Plants provide a desirable source of nutrients in their nectar and encourage multiple visitors to their flowers. The pollinators they attract could carry unwanted hitchhikers in the form of microbes, yet nectar is not regularly infected. Aura Parra and colleagues in Brazil, Canada and the USA, have found plants keep their flowers infection-free by producing antimicrobial compounds. Their research, published in Plant Science, has identified six antimicrobial peptides in the nectar of ornamental tobacco.
Reproduction could be a hazardous process for a plant. A plant wants to bring in pollen from other plants on the bodies of pollinators, but it has no control over where those pollinators have been. They could have been anywhere picking up bacteria and fungi. When the pollinators arrive, they’ll look for nectar. This soup of sugars isn’t just attractive to insects. It’s also a source of nutrients for any travelling bacteria or fungi that could use the nectar to build up energy for a severe attack on the plant’s reproductive organs. It makes sense to look for a defence in the nectar, so Parra and colleagues investigated peptides found in nectar for antimicrobial properties.
The scientists used nectar harvested from Nicotiana sanderae and N. langsdorffii. The attraction of these flowers for botanists is that the flowers have large nectaries and produce a lot of nectar, making the samples easier to take. The team used micropipettes to extract the nectar and then analysed it to see what peptides were within the nectar.
There were plenty of peptides found within the nectar, 793 in total. Of these, around 20% could be antimicrobial peptides. This is a lot to examine, so the team chose six to synthesise and analyse. The scientists chose the peptides most amenable to analysis, so the selected compounds had a low molecular weight and other factors that made them more likely to identify antimicrobial compounds.
Analysis showed that the peptides did indeed have antimicrobial properties, but some peptides had different effects on different microbes. For example, only some of the peptides inhibited the growth of the bacteria Pseudomonas syringae and Xanthomonas perforans. In contrast, all the peptides inhibited the growth of fungi to some extent.
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Despite having a defensive purpose, not all peptides harm microbes. Parra and colleagues also note that some peptides interact with yeasts and bacteria to produce new aromas. The scent can prove attractive to pollinators. As a result, peptides can act both as a sexual attractant and a defence against unwanted infection.
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Parra, A.L.C., Freitas, C.D.T., Souza, P.F.N., von Aderkas, P., Borchers, C.H., Beattie, G.A., Silva, F.D.A. and Thornburg, R.W. (2022) “Ornamental tobacco floral nectar is a rich source of antimicrobial peptides,” Plant Science, 324(111427), https://doi.org/10.1016/j.plantsci.2022.111427