New research reveals that plants have the ability to detect their neighbours' growth rates through aromatic cues called volatile organic compounds (VOCs) and subsequently adjust how much energy they invest into their own growth or defence strategies through responsive gene expression.
VOCs are carbon-based chemicals that evaporate easily into the air and are commonly produced by plants to communicate with herbivores, pollinators and even other plants. They are also responsible for the wide variety of fragrances that plants generate, and are often used in the manufacturing of perfumes, cosmetics, food and cleaning products.
Up until now, most of the research on plant-produced VOCs has focused on damaged plants releasing alarm-like chemical signals that warn neighbors to activate their anti-herbivore defenses. This new study published in the Journal of Experimental Botany describes a previously unrecognised role for healthy plant VOCs in competitive growing environments.
"Healthy non-damaged plants are constantly releasing their own chemical 'fingerprint' into the air, and their neighbors actively read these signals to adjust not only their defenses, but their entire growth strategy," says author Dr Velemir Ninkovic from the Swedish University of Agricultural Sciences. "This is like a continuous conversation between neighbors, and the finding that these background VOCs can reshape growth and gene activity opens up a new dimension in how we understand plant communication."
To better understand the role of these VOCs from healthy plants, Dr Ninkovic and his team conducted two laboratory experiments using three distinct cultivars of barley (Hordeum vulgare) that grow at different rates (slow "Fairytale", intermediate "Luhkas" and fast "Salome") and express different VOC profiles. Barley is one of the world's most important cereal crops, so insights into how it grows, defends itself, and interacts with neighboring plants have direct practical relevance for agriculture.
The slow-growing Fairytale cultivar and fast-growing Salome cultivar were exposed to VOCs from all three cultivars, and the effects on their growth and defence strategies were measured by analysing the physical properties of the plants and changes in gene expression after 25 days.
The researchers found that exposure to different VOC profiles triggered shifts in total plant biomass, whereas VOCs from plants with similar growth rates had negligible effects.
"VOC receiver plants adjusted their growth to match the competitive pressure signaled by their neighbor's scent: they grew more when exposed to a fast-growing neighbor and less when exposed to a slow-growing one," says Dr Ninkovic. "This effect was seen consistently across all parts of the plant leaves, stems, and roots rather than the plant simply reshuffling resources between its parts."
Genetic analysis revealed that these shifts in biomass were linked to changes in growth and defence related pathways. Shifts towards the slow-growing Fairytale VOC profile were associated with an up-regulation in stress-response genes that help to protect against herbivores and a down-regulation in cellular transport and DNA replication genes, whereas the opposite pattern was true for shifts towards the fast-growing Salome VOC profile.
This study found that the VOCs most strongly associated with these growth signals include benzyl nitrile, linalool and octanal, which are responsible for a wide variety of iconic floral fragrances such as lavender and citrus, as well as more metallic, and earthy scents.
"Plants release a rich blend of volatile compounds as a normal part of their biology, and it would make evolutionary sense for neighbors to have developed the ability to pick up on each other's chemical signals over millions of years of co-existence," says Dr Ninkovic. "We believe this type of constitutive VOC interaction may likely be widespread across the plant kingdom, though the specific compounds involved and the strength of the response will probably vary greatly between species."
------
Additional authors: André Åbonde, Merlin Rensing, Jannicke Gallinger, Vasti Thamara Juárez-González, Iris Dahlin, Dimitrije Markovic and German Martinez
The Journal of Experimental Botany is a partially open access journal published on behalf of the Society for Experimental Biology by Oxford University Press. The aim of the Journal of Experimental Botany is to publish papers that advance our understanding of plant biology.
