Poplar Extrafloral Nectar Is Protected against Plant and Human Pathogenic Fungus

Plants secrete nectar to attract mutualistic animals, which predominantly function as pollinators, as in the case of floral nectar, or defenders against herbivores, as in the case of extrafloral nectar (Nicolson et al., 2007). Because nectars usually represent aqueous solutions containing sugars and other nutrient metabolites (Baker and Baker, 1983), they are susceptible to infestation by microbial organisms, which can use the nectar-secreting tissues as entry sites to infect the plant. Nectar-secreting tissues thus require an efficient shield against pathogen infections. To date, our knowledge about the way that plants protect their nectar from microorganisms is rather limited. Several reports have focused on ‘defensive chemicals’, such as alkaloids and phenols, or a defense system based on proteins named nectarins (cf. Escalante—Perez and Heil, 2012). In floral nectar of ornamental tobacco, nectarins have been associated with a so-called ‘nectar redox cycle’. In this system, the production of high concentrations of reactive oxygen species (ROS) provides for microbe-free nectar. In addition to the nectarins, hydrolytic enzymes such as ribonucleases, peroxidases, chitinases, glucanases, and other pathogenesis-related (PR) proteins were discovered in both floral and extrafloral nectars (for review, see Escalante—Perez and Heil, 2012). To study the blend of antimicrobial proteins active in poplar extrafloral nectaries, we investigated the secretome and transcriptome of this organ. Thereby, we discovered a set of defense proteins that proved to be effective against a plant and human pathogenic fungus and thus could serve pharmaceutical purposes.