Plant Signaling

Plants are often perceived as passive organisms — rooted in place, silent, unresponsive. This perception is wrong. Plants are active signalers and sign-interpreters, engaged in continuous communicative exchange with their environment and with other organisms. They produce and respond to chemical, electrical, and mechanical signals, coordinating their own development, defending against herbivores, attracting pollinators, and participating in complex ecological networks. From the perspective of biosemiotics, plants are semiotic organisms — beings whose lives are organized around the interpretation of signs.

Within the individual plant, signaling coordinates the activity of spatially dispersed organs. Hormones such as auxin, cytokinin, ethylene, and abscisic acid travel through the phloem and across cell membranes, carrying information about light conditions, water status, pathogen attack, and developmental stage. Electrical signals, analogous in some respects to animal nerve impulses, propagate rapidly through the plant body in response to wounding or environmental stress. These internal signals enable the plant to behave as a coordinated whole despite its modular, decentralized architecture. A wound on one leaf can trigger defensive chemical production in distant, undamaged leaves — a systemic response mediated by both chemical and electrical channels.

Between plants, signaling operates through volatile organic compounds (VOCs) released into the air and chemical exudates secreted into the soil. When a plant is attacked by herbivorous insects, it may release VOCs that serve multiple functions simultaneously: attracting predators of the herbivore, priming neighboring plants to upregulate their own defenses, and even signaling to undamaged parts of the same plant. These airborne signals create a chemical Umwelt — an invisible semiotic environment in which plants are both producers and interpreters of meaning-bearing signs. The plant’s world is not silent; it is saturated with chemical information.

Perhaps the most striking example of plant communication is the mycorrhizal network — sometimes called the “wood wide web.” Most terrestrial plants form symbiotic associations with mycorrhizal fungi, whose threadlike hyphae connect the root systems of multiple plants, often across species. Through these fungal networks, plants exchange not only nutrients (carbon, phosphorus, nitrogen) but also signaling molecules. A tree under attack by pathogens can transmit warning signals through the mycorrhizal network to neighboring trees, which then preemptively activate their defenses. Mother trees preferentially channel resources to their offspring through these networks. The mycorrhizal network is a relational infrastructure that dissolves the boundary between individual organisms, revealing the forest not as a collection of independent trees but as an interconnected, communicating system.

Plant signaling challenges the assumption that communication and semiosis require a nervous system or consciousness. Plants interpret environmental signs — they distinguish self from non-self at the root level, discriminate between different herbivore species based on salivary chemistry, and adjust their phenotype in response to the ratio of red to far-red light (which indicates the proximity of competing plants). These are not mere mechanical reactions but context-dependent interpretive responses — exactly the kind of sign processing that biosemiotics seeks to understand. The study of plant signaling thus contributes to a broader rethinking of what it means to be a communicating, interpreting organism, extending the domain of semiosis well beyond the animal kingdom.

Related

• Biosemiotics — the study of sign processes in living systems
• Umwelt — the organism’s species-specific sign-world
• Symbiosis — the relational entanglements that mycorrhizal networks exemplify
• Phloem — the internal transport system carrying signaling molecules
• Niche Construction — how signaling organisms shape their shared environment