Can Floral Scent Compounds Also Contribute to Plant Defense?

Benzyl alcohol, a component of floral scent, is not solely a substance for producing an odor. Plants also use it as a building material for other aroma compounds and defense-related substances. Which enzymes do plants use to make it?

In 2026, Ji Hee Lee and colleagues identified two enzymes fulfilling this role in petunia. They aimed to find the enzymes responsible for an unresolved step in benzyl alcohol biosynthesis and clarify the pathways producing floral scent and the plant hormone salicylic acid, as well as where the reactions occur inside cells.

From a scent precursor to a plant defense signal

Benzyl alcohol has a mild, sweet odor and occurs in flowers, leaves, and other parts of many plants. It also provides material for making benzyl benzoate, another compound. Benzyl benzoate lies along a pathway that leads, through further reactions, to salicylic acid.

Salicylic acid is a plant hormone with an important role in responses to pathogens. Tracing the synthesis of a scent-related compound therefore connects to how plants produce defense signals. This concerns plant metabolism, not health benefits for people smelling flowers.

Gene similarity alone did not reveal the answer

The researchers first searched for genes resembling known enzymes that might produce benzyl alcohol. Yet when apparently promising candidates were tested, they had little activity for the desired reaction. Sequence similarity alone does not determine a protein’s actual role within a plant.

The researchers therefore progressively separated proteins extracted from petals, tracing which fractions retained the ability to convert benzaldehyde into benzyl alcohol. By comparing reaction activity with the abundance of proteins in those fractions, they identified two candidates.

When the two enzymes, named PhBR1 and PhBR2, were produced and purified, both converted benzaldehyde into benzyl alcohol using the cofactor NADPH. Comparisons included conditions without protein and with heat-inactivated protein.

The two enzymes differ at only three amino-acid positions. Nevertheless, they differed in their preferred substrates. Exchanging one amino acid between them substantially changed substrate selectivity, showing how a small structural difference can alter enzyme properties.

Reducing the enzymes also reduced compounds emitted by flowers

To test whether the enzymes function inside flowers as well as in a test tube, the researchers produced petunias in which both genes were simultaneously suppressed. They selected three independent lines with strong suppression and compared them with ordinary plants and a control accounting for the transformation procedure.

Volatiles collected from flowers two days after opening, between 18:00 and 21:00, showed about 50–92% lower benzyl alcohol emissions depending on the line. Internal petal pools fell by about 35–87%. The main emission comparison used 15 biological replicates for ordinary plants and 10 for the control and each suppressed line.

Emissions and internal pools of the downstream product benzyl benzoate also decreased. Emissions of methyl salicylate, made from salicylic acid, decreased as well. Together, the purified-enzyme reactions and gene-suppression results in plants supported the involvement of these enzymes in biosynthesis.

Total emissions across all volatile compounds, however, did not change significantly, because petunia scent includes many other constituents. A reduction in specific compounds cannot be interpreted as an equally large reduction in the flower’s overall scent. Nor did the study ask people to assess intensity or preference.

Pathogen-induced salicylic acid also decreased

The researchers next applied plant-infecting bacteria to petunia stems and leaves and examined the increase in salicylic acid. Stem levels were high 21 hours after treatment, so the three suppressed lines were compared with controls at that time.

Salicylic-acid levels in infected stems were about 56–84% lower in the lines with both genes suppressed than in controls. This comparison used three biological replicates per condition.

The results support the idea that the identified enzymes are needed not only for floral aroma production but also for salicylic-acid synthesis through benzyl benzoate. The authors describe the discovery as identifying the last previously unknown enzyme in this pathway.

However, these were measurements of salicylic-acid abundance, not experiments establishing how much more susceptible plants became to disease or their survival rates. Plants also have other salicylic-acid pathways; not every plant relies exclusively on the same one.

Gene expression and enzyme activity do not move in step

Unexpectedly, RNA transcribed from the two genes was abundant in buds before flower opening and fell sharply afterward. Enzyme activity nevertheless persisted after opening rather than immediately disappearing.

The authors suggest that enzymes made during the bud stage remain relatively stable and continue working after flowers open. A decrease in RNA does not mean that every reaction stops at that moment.

Enzyme activity also showed no clear cyclical change between day and night. The authors therefore argue that the rhythm of increased nighttime benzyl alcohol production is more likely to depend on benzaldehyde supply than on repeated increases and decreases in enzyme abundance. This also shows how searching only for candidates whose expression closely tracks scent-compound levels can miss important enzymes.

The same reaction, but different cellular locations

The enzymes also differed in where they were located inside cells. Cells contain small membrane-bound compartments, including peroxisomes. The part of the cell’s interior outside these compartments is called the cytosol.

The researchers attached fluorescent markers to the enzymes and observed them in experimental tobacco leaves. Enzymes from petunia and the tobacco species Nicotiana tabacum localized to the cytosol, whereas those from rice and poplar localized to peroxisomes.

Sequence-based location predictions for 241 similar proteins from many plants also divided them roughly in half. Not all 241 were observed experimentally in cells. The broad computational predictions and experiments on selected enzymes need to be distinguished.

Do substrates travel between cellular compartments?

In petunia, the reactions producing the substrate benzaldehyde and the subsequent product benzyl benzoate are thought to occur in peroxisomes. Because the intervening benzyl-alcohol-producing enzyme is in the cytosol, the findings suggest that material exits the compartment, changes into another compound, and returns.

In rice and poplar, by contrast, these reactions are thought to proceed in the same compartment. Why the arrangement differs among plants, and whether transport proteins help compounds move between locations, remain unknown. The authors call for further investigation of the relationship between compound production and cellular organization.

The research received support from a joint framework involving the US National Science Foundation and the French National Research Agency, the US Department of Agriculture, the Natural Sciences and Engineering Research Council of Canada, the US National Institutes of Health, and other sources. The authors declared no competing interests.

Summary

The two enzymes identified in petunia produce benzyl alcohol and connect to the synthesis of downstream aroma compounds and salicylic acid involved in pathogen responses. Research on scent thus filled a gap in the pathway producing a plant defense signal.

Equally important, reactions making the same compound can occupy different cellular locations in different plants. Looking beyond enzyme identity to when enzymes act and where they exchange substrates gives a more concrete understanding of how plants allocate compounds to scent and defense.

References

Lee, J. H., Huang, X.-Q., Bergman, M. E., et al. (2026). Benzyl alcohol biosynthesis and its subcellular compartmentalization enable scent formation and salicylic acid production. Nature Communications, 17, 8584. DOI: 10.1038/s41467-026-75494-8. Full text. © The Author(s) 2026. The original article is licensed under CC BY-NC-ND 4.0.