Abstract
Nectars are rich in primary metabolites and attract mutualistic animals, which serve as pollinators or as an indirect defense against herbivores. Their chemical composition makes nectars prone to microbial infestation. As protective strategy, floral nectar of ornamental tobacco (Nicotiana langsdorffii x Nicotiana sanderae) contains "nectarins," proteins producing reactive oxygen species such as hydrogen peroxide. By contrast, pathogenesis-related (PR) proteins were detected in Acacia extrafloral nectar (EFN), which is secreted in the context of defensive ant-plant mutualisms. We investigated whether these PR proteins protect EFN from phytopathogens. Five sympatric species (Acacia cornigera, A. hindsii, A. collinsii, A. farnesiana, and Prosopis juliflora) were compared that differ in their ant-plant mutualism. EFN of myrmecophytes, which are obligate ant-plants that secrete EFN constitutively to nourish specialized ant inhabitants, significantly inhibited the growth of four out of six tested phytopathogenic microorganisms. By contrast, EFN of nonmyrmecophytes, which is secreted only transiently in response to herbivory, did not exhibit a detectable inhibitory activity. Combining two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis with nanoflow liquid chromatography-tandem mass spectrometry analysis confirmed that PR proteins represented over 90% of all proteins in myrmecophyte EFN. The inhibition of microbial growth was exerted by the protein fraction, but not the small metabolites of this EFN, and disappeared when nectar was heated. In-gel assays demonstrated the activity of acidic and basic chitinases in all EFNs, whereas glucanases were detected only in EFN of myrmecophytes. Our results demonstrate that PR proteins causally underlie the protection of Acacia EFN from microorganisms and that acidic and basic glucanases likely represent the most important prerequisite in this defensive function.
Highlights
Plants secrete nectar to attract mutualistic animals, which mainly function as pollinators in the case of floral nectar or as defenders against herbivores in the case of extrafloral nectar (EFN) (Simpson and Neff, 1981; Heil, 2008; González-Teuber and Heil, 2009a). 80 Because nectars usually represent aqueous solutions of mono- and disaccharides together with amino acids, they are prone to infestation by microbial organisms
Pathogenesis-related (PR) proteins were detected in Acacia extrafloral nectar (EFN, which is secreted in the context of defensive ant-plant mutualisms)
The results demonstrate that the antimicrobial protection of Acacia EFN is caused by the fraction of enzymatically active PR-proteins and independent of small, soluble molecules: an observation that represents a new strategy by which plants can protect nectar from infestation by potentially deleterious microorganisms. 150
Summary
Plants secrete nectar to attract mutualistic animals, which mainly function as pollinators in the case of floral nectar or as defenders against herbivores in the case of extrafloral nectar (EFN) (Simpson and Neff, 1981; Heil, 2008; González-Teuber and Heil, 2009a). 80 Because nectars usually represent aqueous solutions of mono- and disaccharides together with amino acids, they are prone to infestation by microbial organisms. The first reports on nectar proteins date back to the sixties and seventies of last century (Baker and Baker, 1975; Lüttge, 1961), most studies that considered the defensive function of nectar focused on secondary compounds such as alkaloids and phenols. These metabolites commonly protect nectar from consumption by nectar robbers (animals that feed on nectar without providing a mutualistic service to the plant, see Stephenson, 1981; Johnson et al, 2006) or limit the duration of pollinator visits (Kessler et al, 2008). Nectarins serve the protection 100 from microbial infestation through a biochemical pathway called the Nectar Redox
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