Ever since evolutionary propositions about natural selection, development of species and survival of the fittest were introduced 150 years ago, it has become clear that biological theories and perspectives are highly shaped by socio-cultural trends and political streams. Darwin and his followers were children of their time, and so are biologists today.
Terms derived from modern information technology have been introduced to biology and also into plant sciences. We talk about signaling and communication, connectivity and functional networks, information storage, memory and even learning. In the forthcoming series The Secret Life of Plants, I would like to draw your attention to phenomena and processes which we understand incompletely, and maybe seldom reflect on at all, when dealing with plants in the kitchen, living room, garden, or on a walk in the woods.
If you are addicted to nature, you probably keep a small collection of house plants and greens at home. Regular watering is most likely one of your weekly routines to keep them alive. But sometimes, when getting back from an extended weekend trip or in hot periods, you recognize that your favourite tomato plant on the balcony is stunted and showing withering leaves. You immediately grab the water can and pour the precious liquid into the pots. Within seconds the roots start to take up the water, and after a couple of minutes the plant changes shape through rehydration of branches and leaves – and starts to transpire again.
But did you ever think about how the essential process of transpiration is being controlled? The stomata of the leaf can be considered a type of valve responsible for the exchange of water and gases (carbon dioxide and oxygen) with the environment. Their opening and closing, based on the surrounding guard cells, is generally regulated by light stimuli, CO2, humidity and water status, but chemical signals are necessary for the immediate transduction of environmental signals to control the water pressure inside the guard cells (see Stomatal movements and long-distance signaling in plants).
One important group of chemicals involved are the so-called plant hormones. In the narrow sense of the definition of a hormone, they act at very low concentration in cells or tissues other than the production site, and function as chemical messengers. The hormone abscisic acid (ABA) plays a major role in stomatal closure, while cytokinins and indole acetic acid (IAA) act as antagonists, leading to accumulation of solutes (e.g. potassium ions K+) in cytoplasm, increase of water inflow and cell turgor pressure, and stomatal opening. Guard cells are not just locally controlled due to hormonal concentration changes, but also virtually via long distance root-to-shoot signaling.
This process of autosignaling is also known from plant defenses: when a single leaf is damaged by an insect, internal signals might induce defense responses in the entire plant (systemic acquired resistance, SAR). These responses are mediated via signaling pathways (salicylic acid, jasmonic acid and ethylyene), which further trigger the activation of certain genes, finally leading to the production of defense-related chemicals e.g. toxic or repellent compounds, or even insect attractants.
For example, the parasitoid wasp species Cotesia rubecula lays eggs in the larvae of the Small White, a herbivore feeding on the leaves of cabbage and other Brassica plants (See Parasitoid responses to volatile plant attractants). Upon tissue rupture, the damaged plant releases a blend of volatile compounds which are perceived by receptors in the antennae of the parasitoids, and thus attracts them to the infested host. We find similar defence strategies in other plant species, such as broad bean plants infested with pea aphids. Chemical plant signals indicate good egg laying sites for hover flies, and thus the safe development of larval offspring based on aphid predation. The best-known example is probably the lady beetle whose larvae and adults feed on a variety of pest insects and mites. This is another case in which plant chemical cues are involved to guide host plant localization and selection.
Plants are not only capable of signaling and responding in planta, but even communicate with organisms from other kingdoms. So should we take for granted that they also “talk” to members of their own species or other plant varieties, presumed that these can receive and “hear” the signals?
Though plants do not grow ears like animals, it has been shown that they detect and react to external accoustic emissions – but research on the controversial property to produce structured, acoustic sounds as found in corn roots (Plant Bioacoustics), is still in its infancy. Much focus in recent decades has been put on biotic and abiotic plant stresses, especially in crop plants. Factors such as insect pests and fungi, drought and frost might interfere severely with plant productivity and thus be of significance for food production. Moreover, the composition, level and timing of volatile signals released from plants under stress might be modulated.
If these messaging chemicals are received by neighbouring plants, they potentially induce signaling cascades within the plant, further leading to functional changes and adaptive immunity. This process is also called priming and shows great potential for applications in agriculture to improve plant fitness. But plant-plant interactions can even be more complex: instead of using airborne signals, plants can also communicate via the root interface by utilizing underground mycorrhizal networks. Mycorrhizal fungi commonly live on roots in a symbiotic relationsship, providing the plant with minerals, themselves benefiting from the host’s carbohydrates. The direct physical connection established by the mycorrhizal network allows for the fast exchange of signals between plants and warning of neighbours to be prepared for attackers.
Future studies will surely deepen our knowledge on plant communication and interactions in natural ecosystems. Meanwhile you might stick to your daily routines, water your tomatoes frequently, talk pleasantly with them, and grow them together in one pot for even better communication.