Xylan-degrading enzymes in male and female flower nectar of Cucurbita pepo

† Background and Aims Nectar is a very complex mixture of substances. Some components (sugars and aminoacids) are considered primary alimentary rewards for animals and have been investigated and characterized innumerous species for many years. In contrast, nectar proteins have been the subject of few studies and little isknown of their function. Only very recently have detailed studies and characterization of nectar proteins beenundertaken, and then for only a very few species. This current work represents a first step in the identificationof a protein profile for the floral nectar of Cucurbita pepo. In this regard, the species studied is of particular interestin that it is monoecious with unisexual flowers and, consequently, it is possible that nectar proteins derivedfrom male and female flowers may differ.† Methods Manually excised spots from two-dimensional (2-D) electrophoresis were subjected to in-gel proteindigestion. The resulting peptides were sequenced using nanoscale LC-ESI/MS-MS (liquid chromatography-electrospray ionization/tandem mass spectrometry). An MS/MS ions search was carried out in Swiss-Prot andNCBInr databases using MASCOT software.† Key Results Two-dimensional electrophoresis revealed a total of 24 spots and a different protein profile for maleand female flower nectar. Four main proteins recognized by 2-D electrophoresis most closely resemble b-D-xylosidasesfrom Arabidopsis thaliana and have some homology to a b-D-xylosidase from Medicago varia. They werepresent in similar quantities in male and female flowers and had the same molecular weight, but with slightlydifferent isoelectric points.†Conclusions A putative function for xylosidases in floral nectar of C. pepo is proposed, namely that they may beinvolved in degrading the oligosaccharides released by the nectary cell walls in response to hydrolytic enzymesproduced by invading micro-organisms. Several types of oligosaccharides have been reported to increase thepathogenic potential of micro-organisms. Thus, it is possible that such a mechanism may reduce the virulenceof pathogens present in nectar.