Volatile Organic Compounds, the language of plants

Not only humans have the capacity for communication. Queen bees emit pheromones in order to control workers, and abortions can be provoked in pregnant female mice just by their smelling of odors produced by a male distinct to that which fertilized them. Plants also communicate with chemical signals which they release into their surroundings, helping them to interact with other plants and living beings, including microorganisms. 

Plant communication is an undisputable fact. Plants emit volatile organic compounds (VOCs) which they use to attract pollinators, protect against certain environmental stressors, and repel herbivores. CREAF researchers study the close relationship between these compounds and the microorganisms populating the planet. "Knowing about the microbiota living on plants and their interaction with VOCs can improve understanding of the contribution of these emissions to atmospheric composition, and even their possible effects on climate," says Gerard Farré-Armengol, first author of a recent study.

What are volatile organic compounds?

VOCs are chemical substances produced and emitted by plants and other organisms in gaseous form. Composed of carbon, they help plants to attract pollinators, defend against herbivore insects and parasites, and serve as signals to neighbor plants.

The most common type of VOCs is terpenes, oils which can be found in resin and in aromatic compounds, and which are responsible for the characteristic smell of a flower.

VOCs are involved in plant-microorganism symbioses

Plants don't only use VOCs for their own benefit; these substances also help plants interact with the bacteria and fungi covering them.

The above-ground parts of plants (known in microbiology as the phyllosphere in reference to the habitat provided to microorganisms) are mainly colonized by bacteria and to a lesser extent fungi, similar to what also occurs with roots. "There can be up to 10 million bacteria per square cmof leaf surface. If you consider the quantity of bacteria on each plant, and then the huge amount of plants surfaces on the planet, it is possible to imagine the importance of these microorganisms," says Dr. Farré-Armengol.

VOCs emitted by the plant determine what microbiota will live in the phyllosphere: these are microorganisms which able to feed on the compounds and are resistant to certain VOCs with antimicrobial properties. In fact, it is possible that on each kind of plant tissue there may be different kinds of microorganisms, just as is the case with humans. Bacteria living on our skin are different from those in the intestine, similar to the way that microbiota on flowers and leaves are also expected to be different.

Microbiota, in turn, can produce their own VOCs, which mix with those of the plant. For example, flower and fruit microorganisms affect the aroma of each. Also, the gaseous molecules released by fungi and bacteria can aid plant growth, resistance to different types of stress, and prevent attacks by microorganism pathogens.

Regulation of VOCs in agriculture

Considering the importance of microbiota living on the above-ground parts of plants, what happens if the land is fumigated, destroying this microenvironment? "If we apply pesticides to crops to eliminate microbial infections, we are not only eliminating the infectious microorganisms. We are also killing the microorganisms that are naturally on the plant and which modify or participate in important characteristics such as flower odor. With this, the response of pollinators may be different and could end up negatively affecting crop production," says CREAF researcher Gerard Farré-Armengol.

The future of agriculture will be to find pesticides that eliminate pathogenic fungi and bacteria but not the habitual members of the phyllosphere. This way, volatile organic compounds will remain unaltered and pollination and herbivory can carry on as normal, resulting in improved crop production.

ARTICLE

Farré-Armengol G., Filella I., Llusia J. and Peñuelas, J. Bidirectional Interaction between Phyllospheric Microbiotas and Plant Volatile Emissions. (2016) Trends in Plant Science. DOI: 10.1016/j.tplants.2016.06.005