activity. Nectarin V (∼65 kD) was identified as a berberine bridge enzyme-like protein and found to contain a covalently bound molecule of FAD. Both Nectarin III and Nectarin V shared very similar expression patterns with those of Nectarin I.;The hypothesis of this work is that nectar proteins are secreted into nectar to maintain an axenic state and to protect the developing gynoecium from microbial attack and oxidative damage. Specifically, the proposed functions of these nectar proteins include: the production of hydrogen peroxide for floral defense, maintenance of nectar pH, and maintenance of ascorbate levels in nectar. |
